i
Hydraulic Management of SDI Wastewater Dispersal  
in an Alabama Black Belt Soil  
 
by 
 
Jiajie He 
 
 
 
 
A dissertation submitted to the Graduate Faculty of 
Auburn University 
in partial fulfillment of the 
requirements for the Degree of 
Doctor of Philosophy 
 
Auburn, Alabama 
May 14, 2010 
 
 
 
 
Keywords: Blackland prairie, irrigation, nutrient,  
wastewater, soil, GIS, soil moisture 
 
 
Copyright 2009 by Jiajie He 
 
 
Approved by 
 
Mark Dougherty, Co-chair, Assistant Professor, Biosystems Engineering Department 
Clifford R. Lange, Co-chair, Associate Professor, Civil Engineering Department 
Joey N. Shaw, Professor, Soil and Agronomy 
C. Wesley Wood, Professor, Soil and Agronomy 
John P. Fulton, Associate Professor, Biosystems Engineering Department 
 
 
 
 
 
 
 ii 
Abstract 
 
 
Approximately 52% of the 14000 km2 Alabama Black Belt region is unsuitable 
for conventional onsite wastewater treatment systems because of low hydraulic 
permeability, high water table, or other restrictive soil properties. This research evaluates 
an experimental subsurface drip irrigation (SDI) wastewater disposal system designed to 
reduce environmental and health risk of surface ponding and deep percolation in native 
clay soils by dosing wastewater only when field moisture content is at or below field 
capacity. A soil moisture control system was linked to a manufacturer?s SDI wastewater 
dosing panel and successfully tested in the laboratory. Subsequent field testing of the 
system was conducted from September 2006 to June 2008 on an unreplicated 500 m2 
Houston clay soil site in west central Alabama using clean well water (September 2006 to 
June 2007, year one) and a synthetic wastewater (June 2007 to June 2008, year two). A 
seasonal cropping rotation of sorghum-sudangrass (Sorghum bicolor) and winter wheat 
(Triticum aestivum) with rye (Secale cereale) was planted at the site to maximize annual 
water and nutrient crop uptake and mitigate nutrients offsite transport. 
Observed hydraulic dosing rates in the drain field varied from a high of 1.18 cm 
day-1 during summer drought conditions to a low of 0.0 cm day-1 during wet winter 
months. Zero dosing in winter prevented surface ponding from applied wastewater but 
created requirement for at least a two-month waste storage reservoir. Water percolation is 
a necessary component of effluent treatment in an OWTS. However, estimated monthly
iii 
 
water balance indicated that water percolated below 45 cm depth accounted for over 30% 
of dosed water in the warm season of year one which was a 30-year historic drought 
season, but was negligible in year two during a normal rainfall year. Estimated water 
percolation was presumably the result of preferential flows stimulated by dry weather 
clay soil cracking. Over this two-year study, only 4 months out of 12 had an observed 
water balance in favor of soil adsorption. A minimum a two-month onsite wastewater 
storage requirement is estimated for the experimental system due to zero or low hydraulic 
disposal periods during typical wet winter months.  
Even though soil moisture controlled wastewater dosing may temporarily provide 
nutrient loads higher than crop uptake needs in year two, monitored crop uptake and soil 
nutrient profiles provided no direct evidence of drain field nitrogen and phosphorus 
accumulation or percolation below 100 cm depth. However, the field observation cannot 
exclude the possibility that nutrients may have passed through the top 100 cm soil and 
accumulated at deeper soils. Furthermore, it is anticipated that nutrients would be 
transported into deeper soils in year one had synthetic wastewater been used. . Although 
leaching of wastewater is not an environmental issue in the majority of the Alabama 
Black Belt region, improved monitoring of percolation loss to local groundwater is 
recommended for wastewater dispersal in this region. 
 
 
 
 
 iv
 
 
 
 
Acknowledgments 
 
 
I would like to thank my advisor Dr. Mark Dougherty for his support, guidance, 
inspiration, motivation, and friendship. Thanks for his continuous and unconditional help. 
I'd also like to thank my committee, Drs. Clifford Lange, Joey N. Shaw, Charles W. 
Wood, and John P. Fulton for their services on my Ph.D. committee, inspiring comments 
and discussion. It is my pleasure working with them.  
I'd especially like to thank Mr. James Holliman at Alabama Black Belt Research 
and Extension Center for his support on my research. This research could not been 
finished without his help. My sincerest appreciation goes out to Dr. Francisco Arriaga 
from USDA who contributed to my study with great efforts.  I spent the most important 
and precious period of my life in Auburn and I would like to thank everyone that 
accompanied me on this journey. 
v 
 
Table of Contents 
 
 
Abstract ............................................................................................................................... ii 
Acknowledgments.............................................................................................................. iv  
List of Tables .......................................................................................................................x  
List of Illustrations ........................................................................................................... xiii  
Chapter One Introduction ....................................................................................................1 
1.1 Research Background ........................................................................................1 
1.2 Research Objectives ...........................................................................................2 
1.3 Dissertation Organization ..................................................................................3 
Chapter Two Literature Review ...........................................................................................4 
2.1 Decentralized Wastewater Treatment ................................................................4 
2.1.1 Decentralized Wastewater Treatment in the United States .................4 
2.1.2 Environmental Challenge for Conventional Onsite Septic System ....7 
2.1.3 Subsurface Drip Irrigation Applications With Wastewater ..............10 
2. 2 Drain Field Water Movement .........................................................................11 
2. 2.1 Forms of Water Movement ..............................................................11 
2.2.2 Field Capacity Measurement ............................................................13 
2.2.3 Soil Water Movement Modeling ......................................................16 
2.3 Drain Field Nutrient Movement .......................................................................18
vi 
 
2.3.1 Nitrogen Movement ..........................................................................18 
2.3.2 Phosphorus Movement......................................................................18 
2.4 Integrated Cropping System ............................................................................22 
2.4.1 Water and Nutrient Management of Cropping Systems ...................22 
2.4.2 Soil Moisture Controlled Irrigation ..................................................26 
2.4.3 Water and Nutrient Balance Studies .................................................27 
Chapter Three Potential Environmental and Public Health Risk from Onsite Wastewater 
Treatment Systems in the Alabama Blackland Prairie Soil Area ......................................30 
3.1 Abstract ............................................................................................................30 
3.2 Introduction ......................................................................................................31 
3.3 Materials and Methods .....................................................................................35 
3.3.1 Study Area Delineation .....................................................................35 
3.3.2 NRCS Soil Limitation Rating System (NRCS-SLRS) for Septic Tank 
Absorption Fields. ......................................................................................37 
3.3.3 New OWTS Soil Suitability Rating System (OWTS-SSRS) ...........40 
3.3.4 Onsite Wastewater Treatment System Status in the Alabama Black 
Belt Area ....................................................................................................41 
3.3.5 Prioritizing Environmental and Health Risk Areas ..........................42 
3.4 Results and Discussion ....................................................................................43 
3.4.1 OWTS-SSRS and NRCS-SLRS Comparison ...................................43 
3.4.2 Current Status of Conventional Onsite Wastewater Treatment Systems
....................................................................................................................48 
 vii 
3.4.3 Prioritizing Environmental and Health Risk Areas ..........................51 
3.4.4 Application of Risk Assessment Mapping ........................................52 
3.5 Conclusion .......................................................................................................55 
Chapter Four Soil Moisture Controlled Wastewater Dispersal ? Year One: Laboratory and  
Field Testing ......................................................................................................................58 
4.1 Abstract ............................................................................................................58 
4.2 Introduction ......................................................................................................60 
4.3 Materials and Methods .....................................................................................63 
4.3.1 Laboratory Assembly and Testing Methods .....................................63 
4.3.2 Laboratory Testing Results ...............................................................69 
4.3.3 Field installation and Testing ............................................................73 
4.4 Field Testing Results........................................................................................79 
4.4.1 System Operation Results .................................................................79 
4.4.3 System Hydraulic Performance ........................................................83 
4.5 Conclusion .......................................................................................................86 
Chapter Five Soil Moisture Controlled Wastewater Dispersal ? Year Two: System 
Hydraulic and Nutrient Evaluation ....................................................................................88 
5.1 Abstract ............................................................................................................88 
5.2 Introduction ......................................................................................................90 
5.3 Materials and Methods .....................................................................................93 
5.3.1 Experimental Design and Operation .................................................93 
5.3.2 System Water Management Case Study - Years one and Two, 
September 6, 2006 to June 24, 2008. .......................................................101 
 viii
5.3.3 System Nutrient Management Case Study ? Year Two (June 14, 2007 
to June 24, 2008) ......................................................................................103 
5.4 Results ............................................................................................................105 
5.4.1 System Water Management ........................................................................105 
5.4.2 System Nutrient Management .....................................................................117 
5.4.3 System Application Benefits and Limitations ............................................129 
5.5 Conclusion .....................................................................................................130 
Chapter Six Conclusions and Suggestions for Future Research ......................................134 
6.1 Current OWTS Status in the Alabama Black Belt Area ................................134 
6.2 System Design ...............................................................................................135 
6.3 Field Evaluation of System Water and Nutrient Management ......................136 
6.4 System Benefits and Limitations ...................................................................137 
6.5 Suggestions for Future Research ...................................................................138 
References  .......................................................................................................................140 
Appendix I SDI Drip Emitter Hydraulic Uniformity Test ...............................................164 
Appendix II Site Characterization ...................................................................................166 
Appendix III Sizing SID Drain Field ...............................................................................170 
Appendix IV Field Capacity Measurement .....................................................................173 
Appendix V  Soil Phosphorus Isotherm Measurement ....................................................175 
Appendix VI Soil Moisture Sensor Validation ................................................................178 
Appendix VII Evapotranspiration (ET) Calculation by Penman-Monteith Method Defined  
                        by Food and Agricultural Organization ...................................................181 
Appendix VIII NRCS Web Soil Survey ..........................................................................184 
 ix
Appendix  IX   HYDRUS 2D Numerical Modeling ........................................................187 
 
 
x 
 
 
 
List of Tables 
Table 2.1 Defined decentralized system categorization. .......................................................5 
Table 2.2 Effluent survey values compile by Charles et al. (2005) ................................9 
Table 2.3 Common multi-dimensional vadose zone solute transport numerical models . 
 .......................................................................................................................17 
Table 2.4 Water budget components (Based on selected studies) ....................................29 
Table 2.5 Nutrient budget components (Based on selected studies) ................................29 
Table 3.1 NRCS Soil Limitation Rating System (NRCS-SLRS) rating criteria for 
septic tank absorption fields. .........................................................................37 
Table 3.2         New OWTS Suitability Soil Rating System (OWTS-SSRS) rating criteria. 
 .......................................................................................................................40 
Table 3.3 Major soil series in the Alabama Black Belt study area based on SSURGO  
2007 database. ...............................................................................................44 
Table 4.1 Properties of Houston clay and sand media used in laboratory testing. .......67 
Table 4.2 Soil description and texture data to 1.52 m depth at the experimental site, 
Black Belt Research and Extension Center, Marion Junction, Alabama. .....74 
Table 5.1         Soil physical and chemical properties at the experimental site ....................94 
 
Table 5.2 Chemical analyses of clean well water and injected fertilizer solution (mg L-
1) during year two synthetic wastewater experiment, June 14, 2007 to June 
24, 2008.........................................................................................................99 
 xi
Table 5.3 Crop rotation of treatments 1-IV during the second year synthetic 
wastewater experiment, June 14, 2007 to June 24, 2008. ...........................100 
Table 5.4 Monthly wastewater storage requirement based on observed monthly 
hydraulic disposal rates and Alabama Onsite Sewage Disposal Rules 
(ADPH, 2006). ............................................................................................116 
Table 5.5 Crop yield, nutrient application, and uptake of treatments I and III during 
year two, June 14, 2007 to June 24, 2008 ...................................................118 
Table 5.6 Year two partial field nutrient balance of the treatment I drain field for the 
synthetic wastewater application experiment from June 14, 2007 to June 24, 
2008.............................................................................................................126 
Table I.1 Recommended Classification of Manufacturer?s Coefficient of Variation. 165 
Table II.1 Detailed soil description to 1.52m depth at Field 21, Black Belt Research 
and Extension Center, Marion Junction, Alabama.  Side slope, 4%. ..........168 
Table II.2 Chemical properties and organic matter at Field 21, Houston soil field site, 
Black Belt Research and Extension Station, Marion Junction, Alabama ...168 
Table II.3 Chemical characteristics at Field 21, Houston soil field site, Black Belt 
Research and Extension Station, Marion Junction, Alabama. ....................169 
Table II.4 Particle size distribution at Field 21, Houston soil field site, Black Belt 
Research and Extension Station, Marion Junction, Alabama. ....................169 
Table III.1 Design data used to size drain field based on a household loading rate of 270 
gpd.  Adapted from manufacturer recommendations and State of Alabama 
Onsite Sewage Treatment and Disposal regulations (effective November 23, 
2006).   ........................................................................................................171 
 xii 
Table III.2 Flow and organic loading, State of Alabama Onsite Sewage Treatment and 
Disposal regulations (effective November 23, 2006)  ................................172 
Table IV.1 Soil volumetric moisture contents of the soil cores at 1/3 bar air pressure 173 
Table V.1 Regression results of the soil phosphorus adsorption isotherm. .................176 
Table VII.1 Lengths (days) of crop development stages and single crop coefficient (Kc) 
used in this study .........................................................................................182 
Table VII.2 ET calculation example for the experimental field during the month of 
Septmeber 2006 (FAO 56 method). ............................................................183 
Table IX.1       Particle size distribution of the soil horizons of the experimental site.......188 
Table IX.2       2HYDRUS 2D simulation scenarios ..........................................................190 
Table IX.3 Evaluation of the simulated (scenario 1) soil moisture levels compared to  
observed field data at 20 cm and 45 cm depths  from June 14, 2007 to June 
23, 2008.......................................................................................................195 
 
xiii 
 
List of Figures 
Figure 2.1 Forms of water movement in drain field ....................................................13 
Figure 2.2 Nitrogen movement and fate in drain field ................................................19 
Figure 3.1 The Black Belt study area boundary showing STATSGO Black Belt 
mapping units overlaid with CRA 135A.1 (Blackland Prairie) and 135A.2 
(Flatwoods/Blackland Prairie Margins) land resource area data. ..............39 
Figure 3.2 Soil rating results of the new OWTS Soil Suitability Rating System 
(OWTS-SSRS), Alabama Black Belt. (Dallas and Lowndes Counties 
excluded due to unavailability of SSURGO data) .....................................45 
Figure 3.3 Soil rating results of the NRCS Soil Limitation Rating System (NRCS-
SLRS) for septic tank absorption field, Alabama Black Belt. (Dallas and 
Lowndes Counties excluded due to unavailability of SSURGO data) ......46 
Figure 3.4 Average OWTS density and size in rural block groups, Alabama Black 
Belt. ............................................................................................................50 
Figure 3.5 Environmental risk analysis results in the Black Belt study area. Dallas 
and Lowndes Counties excluded due to unavailability of SSURGO data. 
Urban areas excluded due to general availability of public sewer service.52 
Figure 3.6 Environmental risk analysis of the greater Montgomery area, Alabama. 
Areas not rated are either classed as urban areas, or census block group 
extents are completely outside the study area. ...........................................53
xiv 
 
Figure 4.1 Connection diagram illustrating interface of soil moisture sensors and 
control to existing float-based irrigation dosing system. (Soil moisture 
sensors not shown) .....................................................................................65 
Figure 4.2 Schematic of the laboratory SDI system layout. ........................................66 
Figure 4.3 GP1 logical dosing sequence based on volumetric soil moisture, m3 m-3. 68 
Figure 4.4 Sand media with float #2 in a closed (ON) position and suffcient water in 
reservoir. ....................................................................................................69 
Figure 4.5 Sand media with float #2 in open (OFF) position to test low water level 
response......................................................................................................70 
Figure 4.6 Houston clay soil test with float #2 in a closed position and enough water 
in reservoir.     ............................................................................................71 
Figure 4.7 Houston clay soil test with float #2 in an open position to simulate low 
water level in reservoir. ..............................................................................72 
Figure 4.8 Design layout of the experimental wastewater SDI system, including clean 
water reservoir tank and pump, headwords box, control panel, supply and 
return manifolds, and 30 drip laterals at 0.61m spacing. The SDI system 
was divided into two equal subplots. .........................................................76 
Figure 4.9 Field data recorded from September 2006 to June 2007. (a. field 
monitored soil moisture contents and system daily hydraulic disposal 
rates; b. field monitored daily precipitation, soil temperature at 10 cm 
depth. (Calculated daily ET was added to graph for reference)  
 ....................................................................................................................80 
 xv
Figure 4.10 Estimated monthly drain field water balance from September 2006 to June 
2007. (Drain field surface ponding is not considered. Positive Y axis 
represents water inputs; negative Y axis represents water outputs) ..........83 
Figure 5.1       Experimental layout, treatments, and sampling locations ..........................97 
Figure 5.2 Recorded field data for years one and two, September 2006 to June 2008. 
(a. soil moisture content and daily hydraulic dosing rate; b. daily 
precipitation, soil temperature at 10 cm depth, and calculated daily ET. 
System operation was interrupted by unexpected power and water supply 
outages from October 2006 to March 2007. The system was manually shut 
off in May 2008 and October 2008 to facilitate crop harvesting and 
planting.)  .................................................................................................106 
Figure 5.3 Estimated monthly drain field (treatment I) water balance from September 
2006 to June 2008. (No consideration of drain field surface ponding. 
Positive Y axis represents water inputs; negative Y axis represents water 
outputs) ....................................................................................................108 
Figure 5.4 Observed monthly precipitation at experimental site versus 30-year 
precipitation record (1997-2008). (Source: Alabama Black Belt Research 
and Extension Station) .............................................................................109 
Figure 5.5 Spatial variation of (a) field capacity and (b) saturated hydraulic 
conductivity, Ksat, in the SDI drain field (treatments I and II).  Soil 
hydraulic conductivity was measured onsite using a permeameter (Ksat 
Inc., CA) at 45 cm depth. Field capacity was measured using the 
 xvi
laboratory pressure plate method on intact soil cores (SSSA, 2002b) at 18 
cm depth. ..................................................................................................111 
Figure 5.6 Year two soil moisture profiles, May 14-September 8, 2008. I. Synthetic 
wastewater application subplot; II. Clean well water application subplot 
(control); III. Agronomic practice subplot (control); IV. Undisturbed 
(blank) control ..........................................................................................112 
Figure 5.7 Year one and two monthly precipitation, October 2006 to September 2008, 
compared to 30-year normal. September 2006 is excluded since it is a 
system startup period. July 2008 to September 2008 is included as 
comparison to the records in the same period of 2007. Asterisks indicate 
the recorded month and year closest to 30-year normal, subsequently used 
to estimate average monthly dosing rate. .................................................115 
Figure 5.8 Soil water NH4+-N levels in treatments I and II during year two. ...........120 
Figure 5.9 Soil water NO3--N levels in treatments I and II during year two. ............121 
Figure 5.10 Soil water TP levels in treatments I and II during year two. ...................121 
Figure 5.11 Year two total soil nitrogen profiles for field treatments I, II, III, and IV, 
June 24, 2008. (Data represent means of triplicate sub samples.) ...........122 
Figure 5.12 Year two soil crop available nitrogen (NH4+-N and NO3--N) profiles for 
field treatments I, II, III, and IV, June 24, 2008. (Data represent means of 
triplicate sub samples.).............................................................................124 
Figure 5.13 Year two soil pH profiles for field treatments I, II, III, and IV, June 24, 
2008. (Data represent means of triplicate sub samples.) .........................125 
 xvii 
Figure 5.14 Year two total soil phosphorus profiles for field treatments I, II, III, and IV, 
June 24, 2008. (Data represent means of triplicate sub samples.) ...........126 
Figure 5.15 Year two soil crop available phosphorus profiles for field treatments I, II, 
III, and IV, June 24, 2008. (Data represent means of triplicate sub 
samples.) ..................................................................................................127 
Figure 5.16 Year two soil water soluble phosphorus profiles for field treatments I, II, 
III, and IV, June 24, 2008. (Data represent means of triplicate sub 
samples.) ..................................................................................................127 
Figure I.1 Measured flow rates of each emitter. (Adapted from Ducote, 2006) .......165 
Figure II.1 General soil series map, Black Belt Research and Extension Center, 
Marion Junction, Alabama, showing location of study site. ....................167 
Figure IV.1 Field soil core sampling locations within the SDI site on June 24, 2008 ...... 
 ..................................................................................................................174 
Figure IV.2 Interpolated site map based on the measured soil field capacities (1/3 bar) 
at 20 cm depth. (Sampled on June 24, 2008) ...........................................174 
Figure V.1 Composite soil sample preparation for experimental site soil phosphorus 
adsorption isotherm determination.. ........................................................176 
Figure V.2 Phosphorus adsorption on soil vs. phosphorus in water phase of the five 
soil horizons of the experimental site using composite samples from the 
soil cores taken from each field treatment. (a. 0-20 cm, b. 20-40 cm, c. 40-
60 cm, d. 60-80 cm, e. 80-100 cm.). ........................................................177 
Figure VI.1 Sensor performance under calibrated coefficient and un-calibrated 
coefficient. ...............................................................................................180 
 xviii 
Figure VIII.1 Screen capture of the soil report generated for physical soil properties of 
the experimental site. ...............................................................................186 
Figure IX.1 HYDRUS 2D simulation domain and boundary conditions ....................188 
Figure IX.2 Soil moisture content (%, v/v) time course of the free drained simulating 
soil profile ................................................................................................192 
Figure IX.3 Simulated (scenario 1) soil moisture contents at 20 cm and 40 cm depths 
versus actual field monitoring in treatment I during the one-year synthetic 
wastewater application from June 14, 2007 to June 24, 2008 .................196 
Figure IX.4 Simulated (scenarios 1, 2, and 3) NH4+-N, NO3--N, and phosphorus 
outputs within the simulated soil domain after one year synthetic 
wastewater application. (For scenario 2, no NO3--N presences in the soil 
domain since NH4+-N is not nitrified) ....................................................199 
Figure IX.5 Simulated (Scenario 4) monthly water percolation and nutrient 
concentrations in water phase during the 10-year simulated period. (a. 
simulated monthly water percolation and soil moisture content change; b. 
simulated NO3--N and phosphorus concentration variations at simulated 
observation points ) ..................................................................................201 
 
 
1 
 
CHAPTER ONE 
INTRODUCTION 
 
1.1 RESEARCH BACKGROUND 
Conventional onsite septic systems have been used in the Alabama Black Belt 
area for decades in spite of the fact that many soils in the region have a high smectitic 
clay content with severe drainage limitations such as low permeability, high seasonal 
water table, and restrictive layers. Since the 1950s, over 10 million acres of row crops 
have been abandoned in the state of Alabama (McNider, 2005), much of it in the Black 
Belt region. As a result, rural economies in this area have suffered and many existing 
conventional onsite septic systems are in disrepair. The weakened economy in the 
Alabama Black Belt makes the retrofit of individual system difficult, if not cost-
prohibitive for many households (McCoy et al., 2004). In 2002, the Lowndes County, 
Alabama court ruled against 37 families who discharged raw wastewater into their 
backyards and nearby ditches, drawing national attention to onsite system deficiencies in 
the Alabama Black Belt. According to the US EPA (2002), approximately 47% of the 
houses in Alabama are served by OWTSs with an average system failure rate of 20%. 
In order to alleviate the environmental and health threat from conventional onsite 
septic systems in this area, a subsurface drip irrigation (SDI), soil moisture
1 
 
monitoring/control, cropping system were incorporated into an experimental wastewater 
disposal and treatment system. This new system uses real-time drain field moisture 
content to control SDI wastewater disposal while a seasonal cropping system provides 
water and nutrient uptake. SDI was used to achieve uniform wastewater distribution and 
to locate water and nutrients favorable for crop root uptake (Phene and Ruskin, 1995). 
Drain field soil moisture control of wastewater disposal was incorporated since it has 
been used successfully in agricultural irrigation for water conservation (Dukes and 
Scholberg, 2005). Cropping systems have been widely used in agricultural to provide 
enhanced water and nutrient uptake (Askegaard and Eriksen, 2008; Wang et al., 2008) 
and pesticide leaching control (Ferraro et al., 2003; Giupponi, 1998). A seasonal cropping 
system of sorghum-sudangrass (Sorghum bicolor) and winter wheat (Triticum aestivum) 
with rye (Secale cereale) was applied over the SDI wastewater drain field to enhance 
removal of SDI applied wastewater and nutrients. 
 
1.2 RESEARCH OBJECTIVES 
The goal of this research is to evaluate the hydraulic performance of an 
experimental soil moisture based wastewater dosing system and resulting soil nutrient 
profile after one year of synthetic wastewater application. Specific objectives of this 
research are to: 
1) Evaluate the potential environmental and health threat from conventional onsite 
wastewater treatment systems in the Alabama Black Belt.  
2) Develop, test in the laboratory, and field install a soil moisture based SDI wastewater 
 2
disposal control system.  
3) Evaluate system control over drain field water movement through determination of 
two-year monthly hydraulic disposal rates and water balances.  
4) Evaluate system nutrient impact to the drain field from one-year of synthetic 
wastewater field application through lysimeter sampling, soil nutrient profile, and 
numerical simulation.  
 
1.3 DISSERTATION ORGANIZATION 
This dissertation includes seven chapters. Except for Chapter One (General 
Introduction), Chapter Two (Literature Review), and Chapter Six (Conclusions and 
Suggestions for Future Research), each other chapter of this dissertation is formatted as a 
stand-alone journal paper. 
 Chapter One provides a background and introduction for this dissertation. 
Chapter Two provides a literature review of decentralized systems related to water and 
nutrient management. Chapter Three presents the current status of rural wastewater 
disposal systems in the Alabama Black Belt area and provides results of a GIS risk 
assessment tool developed as justification for this research. Chapter Four describes the 
laboratory and first year field testing of the experimental wastewater disposal system 
designed to overcome specific deficiencies of conventional septic systems in the high 
clay soils of the Black Belt. Chapter Five presents the results of the system evaluation 
over its water and nutrient management over its drain field using combined experimental 
results from first year clean water study and the second year synthetic wastewater study. 
Recommendations are made on system application based on the advantages and 
 3
disadvantages of the experimental system. Chapter Six provides conclusions as well as 
recommendations for future research. 
4 
 
CHAPTER TWO 
 LITERATURE REVIEW 
 
2.1 DECENTRALIZED WASTEWATER TREATMENT  
2.1.1 DECENTRALIZED WASTEWATER TREATMENT IN THE UNITED STATES 
The decision to use centralized or decentralized wastewater treatment for a site or 
region is a cost?benefit analysis (Prihandrijanti et al., 2008). Decentralized wastewater 
treatment methods are used when connecting isolated households to the existing sewer 
system is not practical and/or economical (Viraraghavan, 1986; US Census Bureau, 
1990).  
The Consortium of Institutes for Decentralized Wastewater Treatment, also 
known as ?The Onsite Consortium" defines commonly used terms for decentralized 
wastewater treatment systems (The Onsite Consortium, 2007). A decentralized 
wastewater treatment system is defined as a wastewater treatment system for collection, 
treatment, and dispersal/reuse of wastewater from individual homes, clusters of homes, 
isolated communities, industries, or institutional facilities, at or near the point of waste 
generation. Based on the serving population and system size, a decentralized wastewater 
treatment system can be further divided into a community system, cluster system, onsite 
system, or individual system (Table 2.1). 
5 
 
A community wastewater treatment system is defined as a publicly owned 
wastewater treatment system for collection, treatment and dispersal of wastewater from 
two or more lots, or two or more equivalent dwelling units. A cluster wastewater 
treatment system is defined as wastewater treatment systems designed to serve two or 
more sewage-generating dwellings or facilities with multiple ownership; typically to 
include a comprehensive, sequential land-use planning component and private ownership. 
An onsite wastewater treatment system (OWTS) is defined as a wastewater treatment 
system that relies on natural processes and/or mechanical components to collect and treat 
sewage from one or more dwellings, buildings, or structures and disperse the resulting 
effluent on property owned by the individual or entity. An individual wastewater 
treatment system is defined as a wastewater treatment system designed to serve one 
sewage-generating dwelling or facility.  
Table 2.1 Defined decentralized system categorization. 
 Community system Cluster 
system 
Individual system Onsite system 
Serving 
size 
 two or more lots, 
or two or more 
equivalent dwelling 
units 
two or more 
sewage-
generating 
dwellings or 
facilities  
one sewage-
generating 
dwelling or 
facility. 
 
one or more 
dwellings, 
buildings, or 
structures  
Ownership Publicly Multiple 
ownership 
Individual Individual 
Source: The Onsite Consortium (2007) 
In the United States, about 30% of households are using onsite sewage disposal, 
while in Alabama this number is 47% (AOWTC, 2005). Among current onsite sewage 
disposal methods, conventional septic tank systems have the longest history and the most 
common application (US EPA, 2002). In a conventional septic system, sewage goes 
 6
through primary settling and biological reaction during its retention (around 48 hours) in 
the septic tank. Upon reaching a preset overflow level, the supernatant from the septic 
tank is disposed by gravity to the drain field where percolation through an unsaturated 
soil zone provides various levels of treatment of the effluent (AOWTC, 2005).  
US EPA (2002) defines a conventional septic system as a highly efficient, self-
contained, underground wastewater treatment system. Because septic systems treat and 
dispose of household wastewater onsite, they are often more economical than centralized 
wastewater treatment systems in rural areas where lot sizes are larger and houses are 
widely spaced. Septic systems are simple in design, which make them less expensive to 
install and maintain than other systems (US EPA, 2002; Prihandrijanti et al., 2008). By 
using natural processes to treat wastewater onsite, usually in a homeowner's backyard, 
septic systems do not require the installation of miles of sewer lines, making them less 
disruptive to the environment (US EPA, 2002; AOWTC, 2005).  
Onsite wastewater treatment industry scientists, engineers, and manufacturers 
have developed a wide range of alternative technologies such as recirculation sand filters, 
peat-based systems, mound systems, and package aeration units to address risks from 
increased hydraulic load and nutrient and pathogen contamination (US EPA, 2002). With 
proper management and oversight, alternative systems can be installed in areas where 
soils, bedrock, fluctuating ground water tables, or lot size limit the use of conventional 
septic systems (US EPA, 2002). According to the EPA Onsite Wastewater Treatment 
Systems Manual (US EPA, 2002), there are currently thirteen types of advanced onsite 
wastewater treatment technologies: 1) Continuous-Flow, Suspended-Growth Aerobic 
Systems (CFSGAS); 2) Fixed-film processes; 3) Sequencing batch reactor systems; 4) 
 7
Effluent disinfection processes; 5) Vegetated submerged beds and other high-specific-
surface anaerobic reactors; 6) Evapotranspiration and evapotranspiration/infiltration; 7) 
Stabilization ponds, constructed wetlands, and other aquatic systems; 8) Enhanced 
nutrient removal--phosphorus; 9) Enhanced nutrient removal; 10) Intermittent sand/media 
filters; 11) Recirculating sand/media filters; 12) Land treatment systems; and 13) 
Renovation/restoration of subsurface wastewater infiltration systems (SWIS).  
 
