Interaction of Groundwater, Surface Water and Seawater in 
 Wolf Bay, Weeks Bay, and Dauphin Island Coastal Watersheds, Alabama 
 
 
by 
 
Lee Russell Beasley 
 
 
 
 
A thesis submitted to the Graduate Faculty of 
Auburn University 
in partial fulfillment of the 
requirements for the Degree of 
Master of Science 
 
Auburn, Alabama 
August 9, 2010 
 
 
 
 
 
Copyright 2010 by Lee Russell Beasley 
 
 
Approved by 
 
Ming-Kuo Lee, Chair, Professor, Department of Geology and Geography 
 James A. Saunders, Professor, Department of Geology and Geography 
Lorraine W. Wolf, Professor, Department of Geology and Geography 
Luke J. Marzen, Associate Professor, Department of Geology and Geography 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Abstract 
 
 
Freshwater residing in coastal plain aquifers and watersheds represents one of our 
nation?s most important natural resources. Globally, the distribution and fluxes of 
freshwater in many coastal settings remains poorly understood. As population, 
agricultural, and industrial centers have expanded along sea coasts, demands for 
freshwater resources have resulted in widespread water depletion and contamination in 
coastal regions. Integrative models rooted in science are needed to characterize surface 
water and groundwater quality and quantity in estuarine and coastal environments.  This 
research used the Wolf Bay watershed, an EPA classified ?Outstanding Alabama Water?, 
and Weeks Bay, a coastal watershed with high-risk of mercury methylation, as a natural 
laboratory to gain an understanding of the hydrologic variables that affect water supply 
and water quality.  To understand the hydrochemical conditions in which mercury 
methylates, water quality measurements of temperature, pH, oxidation reduction potential 
(ORP), dissolved oxygen (DO), turbidity, and electrical conductivity were collected at 
more than 60 locations in Wolf Bay and Weeks Bay.  A bay cruise was conducted in July 
2008 to sample bay water and measure water quality parameters. Major ion and stable 
isotope (oxygen and hydrogen) concentrations were analyzed in the laboratory to 
investigate the mixing of seawater and freshwater.  The results indicated elevated 
concentrations of chloride (Cl) and sodium (Na) are high in bay water.  Oxygen and 
 ii
hydrogen isotope analysis provides additional information on the degree of evaporation 
and water mixing in bays. Wolf Bay water is enriched in 
18
O and 
2
H relative to Weeks 
Bay water, river water, and shallow groundwater, indicating that its has received less 
freshwater input, or undergone greater evaporation and mixing with isotopically heavier 
seawater. In Weeks Bay high salinity seawater invades below acidic, low salinity water in 
the bay to form a wedge interface.  Low DO and ORP values observed in this mixing 
zone indicate high microbial activities that may initialize Hg methylation. In Wolf Bay, 
by contrast, less freshwater inflow produces high salinity water, which may prevent key 
microbial processes that initialize Hg methylation and bioaccumulation. The results 
imply that Hg biotransformation is strongly influenced by hydrochemical conditions in 
coastal watersheds. 
Regional scale groundwater flow models of southern Baldwin County were 
developed in a cross section extending from the northern recharge areas (near Bay 
Minette) to the Gulf Coast.  The models predicted two flow regimes in major aquifer 
zones.  Both local and regional flow regimes are present in Aquifer A2 due to local 
variations in topography and water table undulations.  In the deeper Aquifer A3, a 
regional flow regime dominates in which flow directions are more consistent (i.e., from 
north to south) and controlled by the net topographic slope.  Groundwater discharges 
southwards into the coastal estuaries (e.g., Wolf and Weeks bays) and Gulf of Mexico. 
Calculated groundwater flow velocities in major aquifers range from a few to tens of 
meters per year.   The model calculated that groundwater residence time of major 
aquifers ranges from 0 near the recharge area to about 7000 years near the Gulf Coast 
 iii
along a 70 km flow path.  The calculated groundwater residence time is consistent with 
14
C and 
4
He ages measured by Carey et al. (2004). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 iv
 
 
 
 
 