2.1.2 ENVIRONMENTAL CHALLENGE FOR CONVENTIONAL ONSITE SEPTIC SYSTEM 
The environmental challenges for conventional septic systems come from the 
almost complete reliance on soil properties (Oron, 1996). The soil in a drain field should 
have a percolation rate that allows wastewater to penetrate into soil at a rate to provide 
adequate treatment of nutrients and contaminants (Venhuizen, 1995). Unsaturated flow is 
necessary to obtain efficient aerobic treatment while providing sufficient time for effluent 
to stay in contact with the soil (Venhuizen, 1995). Tyler et al. (1977) emphasized that it is 
essential, particularly in coarser soils, that a "clogging mat" form at the top of the 
gravel/soil interface of a conventional gravity trench to create a zone of partially 
restricted flow, helping to assure that flow through the soil beyond the clogged zone is 
unsaturated.  
In the United States, by the late 1800s the Massachusetts State Board of Health 
and other state health agencies had documented the linkage between disease and poorly 
treated sewage, recommending treatment of wastewater through intermittent sand 
filtration, with land application of the resulting sludge (US EPA, 2002). Septic tanks for 
primary treatment (settlement, with limited biological nutrient and contaminant removal) 
 8
of wastewater appeared in the late 1800s. Discharge of septic tank effluent into gravel-
lined subsurface drains became common practice during the middle of the 20th century 
(Kreissl, 2000). Although the use of septic systems significantly improved public health 
and water quality in urban areas, the technology is old for homes and businesses not 
connected to a centralized collection and treatment system (Kreissl, 1982).  
The average phosphorus concentrations in a septic tank effluent were reported to 
be 3 to 20 mg/L (Robertson et al., 1998; Whelan, 1988), with about 85% as 
orthophosphate (Reneau and Pettry, 1976). Charles et al. (2005) statistically analyzed the 
results of several intensive septic tank effluent field surveys from 1976 to 1999 in 
Australia and the United States (Table 2.2), including his own field survey on 200 septic 
tank effluents in Australia. They compared the results to published regulations in 
Australia and the United States and concluded that current septic tank effluent quality 
standards do not ensure safe system design. Charles et al. (2005) observed that the 80th 
percentile of their effluent survey values, TN (250 mg/L) and TP (36 mg/L), should be 
used in new regulations to minimize overloading that is associated most with onsite 
system failure. The more commonly used 50th percentile allows that 50 percent of 
systems designed are exposed to potential drain field overloading and subsequent failure 
(Lesikar, 2007). 
Over 50% of the United States population draws ground water for its potable 
water supply, while 98% of self-supplied domestic and rural households depend on 
ground water (US EPA, 1990). During the mid 1940s, infant mortality was reported to be 
related to NO3--N concentrations in drinking water, especially in rural areas of the United 
States (Hergert, 1986). Public hygiene issues related to onsite septic systems in urban 
 9
fringes has received attention since 1980 (Boyle and Otis, 1981; DeWalle and Schaff, 
1980). Recent medical research has indicated that infant mortality is attributed to NO3--N 
concentrations from improper disposal of human and animal waste (Fare, 1993). Carroll 
and Goonetilleke (2005) confirmed that a high onsite system density of 290 unit km-2 can 
significantly impact shallow groundwater with increased nutrient and pathogen levels.  
Another challenge of conventional onsite septic systems is dependence upon 
gravity flow through the drain field before assimilation by the soil (Venhuizen, 1995). 
Typically, gravity dosing alone cannot achieve a uniform wastewater distribution and the 
result is localized loading rates that are far higher than design loading rates, a 
circumstance which has been reported in several investigations (Reneau et al., 1989, 
Harper et al., 1982) and which EPA states is the main cause of system failure (US EPA, 
2002).  
Table 2.2 Effluent survey values compiled by Charles et al. (2005) 
Value SS BOD TN TP 
Thermotolerant 
coliforms Guideline 
  mg/L mg/L mg/L mg/L cfu. 100/ml   
International             
Average1  94 44 8.6 3.7?104 USA 
Range1  46-156 19-53 7-17 4?103-3?105 USA 
Range2 50-100 140-200 40-100 5-15 10^5-10^8 USA 
Average3 165 280 92 10.5     
         
85th 
percentile3 250 350 105 14     
Average4 54 158 55 15    
Range4 11-695 20-480 10-125 4-90    
Range5 44-54 129-147 41-49 12-14     
         
Australia             
Average6 448.5 365.7 75.7 21.3  NSW 
Range7 17-6970 22-2133 30-60 10-20    
Range8 40-250 120-280 30-60 10-20     
Note: The number of each superscript represents a field survey on septic effluent. 
 10
2.1.3 SUBSURFACE DRIP IRRIGATION APPLICATIONS WITH WASTEWATER  
Subsurface drip irrigation first gained popularity in agriculture about 1980 (Beggs 
et al., 2004) and the uniform hydraulic performance of SDI has been advanced since then 
(Provenzano et al., 2005; Puig-Bargues et al., 2005; Sahin et al., 2005; Shaviv and Sinai, 
2004).  SDI application of wastewater was investigated by researchers studying fate and 
transport of pesticides (Boyd et al., 2003; Jebellie and Prasher, 1999), nutrients (Bakhsn 
et al., 2000) and viruses (Assadian et al., 2005).  
Ruskin (1992) first discussed the potential of using SDI for wastewater 
reclamation. Phene and Ruskin (1995) present the concept of using SDI to prevent water 
and nutrients from leaving root zones to maximize nutrient and water utilization. Jnad et 
al. (2001) studied the impact of SDI wastewater application on soil chemical 
characteristics over silty clay loam and fine sandy loam and concluded that sodium 
should be considered if sodium loading to the field is high since sodium accumulation 
can decrease soil hydraulic conductivity and deteriorate soil physical properties. They 
also found that phosphorus concentrations were significantly greater near the emitter and 
close to the soil surface where the drip lateral was installed at a shallow depth. They 
found no drastic changes along the soil profile in soil TN, Ca, Mg, K, EC, and TOC. 
Assadian et al. (2005) studied wastewater SDI application on loamy sand and clay loam 
under arid and semi-arid weather conditions in Texas. They found that virus movement in 
soil under unsaturated SDI flows was limited to areas around the subsurface emitter. 
However, bacteriophage persisted in both sandy and loamy soils for a 28-day period after 
the last irrigation. A successful and safe reclamation of sodic/saline wastewater for 
subsurface drip irrigation will depend on management strategies that focus on irrigation 
 11
pretreatment, virus monitoring, field and crop selection, and periodic leaching of salts 
(Assadian et al., 2005). Li et al. (2007) found that nitrate accumulated toward the 
boundary of the wetted soil volume for uniform and layered soils (sandy to sandy loam) 
under drip irrigation.  
In general, SDI application of wastewater has the benefit of creating a uniform 
hydraulic distribution in the drain field (Ruskin, 1992; 1995) and has the potential to 
restrict nutrient movement by adjusting disposal amount and frequency to a confined 
wetting front (Jnad et al., 2001). Design dosing is based on the soil texture and specific 
environmental protection requirement. 
 
2. 2 DRAIN FIELD WATER MOVEMENT  
2. 2.1 FORMS OF WATER MOVEMENT  
For a drain field, water inputs and outputs are normally composed of 
precipitation, wastewater application, infiltration, surface runoff, percolation, soil 
evaporation, and plant transpiration. The forms of these water movements are 
demonstrated in Figure 2.1. 
Infiltration is the process of water entering the soil from the surface. Infiltration 
under non-saturated flow was first numerically represented by Richards in the 1920s 
(Raats, 2001), and has been developed into several different classes that hold somewhat 
different views on the relationship between soil water content, diffusivity and hydraulic 
conductivity (Raats, 2001). Nevertheless, an appropriate infiltration rate is important to 
maintain unsaturated flow inside the drain field for surface wastewater application and 
treatment (Venhuizen, 1995). 
 12
Surface runoff is a result of storm water excess above the water holding capacity 
of the soil (Horton, 1933). The adverse environmental impact from surface runoff is its 
potential to take with it nutrients and contaminants that were formerly immobilized in the 
field (Rhode et al., 1981; Arora et al., 1996; Boyd et al., 2003). Soil moisture, surface 
porosity, and vegetative growth and covering rate affect the runoff from a soil surface 
(Lowrance et al., 1997). Smith et al. (2001a) found that surface runoff is the main cause 
of ammonia nitrogen loss during liquid manure application. Storm water applied to the 
soil where nutrients were recently applied presents a higher risk of nutrient loss (Jarvis et 
al., 1987; Edwards and Daniel, 1993). 
Soil evaporation and plant transpiration are related to local weather conditions, 
plant growth status, and plant covering rate over the field (Savabi and Williams, 1995; 
Arnold et al., 1995). Soil evapotranspiration is a major pathway for applied water and is 
directly related to plant nutrient uptake, thus also an important nutrient pathway out of 
the soil (Venhuizen, 1995). 
Percolation is the process of water movement through soil layers by gravity and 
capillary forces. Deep percolation occurs when water moves down through the soil 
profile below the root zone and cannot be utilized by plants (SSSA, 2002). It is important 
for applied wastewater to pass through soil in order to obtain adequate treatment from 
soil and plant root uptake (Venhuizen, 1995), but deep percolation is a phenomenon that 
is not desirable for water conservation in agriculture (Mermound et al., 2005; Ndiaye et 
al., 2007), nor for controlling contaminant movement (Jebellie and Prasher, 1999; Bakhsh 
et al., 2000; Boyd et al., 2003). Significant deep percolation indicates a waste of water for 
 13
agriculture (Patel and Rajput, 2007; Vishnu et al., 2008) as well as potential underground 
water pollution (US EPA, 2002). 
 
Figure 2.1 Forms of water movement in drain field 
 
For crop agriculture, a water budget that minimizes water to satisfy crop needs is 
often a target (Ayars et al., 1999; Alam et al., 2002; Wysocki et al., 2005). However, for 
wastewater land application, the purpose is to dispose as much water as possible without 
exceeding regulations and endangering the environment (Jnad et al., 2001). For this 
reason, numerous studies on agriculture irrigation and wastewater land disposal have 
focused on controlling the fate and movement of nitrogen (Bakhsh et al., 2000; Smith et 
al. 2000a; Beggs et al., 2004), phosphorus (Reneau et al., 1989, Smith et al., 2000b), 
pesticide (Jebellie and Prasher, 1999; Boyd et al., 2003), and pathogens (Yavuz-
Corapcioglu and Haridas, 1984; Damgaard-Larsen et al., 1977; Foppen et al., 2007). 
 
2.2.2 FIELD CAPACITY MEASUREMENT 
Veihmeyer and Hendrickson (1931) first introduce the field capacity (FC) concept 
and define it as ?the amount of water held in the soil after excess gravitational water has 
drained away and after the rate of the downward movement of water has materially 
 14
decreased.? Several assumptions not stated in this FC definition are that the soil is deep 
and permeable, no evaporation occurs from the soil surface, and no water table or slowly 
permeable barriers occur at shallow depths in the profile (SSSA, 1986). 
Gardner (1960) defined FC as that water content below which the hydraulic 
conductivity is sufficiently small that redistribution of moisture in the soil profile due to a 
hydraulic head gradient can usually be neglected. However, different soils take different 
time periods to achieve the ?equilibrium? point. FC is not truly an equilibrium water 
content but instead is that water content at which the soil water flux out of the rooting 
zone becomes negligible and no significant change in water content occurs with time 
(SSSA, 1986). 
In 1997, the Glossary of Soil Science Terms (SSSA, 1997) defined FC as ?the 
content of water, on a mass or volume basis, remaining in a soil 2 or 3 days after having 
been wetted with water and after free drainage is negligible?. This definition assumes a 
uniform soil profile and zero evaporation from the soil surface.  
It is emphasized in the two generations of Soil Science Society of American 
(SSSA) published soil analysis manual (SSSA, 1986; 2002) that field capacity represents 
a practical and readily understandable coefficient to measure the ability of a soil to retain 
water and it is of significant importance to identify a suitable procedure to determine its 
value. If at all possible, the test should be representative of actual conditions. Only a field 
experiment can take into account the various factors affecting the soil water regime at 
FC, and preference should be given to field tests for determining the FC (SSSA, 2002). 
The principle of an in situ FC test is to add water to a field soil, wetting the soil to 
a desired depth. After the water has redistributed into the drier underlying soil and 
 15
drainage from the initial wetted zone becomes negligible, the water content is taken as in 
situ FC (SSSA, 2002). For practical reasons, an arbitrary drainage time of 48 h has been 
used in the field capacity definition. However, it is important to emphasize that, in reality, 
while the hydraulic conductivity of coarser-textured soils may become ?negligible? in 
less than 24 h, the hydraulic conductivity for finer-textured soils may continue to drain at 
a ?non-negligible rate? for weeks (SSSA, 1986; 2002). Furthermore, field capacity can be 
determined for the same plot during different times of the year to allow for possible 
temporal variations since wetting history and changes in rooting depth with time can 
affect the FC (SSSA, 2002).  
There are attempts (Cassel and Sweeney, 1974; Jamison and Kroth, 1958) to 
relate FC to water retention characteristics. It is worth noting that water retention is a soil 
property, whereas FC is a process-dependent value of the water content of the profile 
(SSSA, 2002). Jamison and Kroth (1958) selected the equilibrium water content at 33 
kPa to estimate FC for their soils even though the pressure values ranged from > 10 kPa 
to >100 kPa. Over time, the use of the 33-kPa water content value has been adopted as 
field capacity (The value of FC is much affected by field conditions, so lab tests must be 
considered only as approximate methods (SSSA, 2002). Gardner (1968) pointed out that 
FC should be more related to the hydraulic conductivity of a soil than to the soil water 
matrix potential.  
Even though there is disagreement about the laboratory approximation methods at 
appropriate pressures for specific soil textures, the selection of the appropriate pressure 
for desorption of a particular soil sample is also not straightforward (Cassel and Sweeney, 
1974). The 33-kPa water content may correlate well with a particular coarse-textured soil 
 16
rather than the 10-kPa water content. However, for coarse-textured soils as a group, the 
10-kPa water content is usually considered to be the better choice. Some other reasons for 
this observed deviation are difference in organic matter content, soil structure, degree of 
compaction, degree of wetting, and percent sand, silt and clay (Reeve et al., 1973). FC 
values for different soils were found to vary from -50 to less than -350 cm (SSSA, 2002). 
Actually, -330 cm of water pressure (~1/3 bar) represents somewhat of a compromise 
between these laboratory results (McIntyre, 1974; Addiscott and Whitmotr, 1991; SSSA, 
1986). 
Numerous studies found that FC changes with seasons as field condition changes. 
However, typically, the FC for a given soil is treated as a ?constant,? as a specific value 
which never changes. The concept of a constant value of FC is criticized as ?people who 
do not really understand the soil physics? from a scientific perspective (SSSA, 2002). 
Due to the difficulty in applying a defined FC to field measurement and the 
complications associated with selecting a lab approximation (SSSA, 1986; 2002), FC can 
be either field or lab measured based on available literature to determine a certain 
working range. However, the resulting soil moisture thresholds should be tested over time 
to evaluate the influence of FC on the specific engineering application. 
 
2.2.3 SOIL WATER MOVEMENT MODELING  
Current multi-dimensional vadose zone solute transport numerical models are 
BioF&T, Chemflux, SUTRA, and HYDRUS (Table 2.3). HYDRUS is notable among 
them in that it incorporates much of the power of the multi-dimensional general pollutant 
 17
transport models while having features oriented specifically to modeling irrigation and 
drainage (Beggs et al., 2004). 
HYDRUS has been used to simulate and predict water movement (Skaggs et al., 
2004; Mermound, 2005; Stillman et al., 2006) and solute movement (Hanson et al., 2006; 
Ndiaye, 2007; Hu et al., 2007) in field and laboratory experiments. Due to the 
heterogeneity in field conditions, validating a model is not always feasible (Hopmans and 
?im?nek, 1997; Deurer et al., 2003; Haws et al., 2005). 
Table 2.3 Common multi-dimensional vadose zone solute transport numerical models 
Model Maximum dimensions Organization Major features or applications 
BioF&T 3 Scientific Software Sandy, UT 
Multi-component biodegradation and 
transport including oxygen limited 
and Monod-type. 
Chemflux 3 Rockware, Inc. Golden, CO Chemical transport, including decay; self-adjusting element mesh. 
SUTRA 2 USGS Saturated and vadose transport 
HYDRUS 3 USDA Salinity Lab, Riverside, CA Multi-component chain reactions, irrigation and drainage features. 
RZQM 2 USDA-ARS 
Chemical transport over a wide range 
of topographies, soil types, climatic 
conditions, and management 
practices. 
Note: Adapted from Beggs et al. (2004) 
Royer et al. (2002) mentioned that because it is typically infeasible to make 
detailed distributed measurements of soil properties at a study site, parameters must be 
estimated based on one hydrograph, which leads to a higher likelihood of non-unique 
parameter sets and an ill-posed inverse calibration.  In a deterministic model, 
representative values of soil properties are often accomplished by assuming a 
representative elementary volume (REV) of the study site, where that medium is 
represented by homogenized parameters. The REV assumption holds if there is a 
 18
separation of scales such that the volume of the spatial heterogeneities is significantly 
smaller than the macroscopic transportation domain being modeled (Haws et al., 2005). 
Successful HYDRUS calibrations are either carefully controlled laboratory soil column 
tests (Pernyeszi et al., 2006; Dousset et al., 2007; Suarez et al., 2007) or field tests that 
treat the whole field as a single unit (Moradi et al., 2005; Pot et al., 2005; Fernandez-
Galvez and Simmonds, 2006; Gardenas et al., 2006; Ajdary et al., 2007; Foppen et al., 
2007).   
 
2.3 DRAIN FIELD NUTRIENT MOVEMENT 
2.3.1 NITROGEN MOVEMENT  
Nitrogen removal in the soil is mainly through the pathways of adsorption, 
fixation, volatilization, biological uptake, and denitrification (Figure 2.2) (Broadbent and 
Reisenauer, 1988; Laak, 1982; Lance, 1972; Petrovic, 1990; Tyler et al., 1977; US EPA, 
2002). Venhuizen (1995) did an extensive review of nitrogen fate in the soil and 
concluded that much of the ammonium will be nitrified after entering the soil. Even 
though nitrification most readily occurs when the soil is warm, few sections of Alabama 
stays cool enough to prevent nitrification and subsequent loss of N through leaching and 
nitrification (Reiter, 2003; Scarsbrook and Cope, 1958). Unless conditions are favorable 
for denitrification, such as saturated soil or no air circulation, nitrate will tend to stay in 
the soil and become a potential nitrate pollution source. Nitrate is of negative charge 
which makes it more mobile in soils than ammonia (US EPA, 2002), Furthermore, since 
most ammonia nitrogen can be easily nitrified into nitrate, the chances of observing 
nitrate leaching for a drain field is greater than that of ammonia, (US EPA, 2002).  
 19
 
Figure 2.2 Nitrogen movement and fate in wastewater drain field. 
Ammonia fixation occurs when ammonium ions become trapped in the 
intermicellar layers of clay minerals and fixation by organic components of the soil may 
also occur (Laak, 1982; Lance, 1972; Tyler et al., 1977). However, the potential for 
ammonium fixation is limited and not likely to be important in the long-term nitrogen 
budget of a soil system (Tyler et al., 1977; Broadbent and Reisenauer, 1988).  
Tyler et al. (1977) and Lance (1972) both mentioned that significant volatilization 
losses require considerable air and water contact, which is unlikely during subsurface 
wastewater disposal, since wastewater will be adsorbed quickly by soil particles for 
subsequent nitrification. Also, the equilibrium between ammonium and ammonia gas is 
pH dependent. Significant volatilization can only be expected at a fairly high pH. 
Comparably low pH in the soil will not work in favor of volatilization of ammonium to 
ammonia gas (Lance, 1972). Consequently, nitrogen loss through non-biological 
volatilization of ammonium to ammonia gas is believed to be an insignificant factor for 
subsurface wastewater disposal systems (Venhuizen, 1995). 
Applied to a drainf ield 
Nitrification 
Protein (N) in wastewater 
Discharged to a drain field 
NH4+ NO3- 
Lost to underground water 
Lost due to 
surface runoff 
Lost due to 
surface runoff 
Lost due to surface runoff 
Bound with soil particles Bound with soil particles 
Evaporation in the form of NH3 
Nitrification 
Denitrification to N2 
Assimilated by plants Decomposed and assimilated 
by bacteria 
NH3-N and NH4+ in wastewater 
wastewater 
 20
Soil adsorption has been proven to be the major mechanism in the removal of 
large quantities of ammonium from solution, (Broadbent and Reisenauer, 1988; Lance, 
1972; Tyler et al., 1977). However, since ammonium is most easily nitrified under 
aerobic conditions, significant adsorption will not typically occur after nitrification due to 
the weak affinity between nitrate and soil particles (Venhuizen, 1995).  
Biological uptake and denitrification are also important pathways for applied or 
disposed nitrogen to be removed from a soil system (Venhuizen, 1995). Nitrogen is 
absorbed by plants primarily as ammonium and nitrate, but can also be absorbed as urea 
(Co(CN2)2) (Scarsbrook and Cope, 1958; Tisdale et al.,1993). Tyler et al. (1977) 
mentioned that extensive root systems are not present in effluent drain fields and 
therefore only limited amounts of N would be taken up. Also, in commenting on the 
general uptake potential of plants, Laak (1982) noted that wastewater from subsurface 
effluent drain fields is not as available to plants due to rapid leaching loss. However, 
some leached nitrogen can be scavenged if roots are deep. Although not all of the 
nitrogen applied to the soil can be removed by plant uptake, 50% or less nitrogen uptake 
rate by perennial grasses is generally expected (Broadbent and Reisenauer, 1988). 
Lance (1972) mentioned three conditions that are necessary for denitrification to 
proceed: (1) oxidation of ammonium to nitrate, since ammonia will not be transformed 
directly to nitrogen gas; (2) passage through an anaerobic zone after nitrification has 
occurred, when nitrate is ready for denitrification; and (3) provision of an adequate 
energy source for denitrifying bacteria in the anaerobic zone, which means a carbon 
source is required. The potential total nitrogen reduction by denitrification in land applied 
wastewater effluent has been estimated at 10-15% (Lance, 1972) and 15-25% (US EPA, 
 21
1977). Broadbent and Reisenauer (1988) noted that temporary anoxia can be created in 
the soil by a passing wetting front. Venhuizen (1995) stated that since effluent is 
intermittently dosed in a low pressure disposal system, alternating cycles of higher 
saturation (anoxia) and lower saturation (oxic) will be created. As a result, in an 
intermittent wastewater disposal strategy, alternating nitrification and denitrification will 
occur (Beggs et al., 2004; Venhuizen, 1995).  
 
2.3.2 PHOSPHORUS MOVEMENT  
The mechanisms for removing phosphorus removal from the soil include plant 
uptake, biological immobilization, adsorption, chemical precipitation, surface and 
subsurface runoff (Reneau et al., 1989). These mechanisms are complex and have not 
been completely understood (Venhuizen, 1995). Crops uptake of N: P is approximately 
8:1 (Sharpley, 1996) which often leaves P in surplus in animal and human waste land 
disposal (Faulkner, 2001).  
For wastewater applied phosphorus, it is uncertain whether phosphorus is 
adsorbed or precipitated in any given instance unless the soil is analyzed (US EPA, 
1977). Removal and immobilization of phosphorus is dependent on availability of 
adsorption sites to bind phosphorus, with the majority of sorption sites provided by clay 
and organic soil fractions (Venhuizen, 1995). The phosphorus removal process typically 
starts with a relatively fast adsorption reaction, followed by slower immobilization due to 
the formation of low solubility precipitates (Sawhney and Hill, 1975). Roose and Fowler 
(2004) found that phosphorus remains relatively immobile in the soil once it is adsorbed 
onto soil particles, even under large scale water movement. Although adsorption is 
 22
theoretically limited, soils generally have a greater capacity for phosphorus retention than 
is predicted by adsorption theories, since adsorption sites "regenerate" as precipitation 
proceeds (Broadbent and Reisenauer, 1988; Canter and Knox, 1985; Sawhney and Hill, 
1975; Tyler et al., 1977; US EPA, 1977; Venhuizen, 1995).  
Phosphorus sorption capacities vary in different soils and disposal systems which 
will impact the length of time required to exhaust the phosphorus adsorption capacity to 
any given depth (Sawhney and Hill, 1975; Venhuizen, 1995). The contact area of soil and 
wastewater should be increased in order to fully utilize the adsorption capacity of the 
applied drain field (Sawhney and Hill, 1975). Organic matter also has the potential to 
reduce phosphorus loss by surface runoff by offering additional adsorption sites for 
phosphorus (Roberts and Clanton, 1991). Sawhney and Hill (1975) stated that deeper soil 
layers generally have a lower phosphorus sorption capacity due to decreased cation 
exchange capacity (CEC) and organic matter content. Consequently, phosphorus removal 
should be completed within a certain depth of wastewater dosing locations, otherwise 
phosphorus leaching may occur.  
 
2.4 INTEGRATED CROPPING SYSTEM 
2.4.1 WATER AND NUTRIENT MANAGEMENT OF CROPPING SYSTEMS 
Depending on the specific objective, a well planned cropping system can either 
reduce nutrient loss between seasons (Askegaard and Eriksen, 2008), increase long term 
soil fertility (McNair Bostick et al., 2007; Jagadamma et al., 2007), or control negative 
environment impact from fertilizer application (Ferraro et al., 2003; Giupponi, 1997; Zhu 
et al., 2005). 
 23
A cropping system may be defined as a community of plants which is managed by 
a farm unit to achieve various human goals, which include food, fibre, other raw 
materials, wealth and satisfaction (FAO, 1995). The international Rice Research Institute 
(IRRI) (1978) gives a more detailed definition of a cropping system: "....the crop 
production activity of a farm. It comprises all cropping patterns grown on the farm and 
their interaction with farm resources, other household enterprises and the physical, 
biological, technological and sociological factors or environments".  
Besides providing agronomic value, research in cropping systems also contributes 
to irrigation management (Wang et al., 2008), nutrient control (Askegaard and Eriksen, 
2008; Colomb et al., 2007; Jagadamma et al., 2007; Ju et al., 2006; Liu et al., 2003; Ohno 
et al., 2005; Verma and Sharma, 2008; Wright et al., 2007; Zhu et al., 2005; Zougmore et 
al., 2004), micronutrient control (Wright et al., 2007), and increased soil organic matter 
(Hamza and Anderson, 2005; McNair Bostick et al., 2007; Jagadamma et al., 2008). The 
environmental impact of cropping systems with regard to management of pesticides 
control has also been studied (Ferraro et al., 2003; Giupponi, 1998). 
Wang et al. (2008) studied summer forage cropping as an effective way to control 
deep drainage in a sandy clay loam in southern Australia. Summer forage cropping 
increased evapotranspiration (ET) 40% and lowered soil water content 53%. Drainage 
loss was reduced 62% at the cost of a 13% yield loss in the next winter wheat yield.  
Shan et al. (2005) studied nutrient control and management in cropping systems 
through a 40-year rice growing test in a silty sandy clay loam site in China. They found a 
persistent downward movement of phosphorus , but did not show a correlation between 
phosphorus leachate and phosphorus fertilizer application. Higher phosphorus 
 24
fertilization application rates did result in accumulation of phosphorus in the top soil. 
Ram et al. (2006) showed that adequate soil moisture maintained during summer 
provided an effective environment for efficient water and nutrient uptake for menthol 
mint in a sandy loam soil in a semi-arid, subtropical climate. Fuentes et al. (2003) was 
surprised to observe nitrate accumulated in a deep soil at an arid test site. The expectation 
was that water consumption was relatively high so nutrient movement downward would 
be relatively low. Zougmore et al. (2004) showed that water and nutrient management 
can help to control nitrogen loss from a field in a semiarid sandy loam site, where soil 
erosion and surface runoff were the main causes of nutrient loss. Zhu et al. (2005) studied 
nitrogen transport under an excessively fertilized sandy loam with hot peppers, a crop 
that provides low nitrogen use efficiency. They indicated that unplanned fertilizer 
applications creates the potential for atmosphere and underground water pollution. 
Further emphasis was laid on the importance of choosing an appropriate cropping system 
to match fertilizer application in order to minimize the adverse environmental impact 
from fertilizer over application.  
Jagadamma et al. (2007) studied the effects of nitrogen fertilization and cropping 
systems on soil organic carbon and total nitrogen pools in a 32-year field experiment. The 
results showed the top 30 cm of soil organic carbon was significantly increased by 
fertilization and cropping systems, while soil bulk density was decreased. However, no 
significant correlation was found between soil properties, fertilization rate, and cropping 
systems below 30 cm soil depth. Wright et al. (2007) studied nutrient stratification for 5 
years on a silty clay loam site in Texas. Results indicate that crop rotation increased 
micronutrient levels relative to monoculture cropping in unfertilized soils. Phosphorus 
 25
stratification was found to be significant with the top soil at higher phosphorus levels 
than lower layers. Decreasing plant-available nutrient concentrations with increasing N 
fertilizer rates was also found as a result of increased crop yields that ultimately led to 
higher nutrient removal by crops.  
McNair Bostick et al. (2007) demonstrated that soil organic carbon sequestration 
in the top 20 cm was increased through a 10-year cropping system on loamy sand. 
Rotation cropping systems tested were Sorghum-Sorghum-Sorghum, Groundnut-
Sorghum-Cotton, Groundnut-Cotton-Sorghum, Cotton-Cotton-Cotton, Groundnut-
Groundnut -Groundnut, Cotton-Maize-Sorghum, Sorghum-Fallow, and Continuous 
fallow. Colomb et al. (2007) studied phosphorus management under different crop 
rotation systems in a 36-year field experiment on a silty-clay, clay soil. The site was used 
for a variety of other studies including crop response to water availability, irrigation, and 
nitrogen. Their conclusion was that selective cropping systems can help to minimize the 
transport of phosphorus from soil to water thus preserving global resources of 
phosphorus.  
Ju et al. (2006) studied nitrogen balance and groundwater nitrate contamination 
on a loamy silty alluvial soil under three intensive cropping systems (wheat-maize, 
greenhouse vegetable, and apple orchards). They concluded that farm management 
should be improved from time to time based on field nutrient requirements so as to lower 
the accumulation of nitrate in soils and groundwater while maintaining or improving 
agricultural productivity. Tonitto et al. (2006) studied 36 published metal and/or nutrient 
related agricultural studies and concluded that diversified cropping systems have the 
potential to provide adequate nutrition for an increased population, as well as insure that 
 26
agricultural practices do not contaminate potable water supplies or other food sources 
such as regional fisheries.  
 
2.4.2 SOIL MOISTURE CONTROLLED IRRIGATION 
Soil moisture controlled irrigation supplies plant water requirement indirectly by 
sensing water status of the soil matrix. Soil moisture control systems manipulate 
irrigation frequency and amount according to actual soil conditions throughout the 
growing season. This automated technology provides not only a convenience for the 
operator but also substantial water savings compared to conventional irrigation 
management that is based on current or historical weather (Dukes and Scholberg, 2005). 
Phene and Howell (1984) first used a custom made soil matric potential sensor to control 
SDI for processing tomatoes. Phene et al. (1992) used an evaporation pan to estimate ET 
and used the information as an irrigation scheduling tool for cotton. As electronic 
technology has developed, Granular Matric Sensors (GMS) and Time Domain 
Reflectometry (TDR) sensors have come into use for automated irrigation scheduling 
(Meron et al., 1996; Mu?oz-Carpena et al., 2003). By using TDR to limit the soil 
moisture level to around field capacity, Dukes and Scholberg (2005) reported at least 
11% water savings and 24% reduction in deep percolation. Furthermore, they 
demonstrated that a smaller high/low soil moisture operation window further increased 
the percentage of applied water taken up by crop.  
Beggs et al. (2004) modeled water and nitrogen movement under a time 
controlled SDI and suggested the potential for improved nitrogen removal if soil moisture 
content and nitrification/denitrification reaction rates were coupled. Their proposed 
 27
system would create an optimized wastewater disposal strategy based on a synchronized 
aerobic/anaerobic cycle for maximum nitrogen reduction (Venhuizen, 1995).  
 
2.4.3 WATER AND NUTRIENT BALANCE STUDIES 
Water provides the medium for transport of plant nutrients and other growth 
regulatory compounds and is fundamental to the maintenance of normal physiological 
activity and membrane transport process (Jones and Tardieu, 1998). Water budgets 
during agricultural water management have been extensively studied for both nutrient 
management (Alva et al., 2006; Guo et al., 2008; Kibe et al., 2006; Ram et al., 2006; 
Rego et al., 2008) and water conservation (Abid Karray et al., 2008; Berg and Driessen, 
2002; Cameira et al., 2005; Eitzinger et al., 2003; Ji et al., 2007; Jones and Tardieu, 1998; 
Loos et al., 2007;). 
Berg and Driessen (2002) reviewed approaches that relate soil water potential to 
crop yield. They reported that current approaches to estimate or calculate crop water 
needs are focused on soil water potential, relative soil water content, and evaporative 
demand. Water balance components proposed by several studies are listed in Table 2.4. 
To quantify each component of the water balance, focus was placed on determining soil 
evaporation and crop transpiration or combined as evapotranspiration (ET) since the 
other components of the water balance can be directly measured with adequate resources 
(Abid Karray et al., 2008; Cameira et al., 2005; Jones and Tardieu, 1998; Loos et al., 
2007).  
Nutrient budget calculations are often coupled to a water budget in the calculation 
of nutrient input, runoff, and leaching loss. (Abid Karray et al., 2008 ; Berg and Driessen, 
 28
2002; Cameira et al., 2005; Eitzinger et al., 2003; Ji et al., 2007; Jones and Tardieu, 1998; 
Loos et al., 2007). Nutrient budget components taken from selected studies are listed in 
Table 2.5. Due to the fact that sampling leaching sample is labor intensive (Alva et al., 
2006; Rego et al., 2003), prediction methods were used when no direct measurement is 
available. The prediction method used to obtain the nutrient leaching loss in literature is 
by Darcy?s law to estimate of water flux, multiplied by soil extractable nutrient 
concentration (Alva et al., 2006; Rego et al., 2003). 
 
29 
 
             Table 2.4 Water budget components. (Based on selected studies) 
 Rain fall Irrigation Soil evaporation Crop 
transpiration 
Deep percolation Surface 
runoff 
Soil water content 
change 
Arnold et al. 
(1995) X X X X X X X 
Loos et al. 
(2007) X X X X X  X 
Karry et al. 
(2008) X X X X X  X 
 
             Table 2.5 Nutrient budget components. (Based on selected studies) 
 Fertilizer Mineralization Atmospheric  
deposition 
Non-symb. 
N fixation 
Plant 
uptake 
Surface 
runoff 
Soil nutrient 
change 
Gaseous 
loss 
Water 
erosion 
Deep  
percolation 
Alva et al. 
(2006) X X X X X  X   X 
Guo et al. 
(2008) X    X  X    
Rego et al. 
(2008) X X X X X X X X X X 
 
 
30 
 
CHAPTER THREE 
POTENTIAL ENVIRONMENTAL AND PUBLIC HEALTH RISK FROM 
ONSITE WASTEWATER TREATMENT SYSTEMS IN THE ALABAMA 
BLACKLAND PRAIRIE SOIL AREA 
 
3.1 ABSTRACT 
The Alabama Black Belt, a 14000 km2 area of widespread clayey soils that make 
up part of the larger Blackland Prairie soil area in central Alabama and eastern 
Mississippi, presents an environmental challenge for onsite wastewater treatment systems 
(OWTS) that rely on soil for contaminant dispersal. In this study, a new OWTS soil 
suitability rating system (OWTS-SSRS) was developed as an interpretation of the current 
Alabama OWTS regulations. The new soil rating system was applied to the Alabama 
Black Belt area using soil information extracted from USDA-NRCS digital soil survey 
data (SSURGO). The existing NRCS soil limitation rating system (NRCS-SLRS) was 
provided as an available nationwide assessment tool of the study area. The OWTS-SSRS 
rated approximately 52% of the Alabama Black Belt area as ?Unsuitable? for OWTS, 
while the existing NRCS soil limitation rating system rated 89% of the study area as 
?Limiting? for OWTS.  Spatial analysis of results indicates that the new OWTS-SSRS is 
less conservative than the NRCS rating system.  Demographic analysis based on US 
Census 2000 data reveals that rural areas represent approximately 95% of this region with
31 
 
an average house age of over 20 years. Subsequent raster-based analysis was used to 
prioritize the environmental and health risk in the region using a weighted combination of 
OWTS-SSRS results and US Census derived demographics. City fringes were found to 
be generally at higher risk from OWTS than rural areas, suggesting the following two 
mitigation strategies in the Alabama Black Belt region. For isolated rural households 
outside the range of municipal sewer service, retrofit or replacement of aged OWTS with 
more advanced dispersal systems is recommended. For certain city fringe communities at 
high risk, proactive extension of municipal sewer service is recommended in advance of 
widespread OWTS malfunction. Findings of this study indicate the need for continued 
development of planning and assessment tools to protect vital soil and water resources 
and public health in the Alabama Blackland Prairie soil area.  
 
3.2 INTRODUCTION 
Onsite wastewater treatment systems (OWTS) currently disperse 15 million 
metric tons of wastewater per day, serving about 30% of households in the US (Spicer, 
2002; AOWTC, 2005) and functioning as an important supplement to centralized public 
wastewater treatment plants (WWTP). Despite wide adoption, conventional OWTS 
consisting of a septic tank and a gravity fed effluent dispersal field (where effluent is 
treated) can pose a significant threat to the environment (US EPA, 2002).  
Drain field nutrient overload leading to nonpoint source pollution is a recognized 
source of environmental and health risk from OWTS (US EPA, 2002). Typical nitrogen 
concentrations in septic tank effluent range from 40-80 mg L-1 (Walker et al., 1973), of 
which approximately 75% is ammonium nitrogen and 25% is organic nitrogen (Otis et 
32 
 
al., 1974). Reported total phosphorus concentrations in septic tank effluent range from 3 
to 20 mg L-1 (Robertson et al., 1998; Whelan, 1988), with about 85% as orthophosphate 
(Reneau and Pettry, 1976). Charles et al. (2005) analyzed the results of several intensive 
septic tank effluent field surveys from 1976 to 1999 in Australia and the US, including 
their own field survey of 200 convential OWTS in Australia. They compared results to 
published regulations in both countries and concluded that prevailing septic tank effluent 
quality estimates of drain field nutrient load (50-60 mg TN L-1 and 10-15 mg TP L-1) are 
too low to ensure safe environmental performance and may underestimate actual drain 
field nutrient loads. Charles et al. (2005) concluded that the 80th percentile of their 
effluent survey values, 250 mg TN L-1 and 36 mg TP L-1 should be used to estimate the 
nutrient loads associated with conventional OWTS operation. Their research concluded 
that nutrient overloading may be occurring in a significant number of conventional 
OWTS designed under current Australian and US regulations (Charles et al., 2005).  
Septic tank effluent can contain a significant number of pathogens that are a 
potential threat to local and regional environmental health if not properly controlled. 
Although conventional OWTS can provide 50-90% removal of pathogens (Gerba and 
Goyal, 1985; Siegrist, 2007), concerns exist on how far those pathogens can move and 
how long they can survive after they have entered the soil-water system (Jamieson et al. 
2002). It is recognized that physical filtration of soil media is the main mechanism 
limiting pathogen travel (Canter and Knox, 1985; Hagedorn et al., 1981) and that removal 
efficiency is typically inversely proportional to soil particle size (Tanik and Comakoglu, 
1997; Venhuizen, 1995). Nevertheless, despite reports that significant pathogen removal 
in soil can be achieved within a relatively short distance under proper OWTS 
33 
 
management (Tyler et al., 1977; Converse et al., 1991), pathogen translocation up to 830 
m in malfunctioning OWTS is not rare (Venhuizen, 1995; Dowd et al., 1998; Schijven 
and Hassanizadeh, 2000; Charles et al., 2004). Furthermore, since soil moisture sustains 
pathogen survival, even pathogens retained by soil media still remain a threat until they 
die off (Venhuizen, 1995). In the worst case, as a malfunctioning OWTS loses its nutrient 
and pathogen removal capability, the environmental threat gradually expands beyond the 
drain field (Carroll and Goonetilleke, 2005; Charles et al., 2007).  
It is currently understood that although the lot size of an OWTS is designed to 
achieve adequate removal of nutrients and pathogens within a fairly short length of soil 
(Frankenberger, 1988; Green and Cliver, 1975; Venhuizen, 1995), the cumulative impact 
of large numbers of OWTS in a locality can create a threat to both the local environment 
and public health (Carroll and Goonetilleke, 2005).  The spatial density of OWTS 
influences surface and ground water environmental quality due to 1) increased 
probability of system malfunction and 2) cumulative nutrient and pathogen load that 
exceeds the capacity of local soils to assimilate (US EPA, 1977; US EPA, 2002). 
Literature provides the following environmental impact density classes; low (1-4 unit km-
2), medium (5-15 unit km-2), high (16-100 unit km-2), and extremely high (> 100 unit km-
2) (US EPA, 1977; Yates, 1985). Carroll and Goonetilleke (2005), using principal 
component analysis (PCA) on temporally and spatially monitored well-water systems in 
Queensland, Australia, confirmed that a system density of 290 units km-2 significantly 
increased both nutrient and pathogen concentrations in shallow groundwater systems.  
Conventional OWTS have been used in the Alabama Black Belt area for decades 
in spite of the fact that many soils in this region, although rich agriculturally, have a high 
34 
 
smectitic clay content with severe hydraulic limitations. Soil hydraulic limitations to 
wastewater absorption include low permeability, high seasonal water table, presence of 
restrictive layers, and likelihood of flooding. According to the Geological Survey of 
Alabama (1993), Alabama Black Belt soils are underlain at a general depth of 
approximately 6 m, with shallower formations found at 12 cm to 2 m, by a relatively 
impermeable layer of fossiliferous clayey chalk and chalky marl to a depth of 
approximately 122 m. Below that are the Eutaw and Tuscaloosa aquifers, the only 
significant groundwater sources in the Alabama Black Belt region. When top soil layers 
become saturated, the low permeability of the underlying chalk limits deep percolation to 
underground aquifers. Thus, surface ponding and runoff from conventional OWTS drain 
fields is the more common environmental and health concern from malfunctioning 
OWTS in the Alabama Black Belt. 
Since the 1950s, over 10 million acres of row crops have been abandoned in 
Alabama (McNider et al., 2005), much of it in the Black Belt region. As a result, rural 
economies in this region have suffered and many existing conventional OWTS are in 
disrepair. The weakened economy in the Alabama Black Belt makes the retrofit of 
individual OWTS cost-prohibitive for many households (McCoy et al., 2004). In 2002, in 
Lowndes County, an Alabama court ruled against 37 families who discharged raw 
wastewater into their backyards and nearby ditches, drawing national attention to onsite 
system deficiencies in the Alabama Black Belt (McCoy et al., 2004). According to the 
US EPA (2002), approximately 44% of the houses in Alabama are served by OWTSs 
with an average system malfunction rate of 20%. The relatively high OWTS usage rate in 
Alabama along with the geographical, economical, and political uniqueness of the Black 
35 
 
Belt area indicate the need for an assessment of general soil conditions for wastewater 
dispersal in the region. 
The present study uses spatial analysis to assess environmental and health risk of 
conventional OWTS in the Alabama Black Belt. The first objective was to indicate 
suitability of soils in the Alabama Black Belt for conventional OWTS..  A GIS based Soil 
Suitability Rating System (OWTS-SSRS) was developed as an interpretation of the 
current Alabama Onsite Sewage Disposal Rules (ADPH, 2006) over the soils within the 
Alabama Black Belt area using  Soil Survey Geographic (SSURGO) digital soils data 
(NRCS, 2005). The existing Natural Resources Conservation Service soil limitation 
rating system (NRCS-SLRS) for septic tank absorption fields was presented as an 
availble national assessment of the study area. The second objective was to rate the Black 
Belt study area with respect to conventional OWTS environmental and health risk using 
spatial results from the new OWTS-SSRS and derived demographics such as OWTS age, 
size, and density. Based on results, two strategies to mitigate OWTS related 
environmental and health threats in the Alabama Black Belt are proposed. 
 
3.3 MATERIALS AND METHODS 
3.3.1 STUDY AREA DELINEATION  
The Black Belt study area boundary was defined within a GIS (ArcMap 9.2, ESRI, CA) 
using two Common Resource Area (CRA) maps, spatial resolution 400 km2, version 1.1 
(NRCS, 2005), and the Alabama State Soil Geographic Database (STATSGO), spatial 
resolution 6.25 km2, version 1.0 (NRCS, 2005).  The two CRAs used were 135A.1 - 
Blackland Prairie and 135A.2 - Flatwoods/Blackland Prairie Margins. The majority of the 
36 
 
study area lies within Blackland Prairie major land resource areas made up of the 
following Alabama STATSGO mapping units; AL119 - Vaiden-Sumter-Oktibbeha-
Marietta, AL121 - Savannah-Ora-Faceville, AL143 - Vaiden-Sumter-Oktibbeha, AL147 - 
Vaiden-Minter-Kipling-Angie, AL148 - Vaiden-Sumter-Sucarnoochee-Kipling-
Demopolis, AL166 - Sumter-Searcy-Oktibbeha-Demopolis-Congaree-Brantley, AL168 - 
Sumter-Oktibbeha-Luverne-Conecuh, AL173 - Vaiden-Sumter-Leeper, AL238 - Sumter-
Rock outcrop-Oktibbeha-Kipling-Demopolis-Binnsville, and AL239 - Vaiden-Okolona-
Kipling.  
In an effort to delineate a continuous study area across central Alabama, non-
continuous CRAs were redefined to include narrow areas of CRA 133A.7 - Coastal Plain 
Floodplains and Low Terraces which bisect the study area. CRAs 135A.1 and 135A.2 
were first overlaid and unioned with STATSGO delineations, then a 1500-meter buffer 
was applied to the union. Next, CRA 133A.1 - Southern Hilly Gulf Coastal Plain and 
CRA 133A.7 were overlaid and unioned with the generated 1500-meter buffer. The 
polygon from this delineation became the study area boundary (Figure 3.1). The resulting 
study area extends across central Alabama, including parts of Sumter, Greene, Pickens, 
Hale, Marengo, Perry, Dallas, Lowndes, Butler, Wilcox, Montgomery, Macon, Bullock 
and Russell counties, with a total area of 13981 km2 and an estimated population of 
394,000 (US Census, 2000). 
37 
 
 
Figure 3.1 The Black Belt study area boundary showing STATSGO Black Belt mapping 
units overlaid with CRA 135A.1 (Blackland Prairie) and 135A.2 (Flatwoods/Blackland 
Prairie Margins) land resource area data.  
 
 
3.3.2 NRCS SOIL LIMITATION RATING SYSTEM (NRCS-SLRS) FOR SEPTIC TANK 
ABSORPTION FIELDS.  
Based on available county soil survey information, NRCS developed a national septic 
tank absorption field limitation rating system for guiding both heavily and sparsely 
populated areas in site selection for safe disposal of household effluent (NRCS, 1993 
2007; USDH, 1969). The NRCS Limitation Ratings indicate that the soil has properties 
that may limit the functionality for the intended use and do not indicate whether the soil 
is unsuited for that use (NRCS, 1993). Thirteen soil and site condition criteria (Table 3.1) 
from SSURGO (NRCS, 2007) are considered in the NRCS soil limitation rating system 
38 
 
(NRCS-SLRS). Soil mapping units are rated as Limiting, Somewhat Limiting, or Not 
Limiting for OWTS siting based on the dominant soil series for each soil mapping unit. 
Dominant soil series is defined by NRCS as the associated soil series with the highest 
percentage within the mapping unit. Soil mapping units covered by water or otherwise 
inaccessible or undevelopable such as military areas are listed by NRCS as Not Rated.  
In this study, the NRCS-SLRS result was extracted directly from digital SSURGO 
soil mapping units (spatial resolution 0.02 km2) using Soil Data Viewer (Version 5.1, 
USDA) for spatial display within the study area. SSURGO data for Dallas and Lowndes 
Counties were not available at the time of analysis; therefore, soil information and rating 
system comparison for these two counties is not included. 
39 
 
Table 3.1 NRCS Soil Limitation Rating System (NRCS-SLRS) rating criteria for septic tank absorption fields.  
Source: Derived from SSURGO database (NRCS, 2007). 
          Rating  
          criteria 
 
NRCS 
Ranking 
Maximum 
Ksat in 60-
150 cm (um 
s-1) 
Flooding 
occurrenc
e 
Content of 
large 
stones 
Minimum Ksat 
in 60 cm-
restrictive layer 
(um s-1) 
Depth to 
permafrost 
(cm) 
Ponding 
duration 
Depth to 
bedrock 
(cm) 
Depth to 
cemented 
pan (cm) 
Slope Subsidence (cm) 
Depth to 
saturation 
zone (cm) 
Seepage 
( min 
cm-1) 
Unstable 
fill 
Not Limiting ?41.6 "none" <25% <41.6 >100 others ?182 ?182 <8% <60 ?180 >12 Others 
Somewhat 
Limiting ?41.6 
"very 
rare" or 
"rare" 
25-50% 9.80-41.6 50-100 others 100-182 100-182 8-15% <60 120-180 >12 Others 
Limiting <41.6 
"occasion
al" or 
"very 
frequent" 
or 
"frequent 
>50% >9.80 <50 
?very 
brief? or 
?brief? 
?long? 
or ?very 
long? 
?100 ?100 >15% ?60 <120 ?12 Unstable fill 
40 
 
3.3.3 NEW OWTS SOIL SUITABILITY RATING SYSTEM (OWTS-SSRS) 
To develop a new soil rating system for subsurface gravity dispersal of septic tank 
effluent, each soil mapping unit from the SSURGO database (NRCS, 2007) in the study 
area was ranked for suitability according to current Site Evaluation Criteria from the 
Alabama Onsite Sewage Disposal Rules (ADPH, 2006). A rating of Suitable, Marginally 
Suitable, or Unsuitable (Table 3.2) was assigned to each mapping unit based on 
properties of the dominant soil series. These three ratings indicate whether or not an 
OWTS system can be saftely located on a particular site. In the SSURGO database, soil 
mapping units within military areas, water bodies, or other unavailable areas carry no soil 
information except for land area and thus were not rated. The following numeric code; 
(1) for Suitable, (2) for Marginally Suitable, (3) for Unsuitable, and (4) for Not Rated was 
applied to each soil mapping unit for subsequent spatial analysis and environmental and 
health risk ranking.  
The OWTS-SSRS result was extracted directly from digital SSURGO soil 
mapping units in the study area for spatial display and further analysis. As in NRCS-
SLRS, digital soil information for Dallas and Lowndes Counties is not reported because 
county soil survey information was not completed at the time of this study. 
Table 3.2 New OWTS Suitability Soil Rating System (OWTS-SSRS) rating criteria. 
Based on: Site Evaluation Criteria, Alabama Onsite Sewage Disposal Rules (ADPH, 2006). 
       Rating    
       criteria 
 
OWTS 
Ranking 
Percolation rate 
from 60 cm depth 
to restrictive layer 
 (min cm-1) 
Depth to 
restrictive layer 
(cm) 
Depth to 
seasonal 
water table 
(cm) 
Slope 
(%) 
Flooding 
Suitable <35 >91 >91 0-25 None, rare 
Marginally 
Suitable 35-47 >91 >91 26-40 Occasionally 
Unsuitable >47 <91 <91 >40 Frequent 
41 
 
3.3.4 ONSITE WASTEWATER TREATMENT SYSTEM STATUS IN THE ALABAMA BLACK 
BELT AREA  
The most recent US Census does not provide individual household sewer disposal 
information. Due to legislative reasons, the last year such information was provided was 
1990 (Department of Housing and Urban Development, US Census Bureau, personal 
communication, May 11, 2009). Consequently, OWTS demographics used for this study 
were derived from available 2000 US Census block group data, the second smallest 
mapping unit for which US Census data is available. The spatial resolution of the US 
Census data used for the study area was determined to be 1.70 km2. 
Census block groups in this study were categorized as rural if over 70% of the 
block group population was rated rural by the US Census. Otherwise, the block group 
was rated as urban. Households classed as urban were assumed to be connected to a 
public sewer system. The assumption was made that if a household fell within a rural 
block area, the household was served by an OWTS. The method used to extract US 
Census information did not distinguish between single- and multi-family units or 
commercial and industrial units, so may not accurately represent individual OWTS status 
in each Census block group. 
The lifetime of an OWTS depends on its design, installation, and maintenance. If 
all other things were equal, a higher OWTS age suggests a higher probability of 
malfunction (US EPA, 2002). In the present study, the average house age in each rural 
block was used to represent average OWTS age. For example, an average house age of 
20 years in a block group indicated an average OWTS age of 20 years. The ratio of total 
42 
 
population to the total number of household units in a rural block group was used to 
represent the average OWTS size (person unit-1) in a block group. The ratio of the 
number of household units to the area of the corresponding rural block group was used to 
represent average OWTS density (unit km-2). A higher OWTS size and density suggests a 
higher septic effluent load. It has been demonstrated that the higher the average OWTS 
size or density in an area, the higher the probability of malfunctioning OWTS (US EPA, 
1977).  
 
3.3.5 PRIORITIZING ENVIRONMENTAL AND HEALTH RISK AREAS  
An environmental and health risk map was developed to prioritize the potential 
risk from OWTS in the 14-county Black Belt study area. OWTS-SSRS ratings and 
corresponding metrics of OWTS age, size, and density from US Census data were 
normalized, weighted, and summed to indicate the spatial distribution of environmental 
and health risk in the study area. Data files containing OWTS-SSRS ratings, OWTS age, 
size, and density were each converted into a 141 m ? 141 m raster dataset, then into 
numeric ASCII format. A total of 1053436 raster cells were used in the study area and 
subsequent analysis. The 141 m resolution was used rather than the 0.02 km2 resolution 
of the SSURGO dataset because it represents the smallest mapping unit of the spatial 
datasets used in the study.  
Since there are no direct field measurements of potential environmental and 
health risk of OWTS in the study area, the relative weighting of the four variables with 
respect to the potential of environmental and health risk from OWTS were assumed equal. 
43 
 
Thus, the potential environmental and health risk from a OWTS within each individual 
raster cell of the study area was calculated as shown in Eq. 3.1. The resulting values from 
Eq. 3.l over the entire study area were then normalized from 0 to 100 with higher values 
indicating a higher potential environmental and health risk.  
Environmental and health risk rating = [OWTS density] + [OWTS-SSRS] + [OWTS size] 
+ [OWTS age]?????????.Eq. 3.1       
 
3.4 RESULTS AND DISCUSSION 
3.4.1 SOIL CONDITIONS OF THE ALABAMA BLACK BELT AREA. 
Based on analysis of the SSURGO database, 173 soil series occur within the 
Black Belt study area. Excluding missing Dallas and Lowndes counties, the fifteen most 
widely occurring soil series are listed in Table 3.3. Each soil represents at least 2% of the 
study area, and in combination 51% of the total study area. It is apparent that clayey soils 
dominate, with Conecuh, Kipling, Luverne, Sucarnoochee, Sumter, Vaiden, and Wilcox 
being the most prominent clay soil series. These widespread clay soils are generally 
unsuitable for conventional OWTS.  
Mapped results from the newly developed OWTS-SSRS (Figure 3.2) indicate that 
approximately 52% of the study area (Dallas and Lowndes Counties excluded) is rated 
Unsuitable for conventional OWTS, 31% is rated as Marginally Suitable, and 15% is 
rated as Suitable. The remaining 2% land or water area is Not Rated. The corresponding 
NRCS-SLRS rating map (Figure 3.3) indicates that approximately 89% of the study area  
 
 
44 
 
Table 3.3 Major soil series in the Alabama Black Belt study area based on SSURGO 2007 
database.  
Soil series 
name 
Area 
(km2) 
Percentage of 
total study area  
NRCS description 
Luverne 2786 13 Mixed, semiactive, thermic Typic Hapludults 
Smithdale 1600 7 Fine-loamy, siliceous, subactive, thermic Typic Hapludults  
Oktibbeha 702 3 Very-fine, smectitic, thermic Chromic Dystruderts  
Conecuh 702 3 Fine, smectitic, thermic Vertic Hapludults  
Sumter 642 3 Fine-silty, carbonatic, thermic Rendollic Eutrudepts  
Sucarnoochee 579 3 Fine, smectitic, thermic Chromic Epiaquerts  
Mantachie 515 2 Fine-loamy, siliceous, active, acid, thermic Fluventic Endoaquepts  
Kinston 509 2 Fine-loamy, siliceous, semiactive, acid, thermic Fluvaquentic Endoaquepts  
Kipling 483 2 Fine, smectitic, thermic Vertic Paleudalfs  
Vaiden 460 2 Very-fine, smectitic, thermic Aquic Dystruderts  
Mooreville 420 2 Fine-loamy, siliceous, active, thermic Fluvaquentic Drystrudepts  
Demopolis 417 2 Loamy, carbonatic, thermic, shallow Typic Udorthents  
Savannah 406 2 Fine-loamy, siliceous, semiactive, thermic Typic Fragiudults  
Wilcox 396 2 Very-fine, smectitic, thermic Chromic Dystruderts  
Halso 323 2 Fine, smectitic, thermic Vertic Hapludults  
Subtotal 10940 51  
Remaining 
area* 10565 49 
Total 21505 100 
Note: 1. Dallas and Lowndes counties excluded due to unavailability of SSURGO data. 
          2. Includes remaining 1580soil series, water body, and military areas. 
 
is Limiting for conventional OWTS, 8% is Somewhat Limiting, 1% is Not Limiting, and 
the remaining 2% is Not Rated. Approximately 5% of the area that is rated as Limiting by 
NRCS-SLRS is rated as Suitable for conventional OWTS by the new OWTS-SSRS, 
while 43% of the area that is rated Suitable by the OWTS-SSRS is rated Not Limiting by 
NRCS-SLRS.  
45 
 
 
Figure 3.2 Soil rating results of the new OWTS Soil Suitability Rating System (OWTS-
SSRS), Alabama Black Belt. (Dallas and Lowndes Counties excluded due to unavailability 
of SSURGO data) 
 
 
46 
 
 
Figure 3.3 Soil rating results of the NRCS Soil Limitation Rating System (NRCS-SLRS) for 
septic tank absorption field, Alabama Black Belt. (Dallas and Lowndes Counties excluded 
due to unavailability of SSURGO data) 
 
Although the two soil rating systems share some similar site rating criteria, such 
as depth to restrictive layer, depth to seasonal water table, slope, and flooding frequency, 
the major difference is that they use two different measurements to characterize water 
movement in the soils. The NRCS-SLRS uses Ksat while the OWTS-SSRS uses 
percolation rate. Even though statistic relationships were developed between Ksat and 
percoaltion rate (Fritton et al., 1986), it should be noted that Ksat is different from 
percolation rate and not directly comparable. Ksat is a measurement of how fast water can 
pass through a one-dimensional saturated soil medium under one unit hydraulic gradient, 
47 
 
while percolation rate is a three-dimentional infiltration measurement for a generally 
unsaturated soil under a variable hydraulic head (SSSA, 2002b). Therefore, the two soil 
rating systems can not be quantitatively compared. 
Furthermore, it is important to note when attempting to compare the two rating 
systems that the term ?Limited? does not equate to the rating ?Unsuitable.?  Although 
both soil rating systems are based on long term field experience (NRCS, 1993; USDH, 
1969; ADPH, 2006), the NRCS-SLRS was developed for national level guidance on sites 
being evaluated for a wide range of potential land uses, while the newer OWTS-SSRS 
was based on a specific environmental rule (ADPH, 2006) that regulates the design and 
permitting of conventional OWTS within the state of Alabama. The NRCS Limitation 
Ratings indicate soil properties that may limit the functionality for the intended use, but 
do not indicate whether the soil is unsuited for that use. Conversely, the ?Unsuitable? 
rating indicates that the soil limitations are so severe as to make the soil unsuitable for the 
intended use. Soils identified as ?Limited? in the NRCS system could fall in either 
the ?Suitable?, ?Marginally suitable? or ?Unsuitable? categories in the new Suitability 
Rating system.  
Despite the differences between the two soil rating systems, the general findings 
from both systems is that a large percentage of land within the Alabama Black Belt is not 
recommended for conventional OWTS. This situation calls for alternative systems that 
can function properly on clayey soils. To make the developed OWTS-SSRS more 
beneficial for the Alabama Black Belt, the new OWTS-SSRS could be expanded to 
include ratings for alternative engineered systems such as aeration treatment units, 
48 
 
packed-bed media filters, mounds, or subsurface drip irrigation. Engineered systems such 
as these can exert less burden on native soils for contaminant dispersal by providing 
secondary in-line wastewater treatment, additional treatment media, or improved dosing 
strategies. Although beyond the scope of the present work, an expanded soil rating 
system that includes advanced engineered systems would benefit regional decision 
makers in their evaluation of decentralized versus centralized systems in the Black Land 
Prairie soils area.  
 
3.4.2 CURRENT STATUS OF CONVENTIONAL ONSITE WASTEWATER TREATMENT 
SYSTEMS 
Spatial analysis of US Census block groups in the study area indicates that rural 
(non-urban) areas represent approximately 95% of the Alabama Black Belt area. The 
average size and density of OWTS in rural block groups is illustrated in Figure 3.4. 
Approximately 12% of rural block groups have an estimated OWTS density higher than 
15 unit km-2, posing a potentially high risk to the environment (US EPA, 1977). These 
12% at-risk rural block groups are found generally clustered around city fringes such as 
Montgomery, Selma, and Uniontown, with the highest single block group OWTS density 
(212 unit km-2) found at Uniontown. In 2004, the city of Uniontown in Perry County was 
awarded $350,000 from the Alabama state government to subsidize a public sewer 
service package to eliminate malfunctioning septic tanks on the south side of town and 
extend sewer service to 74 households. This project benefitted more than 200 households 
(Alabama Department of Economic and Community Affairs, 2004). Uniontown currently 
has a lagoon system serving approximately 2000 households that treats between 0.5 and 
49 
 
1.0 MGD of public wastewater. The municipality is currently seeking resources to 
expand its public sewer service to benefit more residences (Uniontown Public Works 
Department, personal communication, May 27, 2009). 
OWTS size in the study area ranges from 1 to 18 persons system-1, with 
approximately 73% of rural block groups having an OWTS size of 2 persons system-1 or 
less. Approximately 99% of rural block groups have an estimated OWTS size lower than 
3 persons system-1. The remaining 1% of rural block groups that have an estimated 
OWTS size greater than 3 persons system-1 are found around the city fringe of 
Montgomery, a major urban center in the Black Belt area and the capital of the state 
(Figure 3.4). The finding that rural block groups in close proximity to urban areas have 
high OWTS density and size is expected given the higher population concentrations in 
these urban and surrounding fringe areas. 
50 
 
 
Figure 3.4 Average OWTS density and size in rural block groups, Alabama Black Belt.  
 