 
Acknowledgments 
 
 
 Without grants from Gulf Coast Association of Geological Societies and the 
Alabama Geological Society, this study would not have been possible. I would like to 
thank Dr. Ming-Kuo Lee, Dr. James Saunders, Dr. Lorraine Wolf and Dr. Luke Marzen 
for their guidance and assistance throughout this project. The author would also like to 
thank Stan Mahoney and Rick Odess of the Wolf Bay Water Watch for their knowledge 
and use of equipment as well as Scott Phipps and Mike Shelton of the Weeks Bay 
National Estuary Research Reserve for allowing the use of their facilities and equipment. 
Finally, I would like to thank my family and friends for their support during my time at 
Auburn University.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 v
 
 
 
 
 
 
Table of Contents 
 
 
Abstract......................................................................................................................... ii 
Acknowledgments .........................................................................................................v 
List of Tables ............................................................................................................. viii 
List of Figures.............................................................................................................. ix 
Chapter 1. Introduction ..................................................................................................1 
Chapter 2. Geology and Hydrogeology .........................................................................6 
 Hydrogeology of Southern Baldwin County  .........................................................9 
Chapter 3. Methodology .............................................................................................14 
 Field Water Quality Data.......................................................................................14 
 Laboratory Geochemistry Data..............................................................................14 
 Geophysical Survey  .............................................................................................17 
 Groundwater Flow and Solute Transport Modeling  ............................................17 
 Geographic Information Systems (G.I.S.) Models  ..............................................18 
Chapter 4. Water Chemistry in Baldwin County and Dauphin Island ........................19 
 Well Locations and Specifics.................................................................................19 
 Groundwater Geochemistry of Baldwin Country..................................................23 
 Saltwater Intrusion in Southern Baldwin Country.................................................27 
 Groundwater Geochemistry of Dauphin Island  ...................................................29 
 Surface Water Chemistry of Wolf and Weeks Bay ...............................................29 
 vi
 
Chapter 5. Discussion .................................................................................................37 
 Mercury Deposition and Precipitation...................................................................37 
 Surface Water Chemistry.......................................................................................40 
 Stable Isotope Signatures of Stream Water, Bay Water and Groundwater ...........64 
Chapter 6. Subsurface Geophysical Survey.................................................................67 
 Electrical Resistivity ..............................................................................................67 
Chapter 7. Hydrologic Models.....................................................................................74 
 Surface Flow of Wolf Creek and Fish River  .......................................................74  
 Regional Groundwater Flow and Groundwater Residence Time..........................76 
Chapter 8. Conclusions ...............................................................................................86 
References  ..................................................................................................................90 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 vii
 
 
 
 
 
 
 
 
List of Tables 
 
 
Table 1. Well Specifics, Rivera Utility wells, Foley, Alabama...................................20 
Table 2. Well Specifics, Orange Beach Water Authority wells, Orange Beach,  
              Alabama .........................................................................................................21 
Table 3. Major Ion Analysis, Riviera Utility Wells, Foley, Alabama .........................24 
Table 4. Major Ion Analysis, Orange Beach Water Authority wells, Orange Beach,  
              Alabama .........................................................................................................25 
Table 5. Dauphin Island Wells Chloride Levels..........................................................31 
Table 6. Well Specifics, Dauphin Island Water Authority wells,  
               Dauphin Island, Alabama..............................................................................32 
Table 7. Surface Water Sample Analysis Wolf Bay, Alabama ..................................34 
Table 8. Surface Water Sample Analysis Weeks Bay, Alabama ................................35 
Table 9. Major Ion Analysis Wolf Bay, Alabama ......................................................36 
Table 10. Isotope Data from Wolf Bay, Alabama ......................................................63 
 
 
 
 
 
 
 
 
 
 viii
 
 
 
 
 