The estimated home age in rural census block groups of the study area (data not 
shown) indicates that approximately 97% of rural block groups in 2000 had an average 
house age over 20 years; and approximately 93% among them had an average house age 
between 20 to 30 years. In 1990, Alabama was ranked fifth in states that rely on OWTS 
for household wastewater treatment and dispersal (44% households relying on OWTS 
compared to the highest, Vermont, with 55%) (US EPA, 2002). Maintenance of 
individual septic systems has already been recognized as a financial difficulty for many 
households in the Alabama Black Belt (McCoy et al. 2004), a burden that is increased 
due to widespread aging of conventional OWTS. 
51 
 
OWTS status in the study area is summarized as follows: 1) OWTS are widely 
used in the Alabama Black Belt; 2) a significant number of existing OWTS in the study 
area have been operating for more than 20 years; 3) system size is generally maintained 
below 2 persons unit-1 except around major urban centers; and 4) 12% of census block 
groups have an estimated OWTS density higher than the EPA regulated threshold for 
negative environmental impact (15 unit km-2) and are generally found clustered around 
city fringes. 
 
3.4.3 PRIORITIZING ENVIRONMENTAL AND HEALTH RISK AREAS 
The spatial distribution of potential environmental and health risk using the new 
soil suitability ratings, OWTS age, size, and density in each 141 m?141 m raster cell of 
the study area was mapped and is illustrated in Figure 3.5. Lowndes and Dallas counties 
were excluded due to unavailability of SSURGO data. Relatively higher potential 
environmental and health risks are observed around city fringe areas such as Montgomery 
and around smaller towns such as Selma and Uniontown where OWTS densities are 
relatively high (>100 units km-2).  This finding is not unexpected since city fringe areas 
with high OWTS densities have in fact been the focus of public health concern for 
decades as public sewer systems have continuously expanded to keep up with urban and 
suburban sprawl (Boyle and Otis, 1981; DeWalle and Schaff, 1980). This study confirms 
through both PCA and spatial analysis what has been reported by previous researchers 
that OWTS density is the factor that most influences local environmental and sanitary 
conditions (Lipp et al., 2001; Carroll and Goonetilleke, 2005).  
52 
 
 
Figure 3.5 Environmental risk analysis results in the Black Belt study area. Dallas and 
Lowndes Counties excluded due to unavailability of SSURGO data. Urban areas excluded 
due to general availability of public sewer service. 
 
 
3.4.4 APPLICATION OF RISK ASSESSMENT MAPPING 
To illustrate the application of the risk analysis map, rural environmental and 
health risk ratings were overlaid with a 30 m false color Orthoimage (USGS, 2003) of 
Montgomery, Alabama. On the enlarged environmental and health risk map (Figure 3.6), 
the city boundary (thin black line) is shown along with current public WWTP service 
extents (green with black centerline).  
 
53 
 
 
Figure 3.6 Environmental risk analysis of the greater Montgomery area, Alabama. Areas 
not rated are either classed as urban areas, or census block group extents are completely 
outside the study area.  
 
Figure 3.6 indicates that public WWTPs are currently serving the general 
Montgomery metropolitan area, including numerous areas indicated as high 
environmental and health risk around the city fringe. However, several potential high risk 
areas within the city boundary to the west and south remain unsewered. The awareness 
that clayey soils and high OWTS densities are prevalent within the greater Montgomery 
municipal area has prompted the Montgomery Water Works & Sanitary Sewer Board to 
extend service to certain clayey soil areas in advance of OWTS malfunction 
(Montgomery Water Works & Sanitary Sewer Board, personal communication, February 
54 
 
12, 2009). For example, the Oak Hills WWTP located south of the city boundary 
currently serves an area indicated as having relative low risk, but provides potential 
future sewer service to surrounding high risk, unsewered areas to the north and west. 
Likewise, the Milleys Creek and Catoma WWTPs on the east and west sides of the city 
may be able to expand their respective service areas to adjacent high risk areas as 
necessary. In general, in the case of Montgomery, OWTS related environmental and 
health risks have been recognized and government efforts have been made to limit that 
risk.  It is hoped that city fringe and other areas within Alabama can benefit from spatial 
assessment of environment and health risk from OWTS and respond with similar 
proactive planning strategies. 
Although city fringes are found to have generally higher environmental and health 
risk from OWTS, especially in clayey soils, the result of this study does not suggest that 
the risk in rural areas in the Alabama Black Belt are insignificant. Prevailing system age 
greater than 20 years throughout this predominantly rural region strongly suggests the 
need to recondition aging OWTS. Since over half of rural sites in the Alabama Black Belt 
area are not suitable for conventional OWTS, alternative engineered systems such as 
mound systems or secondary treated drip irrigation systems approved by the Alabama 
Department of Public Health (ADPH, 2006) become the practical option for more than 
62000 individual households in the region. Subsidized septic system retrofits may be 
recommended in certain rural communities facing higher public health threat. Assessment 
mapping of the type provided in this study can be used to target limited public resources. 
55 
 
Across the Alabama Black Belt region, continued targeted governmental efforts may be 
needed to successfully manage OWTS related environmental and health threats. 
 
3.5 CONCLUSION 
This study developed a new OWTS soil suitability rating system (OWTS-SSRS) 
based on current Alabama state OWTS regulations (ADPH, 2006) and on site conditions 
derived from SSURGO digital soil information (NRCS, 2007). The new OWTS-SSRS 
was compared to the existing NRCS soil limitation rating system (NRCS-SLRS) for 
siting of septic tank adsorption fields. The older NRCS rating system, a more 
conservative rating system, was also based on county soil survey data. Both assessment 
tools indicate that soil properties within the Black Belt study area of Alabama are 
generally unsuitable for conventional OWTS due to the prevalence of low permeability 
clayey soils, shallow ground water table, underground restrictive layers, steep slope, 
and/or flooding frequency. The new OWTS-SSRS rated 52% of the Alabama Black Belt 
study area as Unsuitable for OWTS, while the NRCS-SLRS rated 89% of the study area 
as Limiting for septic tank absorption fields. The difference between the two soil rating 
systems derives mainly from threshold values used by each to classify similar site 
conditions. The new OWTS-SSRS uses less restrictive values than the nationally 
distributed NRCS-SLRS but follows current Alabama Department of Public Health 
regulations. The new OWTS-SSRS is considered more practical for the state of Alabama 
because all potential OWTS sites in Alabama are verified by field reconnaissance before 
approval for installation.  In those cases where soil site conditions prohibit the use of 
conventional OWTS, alternative engineered systems are mandated. With generally 
56 
 
unfavorable soil conditions for conventional OWTS in the Alabama Black Belt, the 
expansion of the new OWTS-SSRS to include alternative engineered systems is 
recommended to further benefit the Alabama Black Belt in terms of wastewater dispersal.  
Mapped results presented indicate that areas around city fringes have a higher 
environmental and health threat as a consequence of older OWTS of larger size and 
higher density. Consequently, city fringe areas associated with system densities greater 
than 100 unit km-2 should receive timely attention to mitigate risk from critical 
wastewater loading.  However, because rural areas also need to assess and manage the 
potential risk of OWTS loading, two strategies to limit the potential environmental and 
health risk from OWTS malfunction in the Alabama Black Belt area are suggested. For 
city fringe communities, the proactive response is to extend municipal sewer service to 
high risk clay soil areas in advance of widespread OWTS malfunction. For isolated rural 
households outside the practical range of municipal sewer service or decentralized 
community systems, subsidized retrofitting, repair, or replacement of aged OWTS with 
alternative engineered systems is recommended.  
Finally, this study demonstrates how spatial technologies and planning strategies 
can be used to target potentially serious regional non-point source pollution threats from 
aging and malfunctioning OWTS. OWTS risk assessment tools such as regional mapping 
products can be used to educate stakeholders about the direct link between soil and water 
stewardship, local environment, and regional public health. The GIS and demographic 
methods presented in this paper can be replicated to generate soil rating maps for the 
remaining counties of the Alabama Black Belt area or the entire state once digital 
57 
 
SSURGO soil data is made available. An expanded soil rating system that includes an 
evaluation of alternative engineered systems is recommended to facilitate individual, 
community, and government response to targeted critical risk areas. 
58 
 
CHAPTER FOUR  
SOIL MOISTURE CONTROLLED WASTEWATER DISPERSAL ? YEAR ONE: 
LABORATORY AND FIELD TESTING 
 
4.1 ABSTRACT 
Conventional onsite wastewater treatment systems (OWTS) in the Alabama Black 
Belt often rely on clayey, smectitic (shrink-swell) Vertisols for effluent dispersal. This 
study explores an alternative wastewater dispersal method for soils of this region. A field 
moisture controlled subsurface drip irrigation (SDI) system was designed, assembled, and 
tested under laboratory and field conditions. The objectives of this study were to: 1) 
design an automated control system that incorporates soil moisture monitoring into a 
conventional wastewater SDI system; 2) complete laboratory testing of the control 
system using soils of dissimilar permeability; 3) test automated system response to 
seasonal field conditions for a one year period using preprogrammed field moisture 
control set points; and 4) test system water balance with a seasonal cropping system in 
the drain field. A soil moisture monitor/control data logger (Delta-T, UK) was wired in 
series with a commercial wastewater SDI system (Geoflow, CA) to achieve soil moisture 
controlled SDI dosing. The experimental system was designed to disperse wastewater 
only when field moisture within the upper 45 cm was between 0.40 m3 m-3 and 0.45 m3 
59 
 
m-3. Field testing took place from August 2006 to June 2007 at the Alabama Black Belt 
Research and Extension Center in Marion Junction, AL. A 500 m2 experimental drain 
field was sized for a family of three producing approximately 1 m3 of wastewater per day 
(ADPH, 2006). Soils are classified as very-fine, smectitic, thermic Oxyaquic Hapluderts. 
A warm season sorghum-sudangrass (Sorghum bicolor) and a cool season mix of winter 
wheat (Triticum aestivum) and rye (Secale cereale) was planted in rotation in the drain 
field. SDI wastewater tubing was installed at 20-25 cm depth, with two capacitance type 
soil moisture sensors buried at 20 cm and 45 cm depths near the center of the irrigated 
field. One year operation indicated that although the system was internally robust, 
unexpected power and water outages curtailed operation on occasion. The system 
effectively withheld water during wet soil conditions as designed and provided seasonally 
varying water dosing rates over periods of dryer soil conditions. Zero water dosing during 
wet soil conditions and during field crop harvesting and planting seasons indicates that at 
least two months of wastewater storage is required for this experimental system. 
Although percolation is a necessary component of effluent treatment in an OWTS, a 
monthly water balance of the experimental site estimates that over 30% of water applied 
during the drought period from March 2007 to June 2007 was lost to percolation below 
45 cm, presumably as a result of dry weather clay soil cracking. Improved system control 
over percolation is recommended by placing multiple soil moisture sensors vertically and 
horizontally or by reducing the emitter and drip line spacing.  
 
 
60 
 
4.2 INTRODUCTION 
Conventional onsite septic tank systems include an underground septic tank with 
a gravity drain field for the effluent. These systems, also called conventional onsite 
wastewater treatment systems (OWTS) (The Onsite Consortium, 2007), are the most 
common decentralized wastewater disposal method in the Alabama Black Belt area 
because of the relatively low cost of installation, operation, and maintenance (ADPH, 
2006). Collected residential sewage goes through primary settling and biological reaction 
in the septic tank. Upon reaching a preset overflow, supernatant is dispersed (where 
effluent is getting treated) by gravity to a drain field where percolation through an 
unsaturated soil zone provides aerobic treatment of the effluent (ADPH, 2006). The 
environmental challenge for conventional OWTS comes from the almost complete 
reliance on soil properties for proper waste treatment (Oron, 1996). Soils having too high 
or too low a percolation rate are generally not suitable for conventional onsite septic 
systems (US EPA, 2002). In the shrink-swell clay soils that dominate the Black Belt 
region of Alabama, conventional OWTS can pose a genuine environmental and health 
threat if not designed and operated properly (McCoy et al., 2004).   
Drain field failure and subsequent nutrient overload is a recognized risk from 
OWTS. Typical nitrogen concentrations in septic tank effluent range from 40-80 mg L-1 
(Walker et al., 1973), of which approximately 75% is ammonium nitrogen and 25% is 
organic nitrogen (Otis et al., 1974). Reported total phosphorus concentration in septic 
tank effluent ranges from 3 to 20 mg L-1 (Robertson et al., 1998; Whelan, 1988), with 
about 85% as orthophosphate (Reneau and Pettry, 1976). Charles et al. (2005) analyzed 
61 
 
several intensive septic tank effluent field surveys from 1976 to 1999 in Australia and the 
US and concluded that nutrient overloading may be occurring in a significant number of 
OWTS designed under current Australian and US regulations. They recommended that 
the 80th percentile of effluent survey values, 250 mg TN L-1 and 36 mg TP L-1, should be 
used in OWTS design to minimize the nutrient overloading associated with onsite drain 
field failure.  
Incidences of poor treatment performance from onsite treatment systems, 
particularly onsite septic systems, are common in the US and worldwide (US EPA, 2002; 
Carroll and Goonetilleke, 2005), and are a significant source of water pollution (Beggs et 
al., 2004). Lipp et al. (2001) demonstrated the adverse pathogen impact from onsite 
sewage systems to a coastal community in Sarasota Bay, FL. Carroll and Goonetilleke 
(2005) confirmed that a high system density (290 systems km-2) significantly impacted 
shallow groundwater systems due to the cumulative nutrient and pathogen load that 
exceeds the capacity of local soils to assimilate (US EPA, 1977; US EPA, 2002).  
A series of GIS analyses conducted by He et al. (2007) evaluated environmental 
and health risk to ground and surface water from conventional onsite septic systems in 
the Alabama Black Belt soil area.  In 2000, more than 97% of the rural census block 
groups in this region had onsite systems with an average age of over 20 years. This data 
confirms the widespread use and aging of conventional onsite septic systems in the area. 
Subsequent risk analysis and ranking revealed that in absence of centralized municipal 
wastewater collection, ground and surface water resources immediately surrounding city 
fringes are at higher risk of being impaired by high OWTS densities over 15 units km-2.  
62 
 
Subsurface drip irrigation (SDI) has the potential to address some of the issues 
with onsite septic systems because the application of water is below the soil surface 
through emitters. Additionally, discharge can be rated by standard methods of uniformity 
and efficiency (Camp, 1998). Wastewater disposal through SDI can provide improved 
application efficiency and more uniform distribution of effluent throughout the reuse 
area, reducing the risk of ground and surface water contamination (Jnad et al., 2001).  
Commercially available wastewater SDI systems designed for onsite septic tank 
effluent are usually coupled with a small, advanced or secondary treatment system with 
sufficient treatment and filtering capacity to prevent clogging of SDI emitters. Most SDI 
control panels utilize a preset time interval for hydraulic dosing of drain fields but do not 
incorporate any automated control besides high and low water cut-off and alarm (Jnad et 
al., 2001). Soil moisture controlled SDI systems have been shown to improve water use 
efficiency by reducing evapotranspiration (Dukes et al., 2005). For domestic wastewater 
dosing in the Alabama Black Belt region, automated dispersal systems have potential to 
reduce the risk of drain field hydraulic overloading by not exceeding the field capacity in 
native clay and other heavy soils. 
 In this study, a pilot scale SDI wastewater dispersal system controlled by 
volumetric soil moisture content was built and evaluated in the laboratory for subsequent 
field installation and testing. This study completes the retrofit of a commercially 
available SDI wastewater dispersal system with independent soil moisture feedback to 
prevent drain field hydraulic overload. The concept is to allow wastewater dispersal only 
when field moisture below field capacity so as to limit ponding and deep percolation 
63 
 
while enhancing crop water uptake and aerobic soil treatment. Although this 
experimental wastewater SDI dispersal system may not be cost effective for the majority 
of rural home owners in the Black Belt, water balance observations in this study provide 
information regarding system feasibility. 
The goal of the study was to evaluate the hydraulic management of an 
experimental wastewater SDI system in a clay soil site. The objectives were to: 1) design 
an automated control system that incorporates soil moisture monitoring into a 
conventional wastewater SDI system; 2) complete laboratory testing of the control 
system using soils of dissimilar permeability; 3) test the automated system response to 
seasonal field conditions for a one year period using preprogrammed field moisture 
control set points; and 4) evaluate system water balance with a seasonal cropping system 
in the drain field. 
 
4.3 MATERIALS AND METHODS  
The experimental system was assembled in the laboratory to test operation of 
electrical and hydraulic components. The system was then installed and evaluated with 
clean well water on a 500 m2 natural Houston clay site from September 06, 2006 to June 
14, 2007.  
 
4.3.1 LABORATORY ASSEMBLY AND TESTING METHODS 
CONTROLLER INTERFACE  
A wastewater SDI controller (GEO1, Geoflow, CA) was wired to a data 
logger/controller (GP1, Delta-T, UK) to provide both real-time soil moisture control and 
64 
 
data logging capabilities for the experimental system. The GEO1 is an elapsed time meter 
(ETM) controller that uses only low reservoir water level to interrupt a programmable 
on/off wastewater dosing sequence. The GP1 data logger/controller is manufactured for 
research with the capability to collect and archive data from the following test devices; 2 
capacitance type soil moisture sensors (ML2 ThetaProbe, Delta-T, UK) with typical 
errors of ? 0.01 m3 m-3 after being validated with intact soil cores, 1 soil temperature 
sensor, 1 flow sensor, and 1 tipping bucket rain gauge. Logged data from the GP1 can be 
programmed, viewed, retrieved, and archived for selected time intervals using 
accompanying software (DeltaLINK-PC, Delta-T, UK). The GP1 data logger/controller 
was connected to the two soil moisture sensors. The programmable GP1 controlled the 
opening and closing of an external 12V circuit based on preset thresholds.  
The electrical schematic in Figure 4.1 depicts the interface of the GEO1 control 
panel with the GP1 data logger/controller. GP1 12V output controls a 12V/115V 
intermediate relay. The 115V side of the intermediate relay was installed in series with 
the existing low water float switch (#2) circuit. A 12V light-emitting diode (LED) was 
installed between terminals 14-13 of the intermediate relay as a visual indicator of the 
status of relay A.  
Two water float switches (#2 and #4) out of four typically provided with the 
GEO1 controller (10 amps, 120/230V AC) were used in this study. Float 4 is a high water 
level alarm, used to indicate excessive effluent in the reservoir. Float 2 signals low water 
level, in which case the ETM is interrupted until a safe liquid pumping level is restored. 
Terminals 2 and 4 in the GEO1 control panel correspond to the positive side of the two 
65 
 
float switches, with two #5 terminals provided as ground terminals. Terminal 12 in the 
intermediate relay was wired into terminal 2 of the GEO1 control panel while terminal 8 
was piggybacked onto the positive side of float 2. The negative side of float #2 was 
grounded to terminal 5. With this electrical design, the dosing sequence is activated only 
when both: 1) the intermediate relay is closed, indicating that soil moisture readings are 
within the designated range, and 2) the circuit for float #2 is closed, indicating an 
adequate water level for pumping.  
 
Figure 4.1 Connection diagram illustrating interface of soil moisture sensors and control to 
existing float-based irrigation dosing system. (Soil moisture sensors not shown) 
 
LABORATORY DESIGN AND TESTING 
The integrated control and data logging system provided by the GP1 and GEO1 
was laboratory tested using an SDI system consisting of two lengths of wastewater drip 
tubing (Figure 4.2). A 0.37 kW submersible effluent pump (Myers, OH) was wired into 
the GEO1 control panel and placed into a plastic 1250 liter water tank. Pumped water 
flowed through a headworks box equipped with a vortex filter and a 240 kPa (35 psi) 
66 
 
pressure regulator before flowing into two parallel, 6.1 m long SDI drip tubes. The drip 
tube was WFPC16?2?24, 16 mm diameter (Geoflow, CA) with 0.61 m emitters spacing. 
The maximum allowable particle size that can pass through emitters is 100 ?m. The 
downstream end of each drip tube was attached to a 2.54 cm return manifold, equipped 
with an air vacuum breaker. The 7.62 m return line returned water through the headworks 
box and into the water tank. 
The GEO1 operating sequence, based on the SDI manufacturer?s recommendation 
(Geoflow, 2003), was set to a 5-minute dosing period followed by a 25-minute resting 
period, providing for approximately two dosing cycles each hour. A forward field flush 
operation that flushes drip tubes to clean potentially clogged emitters was programmed to 
occur every 10th dosing cycle. The 5-minute field flush valve was manually set to 
provide a system pressure (241 kPa) sufficient to maintain a flush velocity of 0.61 m s-1. 
Each dosing and field flush was followed by a manufacturer programmed 15-second filter 
flush, 5-second pump delay, and 1-minute drip drain period.  
 
Figure 4.2 Schematic of the laboratory SDI system layout. 
 
Measured drip tube emitter flows in the laboratory layout (n = 60) ranged from 
1.80 to 2.30 lph, with an average of 2.02 lph. Observed coefficient of variance (Cv) was 
0.067, indicating average emitter performance. (ASAE EP 405.1, 2003) 
67 
 
Real time soil moisture feedback-control was tested using two different soil 
textures. Bagged playground sand (American Countryside, AL) and surface horizon 
Houston clay at the field site were used to evaluate system response to very low and very 
high percolation soils. Particle size distribution, porosity, saturated hydraulic conductivity 
(Ksat), and field capacity of the two media were measured using standard methods (Table 
4.1).  
Table 4.1 Properties of Houston clay and sand media used in laboratory testing. 
 Particle Size Distribution1 Porosity2 Ksat3 Field 
Capacity4 Sand Silt Clay 
% % ?m s-1 %, vol 
Houston clay 7 40 53 63 4.2 41 
Fine sand 100 0 0 37 165 10 
1. Using the pipet analysis method (Soil Survey Investigation Staff, 2004) 
2. Calculated from measured bulk density (Gravimetric method (SSSA, 2002b) using intact soil 
cores) and particle density (Liquid displacement method (SSSA, 2002b)). 
3. Houston clay was measured with permeameter (Ksat Inc., CA) at 45 cm depth, playground     
sand was laboratory measured with constant head method (SSSA, 2002b). 
4. Measured with laboratory pressure plate method (SSSA, 2002b). 
 
Based on NRCS Web Soil Survey information (NRCS, 2008), the field capacity 
(1/3 bar) of the Houston clay soil site (site information will be provided later) is 
approximately 0.42 m3 m-3. Consequently, the soil moisture thresholds used for control 
testing were set at 0.40 m3 m-3 (on) and 0.45 m3 m-3 (off). The intent with these 
thresholds is to avoid hydraulic overloading at the experimental site, while providing 
sufficient moisture for plant uptake and aerobic soil treatment of effluent. The sand media 
was tested using the same thresholds as the clay in order to: 1) observe the more frequent 
response of the GP1 controller/data logger over changing soil moisture conditions within 
a limited time frame; and 2) evaluate the corresponding effectiveness of the control 
system in a highly permeable medium. 
68 
 
The GP1 logical criteria used in the laboratory study are illustrated in Figure 4.3. 
Sensors #1 and #2 represent hypothetical soil moisture readings at 20 cm and 45 cm 
depth, respectively. If there is sufficient water above the pump intake, initial soil moisture 
readings (Condition A) allow normal dosing pump operation. Dosing increases soil 
moisture readings until either one of the two soil moisture sensors reads above 0.45 m3 m-
3 (Condition B). The dosing system will remain idle until free drainage or 
evapotranspiration lowers the volumetric moisture reading of one of the two soil moisture 
sensors below 0.40 m3 m-3 and the other one to below 0.45 m3 m-3 (Condition D). The 
GEO1 dosing sequence will initiate, increasing soil moisture level back to a system cut 
off level (Condition F). System operation subsequently cycles between conditions B and 
F. The dosing system remains idle at any time there is insufficient water in the reservoir 
(#2 float circuit is opened) or if the electric supply is interrupted.  
 
Figure 4.3 GP1 logical dosing sequence based on volumetric soil moisture, m3 m-3.  
 
69 
 
For each test, selected media was placed in two 2-liter free-draining containers. 
Each container had one buried moisture sensor and was placed under one operating drip 
tube emitter along each drip tube. The tipping bucket rain gauge was positioned under a 
third emitter to simulate rain gauge data logging. Laboratory evaluation included readings 
for the two soil moisture sensors, one soil temperature sensor, a flow meter, rain gage, as 
well as system operating voltage.  
 
4.3.2 LABORATORY TESTING RESULTS 
SAND MEDIA 
In the container filled with sand water from the emitter was drained so quickly 
that the moisture content of the media never rose above 0.45 m3 m-3 (Figure 4.4). 
Consequently, dosing was never interrupted, providing continuous dosing as expected.  
 
Figure 4.4 Sand media with float #2 in a closed (ON) position and suffcient water in 
reservoir. 
 
70 
 
To verify that GEO1 float level interruption would curtail dosing even when 
moisture content allowed, the low water float switch (#2) was lifted out of the water tank 
to open the float circuit interrupting the GEO1 operating sequence (Figure 4.5). As 
designed, the simulated low water level stopped pump operation (no flow recorded), even 
though moisture sensors control continued to call for irrigation. As a result, no water was 
emitted by the drip tube (Figure 4.5). The readings of the two soil moisture sensors 
thereafter decreased as the water drained from the containers. 
 
 
Figure 4.5 Sand media with float #2 in open (OFF) position to test low water level response. 
 
CLAY SOIL 
A comparable test of the Houston clay soil indicated that the system responded as 
designed with a significantly lower dosing frequency (no dosing for two days after initial 
dosing) compared to the sand media (average 2 dosings per hour). As indicated in Figure 
4.6, when there was sufficient water in the reservoir the soil moisture level rose to above 
0.45 m3 m-3 after only two dosing cycles, effectively shutting down the pump. After 
71 
 
approximately 14 hours, the pump was activated when 0.40 m3 m-3 was measured by soil 
moisture sensor #2. Since the dosing cycle occurred at the same time that a forward field 
flush cycle was preprogrammed, a higher system flow was recorded as noted in Figure 
4.6. The soil moisture level was brought to and maintained above 0.40 m3 m-3 by this 
dosing cycle. This observation was expected since the water holding capacity (as porosity 
in Table 4.1) for the Houston clay soil is almost twice as much as the sand media, while 
its saturated hydraulic conductivity is about 40 times lower.  
 
Figure 4.6 Houston clay soil test with float #2 in a closed position and enough water in 
reservoir.      
72 
 
 
Figure 4.7 Houston clay soil test with float #2 in an open position to simulate low water level 
in reservoir. 
 
The two soil moisture sensors were taken out of the two containers filled with 
clay soil to intentionally lower moisture readings below 0.40 m3 m-3 and allow activation 
of relay A (Figure 4.7). At the same time, to verify if GEO1 interruption would stop 
dosing even when soil moisture allowed, the #2 float (low water switch) in the water tank 
was manually opened. As designed, the open #2 float interrupted water pumping, and no 
dosing occurred in the SDI wastewater system, even when soil moisture dropped below 
0.40 m3 m-3 (Figure 4.7).  
Consequently, laboratory results indicated that the soil moisture control system 
operated as designed under the soil moisture thresholds evaluated. In order to prevent 
hydraulic overloading in sandy fields, lower thresholds would need to be applied to 
account for much lower field capacity and water holding capacity. The experimental 
system was subsequently upscaled to a Houston soil field site to evaluate hydraulic 
performance in a clayey soil. 
73 
 
4.3.3 FIELD INSTALLATION AND TESTING 
SITE SELECTION AND CHARACTERIZATION 
The site selected for the field study is in Marion Junction, Alabama, at the 
Alabama Black Belt Research and Extension Center (ABBREC), approximately 10 miles 
west of Selma, Alabama. A Houston clay soil site with 1% slope was selected because it 
provided the fewest impediments to year-round SDI wastewater dosing, while providing 
low permeability and high shrink-swell features common to the region. An electric 
service (220v/110v, max. 60A) and a water supply well (max. 9000 lph) provided 
necessary utility connections. A truck-mounted Giddings? probe and sleeve was used to 
retrieve core samples from the site for soil profile characterization for the Houston soil 
(Table 4.2). Five soil horizons (Ap1, Ap2, BA, Bkss1, Bkss2) were identified to 1.52 m. 
Dark clay was prominent at the surface to approximately 42 cm depth, with 
redoximorphic features at 88 cm indicating significant periods of saturated or anaerobic 
conditions during most years. Particle size distribution indicates increasing clay content 
with depth, up to 71% at 152 cm. According to the Geological Survey of Alabama 
(1993), the experimental site is underlain by a relatively impermeable layer of 
fossiliferous clayey chalk of greater than 100 m thick at a general depth of 6 m, with 
shallower formations found at 12 cm to 2 m as well. 
74 
 
Table 4.2 Soil description and texture data to 1.52 m depth at the experimental site, Black Belt Research and Extension Center, Marion 
Junction, Alabama.   
Horizon Depth (cm) Color  Description* Particle Size Distribution (%) 
Sand Silt Clay 
Ap1 (0-23) very dark gray (2.5Y 3/1) clay 7.09 39.63 53.28 
Ap2 (23-42) dark grayish brown (2.5Y 
4/2) 
clay 8.27 38.04 53.7 
BA (42-63) olive brown (2.5Y 4/3) clay; few fine black manganese oxide 
concretions; common medium and 
coarse calcium carbonate concretions 
and soft accumulations 
10.17 33.38 56.45 
Bkss1 (63-88) olive brown (2.5Y 4/3) clay; large wedge-shaped aggregates that 
are bordered by intersecting slickensides; 
very plastic, sticky; few fine black 
manganese oxide concretions; common 
medium and coarse calcium carbonate 
concretions and soft accumulations; 
calcareous 
3.50 35.93 60.57 
Bkss2 (88-152+) light olive brown (2.5Y 5/6); 
common light brownish gray 
(10YR 6/2) redox depletions; 
common dark yellowish 
brown (10YR 4/6) redox 
concentrations 
clay; large wedge-shaped aggregates that 
are bordered by intersecting slickensides; 
very plastic, sticky; few fine black 
manganese oxide concretions; common 
medium and coarse calcium carbonate 
concretions and soft accumulations; 
calcareous 
3.10 25.80 71.10 
*Soil described as per National Cooperative Soil Survey Standard. 
75 
 
FIELD EXPERIMENT DESIGN 
Based on Alabama Department of Health (2006) regulations for onsite sewage 
disposal, the allowable hydraulic loading (dosing) rates for a Houston clay soil is 2.04 lpd 
m-2.  The design flow rate of the experimental system was set at 1022 liters per day, 
equivalent to the daily wastewater flow of a 3-person home in a decentralized subdivision 
system (Alabama Department of Public Health, June 14, 2005, personal communication). 
Consequently, a total drainage or disposal soil area of 500 m2 was required for the design. 
Based on standard practice of 0.61 m spacing between drip lines and 0.61 m spacing 
between emitters (Geoflow, 2003), a total of 823 m of drip line was required. 
Figure 4.8 presents the field layout consisting of 30 drip laterals 27.43 m in length. 
The SDI system was hydraulically divided into two subplots of 15 drip tubes each, 
designed to accommodate two matching irrigation treatments. In this current study, both 
treatment sub plots were supplied with clean well water and identical cultural practices, 
and were subsequently were subsequently analyzed as one field. 
 
 
 
 
 
 
 
 
 
 
 
 
 
76 
 
 
 
Figure 4.8 Design layout of the experimental wastewater SDI system, including clean water 
reservoir tank and pump, headwords box, control panel, supply and return manifolds, and 
30 drip laterals at 0.61m spacing. The SDI system was divided into two equal subplots.  
 
FIELD INSTALLATION AND SETUP 
On June 26, 2006, three weeks prior to SDI system installation, field tillage was 
conducted to loosen extremely dry soil at the site. On July 19, 2006, installation began 
and was completed in two days. Headworks box, SDI control panel, soil moisture data 
logger/controller, and SDI tubing were the same as in the laboratory test. A 7571 L (2000 
gallon) plastic septic tank (FRALO, NY) was used as the water reservoir. A 0.37 kW 
submersible pump was installed inside the plastic septic tank and wired to the GEO1, 
with power supplied from an existing meter near a water supply well 60 m away from the 
tank. The GP1 and GEO1 were mounted on a single panel next to the pump and 
headworks box (Figure 4.8). The tank was automatically filled by the well and a pressure 
77 
 
tank activated by a mechanical float valve in the tank. All SDI main supply and return 
lines were Sch 40 PVC. Drip tubes were installed at a depth of 20-25 cm. Necessary 
electric components were field grounded, exposed wires were waterproofed, and control 
panel openings were sealed to prevent damage from animals and insects. The two 
capacitance soil moisture sensors were buried in one location at 20 and 45-cm depth to 
provide soil moisture monitoring during system operation (Figure 4.8).  
Field crops were planted two weeks after SDI installation was completed. Crops 
grown during the one year study included sorghum-sudangrass (Sorghum bicolor (L.) 
Moench) from August 3, 2006 to November 1, 2006 and a mixture of winter wheat 
(Triticum aestivum) and rye (Secale cereale) from November 1, 2006 to June 08, 2007. 
Sorghum-sudangrass was planted with a grain drill at 33.6 kg seed per hectare on 18 cm 
row spacing. Winter wheat was planted with a grain drill on 18 cm row spacing at 67.2 
kg per hectare; and ryegrass was broadcast at 22.4 kg per hectare. 
The soil moisture operating thresholds were 0.40 to 0.45 m3 m-3, identical to the 
laboratory test. The SDI wastewater controller was set to a 5-minute dosing period 
followed by a 55-minute resting period, providing approximately one dosing cycle per 
hour. Since only clean well water was used in this study, a forward field flush was 
programmed to occur only every 1000th cycle. Dosing and field flushing were followed 
by a 15-second filter flush and a 5-second pump delay, per manufacturer?s 
recommendations. The GP1 data logger/controller was set to record rainfall (mm), soil 
moisture (v/v), flow volume (gallon), voltage (v), and temperature (degree C)  every 15 
minutes.  
78 
 
SYSTEM HYDRAULIC PERFORMANCE EVALUATION 
A monthly water balance was developed for the drain field from September 2006 
to June 2007 to evaluate the impact of automatic system control on soil water. 
Components of the water balance included depth of disposed water (D), precipitation (P), 
evapotranspiration (ET), percolation below 45 cm depth (?), and water content change in 
the upper 45 cm (??).  
Drain field surface runoff and soil lateral flow was neglected because the 
experimental site was relatively level. Percolation below 45 cm depth was estimated by 
mass balance difference between water balance components, including water inputs, 
estimated monthly drain field soil moisture change, and calculated field ET (Eq. 4.1). If 
calculated net monthly percolation indicated a positive moisture gain to soil above 45 cm 
depth (ie. From surrounding soil), then this value was identified as an error term to 
properly balance water input and outputs.  
 ? = D + P ? ET ? ?? ?????????????????????? Eq. 4.1 
The change in monthly soil moisture content within the upper 45 cm of the soil 
(??) was estimated as the difference between weighted field water content (Eq. 4.2) at the 
beginning and ending dates of each month based on in situ soil moisture readings. Since 
soil moisture content was measured at 20 and 45 cm depth, the assumption was made that 
1) soil moisture content varied linearly between 20 and 45 cm depth, and 2) soil moisture 
content in the upper 20 cm depth was represented by the reading at 20 cm. 
?upper 45 cm = (?20cm ? 20 cm) + [ (?20cm+ ?45cm) ? 25 cm/2 ]????????? Eq. 4.2 
        
Field evapotranspiration (ET) was estimated using the Penman-Monteith method 
(FAO, 2006). Data for ET calculation was obtained from the Alabama Agricultural 
79 
 
Weather Information Service (AWIS). Since all necessary weather data for the Penman-
Monteith method was not available at the Black Belt station, selected weather data from 
Thorsby weather station approximately 77 km from the experimental site was used. 
Weather data at 1.52 m height above the surface included daily maximum, minimum, and 
average air temperature, daily maximum and minimum relative humidity, daily solar 
radiation, and daily maximum wind speed.  
 
4.4 FIELD TESTING RESULTS 
4.4.1 SYSTEM OPERATION RESULTS 
Daily rainfall, soil moisture, daily system hydraulic disposal rate, soil 
temperature, and calculated daily ET are illustrated in Figure 4.9 from September 6, 2006 
to June 14, 2007. After the system was placed into operation, an automatically controlled 
hydraulic dosing rate of approximately 1.4 cm d-1 was maintained until soil moisture 
stabilized at higher levels around September 17, 2006 when the hydraulic dosing rate 
dropped to below 1.0 cm d-1. On September 12, a 12 mm precipitation event increased 
field moisture, and soil moisture readings thereafter stabilized at approximately 0.30 m3 
m-3 at 20 cm depth and 0.45 m3 m-3 at 45 cm depth. The hydraulic dosing rate was 
stabilized at 0.7-1.0 cm d-1. The water supply to the tank was abruptly cut off on the last 
day of September due to a reservoir float malfunction. As designed, the water dosing 
pump automatically stopped although soil moisture levels called for irrigation. After the 
water supply problem was corrected the next day, the system resumed soil moisture  
 
80 
 
 
Figure 4.9 Field data recorded from September 2006 to June 2007. (a. field monitored soil moisture contents and system daily hydraulic 
disposal rates; b. field monitored daily precipitation, soil temperature at 10 cm depth. (Calculated daily ET was added to graph for 
reference)  
81 
 
controlled water dosing. This water supply interruption afforded the opportunity to verify 
a successful low water level control in tandem with soil moisture control in the system. 
The experimental system was left unattended for two months during the winter of 
2006/07. The data logger indicated that an external power supply interruption occurred 
on October 3, 2006 until December 5, 2006 (Figure 4.9), curtailing pump operation for 
more than two months. After the power outage was corrected on December 5, 2006, the 
system resumed normal operation (Figure 4.9). During the power outage, data logging 
backed up by an on-board battery provided a time series of soil moisture under natural 
rainfall at the site.  
As indicated from Figure 4.9, there were likely several opportunities for water 
dosing during November and December 2007 if the external power supply had not been 
interrupted. Nevertheless, soil moisture content increased rapidly and maintained 
conditions >0.50 m3 m-3 after a 61 mm precipitation event on December 20-21, 2006, 
followed by several other large events and additional precipitation until the end of 
February 2007. Field records indicate that the system cutoff threshold of 0.45 m3 m-3 was 
exceeded until mid-February 2007 due to low temperatures (average 12 oC), low 
evapotranspiration, and continued rainfall that kept the site from draining. Rural 
households adopting this type of OWTS would require an alternative dispersal method or 
additional 2-month storage during periods of power interruption or lengthy wet field 
conditions.  
It was also observed that the experimental system never initiated water dosing 
when either of the two soil moisture sensors gave a reading higher than 0.45 m3 m-3. The 
82 
 
only surface ponding that was likely was during wet winter months when the soil 
moisture sensors indicated higher than 0.45 m3 m-3. Consequently, surface ponding is not 
considered a major concern of the experimental system. 
During the first week of March 2007, recorded soil moisture levels at 20 cm depth 
began to drop, followed by a drop in 45 cm depth readings due to increased soil warming 
and ET from the growing winter wheat/rye crop (Figure 4.9). On March 7, 2007, the 
GEO1 wastewater dosing control system was successfully activated by the GP1 data 
logger/controller when the recorded 20 cm soil moisture readings decreased below 0.40 
m3 m-3, even though 45 cm soil moisture was still above 0.40 m3 m-3. Operation of the 
wastewater SDI system continued thereafter amid increasing soil temperatures, storm 
events, and crop growth until May 25, 2007 when the winter wheat/rye mix was 
harvested (Figure 4.9). Since crop harvesting and planting relies on heavy machinery that 
cannot withstand wet field conditions, water dosing was manually disabled from May 25, 
2007 to June 14, 2007 to ease harvesting of winter wheat and rye and planting of 
sorghum-sudangrass. After that, the experimental system was placed back on automatic 
control and prepared for the second year of the study (not included in this paper). The 
nearly one month cut off period required for agronomic crop management would be 
difficult to justify in a single household application, but may find application in a 
community sized decentralized system with larger available dispersal area. Considering 
the likelihood of a 2-month zero dosing period during wet winter conditions, at least a 2-
month septic tank storage requirement is anticipated.  
 
83 
 
4.4.3 SYSTEM HYDRAULIC PERFORMANCE  
An estimated monthly water balance is presented in Figure 4.10. Water was dosed 
during spring through fall, with a peak value of approximately 23.32 cm month-1 in April 
of 2007 during drought conditions. This peak dosing rate is almost four times higher than 
Alabama recommendations for hydraulic loading (6.00 cm month-1) of clayey soils 
similar to the test site (ADPH, 2006). Since the experimental system was never limited by 
water supply except during malfunction from October 2006 to January 2007, recorded 
hydraulic dosing rates represent the maximum system hydraulic dosing rates can be 
achieved by the system under the pre-set soil moisture control thresholds. Advantages 
demonstrated by this experimental system include: 1) avoidance of wet soil conditions by 
withholding wastewater dosing until field moisture content drops to a pre-determined 
?operational? window; 2) temporary increase in wastewater hydraulic dosing with real-
time soil moisture sensing under favorable field conditions.  
Figure 4.10 Estimated monthly drain field water balance from September 2006 to June 
2007. (Drain field surface ponding is not considered. Positive Y axis represents water 
inputs; negative Y axis represents water outputs) 
84 
 
According to the monthly water balance, more than 30% of applied water 
percolated below 45 cm depth during the September 2006, November 2006 to December 
2006, and March 2007 to June 2007. The estimated percolation below 45 cm depth 
during November 2006 to December of 2006 when the experimental system did not dose 
any water into the field indicates that the drain field is not suitable for wastewater dosing 
during normal winter months. On the other hand, the experimental system did not 
aggravate the already saturated soil conditions in the drain field during winter wet periods 
when most conventional septic systems would be experiencing drain field surface 
ponding.  
The period from March 2007 to June 2007 coincided with a historic drought 
March through June precipitation was 248 mm versus 492 mm in an average year. During  
system startup in September 2006, the test site was also dry in the upper 45 cm (< 0.20 
m3 m-3, Figure 4.9). It is recognized that shrinking and swelling of clay-rich smectitic 
soils create dynamic crack formations that change soil physical and hydraulic properties 
(Bouma et al., 1981). Preferential channels can form which alter the landscape hydrology 
and facilitate rapid transport of water into the soil (Bouma et al., 1981; Youngs, 1995; 
Kishne et al., 2009). Although the cracking extent of the clay soil at the test site was not 
quantified during this study, surface cracking was consistently observed. Cracking 
development to a depth of around 50 cm is normal for Vertisols (Amidu et al., 2007) and 
more than 100 cm depth crack development has been reported in Houston clays (Kishne 
et al., 2009).  
85 
 
Since the test site is a low permeable Houston clay soil, a possible explanation for 
the estimated percolation loss during the dry period of 2007 is that dosed water did not 
adequately moisten the upper soils so as to curtail soil crack development. Presumably, 
when the wetting front reached the soil cracks between the soil moisture sensors and the 
drip emitters, much of the water moved by preferential flow away from the soil moisture 
sensors, draining the soil profile at a higher rate than would have occurred in a more 
homogenized soil structure. 
In order for soils to provide effective effluent treatment, water has to percolate 
through soil horizons above underground restrictive layers (AWOCT, 2005). For the 
experimental site, evidence of a seasonal water table exists at around 88 cm depth (Table 
4.2), indicating that there is an additional 43 cm of soil below the 45 cm depth to mitigate 
environmental pollution at the site. Water percolation below 45 cm would be of more 
serious concern if a local water table existed. 
The estimated magnitude of percolation at the experimental site suggests that the 
experimental soil moisture controlled hydraulic dosing system as designed was not 
effective in preventing clay soil shrinking during dry soil conditions. One reason for the 
experimental system?s ineffectiveness in controlling soil cracking could be the 0.61 m 
spacing between emitters and drip lines. It is likely that the wetting diameter of the 
emitters did not fully moisten the field area leaving dry areas susceptible to cracking 
during dry soil conditions. Consequently, reduced emitter and drip line spacing may 
enhance water distribution, and limit soil cracking. Another possible reason that the 
system did not effectively prevent soil cracking is that there were limited field moisture 
86 
 
monitoring locations used in this study. If the wetting influence from percolated water 
were monitored by soil moisture sensors at more frequent spacing and at deeper depths, 
preferential flow may be more effectively controlled. Recommendations for system 
improvements include the use of multiple soil moisture monitors vertically and 
horizontally to more adequately reflect drain field moisture conditions and site 
heterogeneity. Emitter and drip line spacing could also be reduced to increase soil 
moisture uniformity and limit crack development in the field. Above suggestions require 
additional field testing to evaluate their impact on system hydraulic performance, 
including water loss to preferential flow. 
 
 
4.5 CONCLUSION 
Assembly of an experimental soil-moisture based wastewater dosing system was 
completed by integrating two commercially available systems, a research grade soil 
moisture data logger/controller and a wastewater SDI control system. Wastewater SDI 
dosing was laboratory tested using two soil moisture sensors in a clay and sand media. 
During the laboratory test, the system responded to real-time readings of soil moisture as 
designed by 1) withholding water dosing during wet soil conditions of ?  0.45 m3 m-3 or 
low reservoir condition, and 2) initiating and continuing water dosing under allowable 
soil conditions between 0.40-0.45 m3 m-3.  
Once installed in the field, the experimental system responded to seasonally 
changing field conditions, effectively adjusting water disposal according to real-time soil 
moisture. Although the experimental system was internally robust, unexpected power 
outages shut down the system periodically, emphasizing the need to conduct regular 
87 
 
system checks for successful operation. The experimental system effectively stopped 
water dosing during wet soil conditions and prevented surface ponding from system 
water dosing. The observed zero water dosing period during wet winter conditions 
indicates the need for a wastewater storage capacity of at least two months. This 
constraint likely creates an insurmountable challenge for application of this system for 
individual rural homeowners. 
Observed water management of the experimental system indicated that more than 
30% of applied water was lost to percolation below 45 cm during dry soil conditions, 
presumably a result of soil cracking. Water percolation loss indicates that the 
experimental system, including lateral and emitter spacing configurations and soil 
moisture monitoring and feedback control, is not able to effectively limit water 
percolation during dry soil conditions. This finding suggests that clay shrinkage left the 
system unable to control water percolation. In spite of the successful operation of the soil 
moisture based dosing control, the current experimental system did not overcome the 
severe hydraulic limitations inherent in the Houston clay soil. Recommendations for 
system improvement include the use of multiple soil moisture sensorsvertically and 
horizontally to more adequately reflect drain field moisture conditions and site 
heterogeneity. Emitter and drip line spacing can also be reduced in an attempt to limit soil 
crack development in the field. Above recommendations require further field testing to 
evaluate their impact on system hydraulic performance. 
88 
 
CHAPTER FIVE 
SOIL MOISTURE CONTROLLED WASTEWATER DISPERSAL-YEAR TWO: 
SYSTEM HYDRAULIC AND NUTRIENT EVALUATION  
 
5.1 ABSTRACT 
Rural areas represent approximately 95% of the 14000 km2 Alabama Black Belt, 
an area of widespread Vertisols, dominated by clayey, smectitic, shrink-swell soils. The 
area is characterized by widespread use of onsite wastewater treatment systems (OWTS) 
that rely on soil for wastewater dispersal. An experimental field moisture controlled 
subsurface drip irrigation (SDI) system designed for integrated use with a seasonal 
cropping system was installed and evaluated for two years on a 500 m2 Houston clay site 
in west central Alabama. From September 2006 to June 2007 (year one) clean well water 
was applied; from June 2007 to September 2008 (year two) a synthetic wastewater was 
used. The objectives of the study were to: 1) evaluate two-year system hydraulic 
management in terms of seasonal water disposal, water percolation control, soil moisture 
profile, and annual water budget; 2) evaluate system nutrient management in year-two in 
terms of monthly soil water nutrient level, seasonal and annual field crop nutrient uptake, 
and annual soil nutrient profile. System feasibility is addressed based on results of the 
two-year field experiment. 
89 
 
Hydraulic dosing rates during the two year study fluctuated as expected with higher 
dosing rates during warm season and lower dosing rates during cold season. Drain field 
surface ponding was not observed during dry warm seasons and was not aggravated by 
hydraulic dosing during the cold season. Estimated water percolation loss below 45 cm 
occurred in warm season during which time approximately 30% of dosed water was lost 
to percolation. Average hydraulic dosing rate during the warm season of year two was 
0.17 cm d-1, more than half the 0.40 cm d-1 rate observed during the same period of year 
one. Differences are likely due to the higher, more normal precipitation in year two. The 
estimated annual water balance based on two years of experimental data indicates the 
need for a minimum 2-month wastewater storage requirement, verifying that the system 
is not suitable as a stand-alone wastewater disposal and treatment method. Soil moisture 
profiles monitored during year two from May 2008 to September 2008 suggest that 
significant percolation did not occur below 100 cm.  
Annual crop nutrient uptake represented approximately 32% of applied nitrogen 
and 31% of applied phosphorus during year two. Results suggest there was potential for 
nutrient (N, P) leaching during the experiment as a result of moisture based dosing. 
However, soil cores sampled near the end of the experiment provided no direct evidence 
of drain field N or P accumulation or percolation below 100 cm depth. Additional, 
replicated field sampling is required before definite conclusions can be drawn regarding 
the nutrient impact of wastewater dosing at this site. 
In spite of operation of the soil moisture based dosing system, the experimental 
SDI system is not suitable for direct application of wastewater in clay soils of the 
90 
 
Alabama Black Belt. System limitations which need further study include: 1) near zero or 
zero low hydraulic dosing rates during wet soil conditions resulting in an impractical 
wastewater storage requirement; 2) lack of system response to seasonal soil shrink-swell 
resulting in potentially large dry weather percolation losses below 45 cm depth; 3) 
nutrient loading imbalance with respect to crop uptake due to soil moisture based dosing; 
and 4) field system and crop operation and maintenance requirements that are impractical 
for land owners with limited resources. Based on field observations, the recommended 
application of the experimental system is to supplement other wastewater treatment 
systems that can function during wet periods when this system provides little or no 
hydraulic dosing. Recommendations are made to improve experimental system control 
over water percolation below 45 cm. 
 
5.2 INTRODUCTION 
Onsite wastewater treatment systems (OWTS) in the US currently treat 15 million 
metric tons of wastewater per day, serving approximately 30% of U.S. households 
(Spicer, 2002; AOWTC, 2005). This widespread rural treatment and dispersal method 
functions as an important supplement to centralized public wastewater treatment plants 
(WWTP). Despite wide adoption, conventional OWTS consisting of a septic tank and a 
gravity fed effluent disposal field can pose a significant threat to the environment by 
polluting surface and groundwater with nutrients and pathogens (US EPA, 2002).  
During the mid-1940s, infant mortality was linked to NO3--N concentrations in 
drinking water, especially in rural areas of the United States (Hergert, 1986; Fare, 1993). 
91 
 
Public hygiene issues related to onsite septic systems in urban fringe areas has received 
attention since 1980 (Boyle and Otis, 1981; DeWalle and Schaff, 1980). Typical nitrogen 
concentration in septic tank effluent ranges from 40-80 mg L-1 (Walker et al., 1973), of 
which approximately 75% is ammonium nitrogen and 25% is organic nitrogen (Otis et 
al., 1974). Reported total phosphorus concentration in septic tank effluent ranges from 3 
to 20 mg L-1 (Robertson et al. 1998, Whelan 1988), with approximately 85% as 
orthophosphate (Reneau and Pettry 1976). Charles et al. (2005) analyzed the results of 
several intensive septic tank effluent field surveys from 1976 to 1999 in Australia and the 
US, including their own field survey of 200 septic tanks in Australia. To minimize 
nutrient overloading associated with most conventional OWTS failure, Charles et al. 
(2005) suggested a design value of 250 mg L-1 for total nitrogen and 36 mg L-1 for total 
phosphorus to estimate drain field nutrient load. They concluded that widespread nutrient 
overloading is likely occurring in a significant number of conventional OWTS designed 
under current Australian and the US regulations (Charles et al., 2005).  
The failure of conventional OWTS in the Alabama Black Belt area is a 
widespread and recognized problem (McCoy et al., 2004; He et al., 2007). The most 
common cause of conventional OWTS failure in this region is low permeable smectitic 
soils characterized by shrink-swell (ADPH, 2006). Based on spatial analysis of SSURGO 
(NRCS, 2007) soils data with Alabama Onsite Sewage Disposal Rules (ADPH, 2006), 
over 77% of the Alabama Black Belt area is unsuitable for conventional OWTS mainly 
due to low soil permeability (ADPH, 2006; He et al., 2007). Furthermore, the severe 
shrinking observed in smectitic soils during extended dry periods can stimulate 
92 
 
preferential flows in a field (Hoogmoed and Bouma, 1980; Beven, 1981). Preferential 
flow has been identified as a potential conduit for water and nutrient deep percolation that 
can threaten underground water systems (Weaver et al., 2005; Larsson et al., 2007; 
Muukkonen et al., 2009). Alternative engineered systems such as mound systems and 
drip irrigation systems receiving secondary treated effluent are currently mandated on 
new sites that are hydraulically limited to mitigate public health safety issues in the 
Alabama Black Belt (ADPH, 2006). Nevertheless, conventional OWTS are widely used 
in the Alabama Black Belt region partly because economic conditions in the region do 
not support retrofits or widespread replacement with more advanced systems (McCoy et 
al., 2004).  
This study evaluates a commercially available subsurface drip irrigation (SDI) 
wastewater disposal system retrofitted with soil moisture feedback control.  SDI was used 
to more uniformly distribute wastewater and to supply nutrients favorable for crop uptake 
(Phene and Ruskin, 1995). Drain field soil moisture control for wastewater disposal was 
incorporated as a proven technology for water percolation control in agricultural 
irrigation (Dukes and Scholberg, 2005). A managed cropping system for enhanced water, 
nutrients, and contaminants removal was incorporated into the drain field design to 
provide enhanced water and nutrient uptake (Askegaard and Eriksen, 2008; Ferraro et al., 
2003; Giupponi, 1998; Wang et al., 2008).  
In this study, an experimental soil moisture controlled SDI wastewater dosing 
system was field tested on a 500 m2 Houston clay soil using clean well water from 
September 06, 2006 to June 13, 2007 (year one), and with a synthetic wastewater from 
93 
 
June 14, 2007 to September 8, 2008 (year two). The objectives of the study were to: 1) 
evaluate two-year system hydraulic management in terms of seasonal water disposal, 
water percolation control, soil moisture profile, and annual water budget; 2) evaluate 
system nutrient management in year-two in terms of monthly soil water nutrient level, 
seasonal and annual field crop nutrient uptake, and annual soil nutrient profile. System 
feasibility is addressed based on results of the two-year field experiment.   
 
5.3 MATERIALS AND METHODS 
5.3.1 EXPERIMENTAL DESIGN AND OPERATION 
SITE SOIL PHYSICAL AND CHEMICAL PROPERTIES 
The experimental site is located at the Alabama Black Belt Research and 
Extension Center, 11 miles west of Selma in west central Alabama. Soil at the site is a 
Houston clay (very-fine, smectitic, thermic Oxyaquic Hapludert). Measured soil physical 
and chemistry properties are summarized in Table 5.1. The soil profile is dominated by 
clay with CEC ranging from 23-30 meq 100 g-1. Porosity ranges from 52-64%. The soil 
profile has a phosphorus adsorption capacity of approximately 17000 mg P kg-1 soil. 
94 
 
                 Table 5.1 Soil physical and chemical properties at the experimental site. 
 
 
 
 
 
 
 
 
 
Notes:  
1. Determined by the pipet method (Soil Survey Investigation Staff, 2004). 
2. Determined by the ammonium acetate (pH 7) method (SSSA, 2002a) using composite soil subsamples from the soil cores sampled from all 
four field treatments taken on June 24, 2008. (1 cmolc kg-1 equals to 1 meq 100 g-1). 
3. NRCS Web Soil Survey version 2.1, survey area data version 4, September 16, 2008.  
4. Determined with liquid displacement method (SSSA, 2002b) using composite soil subsamples from the soil cores of all four field 
treatments taken on June 24, 2008.  
5. Calculated from bulk density from NRCS and lab measured particle density.  
6. Determined by the method of Self-Davis et al. (2000) using composite soil subsamples from the soil cores of all four field treatments 
sampled on June 24, 2008.
Soil 
horizon 
Depth 
(cm)1 
Particle Size 
Distribution1, % Ave. CEC2  
cmol kg-
1?1SD 
Bulk density 
from NRCS 
web soil 
survey3   
g cm-1 
Ave. Particle 
density4  
g cm-3?1SD   
Ave. 
Porosity6  
%  
Maximum soil 
phosphorus 
adsorption 
capacity6  
mg P kg-1 soil 
Sand Silt Clay 
Ap1 0-20 7.09 39.63 53.28 29.11?1.82 1.10-1.45 2.62 ?0.01 63?1 17000 
Ap2 20-40 8.27 38.04 53.70 25.48 ?0.13 1.10-1.45 2.59 ?0.01 58 ?4 17000 
BA 40-60 10.17 33.38 56.45 23.81 ?0.61 1.10-1.45 2.59 ?0.01 55?2 17000 
Bkss1 60-80 3.50 35.93 60.57 24.70 ?1.41 1.10-1.45 2.62 ?0.02 59?1 17000 
Bkss2 80-100 3.10 25.80 71.10 25.67 ?0.19 1.10-1.45 2.60 ?0.03 54?2 17000 
95 
 
SDI DESIGN AND FIELD TREATMENTS LAYOUT AND OPERATION 
Based on Alabama Department of Health (2006) regulations for onsite sewage 
disposal, the allowable hydraulic loading (dosing) rates for the Houston clay soil is 2.04 
lpd m-2.  The design flow rate for the experimental field layout was 1022 liters per day, 
equivalent to the daily wastewater flow of a 3-person home in a decentralized subdivision 
system (Alabama Department of Public Health, June 14, 2005, personal communication). 
Consequently, a total drainage or disposal soil area of 500 m2 was required for this design. 
Based on standard practice of a 0.61 m spacing between drip lines and 0.61 m emitter 
spacing (Geoflow, 2003), a total of 823 m of drip line was required. 
The SDI system consists of 30 drip tubes, WFPC16?2?24, 16 mm diameter 
(Geoflow, CA), 27 m long at 0.61 m lateral spacing installed approximately 20-25 cm 
deep (Figure 5.1). The maximum particle size that can pass through emitters without 
obstruction is 100 ?m. The SDI system was supplied by well water stored in a 7600 L 
above-ground plastic septic tank (Fralo, NY). The SDI system was hydraulically divided 
into two subplots (I and II, Figure 5.1), each with 15 drip tubes. The design flow rate of 
the entire SDI system was 76 lpm. The experimental site was disked to a depth of 20-25 
cm before SDI installation to reduce site heterogeneity in the top soil and to provide more 
friable conditions for tube installation.  
The soil moisture operating thresholds were 0.40 to 0.45 m3 m-3, identical to the 
laboratory test. The SDI wastewater controller was set to a 5-minute dosing period 
followed by a 55-minute resting period, providing approximately one dosing cycle per 
hour. Since only clean well water was used in this study, a forward field flush was 
programmed to occur only every 1000th cycle. Dosing and field flushing were followed 
96 
 
by a 15-second filter flush and a 5-second pump delay, per manufacturer?s 
recommendations. The GP1 data logger/controller was set to record rainfall (mm), soil 
moisture (v/v), flow volume (gallon), voltage (v), and temperature (degree C)  every 15 
minutes.  
Based on the NRCS Web Soil Survey (NRCS, 2008), the field capacity (1/3 bar) 
at the 45 cm of the experimental site is 0.42 m3 m-3. Consequently, the soil moisture (m3 
m-3) thresholds used for SDI control were set at 0.40 (on) and 0.45 (off) with the intent to 
avoid hydraulic overloading the experimental site while maintaining adequate soil 
moisture for managed crop uptake and aerobic soil treatment for wastewater. System 
hydraulic disposal occurred when either of the two soil moisture sensors read < 0.40 m3 
m-3. System hydraulic dosing was not enabled when either of the two soil moisture 
sensors read above 0.45 m3 m-3.  
 
 
 
 
 
 
 
 
 
 
 
 
97 
 
 
Figure 5.1 Experimental layout, treatments, and sampling locations. 
98 
 
The experimental SDI system (Treatments I and II) was operated with clean well 
water from September 6, 2006 to June 13, 2007. Starting from June 14, 2007 until June 
24, 2008, Treatment I received synthetic and Treatment II continued to receive clean well 
water. A 100 ?m mesh filter was installed on the main water supply to the SDI system to 
screen well water of particles before entering drip lines. Synthetic wastewater was 
prepared by dissolving a commercial 30-10-10 fertilizer in a 1140 L nurse tank with well 
water at a ratio of 113 kg fertilizer to a full tank. Synthetic wastewater was then screened 
with a second 100 ?m mesh filter before injection into the treatment I supply line. The 
chemical injection pump (Neptune, Japan) for treatment I was operated at a flow rate of 
14.2 lph whenever main SDI dosing pump came on. Synthetic wastewater entered the 
drip lines at a ratio of approximately 1:160 (synthetic wastewater: clean well water). The 
resulting nutrient content was approximately 80 mg TN L-1, 10 mg P L-1, and 100 mg 
TOC L-1 throughout year two. Table 5.2 presents chemical analyses of clean well water 
and prepared fertilizer solutions from five dates throughout the study.  
Treatment I received all crop water and nutrient supply from rainfall and synthetic 
wastewater. Treatment II received rainfall and clean well water to indicate potential soil 
water nutrient increase in treatment I due to wastewater application. Treatment III, 27 m 
? 18 m plot was used as an agronomic control to indicate differences in treatment nutrient 
uptake efficiency and soil nutrient profiles. Treatment III was not irrigated but received 
surface applied fertilizer at the beginning of each crop growing season (2 seasons year-1, 
67 kg N ha-1 season-1). An identical crop rotation, sorghum-sudangrass (Sorghum 
bicolor), winter wheat (Triticum aestivum) and rye (Secale cereale) mix, was rotated in 
99 
 
treatments I, II, and III (Table 5.3). Treatment IV, an undisturbed area of approximately 
242 m2 west of treatments I and II (Figure 5.1), was used to indicate the background soil 
conditions. 
Table 5.2 Chemical analyses of clean well water and injected fertilizer solution (in average, 
mg L-1) during year two synthetic wastewater experiment, June 14, 2007 to June 24, 2008. 
Measured 
ingredients Solution 6/15/2007
1 8/03/20071 9/28/2007 3/20/2008 6/16/20081 
NH4+-N 
Well water 0.27 0.28 0.31 0.29 1.67 
Nutrient 
solution 855 1280 16.2 810 1550 
NO3--N 
Well water N.D.2 N.D. N.D. N.D. 0.44 
Nutrient 
solution 27.7 38.0 N.D. N.D. 24.3 
TKN 
Well water N.D. N.D. N.D. N.D. 0 
Nutrient 
solution 15700 N.A.
3 N.A N.A N.A 
Total 
phosphorus 
Well water N.D. N.D. N.D. N.D. 0.12 
Nutrient 
solution 1920 2420 2800 1280 1890 
TOC 
Well water <1 <1 <1 <1 <1 
Nutrient 
solution 22600 18400 21300 22600 23000 
pH 
Well water 5.32 6.13 6.59 6.51 6.40 
Nutrient 
solution 8.19 8.20 8.50 7.63 8.10 
1. Fresh nutrient solutions prepared on these dates. 
2. None Detectable.  
3. Not Available. TKN was not measured on these dates and assumed approximately close 
to that of 6/15/2007. 
 