 
List of Figures 
 
 
Figure 1. Site Location Map ................................................................................................2 
Figure 2. Hydrogeologic Cross-section of Southern Baldwin County ................................8 
Figure 3. Geologic Map of Southwest Alabama?............................................................10 
Figure 4. Generalized Cross-section of Southern Baldwin County ..................................11 
Figure 5. Sample Locations in Wolf Bay...........................................................................16 
Figure 6. Distribution of municipal wells located in Foley and Orange Beach, 
               Alabama ..............................................................................................................22 
Figure 7. Piper Diagram of Southern Baldwin County Drinking Wells............................26 
Figure 8. Contour Map with Chloride and TDS MCL Limits in Southern Baldwin  
                County, Alabama ...............................................................................................28 
Figure 9. Map of Locations of MDN Sampling Sites in the Southeastern U.S. ................38 
Figure 10. Total Mercury Wet Deposition Graph..............................................................39 
Figure 11. Plot of pH vs. Conductivity?..........................................................................43 
Figure 12. Plot of Conductivity vs. Dissolved Oxygen ....................................................44 
Figure 13. Plot of Temperature vs. pH ..............................................................................45 
Figure 14. Plot of Temperature vs. Conductivity ..............................................................46 
Figure 15. Plot of pH Values Transect...............................................................................47 
Figure 16. Plot of Conductivity Values Transect...............................................................48 
 ix
Figure 17. Contour Maps of pH.........................................................................................49 
Figure 18. Contour Maps of Temperature?. ....................................................................50 
Figure 19. Contour Maps of Conductivity ........................................................................51 
Figure 20. Contour Maps of Dissolved Oxygen ................................................................52 
Figure 21. Contour Maps of Oxidation-Reduction Potential.............................................53 
Figure 22. Piper Diagram of Wolf and Weeks Bay...........................................................55 
Figure 23. Plot of Chloride vs. Bromide............................................................................57 
Figure 24. Plot of Chloride vs. Calcium ............................................................................58 
Figure 25. Plot of Chloride vs. Magnesium.......................................................................59 
Figure 26. Plot of Chloride vs. Sodium .............................................................................60 
Figure 27. Plot of Chloride vs. Sulfate ..............................................................................61 
Figure 28. Plot of Isotope Data for Wolf and Weeks Bay?.............................................66 
Figure 29. Location of Dauphin Island Geophysical Survey ............................................70 
Figure 30. Location of Orange Beach Geophysical Survey ..............................................71 
Figure 31. Dauphin Island Electrical Resistivity Survey...................................................72 
Figure 32. Orange Beach Electrical Resistivity Survey.....................................................73 
Figure 33. Hydrographs of Wolf Creek and Fish River.....................................................75 
Figure 34. Equipotential Map of Southern Baldwin County .............................................78 
Figure 35. Calculated Regional Groundwater Flow Model...............................................82 
Figure 36. Cross-Section of A2 and A3 Aquifers in Southern Baldwin County...............83 
Figure 37. Calculated Residence Time Model...................................................................84 
Figure 38. Plot of Hydrologic Residence Time in the A3 Aquifer vs. Distance ...............85 
 
 
 x
 1
 
INTRODUCTION 
 
Groundwater and surface water are a vital source of fresh water for industrial, 
municipal and private use. Globally and across the nation, the good stewardship of local 
watersheds is essential to the well-being of human communities and ecological systems 
within their boundaries (Alley, 1999). Alabama and the other southeastern states of the 
south Atlantic-Gulf region are the fastest growing areas in the United States. Thus, it is 
inevitable that water supply and quality problems will arise from population growth 
which underscores the need to protect water resources from degradation (Shat, 2005; 
Foster, 2006). Baldwin County lies along the Alabama Gulf Coast, an area where fresh 
groundwater is highly important due to the rapidly expanding development of the region 
and the subsequent increased water use (Chandler et al., 1985).  The increasing use of 
water may cause overdevelopment of the groundwater resources, which in turn may 
cause water depletion, saltwater contamination, and other water quality problems.  The 
lack of surface and subsurface data of the Wolf Bay watershed and the fact that it is an 
EPA Classified ?Outstanding Alabama Water? (Alabama Water Watch, 2007) made this 
the perfect research area for this study. 
 