 
100 
 
Table 5.3 Crop rotation of treatments 1-IV during the second year synthetic wastewater experiment, June 14, 2007 to June 24, 2008. 
Field treatments Warm season1 rotation Cool season 2 rotation Fertilizer application Irrigation 
Treatment I  
(Synthetic wastewater 
application) 
Two seasons of Sorghum-sudan 
grass growth (Jun 07- Aug 07, 
and Aug 07- Nov 07 ) 
One season of wheat/rye mix  
(Nov 07-Jun 08) 
Proportional to 
value of applied 
wastewater 
SDI synthetic 
wastewater  
Treatment II  
(Clean well water 
application) 
Two seasons of Sorghum-sudan 
grass growth (Jun 07- Aug 07, 
and Aug 07- Nov 07 ) 
One season of wheat/rye mix  
(Nov 07-Jun 08) 
None 
SDI clean well 
water disposal 
Treatment III  
(Agronomic control) 
Two seasons of Sorghum-sudan 
grass growth (Jun 07- Aug 07, 
and Aug 07- Nov 07 ) 
One season of wheat/rye mix  
(Nov 07-Jun 08) 
67 kg N ha-1 at the 
beginning of each 
crop growing season 
None  
Treatment IV  
(Undisturbed control) 
No crop No crop None None  
1.  Sorghum-sudan grass was planted at 33.6 kg seeds ha-1 at 18 cm spacing.  
2. Winter wheat was planted at 67.2 kg seeds ha-1 and rye was planted at 22.4 kg seeds ha-1 at 18 cm spacing. 
101 
 
5.3.2 SYSTEM WATER MANAGEMENT CASE STUDY - YEARS ONE AND TWO, SEPTEMBER 
6, 2006 TO JUNE 24, 2008. 
MONTHLY WATER BALANCE DEVELOPMENT- YEARS ONE AND TWO 
A monthly water balance was developed for treatment I from September 6, 2006 
to June 24, 2008 to evaluate the impact of seasonal soil moisture on automatic system 
control. Components of the calculated water balance include depth of disposed water, 
precipitation, evapotranspiration, percolation below 45 cm depth, and water content 
change in the upper 45 cm, as described in Chapter 4, without considering of drain field 
surface ponding. 
 
SITE KSAT AND FIELD CAPACITY VERIFICATION - YEAR TWO 
After the end of year two, two site uniformity of treatments I and II (Figure 5.1) 
was quantified in terms of field capacity and soil saturated hydraulic conductivity (Ksat) 
to determine how closely field conditions conformed to original system design 
parameters. Field capacity was measured on June 24, 2008 by taking intact soil cores at 
20 cm depth from nine uniformly distributed locations. Volumetric soil moisture at field 
capacity (1/3 bar) was measured using the laboratory pressure plate method (SSSA, 
2002b). Ksat was also measured onsite during June-July, 2008 using a permeameter (Ksat 
Inc., CA) at six uniformly distributed locations at 45 cm depth. Resulting site maps of 
field capacity and Ksat  distribution were generated using inverse distance weight (IDW) 
method within a GIS (ArcMap9.2, ESRI, CA). The Christiansen uniformity coefficient 
(Cu) (Soil Conservation Service, 1970) was calculated for both field capacity and Ksat to 
quantify site uniformity for these two important hydraulic parameters. 
102 
 
SOIL MOISTURE PROFILES - YEAR TWO 
A pre-calibrated capacitance soil moisture profiler (PR2, Delta-T, UK) was used 
to record soil moisture profiles at three locations on eight spate dates in each of the four 
treatments (Figure 5.1) at depths of 10, 20, 30, 40, 60, and 100 cm. Soil moisture profiles 
were obtained from May 2008 to September 2008 at approximately bi-weekly intervals. 
The soil moisture profiler provided reliable readings only when soil moisture content was 
below saturation. Profile measurements of volumetric soil moisture were used to identify 
if water moved through the profile as a consequence of hydraulic dosing. 
 
WASTEWATER STORAGE ESTIMATION - YEAR ONE AND YEAR TWO 
A wastewater storage requirement was estimated for the experimental system 
based on observed monthly hydraulic disposal rates in year one and year two. Historic 
precipitation from January 1978 to June 2008 was obtained from the Alabama Black Belt 
Research and Extension Center and compared to precipitation recorded onsite during the 
two-year study period. The allowable hydraulic dosing rate for a calendar month was 
estimated as the dosing rate observed for the month whose monthly precipitation fell 
closet to the 30-year average (50th percentile). Theoretical monthly residential 
wastewater supply was approximately 0.20 cm d-1 (6.00 cm month-1) based on the 
allowable hydraulic dosing rate for Houston clay by Alabama Department of Public 
Health (ADPH, 2006). An annual wastewater storage requirement in terms of equivalent 
months of residential flow was determined as the cumulative difference between 
consistent monthly wastewater flows and allowable monthly wastewater dosing rates. A 
zero annual balance between monthly inflows and outflows does not gurantee a zero 
103 
 
storage requirement. However, a zero balance indicates that the site is capable of 
absorbing all wastewater over the year in spite of prolonged periods of wet weather zero 
dosing. 
 
5.3.3 SYSTEM NUTRIENT MANAGEMENT CASE STUDY ? YEAR TWO (JUNE 14, 2007 TO 
JUNE 24, 2008) 
N AND P LEVELS IN SOIL WATER - YEAR TWO 
Suction lysimeters (Irrometer, CA) were installed at depths of 15, 30, and 45 cm 
at three locations in treatments I and II (Figure 5.1). Soil water samples were collected 
once per month from August 2007 to June 2008 during year two. A 50-60 kPa vacuum 
was applied to each lysimeter and allowed to sit 3-4 hours before the sample was 
collected in a clean 120 ml HDPE bottle. Samples were stored at 4 oC until analyzed. All 
lysimeter samples were filtered through 0.45 um membrane filters before their chemical 
analyses. Ammonium-N (NH4+-N) and nitrate-N (NO3--N) concentrations were 
determined with colorimetric analysis using a microplate reader (Sims et al., 1995). Total 
phosphorus (P) concentration was determined by inductively coupled argon plasma 
spectroscopy (ICAP 9000, Thermo Jarrel Ash, Franklin, MA). 
 
CROP GROWTH AND NUTRIENT UPTAKE - YEAR TWO 
Each season, harvested crops from treatments I and III were measured for total 
fresh and dry matter yield (105 ?C) since they were the only two treatments that received 
supplemental nutrients (Table 5.3). To determine total N and P in plant tissue, dried plant 
samples were ground to pass a 1-mm mesh screen using a Wiley Mill (Thomas Scientific, 
104 
 
PA). Samples were dry-ashed, digested with HCl (Hue and Evans, 1986) and analyzed 
via ICAP (ICAP 9000, Thermo Jarrel Ash, Franklin, MA). Above ground crop uptake of 
N and P was estimated by multiplying plant nutrient content by crop dry matter yield. 
 
SOIL CORE SAMPLING AT THE END OF YEAR TWO 
One meter long soil cores were taken from all four treatments on June 24, 2008 
after winter wheat and rye harvest, approximately one-year from the June 14, 2007 start 
of synthetic wastewater application. Soil cores were collected using a tractor-mounted 
Giddings? hydraulic probe at three locations in each treatment (Figure 5.1). At the 
laboratory, each soil core was divided by depth into five subsamples: 0 to 20, 20 to 40, 40 
to 60, 60-80, and 80-100 cm. Subsamples were dried in a ventilated oven (Heraeus, US) 
at 60 oC for four days, pulverized and screened to pass a 2-mm sieve. Total soil C and N 
was quantified by combustion using a LECO CHN-600 analyzer (LECO Corp., St. 
Joseph, MI). Total soil P was quantified using the perchloric acid procedure of Shelton 
and Harper (1941). Crop available P was determined using the Mississippi extract 
method (Lancaster, 1970) and analyzed by ICAP (ICAP 9000, Thermo Jarrel Ash, 
Franklin, MA). Soil pH was measured using 1:1 soil/water (m/m) slurries with a pH 
meter (Orion,US). Crop available N was determined by extracting soil subsamples with 
1M KCl solution and analyzing the extract for NH4+ and NO3- (Sims et al., 1995). Water 
soluble P for each subsample was measured using the method of Self-Davis and Moore 
(2000). Subsamples of soil cores from all four treatments were composited by horizon to 
measure soil cation exchange capacity (CEC) using the ammonium acetate (pH=7) 
method (SSSA, 2002a). Soil maximum P adsorption capacity and soil P adsorption 
105 
 
coefficient (Kd) were determined using the same composite samples and methods by Self-
Davis et al. (2000). 
 
5.4 RESULTS 
5.4.1 SYSTEM WATER MANAGEMENT  
FIELD OBSERVATION OF SYSTEM HYDRAULIC RESPONSE- YEARS ONE AND TWO 
(SEPTEMBER 6, 2006 TO JUNE 24, 2008) 
Hydraulic disposal rates and soil moisture at two sampled depths from years one 
and two, September 6, 2006 to June 24, 2008, are presented in Figure 5.2a. Soil 
temperature at 10 cm, field ET, and daily precipitation for the same period are illustrated 
in Figure 5.2b. The experimental SDI dosing system did not function due to onsite power 
outage and water supply cutoff from October 2006 to January 2007 (Figure 5.2a). In 
addition, the experimental system was cut off manually for approximately one month in 
May and October to facilitate field crop harvesting and planting. For the remainder of the 
2-year study period, SDI dosing was controlled by the soil moisture feedback system. 
Generally, the experimental system response to changing soil moisture was consistent 
throughout years one and two. 
Throughout years one and two, relatively higher dosing rates and frequencies 
from were observed from late spring to late autumn as expected, with consistent near zero 
dosing periods during wet, winter months. System hydraulic dosing in year one was of 
higher magnitude and frequency than in year two. The highest hydraulic dosing rate, 1.18 
cm d-1 occurred in April 2006. The average hydraulic dosing rate during the period from 
February 2007 to June 2007 was approximately 0.40 cm d-1. There were almost 3 months  
106 
 
 
Figure 5.2 Recorded field data for years one and two, September 2006 to June 2008. (a. soil moisture content and daily hydraulic dosing 
rate; b. daily precipitation, soil temperature at 10 cm depth, and calculated daily ET. System operation was interrupted by unexpected 
power and water supply outages from October 2006 to March 2007. The system was manually shut off in May 2008 and October 2008 to 
facilitate crop harvesting and planting.)  
107 
 
during the same period in 2008 when there was almost no dosing. In fact, the average 
hydraulic dosing rate during this period was only 0.17 cm d-1. The experimental system 
did not aggravate periods of drain field saturation during the wet winter of year two when 
the drain field was naturally saturated. The demonstrated advantages: 1) avoid wet soil 
conditions by withholding wastewater dosing until field moisture content drops to a pre-
determined ?operational? window; and 2) temporary increase wastewater hydraulic 
disposal with real-time soil moisture sensing based on seasonal soil conditions under 
favorable field conditions) of this the soil moisture based water management strategy in 
year one, are still applicable to the year two. However, the experimental system had a 
longer observed wastewater withholding period and a lower seasonal water dosing 
capability in year two. 
As indicated from the monthly water balance table (Figure 5.3), water percolation 
loss during warm seasons below 45 cm was an unexpected consequence of operating the 
system. More than 30% of dosed water was estimated lost to percolation below 45 cm 
depth during year one (September 2006, winter of 2006/2007, March to June 2007). 
However, this large percolation loss was not observed during the same period of year two 
except for June 2008. Similar to the hypothesis proposed in Chapter 4 that it was soil 
cracking during the dry season that caused percolation loss, observed percolation during 
June 2008 is also believed to be caused by the same reason.  
108 
 
 
Figure 5.3 Estimated monthly drain field (treatment I) water balance from September 2006 to June 2008. (No consideration of drain field 
surface ponding. Positive Y axis represents water inputs; negative Y axis represents water outputs)
109 
 
 
Figure 5.4 Observed monthly precipitation at experimental site versus 30-year precipitation 
record (1997-2008). (Source: Alabama Black Belt Research and Extension Station)  
 
Different hydraulic dosing rate and water percolation loss between years one and 
year two requires an explanation. There was a significant difference in precipitation 
between years one and two. The period from March 2007 to June 2007 coincided with a 
historic drought; 248 mm precipitation versus 492 mm in an average year (Figure 5.4). 
The month of September 2006 was the system startup period during which time the test 
site was relatively dry in the upper 45 cm (below 0.2 m3 m-3, Figure 5.2). Observed data 
also indicate percolation below 45 cm depth in June 2008 likely due to increased rainfall 
and correspondingly reduced ET (Figure 5.3). However, more normal and higher rainfall 
returned during the same period in 2008 (Figure 5.4). Therefore, observed higher 
hydraulic dosing rates in the drought season of year one and the lower dosing rates during 
the normal rainfall warm season in year two can be explained by the significant weather 
change between years one and two. 
110 
 
Since multi-year rainfall variability is expected impact soil cracking on natural 
soils (Kishne et al., 2009), it is likely that soil cracking developed during the warm 
season in 2008 was not as severe as during the drought of 2007. As a consequence, 
percolation due to preferential flow would be lower in 2008 than in 2007, as indicated in 
Figure 5.3.  
 
SITE SOIL HYDRAULIC PARAMETERS VERIFICATION -  YEAR TWO 
Measured field capacity in treatments I and II varied from 0.37 to 0.44 (m3 m-3) 
(Figure 5.5a), close to system operational thresholds (0.40 ? 0.45 m3 m-3). The 
Christiansen uniformity coefficient for field capacity was 96.9%, indicating a high 
uniformity for that texture dependent soil parameter. Measured Ksat of the experimental 
site varied from 0.12 to 0.29 ?m s-1 with relatively higher values in the upper slope 
section (Figure 5.5b). The Christiansen uniformity coefficient for Ksat was 76.2%, 
indicating a uniformly low permeability. In fact, the Houston clay is rated ?Extreme? for 
conventional septic systems by the Alabama Department of Public Health, meaning that 
water percolation is extremely low in this type of soil and conventional septic systems are 
not suitable on this type of soil. Field soil testing indicated that permeability and field 
capacity corresponded well to system design and operational thresholds. This result 
provides further evidence that soil cracking likely stimulated preferential flows that 
caused substantial percolation loss during dry seasons.  
111 
 
 
Figure 5.5 Spatial variation of (a) field capacity and (b) saturated hydraulic conductivity, 
Ksat, in the SDI drain field (treatments I and II).  Soil hydraulic conductivity was measured 
onsite using a permeameter (Ksat Inc., CA) at 45 cm depth. Field capacity was measured 
using the laboratory pressure plate method on intact soil cores (SSSA, 2002b) at 18 cm 
depth. 
 
Recommendations for system improvements, similar to those reported in Chapter 
4, are to include the use of multiple soil moisture monitors vertically and horizontally to 
more adequately reflect drain field moisture conditions and site heterogeneity. Emitter 
and drip line spacing could also be reduced to increase soil moisture uniformity and limit 
crack development in the field. Above suggestions require additional field testing to 
evaluate their impact on system hydraulic performance, including water loss to 
preferential flow. 
 
SOIL MOISTURE PROFILE ? YEAR TWO  
As demonstrated by the 30-year historic monthly precipitation record (Figure 5.4), 
during the summer time of year two (May to September 2008), normal to high rainfall 
returned (77.9 - 230 mm month-1), followed by a low rainfall (43 mm) in September 2008. 
112 
 
 
Figure 5.6 Year two soil moisture profiles, May 14-September 8, 2008. I. Synthetic 
wastewater application subplot; II. Clean well water application subplot (control); III. 
Agronomic practice subplot (control); IV. Undisturbed (blank) control. . 
 
113 
 
Soil moisture content in the upper 20-60 cm during this period was at least 0.10 m3 m-3 
greater in irrigated treatments I and II compared to treatments III and IV (Figure 5.6). 
Since water was dispersed in treatments I and II only when field moisture content fell 
below 0.40  m3 m-3, soil moisture observations document that soil moisture controlled 
dosing successfully managed soil moisture levels during the monitoring period. 
Interestingly, even though treatments I and II received the same volume of water, 
treatment I had a average higher soil moisture content (50 m3 m-3 versus 35 m3 m-3  ) than 
treatment II within the 20-60 cm depth. This unexpected difference may have resulted 
from differences between treatments in their field capacity or Ksat (Figure 5.5). 
Since soil moisture profilers provide false volumetric soil moisture readings 
above saturation, obtained profile readings are valid only if below the porosity of the 
monitored soil profile. NRCS web soil survey (NRCS, 2008) was used to estimate the 
porosities of the soil profile at the site, with 0.45-0.58 m3 m-3 determined as the upper and 
lower bounds (Figure 5.6). Recorded soil moisture readings above soil porosity were 
observed in treatments I and II between 20-60 cm depth, possibly caused by water stored 
within soil cracks or at the profile tube soil interface. Numerous soil moisture readings at 
100 cm depth were found higher than estimated soil porosity in treatments II, III, and IV, 
suggesting that free water may have accumulated at the sampled depths (personal 
correspondence, M. McClung, Nov 20, 2008). During the one meter coring, the presence 
of chalk was noted at or near 100 cm in all treatments except treatment I. According to 
the Geological Survey of Alabama (1993), the experimental site is underlain at a depth of 
approximately 6 m by a relatively impermeable layer of fossiliferous clayey chalk and 
114 
 
chalky marl, with shallower formations found reverently at 12 cm to 2 m. Therefore, it is 
possible that a shallow restrictive layer (more restrictive than Houston clay in terms of 
water percolation) of clayey chalk near 100 cm depth in treatment II-IV. The water 
percolated from above layers may accumulate right on top of this restrictive layer and 
formed a water table. The soil moisture profile for treatment I indicates an increasing 
moisture trend to approximately 60 cm followed by a decreasing moisture trend to 100 
cm depth. In the absence of preferential flow, this moisture profile would be expected to 
distribute dosed wastewater effectively throughout the soil profile.   
 
WASTEWATER STORAGE REQUIREMENT- YEAR ONE AND YEAR TWO 
Theoretical hydraulic dosing rates fluctuated throughout years one and two, while 
household wastewater flows were held constant. Consequently, where the monthly water 
balance indicated a negative balance it was necessary to provide wastewater storage 
requirement. Maximum allowable dosing rates for each month were assigned to those 
months with monthly precipitations closest to the 30-year historical normal (Table 5.4, 
Figure 5.7).  
Determination of wastewater storage volume requirement in terms of a monthly 
effluent flow assumed that there was already a two-month wastewater storage 
requirement at the beginning of January from the previous winter. Required monthly 
wastewater storage increased or decreased as maximum allowable wastewater dosing 
rates fluctuated (Table 5.4). Wastewater storage requirement increased annually from 
January to June, indicating a maximum wastewater storage requirement of approximately 
seven months in August. After that, the wastewater storage requirement decreased, but 
115 
 
ended the year with a three-month wastewater storage requirement that will be carried 
into the next calendar year. Table 5.4 indicates a major limitation of the Black Belt clay 
soils that over the two-year study period on average, only 4 months out of 12 had an 
observed positive water balance in favor of soil adsorption. 
 
Figure 5.7 Year one and two monthly precipitation, October 2006 to September 2008, 
compared to 30-year normal. September 2006 is excluded since it is a system startup period. 
July 2008 to September 2008 is included as comparison to the records in the same period of 
2007. Asterisks indicate the recorded month and year closest to 30-year normal, 
subsequently used to estimate average monthly dosing rate.   
 
January and February zero dosing periods observed in year one and year two 
caused by naturally saturated soil conditions that curtailed wastewater disposal. The zero 
dosing period in October was a manual shut down required to facilitate warm season 
harvesting and cool season planting. If the wastewater dosing rate could be increased by 
expanding drain field size to offset the storage requirement, the wastewater storage 
requirement can be reduced and fixed. However, if a zero annual water balance could be 
achieved, a minimum two-month wastewater storage requirement still exists to 
accommodate during wet winters. Advantages of this experimental SDI system can be
116 
 
Table 5.4 Monthly wastewater storage requirement based on observed monthly hydraulic disposal rates and Alabama Onsite Sewage 
Disposal Rules (ADPH, 2006). 
 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Annual summary 
Maximum allowable  
monthly hydraulic  
wastewater dosing rates1  
in cm month-1   
0 0 0.62 0.47 2.47 2.44 14.9 20.1 14.9 0 9.9 0 65.9 cm 
Monthly wastewater 
generation rate  
(cm month-1)2 
6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 72.0 cm 
Wastewater storage  
requirement from  
previous month (month) 
2 3 4 4.9 5.9 6.5 7.1 5.6 3.2 1.7 2.7 2.1  
Wastewater storage  
requirement change from current  
month (month)3  
+1 +1 +0.9 +0.9 +0.6 +0.6 -1.5 -2.4 -1.5 +1 -0.6 +1  
Accumulative wastewater 
storage requirement at the 
end of month (month)4 
3 4 4.9 5.9 6.5 7.1 5.6 3.2 1.7 2.7 2.1 3  
   
Notes: 1. Field observed monthly hydraulic disposal rate for month when recorded precipitation falls closest to 30-year average. 
              2. Defined by the Alabama Onsite Sewage Disposal Rules (ADPH, 2006) for clay soils receiving effluent from conventional onsite septic 
systems. (Depends on soil hydraulic properties and receiving wastewater quality, not quantity). 
              3. Determined by dividing the difference between ?Expected monthly maximum hydraulic wastewater disposal rate? and ?Monthly 
wastewater generation rate? by 6.00 cm month-1. 
              4. Determined by adding the number of ?Wastewater storage from previous month? and ?Wastewater storage generated in current month 
of each column?. 
117 
 
exploited if used in conjunction with other wastewater treatment systems that function 
during periods of low soil hydraulic dosing. For example, this system may be used as a 
supplement to existing municipal or decentralized community wastewater treatment 
facilities having adequate land, labor, and machinery. 
Based on observed data, a seven-month wastewater storage requirement may be 
required (Table 5.4). A worst case analysis indicates that the wastewater storage 
requirement increases each year due to a negative water balance, which means that more 
effluent is produced than can be safely dispersed in a year. Thus, if a minimum 2-month 
wastewater storage is specified, every expectation is that this volume will provide 
sufficient storage in only the driest years.  
 
5.4.2 SYSTEM NUTRIENT MANAGEMENT  
FIELD CROP NUTRIENT UPTAKE - YEAR TWO (JUNE 14, 2007 TO JUNE 24, 2008) 
Nutrient uptake performance and field crop yield in treatments I and III are listed 
in Table 5.5 for each growing season in year two. Dry mass yield of the 1st cutting of 
sorghum and sudangrass in 2007 was 3.7 times higher in treatment I than in treatment III. 
The 2nd cutting of sorghum and sudangrass and the subsequent winter wheat and rye had 
similar crop yields for both treatments. These results may indicate a reduced crop yield 
benefit from synthetic wastewater application into fall and winter as wastewater 
hydraulic dosing rate decreased. Annual crop nutrient uptake in treatment I represented 
approximately 32% of applied nitrogen and 31% of applied phosphorus in year two. 
Approximately 86% of applied nitrogen in treatment III was represented by crop nutrient 
118 
 
uptake in the same period, suggesting nitrogen application in treatment I may not have 
been over supplied compared to the crop growth requirement.  
Table 5.5 Crop yield, nutrient application, and uptake of treatments I and III during year 
two, June 14, 2007 to June 24, 2008. 
 Growing crops 
Dry mass 
yield, 
kg ha-1 
Total N 
applied,  
kg ha-1 
Total N uptake,  
kg ha-1  
(% of total N 
applied) 
Total P 
applied,  
kg ha-1 
Total P uptake,  
kg ha-1  
(% of total P 
applied) 
Treatment I 
(synthetic 
wastewater 
application 
subplot) 
Sorghum-sudan grass 
1st cut 
(Jun 07- Aug 07)  
5910 267 103  (38%) 52 11 (21%) 
Sorghum-sudan grass 
2nd cut 
(Aug 07- Nov 07)  
820 347 14  (4%) 31 2  (6%) 
Winter wheat  
and rye  
(Nov 07-Jun 08)  
13200 186 138  (75%) 11 17  (154%)1 
Annual  
(Jun 07-Jun 08) 19900 800 
256  
(32%) 94 
29  
(31%) 
Treatment III 
(agronomic 
control 
subplot) 
Sorghum-sudan grass 
1st cut 
 (Jun 07- Aug 07)  
1620 67 28  (42%) 0 11 
Sorghum-sudan grass 
2nd cut 
(Aug 07- Nov 07)  
774 67 10  (15%) 0 5 
Winter wheat  
and rye  
(Nov 07-Jun 08)  
11700 67 136 (203%)1 0 107 
Annual  
(Jun 07-Jun 08) 14000 202 
174  
(86%) 0 123 
1. Crop dry mass yields were calculated as the average of three field measurements. 
2.  Total N and total P applied were calculated based synthetic wastewater composition (Table 
5.2) and recorded system daily flow rate (Figure 5.1). 
3. Crop uptake of nitrogen or phosphorus was greater than fertilized, meaning soil provided 
additional nitrogen or phosphorus to meet crop growth requirements. 
 
Higher system hydraulic dosing rates in summer and fall brought higher nutrient 
load to treatment I compared to the spring and winter. Although the nutrient contribution 
from the synthetic wastewater resulted in over three times the crop yield in treatment I 
versus treatment III during the warm season, excess nitrogen and phosphorus in soil 
water was the apparent result (Figures 5.10-5.12). The lack of yield difference between 
treatments I and III after the warm season seems to indicate that unscavenged nitrogen 
119 
 
could potentially be lost from a drain field due to the high mobility of nitrogen in soil 
(Sparks, 2003). These observations indicate that soil moisture controlled wastewater 
disposal may temporarily provide nutrient loads higher than crop uptake needs.  
 
SOIL WATER NUTRIENT CONCENTRATION - YEAR TWO (AUGUST 2007 TO JUNE 2008) 
Field suction lysimeter data from treatments I and II during year two are 
presented for NH4+-N (Figure 5.8), NO3--N (Figure 5.9), and total phosphorus (TP) 
(Figure 5.10) to demonstrate the impact of  applied wastewater on soil water nutrient 
levels under a normal rainfall year. With the measured data of treatment II as a control, 
soluble NH4+-N (all less than 10 mg L-1) and TP levels (all less than 9 mg L-1) in soil 
water samples from treatment I were lower than that of the applied synthetic wastewater 
(approximately 80 mg L-1 of NH4+-N and 10 mg L-1 of TP), suggesting that nutrients with 
synthetic wastewater underwent chemical, physical, or biological transformation and/or 
uptake that removed them from the water phase. Since the negligible water percolation 
below 45 cm depth during the first half of 2008 was only by estimation and there was an 
obvious water percolation below 45 cm depth in June 2008, the possibility exists that 
water did percolate below 45 cm during the lysimeter monitoring period. Measured 
nutrients were generally at relatively higher levels during the warm season compared to 
the cold season, suggesting that the increased nutrients supplied from higher hydraulic 
dosing rates in the warm season may have extended deeper than the 45 cm depth during 
year two. Also, if nutrients were applied during year one when a significant amount of 
water percolation was estimated, the chances of nutrient deep percolation would be 
expected even higher than in year two. 
120 
 
Suction lysimeter measurements provide only a snapshot of the nutrient levels in 
soil solution during year two. Since sampled depth reached only 45 cm depth, definite 
conclusions regarding nutrient leaching status cannot be made. The cumulative effect 
from the synthetic wastewater application (treatment I) was further evaluated by 
comparing soil nutrient profiles of the four treatments (Figures 5.11-5.16).  
 
 
Figure 5.8 Soil water NH4+-N levels in treatments I and II during year two. 
 
121 
 
 
Figure 5.9 Soil water NO3--N levels in treatments I and II during year two. 
 
 
Figure 5.10 Soil water TP levels in treatments I and II during year two. 
 
 
SOIL NUTRIENT PROFILE AT THE END OF SYNTHETIC WASTEWATER APPLICATION ? YEAR 
TWO 
Soil total nitrogen (TN) profiles (Figure 5.11) and soil crop available nitrogen 
profiles (Figure 5.12) for the four treatments after year two indicate gradually reduced 
122 
 
nutrient concentrations with depth in all treatments over the sampled 100 cm soil depth, 
with no discernable differences between the treatments. Comparison of average soil 
cation exchange capacities (CEC) for the five soil horizons (Table 5.1) with crop 
available NH4+-N profiles (Figure 5.12) indicates that crop available NH4+-N in all 
treatments was approximately 1000 times lower than soil CEC. This observation, 
together with the measured soil pH = 8 of treatment I (Figure 5.13) indicating favorable 
conditions for NH3 volatilization (Chang, 2006; Sigunga et al., 2002), suggests that soil 
solution chemistry reduced the potential for NH4+-N deep percolation.  
 
Figure 5.11 Year two total soil nitrogen profiles for field treatments I, II, III, and IV, June 
24, 2008. (Data represent means of triplicate sub samples.) 
 
The observed decreasing trend of crop available NO3--N with soil depth (Figure 5.12) 
indicates there was little likelihood of NO3--N leaching over the soil profile at the time 
the soil cores were sampled, as has been reported by other field studies. P?rez et al. 
(2003) reported that downward movement of a fertilizer NO3--N plume increased crop 
123 
 
available NO3--N and water phase NO3--N levels along its pathway. Although the 
synthetic wastewater in the present study was applied consistently to treatment I when 
soil cores were taken and the undisturbed control plot was barren without any applied 
fertilizers, there is no apparent difference between the two treatments in terms of NO3--N 
leaching. In fact, it appears evident that NO3--N leaching did not occur in any treatment 
during the synthetic wastewater application period, or its influence on soil nitrogen 
profile in treatment I had already passed down to soils deeper thatn 100 cm as indicated 
by the slightly increased of soil total N and crop available NH4-N. This is also an 
indication that soil profile sampling depth should be even deeper in future studies. 
124 
 
 
Figure 5.12 Year two soil crop available nitrogen (NH3+-N and NO3--N) profiles for field 
treatments I, II, III, and IV, June 24, 2008. (Data represent means of triplicate sub 
samples.) 
 
 
 
 
 
 
 
125 
 
 
 
Figure 5.13 Year two soil pH profiles for field treatments I, II, III, and IV, June 24, 2008. 
(Data represent means of triplicate sub samples.) 
 
Total nitrogen applied to treatment I during year two was 800 kg ha-1, which 
represents approximately 12% of the total nitrogen contained in the soil (Table 5.6).  This 
finding further indicates there is a low possibility of nitrogen leaching loss during the 
experimental period. However, as indicated previously, substantial water percolation 
losses during dry soil conditions may have occurred at the site due to preferential flows. 
Consequently, the possibility exists that preferential flows caused by soil cracking may 
have conducted applied synthetic wastewater into deeper (> 100 cm) soils without being 
detected by sampled soil cores. If a restrictive layer exists close to the 100 cm depth as 
suggested by soil moisture profiles (Figure 5.6), then nutrient accumulation might 
occurred at depth close to 100 cm, which however was not observed in this study. 
 
 
 
126 
 
Table 5.6 Year two partial field nutrient balance of the treatment I drain field for the 
synthetic wastewater application experiment from June 14, 2007 to June 24, 2008. 
 Annual contribution 
from synthetic 
wastewater, kg ha-1 
Annual crop 
uptake, kg ha-1 
Total content in the upper 100 cm 
depth at the end of soil core 
sampling, kg ha-1 
Nitrogen 800 256 6783 
Phosphorus 94 29 2000 
1. Reported values are the averages of three field measurements. 
2. Total N, P contents in the upper 100 cm depth at the end of soil core sampling were calculated 
with field measured bulk density and soil total N, P profiles.  
 
 
 
Figure 5.14 Year two total soil phosphorus profiles for field treatments I, II, III, and IV, 
June 24, 2008. (Data represent means of triplicate sub samples.) 
 
127 
 
 
Figure 5.15 Year two soil crop available phosphorus profiles for field treatments I, II, III, 
and IV, June 24, 2008. (Data represent means of triplicate sub samples.) 
 
 
Figure 5.16 Year two soil water soluble phosphorus profiles for field treatments I, II, III, 
and IV, June 24, 2008. (Data represent means of triplicate sub samples.) 
 
Year two soil TP (Figure 5.14), crop available P (Figure 5.15), and water soluble 
P (Figure 5.16) profiles of the four treatments indicate a decreasing trend of 
approximately 80% over the 100 cm soil profile depth, with no discernable difference 
128 
 
between treatments. The measured soil phosphorus adsorption capacity of the 
experimental site was 16.67 g P kg-1 soil (Table 5.1), over 400 times higher than the 
measured crop available P of treatment I. Furthermore, the total amount of phosphorus 
applied to treatment I during the one-year wastewater application period was 94 kg ha-1, 
approximately 4.7% of the total phosphorus contained in the upper 100 cm depth of soil 
(2000 kg P ha-1) (Table 5.6). Thus, it appears that there is a low likelihood of field soil P 
accumulation after one year of synthetic wastewater application. However, it should be 
noted that preferential flows may have carried wastewater into soils deeper than 100 cm 
depth with less resulting influence on the top 100 cm soils. The year two study was 
carried out under a relatively normal rainfall year when water percolation below 45 cm 
was estimated as negligible.  During year one a significant amount water percolation loss 
was estimated under a historic drought. Consequently, if nutrients had been applied in 
year one, the soil profile may have indicated signs of nutrient accumulation or deep 
percolation. 
Previous researchers reported that P leaching is negligible in drain fields due to 
numerous P immobilization mechanisms such as chemical precipitation and 
chemical/physical adsorption (Sawhney and Hill, 1975; Venhuizen, 1995). Field results 
from this study appear to conform to literature regarding negligible concern for short-
term P leaching in spite of the recognized potential for long term P accumulation due to 
annual P input. Findings are by no means conclusive since the nutrient application study 
was carried out for only one years. With considerable water percolation loss below 45 cm 
depth indicated, the possibility of higher P levels in deep soils in treatment I exists, 
129 
 
especially if wastewater dosing were continued. Due to the limited number of soil core 
sampling locations (3 locations per treatment), it is possible that applied wastewater may 
have carried P into deeper soils through preferential flow. 
 
5.4.3 SYSTEM APPLICATION BENEFITS AND LIMITATIONS 
Based on observed data from years one and two of the field study, the 
experimental soil moisture controlled wastewater disposal system as designed and 
installed provides several advantages. The system first and foremost effectively 
withholds wastewater disposal during wet field conditions to mitigate health threats 
associated with drain field hydraulic overloading that leads to surface ponding. Second, 
wastewater may be stored or recycled for use during favorable field conditions, similar to 
agricultural or other liquid waste storage facilities. Third, during certain warm seasons, 
hydraulic disposal rates may be temporarily increased to levels higher than the hydraulic 
loading rate permitted in current OWTS drain field design (ADPH, 2006; AWOTC, 
2005; EPA, 2003).  
However, several operational deficiencies present solid road blocks for a 
successful application of the technology. First, there are extended zero and near zero 
hydraulic dosing rate periods (minimum 2-month) during wet soil conditions that result in 
an excessive (minimum 2-month) wastewater storage requirement. Over the two-year 
study period, on average only 4 months out of 12 had observed system hydraulic dosing 
rates higher than wastewater inflows. The large wastewater storage requirement prevents 
this experimental system from being acceptable as a stand-alone application. However, 
130 
 
opportunities exist for use of this system supplement to other wastewater treatment 
technologies that can function when field conditions do not favor direct soil dispersal. 
Second, as indicated from the two-year monthly water balance (Figure 5.3), it is apparent 
that the experimental system cannot overcome water percolation losses (over 30% of 
dosed water) during severe drought conditions due to seasonal changes in shrink-swell 
clays. The system as designed cannot guarantee effective control over this type of water 
percolation loss in smectitic soils. Thirdly, because this experimental system is 
completely dependent upon soil moisture to control the dispersal of wastewater, 
wastewater applications during certain seasons may exceed crop uptake (approximately 
70% of annual applied N and P were not accounted for by crop uptake), making the drain 
field at least temporarily a potential pollution source for groundwater. Fourth, but not the 
least, routine and frequent field and crop maintenance (three field crop harvesting and 
plantings per year under the seasonal rotation used in this study) is required of this 
system to ensure adequate operation. The combined requirements of land, equipment, and 
labor exclude this experimental system as a viable option for land owners with limited 
resources. Even if the above challenges were adequately resolved, more conclusive 
results would be required to document economic and environmental feasibility through 
adequately replicated field trials.  
 
5.5 CONCLUSIONS 
Over a two-year field study, an experimental wastewater SDI system that 
incorporates real-time soil moisture control and a seasonal cropping system was 
131 
 
evaluated for hydraulic and nutrient management in the Alabama Black Belt clay soil. 
Soil-moisture controlled hydraulic disposal rates in the drain field varied between 1.18 
cm day-1 in April 2007 to a nearly two-month zero disposal period (0.0 cm day-1) during 
the preceding and succeeding winter seasons. Demonstrated advantages of the water 
management strategy of this experimental system include: 1) avoidance of soil moisture 
conditions above field capacity in the absence of consistent rainfall events; 2) seasonally 
increased wastewater disposal rates under favorable dry field conditions; 3) the 
experimental system dependence on soil moisture control for wastewater disposal may 
temporarily create a field nutrient imbalance with respect to crop uptake; and 4) routine 
system and crop maintenance requirements are not practical for land owners with limited 
resources.  
There are several inherent limitations of the system. First, estimated water 
balance suggests that over 30% of dosed water percolated below 45 cm during the 
drought of 2007. Although water percolation is necessary for effluent treatment, this 
result indicates the inadequacy of the experimental system to limit preferential flow in 
smectitic soils during dry soil conditions. Second, a minimum 2-month wastewater 
storage requirement was found necessary, indicating that the experimental system is not 
suitable as a stand-alone wastewater dispersal and treatment method. Nevertheless, the 
experimental system as designed can be used in conjunction with other wastewater 
treatment methods that can function during wet periods when this experimental system 
provides zero or near zero hydraulic dosing. 
Despite the potential environmental risks associated with deep percolation, field 
132 
 
nutrient observations indicate that the experimental system did not result in discernable 
drain field nitrogen and phosphorus accumulation or percolation beyond 100 cm. These 
findings are by no means conclusive since preferential flows caused by clay soil 
shrinking and crack development may have carried nutrients beyond 100 cm without 
being detected in treatment I, where the possible presence of a restrictive layer at the 100 
cm depth in treatments II-IV does not exist. 
This study concludes that soil moisture controlled SDI dispersal of wastewater in 
native clay soils of the Alabama Black Belt is not suitable as a stand-alone disposal and 
treatment method. Major reasons cited include an excessive (2-month minimum) 
wastewater storage requirement and environmental questions related to groundwater that 
are left unresolved by this study. Nevertheless, the system as designed and installed has 
potential as a supplement to existing municipal or decentralized community wastewater 
treatment facilities with adequate land, machinery, and labor. In order to quantifying 
indicated system limitations, the system should be further studied in an attempt to limit 
soil cracking during dry field conditions and adequately control soil water balance. 
Possible improvements to this end include reduced emitter and drip line spacing and a 
more uniform distribution of soil moisture sensors within the field. In addition, the long-
term fate and transport of nutrients (NH4+-N, NO3--N, and TP) in percolated water 
requires further field study utilizing a more intensive spatial and temporal field sampling. 
Replicated experiments on a variety of similar sites are recommended to provide robust 
results regarding the environmental and economic feasibility of soil moisture controlled 
SDI wastewater dispersal in the Alabama Black Belt.  
133 
 
CHAPTER SIX 
CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH  
 
6.1 CURRENT OWTS STATUS IN THE ALABAMA BLACK BELT AREA 
Demographic data from the US Census indicates that 1) OWTS are widely used in 
the Alabama Black Belt; 2) a significant number of existing OWTS in the study area have 
been operating for more than 20 years; 3) system size is generally maintained below 2 
person unit-1 except around major urban centers; and 4) 12% of census block groups have 
an estimated OWTS density higher than the EPA regulated threshold for negative 
environmental impact (15 unit km-2) and are generally found clustered around city fringes.  
A new OWTS soil suitability rating system (OWTS-SSRS) developed based on 
current Alabama OWTS regulations was applied to the Alabama Black Belt area using 
soil information extracted from USDA-NRCS digital soil survey data (SSURGO). The 
new OWTS-SSRS was compared with an existing soil limitation rating system developed 
by NRCS (NRCS-SLRS). The new OWTS-SSRS rated approximately 52% of the 
Alabama Black Belt area as unsuitable for OWTS, while the more conservative NRCS-
SLRS rated 89% of the area as limiting for OWTS.   
Based on the results of this study, two strategies to limit the potential 
environmental and health risk from OWTS failure in the Alabama Black Belt area are 
134 
 
suggested. For city fringe communities, a proactive response is to extend municipal sewer 
service to high risk clay soil areas in advance of widespread OWTS failure. For isolated 
rural households outside the range of municipal sewer service, retrofits or replacement of 
aged OWTS with more advanced dispersal systems is recommended, such as aeration 
treatment units, packed-bed media filters, mounds, or subsurface drip irrigation.. The 
need for alternative wastewater treatment and dispersal systems for clayey soil regions 
within the Alabama Black Belt area is clear. An appropriate alterntive for wastewater 
treatment and dispersal on clayey soils would benefit regional decision makers in targeted 
efforts to retrofit or replace failure systems in the Black Land Prairie soils area. 
 
6.2 SYSTEM DESIGN 
This research developed and tested a soil moisture controlled wastewater SDI 
system for use in a clay adsorption field. The intention of the research was to evaluate an 
alternative wastewater dispersal and treatment system for the study area. The 
experimental system was installed on a clay soil site of 500 m2 and evaluated with respect 
to water and nutrient management over a two-year period from September 2006 to June 
2008. The experimental system included an SDI wastewater dosing system controlled by 
volumetric soil moisture. A seasonal cropping rotation of sorghum-sudangrass (Sorghum 
bicolor) and winter wheat (Triticum aestivum) with rye (Secale cereale) mix was 
integrated into the wastewater dispersal and treatment system to enhance annual water 
and nutrient crop uptake. Wastewater was only dispersed in the drain field when field 
135 
 
moisture content at monitored locations dropped below field capacity (0.40 m3 m-3), to 
limit health threats associated with overloaded drain fields. 
 
6. 3 FIELD EVALUATION OF SYSTEM WATER AND NUTRIENT MANAGEMENT 
Observed hydraulic dosing rates in the drain field varied from a high of 1.18 cm 
day-1 during drought conditions to a low of 0.0 cm day-1 during wet winter months. 
Estimated monthly water balance indicated that over 30% of dosed water percolated 
below 45 cm depth during the historic dry summer of 2007, presumably due to soil 
cracking. Although water percolation below 45 cm was not necessarily an environment 
concern at the site, preferential flows caused by soil cracking during dry soil conditions 
was unexpected. Over the two-year study, only 4 months out of 12 had an observed water 
balance in favor of soil adsorption. A minimum a two-month onsite wastewater storage 
requirement is estimated for the experimental system due to zero or low hydraulic 
disposal periods during typical wet winter months.  
Observed seasonal and annual field nutrient loading and crop nutrient uptake in 
the drain field indicates the following nutrient imbalance. Nutrient supply from 
wastewater was at surplus levels for warm season crops, while cool season crops 
scavenged nutrients due to zero or almost zero wastewater disposal rates. Although soil 
nutrient profiles provided no direct evidence of drain field nitrogen or phosphorus 
accumulation or percolation below 100 cm depth in year two of the study, applied 
wastewater may have been transported into deeper soils (> 100 cm) by preferential flows 
caused by soil cracking if no restrictive layer existed at close to 100 cm to intercept 
136 
 
percolated water and nutrients.  
 
6.4 SYSTEM BENEFITS AND LIMITATIONS 
Based on the two-year field experiment, the obvious advantages of the soil 
moisture controlled wastewater dispersal strategy are: 1) avoidance of soil moisture 
conditions above field capacity in the absence of consistent wet weather; and 2) 
seasonally increased wastewater dosing rates during favorable (dry) field conditions. In 
spite of these benefits, the experimental system cannot be recommended for direct field 
application due to the following limitations: 1) the extended zero or near zero hydraulic 
dosing rate (minimum 2-month) observed during wet winter conditions creates an 
impractical two-month minimum wastewater storage requirement; 2) the combined 
dosing control and SDI system resulted in substantial water percolation loss (over 30% of 
dosed water) during severe drought apparently due to seasonal soil shrinking; 3) the 
system dependence on soil moisture control for wastewater disposal created a nutrient 
imbalance by annually supplying approximately 70% more N and P than measured crop 
harvest); and 4) routine SDI and crop operation and maintenance requirements (three 
times per year of field crop harvest and planting) are not practical for land owners with 
limited resources.  
Although the geologic status of the Alabama Black Belt area indicates that deep 
percolation not a serious environmental issue,  effective control of soil water balance for 
environmentally safe dosing is one of the main goals still to be realized by this 
experimental system.   
137 
 
Based on the observed benefits and limitations of the experimental system, this 
study concludes that soil moisture controlled SDI disposal of wastewater in native clay 
soil of the Alabama Black Belt is not suitable as a stand-alone disposal and treatment 
method. The main limitation of the experimental system is the excessive (two-month 
minimum) wastewater storage requirement. Nevertheless, the system has potential as a 
supplement to certain existing municipal or decentralized community wastewater 
treatment facilities with adequate land, equipment, and labor resources.  Due to the 
limited time frame of the field experiment and inadequate field sampling, extended field 
experiment is suggested to obtain more conclusive results than the current study in order 
to better address the applicability of the proposed soil moisture controlled SDI 
wastewater dispersal in the Alabama Black Belt area and other similar soil regions. 
 
6.5 SUGGESTIONS FOR FUTURE RESEARCH 
The purpose of this study is to obtain quantifiable results regarding system water 
and nutrient management in a high clay drain field. System feasibility components 
including further quantification of wastewater storage requirement, surface ponding, and 
nutrient leaching are needed. Based on results of the present study, a long-term field 
experiment is recommended to include; 1) adequately replicated (three minimum) field 
treatments, similar to the current study: soil moisture controlled wastewater application, 
soil moisture controlled clean water application, agronomic control, and undisturbed 
control, 2) an improved field sampling schedule at deeper locations and higher 
frequencies. Since no significant percolation below the soil moisture controlling zone was 
138 
 
observed during year two of the study (a normal rainfall year), SDI layout can be 
continued per the current study (61 cm of lateral and emitter spacing). Modification of 
SDI lateral and emitter spacing can be based on experimental results regarding system 
effectiveness in controlling soil water balance. Results will quantify system water and 
nutrient management in terms of hydraulic seasonal dosing pattern, monthly water 
percolation loss, wastewater storage requirement, drain field water and nutrient balance, 
monthly soil water nutrient levels, monthly nutrient leaching loss, and seasonal drain 
field nutrient balance. Conclusions can be made regarding system applicability based on 
observed season water and nutrient management cycles. Conclusive results from an 
improved long-term experiment are expected to provide a more thorough understanding 
of soil water balance control of wastewater dispersal on clayey soils with respect to the 
environmental challenges inherent when attempting to control leaching control while 
preventing surface ponding. 
 
139 
 
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162 
 
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163 
 
APPENDIX I  
SDI DRIP TUBE EMITTER HYDRAULIC UNIFORMITY TEST 
System hydraulic performance was evaluated by drip tube emitter hydraulic 
uniformity and system flush velocity. To test emitter hydraulic uniformity, twenty emitter 
flow rates were measured at 240 kPa three times to determine average discharge per 
emitter. A coefficient of variation for the 20 emitters was calculated using Eq. I.1 (ASAE 
EP 405.1, 2003), with results compared to standard values. 
( ) 2/1
2/1222
2
2
1
1?
?????? ?+++
= nq
qnqqq
C nv
K
                      (I.1) 
 
Where: 
      Cv = emitter coefficient of variation; 
       q1, q2, ?, qn = discharge of emission devices tested (l/h); 
 q  = average discharge of all emission devices tested (l/h); 
        n = number of emitters (n=20). 
Measured flow rates of each emitter are plotted in Figure I.1 against the 
manufacturer?s specification of 2 L h-1. The calculated coefficient of variation ( vC ) is 
0.067, indicating that hydraulic uniformity performance is average (Table I.1) (ASAE EP 
405.1, 2003). One of the SDI advantages in agricultural industry is that water can be
164 
 
distributed rather uniform in the field than surface overflow (Beggs et al., 2004). For 
OWTS, a uniform wastewater distribution in drain field can postpone local hydraulic and 
nutrient overloading in the field that causes most OWTS failures (US EPA, 2002). This 
average rated emitter hydraulic performance is expected for future field experiment. 
1.50
1.75
2.00
2.25
2.50
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Emitter number
Fl
ow
 ra
te
, L
/h
0.40
0.45
0.50
0.55
0.60
0.65
Fl
ow
 ra
te
, g
ph
Rep 1 Rep 2 Rep 3 Specification
 
Figure I.1 Measured flow rates of each emitter. (Adapted from Ducote, 2006) 
 
 
Table I.1 Recommended Classification of Manufacturer?s Coefficient of Variation. 
Cv range                                       Emitter Performance                                  
< 0.05                                                  Good 
0.05 to 0.07                                         Average 
0.07 to 0.11                                         Marginal 
0.11 to 0.15                                         Poor 
> 0.15                                                  Unacceptable 
Source: American Society of Agricultural and Biological Engineers (ASAE EP 405.1, 
2003). 
 
165 
 
APPENDIX II  
SITE CHARACTERIZATION 
 
The site selected for the field study is in Marion Junction, Alabama, at the 
Alabama Black Belt Research and Extension Center (ABBREC), located approximately 
10 miles west of Selma, Alabama. The unique value of the ABBREC is the proximity to 
the people, plants, weather, and soils of the region.  
The test site was selected by field investigation after identifying five fields 
reasonably accessible to power, water, and transportation. Figure II.1 shows the 
boundaries and major soil series of the 450-hectare ABBREC and five potential fields for 
the study. A Houston clay soil at Field 21 was ultimately selected because of the fewest 
impediments to year-round SDI wastewater dosing, while providing low permeability and 
high shrink-swell features common to the region. Field 21 also has a low slope and 
relatively high fertility potential. A truck-mounted Giddings? probe and sleeve was used 
to retrieve core samples from Field 21 for soil profile characterization.
166 
 
 
 
Figure II.1 General soil series map, Black Belt Research and Extension Center, Marion 
Junction, Alabama, showing location of study site.   
 
Field and laboratory soil characterization for the Houston soil site at Field 21 was 
completed by the Auburn University Pedology Laboratory (Tables II.1-II.4).  Five soil 
horizons were identified to 1.52 m and corresponding to Figure II.1.  Table II.1 indicates 
that dark clay was prominent at the surface to approximately 42 cm depth, with evidence 
of redox at 88 cm indicating significant periods of saturated or anaerobic conditions 
during most years. Table II.2 indicates a pH favorable for agricultural production and a 
base saturation free of exchangeable acidity (H+1 and Al+3). 
 
 
167 
 
Table II.1 Detailed soil description to 1.52m depth at Field 21, Black Belt Research and 
Extension Center, Marion Junction, Alabama.  Side slope, 4%. 
Horizons Depth 
(cm) 
Color  Detailed description 
Ap1 (0-23) very dark gray (2.5Y 3/1) clay 
Ap2 (23-42) dark grayish brown (2.5Y 
4/2) 
clay 
BA (42-63) olive brown (2.5Y 4/3) clay; few fine black manganese oxide 
concretions; common medium and 
coarse calcium carbonate concretions 
and soft accumulations 
Bkss1 (63-88) olive brown (2.5Y 4/3) clay; large wedge-shaped aggregates 
that are bordered by intersecting 
slickensides; very plastic, sticky; few 
fine black manganese oxide 
concretions; common medium and 
coarse calcium carbonate concretions 
and soft accumulations; calcareous 
Bkss2 (88-152+) light olive brown (2.5Y 5/6); 
common light brownish gray 
(10YR 6/2) redox depletions; 
common dark yellowish 
brown (10YR 4/6) redox 
concentrations 
clay; large wedge-shaped aggregates 
that are bordered by intersecting 
slickensides; very plastic, sticky; few 
fine black manganese oxide 
concretions; common medium and 
coarse calcium carbonate concretions 
and soft accumulations; calcareous 
Described: by P.G. Martin and J.N. Shaw (11/17/05). 
Table II.2 Chemical properties and organic matter at Field 21, Houston soil field site, Black 
Belt Research and Extension Station, Marion Junction, Alabama. 
Horizon Lower depth Base saturation pH (1:1) cm NH
4OAc H2O 0.1N CaCl2 
Ap1 23 100.00 7.34 7.17 
Ap2 42 100.00 7.47 7.23 
BA 63 100.00 7.46 7.27 
Bkss1 88 100.00 7.38 7.25 
Bkss2 152 100.00 7.33 7.30 
Source: Auburn University Pedology Laboratory. 
Table II.3 indicates relatively high CEC, with clay particles predominantly of the 
2:1, shrink-swell type widely associated with the Alabama Black Belt. Table II.4 
168 
 
confirms that the majority of the particle size distribution is in clay fraction, with 
increasing clay percentages with depth up to 71% at 152 cm.  
Table II.3 Chemical characteristics at Field 21, Houston soil field site, Black Belt Research 
and Extension Station, Marion Junction, Alabama. 
Horizon 
Lower 
Depth 
Exchangeable Cations Cation Exchange Capacity 
Ca Mg K Al Na CEC-7 ECEC ECEC CEC-7 
cm meq/100 g soil meq/100 g clay 
Ap1 23 na 0.12 0.15 0.12 0.03 38.84 39.37 72.89 73.88 
Ap2 42 na 0.04 0.12 0.08 0.03 34.15 38.42 63.60 71.55 
BA 63 na 0.04 0.11 0.08 0.04 34.42 37.66 60.98 66.72 
Bkss1 88 na 0.07 0.16 0.04 0.06 35.26 36.48 58.21 60.23 
Bkss2 152 na 0.18 0.18 0.06 0.08 40.62 42.85 57.14 60.27 
Source: Auburn University Pedology Laboratory 
Table II.4 Particle size distribution at Field 21, Houston soil field site, Black Belt Research 
and Extension Station, Marion Junction, Alabama. 
Horizon Lower 
Depth 
Particle Size 
Distribution 
Sand Size Distribution 
Sand Silt Clay 2-1 1-0.5 0.5-
0.25 
0.25-
0.1 
0.1-
0.05 cm % 
Ap1 23 7.09 39.63 53.28 0.64 0.64 1.50 1.93 2.36 
Ap2 42 8.27 38.04 53.7 1.56 1.34 1.56 1.79 2.01 
BA 63 10.17 33.38 56.45 3.10 1.99 1.55 1.77 1.77 
Bkss1 88 3.50 35.93 60.57 0.22 0.66 0.66 0.87 1.09 
Bkss2 152 3.10 25.80 71.10 1.03 0.52 0.26 0.26 1.03 
Source: Auburn University Pedology Laboratory 
169 
 
APPENDIX III  
SIZING SDI DRAIN FIELD  
The most basic design calculation needed to size any septic tank drain field, 
whether a conventional system or an engineered system, is the required disposal area.  
Table III.1, below, presents design data used to determine the field area for the 
experimental wastewater SDI dosing system. Alabama Department of Health (2006) 
regulations provide allowable hydraulic loading (dosing) rates for soils with different 
percolation rates. Percolation rate in the Houston clay soil is known to be much greater 
than 120 minutes per 25.4 cm, so the far right column in Table III.1 (bold) is used 
exclusively for the design of Field 21. What is needed to determine drain field area is an 
estimate of effluent volume generated per day.  
Table III.2 provides flow and organic loading rates for residential generators in 
Alabama, indicating the general requirement to use 1136 liters per day as a flow 
minimum.  The experimental disposal design completed for Field 21 uses a lesser quantity 
of 1022 liters per day because of the recognized allowance for lower generation rates 
when designing for decentralized systems typical of small subdivisions or other rural 
wastewater cooperators (Alabama Department of Public Health, June 14, 2005, personal 
communication). 
 
170 
 
Table III.1 Design data used to size drain field based on a household loading rate of 270 gpd.  Adapted from manufacturer 
recommendations and State of Alabama Onsite Sewage Treatment and Disposal regulations (effective November 23, 2006).   
Soil type Group IV (Sandy Clay, Silty Clay, Clay) 
Percolation rate (min 25.4 mm-1) 90 95 100 105 110 115 120 >120 
Drip field or  
hydraulic loading rate*  
lpd m-2 (gpd sq.ft.-1) 
4.075 
(0.1) 
3.056 
(0.075) 
2.037 
(0.05) 
2.037 
(0.05) 
2.037 
(0.05) 
2.037 
(0.05) 
2.037 
(0.05) 
2.037 
(0.05) 
Quantity of effluent to be disposed 
per day, 
 liters (gallons) 
1022 
(270) 
1022 
(270) 
1022 
(270) 
1022 
(270) 
1022 
(270) 
1022 
(270) 
1022 
(270) 
1022 
(270) 
Determine total area required m2 
(ft2) 
250.8 
(2700) 
334.5 
(3600) 
502 
(5400) 
502 
(5400) 
502 
(5400) 
502 
(5400) 
502 
(5400) 
502 
(5400) 
Spacing between wasteflow lateral 
lines, m (ft) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
Spacing between wasteflow lateral 
emitters, m (ft) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
0.61 
(2) 
Total length of lateral lines m (ft) 
411.5 
(1350) 
548.6 
(1800) 
823 
(2700) 
823 
(2700) 
823 
(2700) 
823 
(2700) 
823 
(2700) 
823 
(2700) 
Total number of emitters 2700 3600 5400 5400 5400 5400 5400 5400 
* Values in this row are set by Alabama state regulation (ADPH, 2006).  Group I, II, and III soils having percolation rates less than 90 
min 25.4 mm-1 have hydraulic dosing rates ranging from 8.149 to 18.34 lpd m-2 (0.2 to 0.45 gpd sq.ft.-1) and corresponding area 
requirements ranging from 125.4 m2 (1350 ft2) down to as low as 55.74 m2 (600 ft2). 
171 
 
Table III.2 Flow and organic loading, State of Alabama Onsite Sewage Treatment and 
Disposal regulations (effective November 23, 2006). 
Generator Design Unit Design BOD/TSS 
day-1 
Design Flow  
day-1 
Dwelling (8 
bedrooms or fewer) 
per bedroom 0.181 kg  568 liters  
9 or more bedrooms 
to a single system 
per person 0.091 kg  
 
284 liters  
Notes: 
1. Organic loadings are prior to septic tank. It may be assumed that the tank will remove 
a maximum of 40% of the BOD & TSS load of sewage and 30% of high-strength sewage. 
This is an assumed loading rate for field sizing and should not necessarily be used for 
treatment design. 
2. Estimated flows for residential systems assume a maximum occupancy of two persons 
per bedroom for systems handling fewer than 9 bedrooms. Large-Flow systems require 
engineer design, and documentation of occupant loading.   
 
 
Using Table III.1, a total area of 502 m2 (5400 ft2) of drainage or disposal soil area 
is required, as follows; effluent generation/allowable hydraulic loading rate (based on 
percolation rate of the site) 
  (1022 liters per day) / (2.037 lpd/m2) = 502 m2 disposal area required. 
Based on standard practice of 0.61 m (2 ft) spacing between drip lines and 0.61 m 
(2 ft) spacing between emitters, a total of 823 m (2700 ft) of drip line is required, 
calculated as follows;   
                         502 m2 / 0.61 m = 823 m total length of drip line required. 
172 
 
APPENDIX IV  
FIELD CAPACITY MEASUREMENT 
 
Soil cores (?: 5.5 cm, H: 6 cm) were taken from nine (9) different locations at a 
20 cm depth within the SDI field (including treatment I (synthetic wastewater 
application) and treatment II (clean well water application)) on June 24, 2008 (Figure 
I.1). Soil cores were lab approximated for its field capacity at 1/3 bar air pressure. The 
measured volumetric moisture contents of the soil cores ranged from 37.19% to 44.39%, 
averaged at 40.60% with a standard deviation of 1.96% (Table I.1). The measured values 
together with their field sampling locations were imported into ArcMap 9.2 (ESRI, CA) 
and interpolated over the entire SDI site using inverse distance weight (IDW) method 
(Figure I.2). 
Table IV.1. Soil volumetric moisture contents of the soil cores at 1/3 bar air 
pressure. 
Soil moisture measurements, %, v/v Average STD 
#1 #2 #3 #4 #5 #6 #7 #8 #9 
41.16 40.46 38.80 40.42 40.90 44.39 37.19 40.51 41.55 40.60 1.96 
173 
 
 
Figure IV.1 Field soil core sampling locations within the SDI site on June 24, 2008. 
 
 
Figure IV.2 Interpolated site map based on the measured soil field capacities (1/3 
bar) at 20 cm depth. (Sampled on June 24, 2008)
174 
 
APPENDIX V 
SOIL PHOSPHORUS ISOTHERM MEASUREMENT 
The soil phosphorus isotherm of the experimental site was obtained by using 
composite soil samples with the method by Sevis-David (2002). The composite soil 
samples were prepared by mixing equal weight of subsamples from the soil cores of the 
four field treatments (I, II, III, and IV) (Figure V.1). Lab obtained isotherm data points 
are presented in Figure V.2. The data points of each soil depth were first fitted with 
Langmuir isotherm to determine maximum soil phosphorus adsorption capacity. Then, 
the adsorption coefficient, Kd, for HYDRUS 2D modeling was obtained by linear 
regression on the data points where water phase phosphorus concentration were lower 
than 20 mg P L-1. This is because the maximum phosphorus concentration in the 
simulated wastewater was 10 mg P L-1 and the data points where water phase phosphorus 
concentration lower than 20 mg P L-1 showed a better linear relationship than that of the 
whole data set. The obtained coefficients for Langmuir type and linear type soil 
phosphorus adsorption isotherms are listed in Table V.1. 
175 
 
 
Figure V.1 Composite soil sample preparation for experimental site soil phosphorus 
adsorption isotherm determination. 
 
Table V.1 Regression results of the soil phosphorus adsorption isotherm. 
        Constant 
 
 
 
Soil horizon  
(cm) 
Langmuir isotherm Linear regression 
K, L 
mg-1 P Smax, mg P kg
-1 soil R2 Kd, cm3 g-1 soil R2 
0-20 0.06 16667 0.804 300 0.51 
20-40 0.06 16667 0.951 472 0.85 
40-60 0.06 16667 0.923 1188 0.86 
60-80 0.06 16667 0.917 793 0.82 
80-100 0.06 16667 0.943 1055 0.81 
Note: 1. Langmuir isotherm ( ), where S = P retained by the solid phase, 
mg P kg-1 soil; Smax = soil maximum P adsorption capacity, mg P kg-1 soil; C = 
concentration of P after 24 h equilibration, mg L-1; K = a constant related to the 
bonding energy, L mg-1 P. 
           2. Linear isotherm (S=Kd?C), where S = P retained by the solid phase, mg P kg-1 
soil; C = concentration of P after 24 h equilibration, mg L-1; Kd= a constant 
related to the bonding energy, L kg-1 soil or cm3 g-1 soil. 
 
176 
 
Figure V.2 Phosphorus adsorption on soil vs. phosphorus in water phase of the five 
soil horizons of the experimental site using composite samples from the soil cores 
taken from each field treatment. (a. 0-20 cm, b. 20-40 cm, c. 40-60 cm, d. 60-80 cm, 
e. 80-100 cm.) 
 
 
 
 
 
 
 
 
177 
 
APPENDIX VI 
SOIL MOISTURE SENSOR VALIDATION 
 
The ThetaProbe (ML2, Theta-T, Dynamax, TX) and PR2 (Theta-T, Dynamax, 
TX) soil moisture sensors are both capacitance type sensors and use the same technology 
to measure soil moisture content. It directly measures the soil dielectric constant ( ) and 
translates the readings into soil volumetric moisture content ( ) through a linear relation 
(Eq. VI.1). Since the soil moisture sensors were used as is during the laboratory and field 
experiment, soil moisture sensors were validated by calibrating the coefficients ( ) 
for the   and  linear equation (Eq. VI.1) using intact soil cores from the experimental 
site. As recommended by the manufacture (Dynamax, TX), the PR2 can be calibrated 
with a pre-calibrated Theta-T sensor. Therefore, the calibrated coefficients for the 
ThetaProbe can be used with high confidence for the PR2 probe since they were 
monitoring the same soil on the same site. In this study, the calibration procedure was 
used as a validation for the used soil moisture sensors. 
Since soil moisture content ( ) is proportional to the refractive index of the soil 
( ) as measured by the ThetaProbe and Profile Probe. The goal of calibration is to 
determine the two coefficients ( ) that were used in a linear equation to convert 
probe reading ( ) into volumetric soil moisture: . ----Eq. VI.1
178 
 
Two intact soil cores (?: 5.5 cm, H: 6 cm) were taken from besides the 
ThetaProbe at 1:30pm on May 09, 2008. The soil cores then lab measured for their 
volumetric moisture contents. The average reading was 39.0 % v/v and the   was 4.98. 
Another two intact soil cores (?: 5.5 cm, H: 6 cm) were taken from besides the 
ThetaProbe at 1:30pm on August 13, 2008. The soil cores then lab measured for their 
volumetric moisture contents. The average reading was 50.6 % v/v and  was 6.01. 
Using the readings of the two observations to solve the equation VI.1.  
                     (Eq. VI.2) 
                   (Eq. VI.3) 
Resulting  
             
The actual field volumetric moisture contents vs. the probe readings is plotted in 
Figure III.1 using the coefficients (before calibration) provided by Dynamax (TX) and 
the calibrated coefficients. It is necessary to emphasize that this calibration curve is 
suppose to work precisely only when the soil is not saturated. Free water that stays within 
soil empty pockets (e.g. water in cracks), if it stays close enough to the probe, can make 
the probe to give a reading higher than soil saturation level. Under such a circumstance, 
site condition where the probe is working should be explored before any solid conclusion 
can be made to interpret any unrealistic readings. Based on the above and Figure VI.1, 
the error between the pre-calibration and after calibration of the used soil moisture 
sensors (ML2, ThetaProbe and PR2) will be maximum 1.0 % v/v when the actual soil 
moisture content is 50%, v/v, and will decrease as actual field moisture content decreases. 
179 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure VI.1 Sensor performance under calibrated coefficient and un-calibrated coefficient.
180 
 
APENDIX VII  
EVAPOTRANSPIRATION (ET) CALCULATION BY PENMAN-MONTEITH 
METHOD DEFINED BY FOOD AND AGRICULTURAL ORGANIZATION 
The ET calculation equation used here is the FAO Irrigation and Drainage Paper 
56 (FAO 56) version Penman-Monteith method (FAO, 2006) and is presented as: 
?
?
? +?
?+
+?
??= )())(()(1 vfeTeRR
LET
asnlnsday
P   (Eq. VII.1) 
The data used for ET calculated was obtained from Alabama Mesonet?s 
Agricultural Weather Information Service (AWIS). Since all necessary weather data at 
the Black Belt station was not available for the FAO 56 version Penman-Monteith 
method, weather data from Thorsby weather station approximately 77 km from the 
experimental site was used. The used parameters from AWIS are daily max (MAX), min 
(MIN), and average (AVG) air temperature (oF) at 5 ft high, daily maximum (RHX) and 
minimum (RHN) relative humidity in percent at 5 ft high, daily solar radiation (SOLR) in 
watt-hours_per_square_meter at 5 ft high, daily maximum wind speed (HGU) in mph at 5 
ft high. Once data was downloaded from AWSI, it was imported into a spread sheet and 
daily ET0 was calculated using the automatic cell calculation function by defining the Eq 
2.5 using the recorded daily weather information. Since the planting date and harvesting 
dates of the testing field crops are known, estimated ET (ETc) at the experimental site
181 
 
was estimated by multiplying the reference ET0 with single crop coefficient (Kc) as 
suggested in FAO 56 for Sorghum, Sudangrass, Rye, and Winter wheat (Table IV.1). One 
calculation example is provided for the month of September 2006 when the field was 
planted with Sorghum and Sudangrass (Table VII.2). 
Table VII.1 Lengths (days) of crop development stages and single crop coefficient (Kc) used 
in this study. 
 Initial Development to Middle Late 
Days Kc Days Kc Days Kc 
Sorghum-grain 20 0 75 1.00-1.10 30 0.55 
Sudan Grass hay (annual) 25 0.50 40 0.90 10 0.85 
Winter Wheat - with non-frozen 
soils 20 0.7 130 1.15 30 0.25-0.4 
Rye Grass hay N/A 0.95 N/A 1.05 N/A 1 
Adapted from FAO Irrigation and Drainage Paper 56 (2006). 
182 
 
       Table VII.2 ET calculation example for the experimental field during the month of Septmeber 2006 (FAO 56 method). 
Date Max. air temperature, Fo Min. air temperature, Fo Ave. air temperature, Fo 
Max. air 
humidity, 
% 
Min. air 
humidity, 
% 
Solar 
radiation, kJ 
m-2 d-1 
Max. wind 
speed at 5 ft 
height,  
mile hr-1 
ET0, 
mm d-1 Kc 
ETc, 
mm d-1 
9/1/2006 89 67 78 93 42 11502.00 15 5.13 1 5.13 
9/2/2006 89 69 79 93 40 11527.20 17 5.39 1 5.39 
9/3/2006 90 69 80 87 36 11998.80 12 5.31 1 5.31 
9/4/2006 90 68 79 94 39 12124.80 12 5.15 1 5.15 
9/5/2006 91 70 81 92 35 10580.40 11 4.82 1 4.82 
9/6/2006 86 67 77 93 51 5572.80 12 2.92 1 2.92 
9/7/2006 89 67 78 95 30 11714.40 12 5.25 1 5.25 
9/8/2006 87 64 76 95 39 10036.80 12 4.44 1 4.44 
9/9/2006 89 67 78 96 40 12106.80 12 5.03 1 5.03 
9/10/2006 86 67 77 96 54 7520.40 9 3.07 1 3.07 
9/11/2006 89 67 78 92 39 12067.20 14 5.31 1 5.31 
9/12/2006 89 71 80 85 38 8107.20 16 4.66 1 4.66 
9/13/2006 78 70 74 97 69 3078.00 20 1.85 1 1.85 
9/14/2006 83 62 73 96 56 7315.20 13 3.14 1 3.14 
9/15/2006 82 60 71 96 38 12744.00 14 5.01 1 5.01 
9/16/2006 84 60 72 96 37 12776.40 11 4.91 1 4.91 
9/17/2006 88 62 75 95 28 12891.60 9 5.19 1 5.19 
9/18/2006 91 67 79 94 34 12272.40 9 5.04 1 5.04 
9/19/2006 89 71 80 97 52 5587.20 31 4.26 1 4.26 
9/20/2006 80 58 69 96 30 11181.60 19 5.19 1 5.19 
9/21/2006 76 52 64 95 35 13122.00 15 4.85 1 4.85 
9/22/2006 80 56 68 86 35 12171.60 16 5.14 1 5.14 
9/23/2006 88 71 80 92 60 7329.60 22 3.80 1 3.80 
9/24/2006 90 71 81 95 52 5778.00 20 3.69 1 3.69 
9/25/2006 85 60 73 96 64 6811.20 16 3.00 1 3.00 
9/26/2006 76 55 66 95 47 12769.20 17 4.52 1 4.52 
9/27/2006 76 53 65 96 39 10623.60 17 4.32 1 4.32 
9/28/2006 82 58 70 96 37 11631.60 13 4.69 1 4.69 
9/29/2006 83 46 65 96 40 11491.20 24 5.49 1 5.49 
9/30/2006 73 49 61 96 32 12304.80 11 4.25 1 4.25 
 
183 
 
APPENDIX VIII  
NRCS WEB SOIL SURVEY 
Method 
NRCS Web Soil Survey (WSS) provides soil data and information produced by 
the National Cooperative Soil Survey. It is operated by the USDA Natural Resources 
Conservation Service (NRCS) and provides access to the largest natural resource 
information system in the world. To access, type the website below and follow the 
specific instructions shown on the web page to locate the area of interest and extract 
desired soil information from available options which is based on current SSURGO 
database (NRCS, 2007). The current soil area data is version 4, September 6, 2008.  
Website: http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx 
For this study, site description and physical soil properties were extracted to assist 
understanding field soil moisture profile monitoring data carried from May 2008 to 
September 2008. The relevant information is listed below. 
 
Site Description 
Map Unit Setting  
? Mean annual precipitation: 48 to 56 inches  
? Mean annual air temperature: 61 to 68 degrees F 
184 
 
? Frost-free period: 200 to 250 days  
Map Unit Composition  
? Houston and similar soils: 85 percent  
? Minor components: 1 percent  
Description of Houston clay 
      Setting  
? Landform: Hillslopes  
? Landform position (two-dimensional): Footslope  
? Landform position (three-dimensional): Base slope  
? Down-slope shape: Linear  
? Across-slope shape: Linear  
? Parent material: Clayey marine deposits derived from chalk  
      Properties and qualities  
? Slope: 1 to 5 percent  
? Depth to restrictive feature: 48 to 72 inches to paralithic bedrock  
? Drainage class: Moderately well drained  
? Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low 
(0.00 to 0.06 in/hr)  
? Depth to water table: About 48 to 72 inches  
? Frequency of flooding: None  
? Frequency of ponding: None  
? Available water capacity: Moderate (about 8.4 inches)  
      Interpretive groups  
? Land capability (nonirrigated): 2e  
      Typical profile  
? 0 to 10 inches: Clay  
? 10 to 42 inches: Clay  
185 
 
? 42 to 80 inches: Clay  
Minor Components  
      Eutaw  
? Percent of map unit: 1 percent  
? Landform: Depressions  
? Landform position (two-dimensional): Footslope  
? Landform position (three-dimensional): Dip  
? Down-slope shape: Concave  
? Across-slope shape: Concave  
 
Physical Soil Properties 
 
FigureVIII.1 Screen capture of the soil report generated for physical soil properties of the 
experimental site. 
186 
 
APPENDIX IX  
HYDRUS 2D NUMERICAL MODELING 
HYDRUS 2D (?im?nek et al., 1999), a PC based modeling environment for 
analysis of water flow and solute transport in variably saturated/unsaturated porous 
media, was used to simulate water and nutrient movement adjacent to drip emitters in 
treatment I using selected field data. The purpose of this simulation is to estimate water 
and nutrient short- and long-term moving trend under the soil moisture controlled SDI 
wastewater disposal over the testing Houston clay. 
 
Model Setup 
The boundary selected for numeric simulation was a rectangular soil profile, 100 
cm deep by 60 cm wide. This control area represents the cross-sectional space between 
two emitters on adjacent drip laterals (Figure IX.1). Facing emitters are represented as 
two 16 mm semi-circles 20 cm deep at the side boundary. Each emitter is set with a time 
variable flux representing daily wastewater application. Soil horizons are characterized in 
the model following the site soil analyses (Table IX.1). The upper boundary of the model 
was set as a time-variable atmospheric surface associated with daily ET and precipitation. 
The bottom boundary of 100 cm was set to permit free drainage. The side boundaries, 
excluding emitters, were set to exclude lateral flux. Soluble nutrients, NH4+-N and
187 
 
phosphorus, entered the modeled soil profile as wastewater flux at the two emitters. Two 
hypothetical observation points for data summary were selected at 20 cm and 45 cm 
depths in the middle of the modeling space to compare with field observed soil moisture 
data (Figure IX.1). Two additional hypothetical observation points were selected at 45 cm 
and 100 cm depths directly beneath one emitter to monitor variation of water phase 
nutrient concentration over time. Required hydraulic parameters for numerical modeling, 
such as soil water retention function, were estimated using the neural prediction function 
embedded in HYDRUS 2D, based on the actual soil texture for each horizon (Table IX.1).  
 
Figure IX.1 HYDRUS 2D simulation domain and boundary conditions.  
Table IX.1 Particle size distribution of the soil horizons of the experimental site. 
Horizon Lower 
Depth 
Particle Size Distribution 
Sand Silt Clay 
cm % 
Ap1 23 7.09 39.63 53.28 
Ap2 42 8.27 38.04 53.7 
BA 63 10.17 33.38 56.45 
Bkss1 88 3.50 35.93 60.57 
Bkss2 152 3.10 25.80 71.10 
Source: Auburn University Pedology Laboratory 
188 
 
Model input included daily system hydraulic disposal rate (mm d-1), precipitation 
(mm), nutrient (N, P) concentration of the applied synthetic wastewater (mg L-1), and 
calculated daily field ET (mm d-1). Initial soil and soil water nutrient levels were set to 
zero to illustrate the net movement of applied nutrients similar to Beggs et al. (2004). 
Nutrient concentrations of the disposed wastewater were set to 80 mg N L-1 for 
ammonium nitrogen (NH4+-N) and 10 mg P L-1 for total phosphorus to match the 
synthetic wastewater. Nitrate nitrogen (NO3--N) was simulated as the only daughter 
product of NH4+-N and was assumed not initially contained in the synthetic wastewater 
since chemical analysis of the used synthetic wastewater contained negligible NO3--N. 
Since crop uptake and soil adsorption for nitrogen and phosphorus are the most 
significant pathways for nitrogen and phosphorus in soils (Venhuizen, 1995), simulated 
chemical/physical reactions in this modeled soil matrix are nitrification/denitrification, 
soil adsorption, and crop uptake for nitrogen; soil adsorption and crop uptake for 
phosphorus. Water phase diffusion coefficients were set to 1.60 cm2 d-1 for NH4+-N, NO3-
-N and phosphorus based on Beggis et al. (2004) on their simulation on soiltesxture. Soil 
adsorption coefficients were set to 6.0? 10-6 cm3 g-1 for NH4+-N and 1? 10-7 cm3 g-1 for 
NO3--N for the entire soil profile based on Beggis et al. (2004) on their simulation on 
loam and clay loam soils. Linear adsorption coefficients (Kd, cm3 g-1) for phosphorus 
were assigned to soil according to lab measurement (Table 5.1); 300 for 0-20 cm, 472 for 
20-40 cm, 1188 for 40-60 cm, 793 for 60-80 cm, and 1055 for 80-100 cm. The 
nitrification coefficient for NH4+-N was set at 0.72 d-1 and the denitrification coefficient 
for NO3--N was set at 0.072 d-1 as used by Misra (1974) for long column of silt loam soil, 
189 
 
low water flow, at 0.5% (m/m) oxygen. Root mass (dimensionless) was simulated as 
linearly decreasing in the 100 cm depth, 1 at the top and 0 at the bottom. Root water 
uptake was modeled using the predefined HYDRUS 2D routine for forage.   
Modeling scenarios included three one-year synthetic wastewater applications 
scenarios and one long term (10-year) synthetic wastewater application scenario (Table 
IX.2). The one-year (June 14, 2007 to June 24, 2008) synthetic wastewater application 
period was simulated with comparative scenarios including: 1) root water and nutrient 
uptake with nitrification/denitrification and soil adsorption for nitrogen, and soil 
adsorption for phosphorus; 2) root water and nutrient uptake only; and 3) No root water 
and nutrient uptake, has nitrification/denitrification and soil adsorption for nitrogen, and 
soil adsorption for phosphorus. A 10-year simulation (scenario 4) repeats the one-year 
synthetic wastewater application from June 14, 2007 to June 24, 2008 with both root 
water and nutrient uptake and solute reactions mode on, similar to scenario 1. This long-
term simulation was used to assess long term water and nutrient movement in the drain 
field. 
Table IX.2 HYDRUS 2D simulation scenarios. 
Notes: 1. Solute reaction includes nitrification of NH4+-N, denitrification of NO3--N, soil 
adsorption for NH4+-N, NO3--N, and phosphorus. 
            2. Verified using HYDRUS 2D as the field capacity of the simulating soil matrix. 
            3. Duration from June 14, 2007 to June 24, 2008 when synthetic wastewater 
application experiment was carried.  
            4. Repeat scenario 1 consecutively ten times. 
Scenario # Simulation period 
Root 
nutrient 
uptake 
Solute 
reaction1 
Initial conditions 
Water content2 Soil nutrient concentration 
1 Annual3 Yes On 
40%, v/v of the 
entire soil profile None 
2 Annual  Yes No 
3 Annual  No On 
4 Ten years4  Yes On 
190 
 
The HYDRUS 2D model was not calibrated over the 12-month simulation period 
because of the experimental setup and ambient environment. For a valid model 
calibration, the differences between simulated and observed data can only be ascribed 
and quantified to intrinsic limitations of field instruments (?im?nek and Hopmans, 2002), 
and the applied process model and selected hydraulic relationships must be an exact 
description of the soil physical behavior (?im?nek and Hopmans, 2002). For this 
particular study, one major difference between the actual field condition and the modeled 
soil profile is that soil cracking processes during hot, dry summer condition breaks soil 
continuity within the field, a scenario that the HYDRUS 2D model is not able to 
simulate. Therefore, a non-perfect 12-month simulation model performance is expected, 
especially, since spatial and temporal variability in parameter values such as Ksat will 
result in uncertain model outputs when describing or predicting a natural event 
(Shirmohammadi et al., 2006). 
 
Simulation Result 
Simulated Field Capacity 
Since the soil moisture controlled SDI wastewater disposal system choose its 
operation window around field capacity of the experimental site, it is necessary to find 
out the simulated ?field capacity? of the modeled soil matrix and use it as the initial status 
for the simulation. The defined condition for field capacity requires no evaporation 
occurs from the soil surface, no water table or slowly permeable barriers occur at shallow 
depths in the profile, and the soil profile should be let free drained until no substantial 
191 
 
drainage occurs (SSSA, 1986). To mimic these conditions, the modeled soil profile 
(Figure IX.1) was initially assigned a volumetric soil moisture content of 50% and then 
let bottom free drained for 60 days with the soil profile of no root uptake, no 
precipitation, and no irrigation. The simulated time course of the volumetric soil moisture 
content of the soil profile is illustrated in Figure IX.2. 
From the soil moisture content?s dropping trend, ?substantial drainage? stops at 
around 40%, v/v (Figure IX.2). Since this number is close to the lab approximated field 
capacity (37%-44%, v/v) of the experimental site at 20 cm depth, it was then used as the 
initial soil moisture content of the soil profile for scenario simulations. 
 
 
Figure IX.2 Soil moisture content (%, v/v) time course of the free drained simulating soil 
profile. 
 
Model Evaluation in Terms of Soil Water Movement (June 14, 2007 to June 24, 2008) 
Simulated soil moisture levels at 20 and 40 cm depth under scenario 1 were 
graphically compared first with field observed field data to illustrate the difference 
between simulation and field observation (Figure IX.3).  It is graphically shown that the 
192 
 
simulation process overestimates soil moisture levels from June 14, 2007 to November 
14, 2008, slightly underestimates from November 15, 2007 to January 9, 2008, and 
obviously underestimates from January 2008 to June 2008. 
Quantified comparison results using Nash-Sutcliffe efficiency (NSE), percent bias 
(PBIAS), and root mean square error (RMSE)-observation standard deviation ratio (RSR) 
as described by Moriasi et al. (2007) are tabulated in Table IX.3. Over the 12 month 
simulation, each of the three indexes indicates a poor correlation with field data. NSE 
rated simulated results ?Unacceptable?, PBIAS rated the simulated results 
?Underestimation?, and RSR rated the simulated results ?Errors? (Table IX.3).  
One explanation for the overestimation period is that the model failed to account 
for seasonal shrinking that caused preferential flows during dry summer conditions. Field 
observed system hydraulic disposal rates, in the actual cracking field conditions, 
substantial of those applied water will be conducted into deep soils much quicker due the 
cracking between soil aggregates. However, if applied in a more homogenized soil 
profile, these water would be expected to exert higher influence on the upper 45 cm soil. 
For the underestimated period, it can be explained with the other feature of the 
clay soil, swelling during wet conditions. The clay soils naturally swelled in wet weather 
as well as in April and May of 2008 when local monthly precipitations were above 
average levels (Figure 5.5). With a negligible hydraulic disposal rate (0-2.44 cm month-1) 
from the SDI dosing system, precipitation was responsible for field wetting. The swelling 
of the clay soils under wet conditions decreased soil water percolation rates (Bouma and 
Loveday, 1987; Weaver et al., 2005) and also held water longer. The HYDRUS 2D 
193 
 
model did not accurately account for these physical and hydraulic property changes, 
resulting lower (underestimated) soil moisture content in the upper 45 cm.  
Simulated results can be improved if related soil physical and hydraulic properties 
are properly reflected during the simulated time frame (Cabidoche and Guiliaume, 1998), 
and the current HYDRUS 2D simulation on SDI irrigation on clay soils is recommended 
for periods, if possible, when clay soil shrink/swell processes are at their minimum extent. 
Nevertheless, comparative results between simulation and observation in this study 
suggest that the experimental SDI system is disposing water faster than can be held by 
the soil during a hot, dry summer, more so during drought conditions. The current field 
experiment setup and field moisture sensing strategy does not adequately address this 
seasonal and spatial change in drain field hydraulic properties, and also soil cracking 
development seems not able to be deminished by those disposed water. 
 
194 
 
           Table IX.3 Evaluation of the simulated (scenario 1) soil moisture levels compared to observed field data at 20 cm and 45 cm depths    
           from June 14, 2007 to June 23, 2008. 
      Evaluation            
               index  
 
Time frame 
Nash-Sutcliffe efficient 
(NSE)1 Percent bias (PBIAS)
2 RMSE-observation standard 
deviation ratio (RSR)3 
20 cm  45 cm 20 cm 45 cm 20 cm 45 cm 
June 14, 2007- 
June 23, 2008 
(whole year) 
-2.126, 
Unacceptable 
-0.167, 
Unacceptable 
6.31, 
Underestimation 
5.78, 
Underestimation 
1.77,  
Errors 
1.08,  
Errors 
June 14, 2007- 
November 14, 
2007 
0.067, 
Unacceptable 
0.44, 
Unacceptable 
0.03, 
Overestimation 
-3.80, 
Overestimation 
0.06,  
Errors 
0.75,  
Errors 
November 15 
2007- January 
9, 2008 
0.998, 
Unacceptable 
0.999, 
Unacceptable 
4.59, 
Underestimation 
0.072, 
Underestimation 
0.04,  
Errors 
0.026,  
Errors 
January 10, 
2008- June 23, 
2008 
0.98, 
Unacceptable 
0.977, 
Unacceptable 
13.98, 
Underestimation 
14.35, 
Underestimation 
0.15,  
Errors 
0.15,  
Errors 
           Notes: 1. Based on the calculated evaluation index (range from -?~1), the ratings are categorized into Optimal (1), Acceptable     
(0-1), and Unacceptable (?0). 
                       2. Based on the calculated evaluation index (range from -?~+?), the ratings are categorized into Optimal, 
Underestimation (>0), and Overestimation (<0), with lower the magnitude of the index the better the simulation 
result. 
                       3. Based on the calculated evaluation index (range from 0~+?), the ratings are categorized into Optimal (0), Errors 
(>0), with lower the index the better the simulation result. 
195 
 
 
Figure IX.3 Simulated (scenario 1) soil moisture contents at 20 cm and 40 cm depths versus actual field monitoring in treatment I during 
the one-year synthetic wastewater application from June 14, 2007 to June 24, 2008. 
196 
 
Scenario Simulated Nutrient Output After One Year Synthetic Wastewater Application  
Annual nutrient output from simulated soil profiles in scenarios 1, 2, and 3 are 
compared in Figure V.4. Simulated nitrification is more effective than crop uptake in 
preventing NH4+-N from leaching (below 100 cm depth) since NH4+-N leaching only 
occurs when there is no nitrification in the soil profile (scenario 2), while nitrification 
itself without crop uptake can prevent NH4+-N leaching (scenario 3). More than 30% of 
annually applied NH4+-N is lost to leaching under scenario 2 when there is no chemical 
mechanism to attenuate nitrogen. Results for NO3--N indicate that both denitrification and 
crop uptake are important in limiting NO3--N from leaching since denitrification cannot 
totally eliminate NO3--N leaching only by itself (scenario 3). Under scenario 2 when there 
is no denitrification but crop uptake for NO3--N, although there is no NO3--N input due to 
no nitrification of NH4+-N, it is also anticipated that NO3--N leaching issue can also be 
magnificent if applied at a comparable level as NH4+-N (80 mg L-1 in this study). 
Although the nitrification/denitrification reaction coefficients are not directly field 
derived and won?t be consistent throughout seasons in a natural drain field, the results of 
the scenario comparison indicate the importance of maintaining intermittent 
nitrification/denitrificaiton conditions within the soil matrix to limit nitrogen deep 
percolation (Beggs et al., 2004; Venhuizen, 1995), especially in case where nitrogen is in 
surplus as a crop nutrient (Broadbent and Reisenauer, 1988).   
Comparable results for phosphorus show that the high soil adsorption capacity for 
phosphorus retains applied phosphorus. As a major phosphorus retaining mechanism, 
phosphorus leaching would be expected to increase if the high soil adsorption capability 
197 
 
were to become ineffective as in scenario 2 when there is only crop uptake to keep 
phosphorus from leaching.  
The simulated phosphorus fate and transport pathways are much less complex 
than that in actual natural field conditions (Reneau et al., 1989, Venhuizen, 1995). 
Nevertheless, simulated results deliver the message that the length of time required to 
exhaust a soil?s phosphorus adsorption capacity depends on the soil, disposal rate, and 
available phosphorus attenuation mechanisms such as adsorption/precipitation (Sawhney 
and Hill, 1975; Venhuizen, 1995).  
Nutrient attenuation mechanisms that take effect within the same or different soil 
zones can decrease nutrient leaching even though deep percolation is substantial. On the 
other hand, poor nutrient removal, whether via crop uptake or chemical/physical 
attenuation, will lead to undesirable nutrient leaching when the field is consistently 
loaded above field capacity.  
198 
 
 
Figure IX.4 Simulated (scenarios 1, 2, and 3) NH4+-N, NO3--N, and phosphorus outputs within the simulated soil domain after 
one year synthetic wastewater application. (For scenario 2, no NO3--N presences in the soil domain since NH4+-N is not 
nitrified) 
 
199 
 
Long Term (10-Year) Simulation of Water and Nutrient Movement in the Drain Field 
Water percolation at 100 cm depth over a 10 year simulation shows a repetitive 
pattern with substantial water percolation in both summer and winter (Figure IX.5a). 
Simulated soil moisture content at 20 and 45 cm depth show a regular fluctuation 
between 38%, v/v and 48%, v/v. This simulated result suggests that the hydraulic 
response to repeated wastewater disposal reached a ?dynamic? steady state starting from 
the first year.  
Over the entire 10 year period, in the simulated soil water phase there is no 
indication of NH3+-N presence at 50 cm and 100 cm depths. However, NO3--N 
concentration in percolates at the 45 cm depth show repetitive surges (Figure IX.5b) 
coincident with high and low water percolation cycles as indicated in Figure IX.5a. While 
NO3--N surges at 45 cm periodically reaches almost 10 mg L-1, it does not extend to 100 
cm depth where NO3--N consistently maintained below 0.05 mg L-1. This result indicaties 
that the simulated denitrification and crop uptake process effectively attenuates NO3--N 
to a negligible level while hydraulic loading moves the NO3--N toward the 100 cm depth.  
Simulated phosphorus levels in the percolates at 50 cm and 100 cm depths 
indicate accumulating phosphorus levels resulting from repetitive water loadings. 
Simulated phosphorus levels in the percolates are relatively stable at 9 mg P L-1 at 45 cm 
depth, and 8 mg P L-1 at 100 cm depth. Since there are only soil adsorption and crop 
uptake are simulated as the mechanisms to remove phosphorus from water phase, the 
initial rising then stabled phosphorus levels in the percolates indicates that consistent 
200 
 
 
Figure IX.5 Simulated (Scenario 4) monthly water percolation and nutrient concentrations in water phase during the 10-year simulated 
period. (a. simulated monthly water percolation and soil moisture content change; b. simulated NO3--N and phosphorus concentration 
variations at simulated observation points ) 
201 
 
phosphorus loading already exhausted the soil?s phosphorus adsorption capacity at 
shallow soil depths, leaving crop uptake as the remaining removal mechanism. This 
simulated phosphorus movement indicates that yearly repetitive drain field hydraulic 
loading into the soil has the potential to carry phosphorus deeper into the soil. 
However, phosphorus is not commonly found present in deep soils since there are 
numerous mechanisms to retain applied phosphorus (Reneau et al., 1989). Sawhney and 
Hill (1975) and Venhuizen (1995) reported phosphorus slowly creeping forward in onsite 
wastewater drain fields as soil phosphorus adsorption sites are depleted. Nevertheless, 
finding it present in deep clay soils due to ineffective phosphorus immobilization 
mechanism, such as preferential flows stimulated by soil cracking development is still 
possible (Larsson et al., 2007; Muukkonen et al., 2009), especially under the actual 
experimental site that is dominant with Huston clay, a Vertisol notorious for its cracking 
development during dry seasons (Amidu et al., 2007; Kishne et al., 2009). 
 
Conclusion 
Short term one year scenario comparisons suggest the necessary to maintain 
effective spatial and temporal nutrient attenuation mechanism along soil profile to 
prevent nutrient leaching. Long term 10-year simulation indicates the accumulated effect 
that repetitive hydraulic overloading can have on ground water nutrients. Even though 
short term adverse effect from wastewater drain field overload may not be discernable, 
cumulative impact has been shown can lead to a noticeable pollutants elevation in ground 
water. Based on simulation results, although less complex than actual field conditions, 
202 
 
the importance of maintaining nutrient attenuation mechanisms such as 
nitrification/denitrification, crop uptake, soil adsorption, and immobilization in the soil to 
reduce nutrient level in percolates should be emphasized. On the other hand, even if drain 
field is hydraulically overloaded, a series of well maintained spatial and temporal nutrient 
attenuation mechanisms can still control nutrient percolation within a certain depth or 
even enhance the drain field capacity for sudden surge flows.