 Wolf Bay is located on the Gulf of Mexico in southern Baldwin County between 
Perdido Bay to the east and Mobile Bay to the west (Figure 1). Wolf Bay is an estuary 
where freshwater and seawater mix and its watershed host a diversity of habitats that 
support several federally listed species including black bears, bald eagles, Florida  
 
 
 
 
Mobile Bay 
Dauphin Island 
Weeks Bay 
Wolf Bay 
Gulf of Mexico
 
 
 
Figure 1: Site location map of Wolf Bay, Weeks Bay and Dauphin Island (modified from 
Monrreal, 2007) 
 
 
 
 2
 3
manatees, sea turtles, Gulf sturgeons, red-cockaded woodpeckers, American alligators, 
Alabama red-bellied turtles, and Eastern indigo snakes (Alabama Water Watch, 2007).  
Major streams flow into Wolf Bay including Wolf Creek, Sandy Creek, Mifflin Creek, 
Graham Creek, Owens Bayou, Moccasin Bayou, and Hammock Creek.  Wolf Bay flows 
into the Intercoastal Waterway, which flows into either Perdido Bay or Mobile Bay, 
depending on the moon, wind, and tide, and ultimately into the Gulf of Mexico (Alabama 
Water Watch, 2007).    
 
Estuaries and coastal watersheds of Alabama Gulf Coast are highly susceptible to 
contamination by mercury (Hg), an element known to be extremely toxic to wildlife and 
humans. Weeks Bay, an estuary of Mobile Bay to the east of Wolf Bay, is located in 
southwestern Alabama?s Baldwin County and has a watershed of 126,000 acres.  Fish, 
such as Largemouth Bass, caught within the Weeks Bay watershed have been found to 
contain Mercury level above Federal Food and Drug Administration standards of 1 
mg/kg.  By contrast, fish consumption advisory related to Hg contaminated has not yet 
been issued in the Wolf Bay watershed.  Although recent studies suggested that direct 
atmospheric deposition and riverine input are the primary sources of Hg to estuaries 
(Mason et al., 1994; 1999; Kim et al., 2004; Monrreal, 2007), the hydrologic controls on 
the fate and transformation of Hg in these estuaries remain poorly understood. The risks 
associated with mercury consumption coupled with the ever-increasing demand for water 
and seafood consumption underscore the necessity to understand the hydrology, water 
chemistry, and fate and biotransformation of mercury in Alabama coastal watersheds.   
 
 4
New tools are needed for accurate assessment of groundwater and surface flow 
conditions as aquifers are subjected to the stress of over-development and increased use. 
This is especially true in the Alabama Gulf Coast region, where increased development in 
local townships require dramatic land use changes and large quantities of groundwater 
from coastal plain aquifers.  A hydrologic model is needed to accurately describe the 
characteristics of the groundwater watershed, the dynamics of basin-wide processes, and 
the impacts of anthropogenic (e.g. groundwater pumping) sources being put on the 
system. 
 
The primary objectives of this study were to (1) measure spatial variations of 
water quality (i.e., temperature, conductivity, dissolved oxygen, oxidation-reduction 
potential) and chemistry (i.e. major ions, and oxygen and hydrogen stable isotopic 
compositions) in Wolf Bay and its major streamflows, (2) compile groundwater data 
(field parameters, major ions, residence time) to construct regional groundwater flow and 
hydrochemistry models, and (3) assess the freshwater/saltwater interfaces in the Wolf 
Bay and Dauphin Island areas using geophysical imaging methods involving the 
electrical resistivity measurements. The surface water measurements of Wolf Bay were 
compared to previous studies on Weeks Bay (Monrreal, 2007) to aid in the understanding 
of the hydrochemical conditions and key microbial processes that initialize mercury 
methylation and bioaccumulation in Alabama coastal watersheds.  
 
 Based on the data obtained from the research around the project areas this study 
defines surface water and groundwater quality and quantity (e.g., groundwater residence 
 5
time and renewal rates) in the southern Baldwin County area; the data can be used to gain 
knowledge of the variables that affect water supply and water quality. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 6
GEOLOGY AND HYDROGEOLOGY 
 
Wolf Bay is located in southern Baldwin County, southwest Alabama and is in the 
Coastal Lowlands physiographic district which consists of saltwater marshes, swamps, 
bays, inlets, beaches, sand dunes, islands, peninsulas, and tidal waters (Chandler et al., 
1996).  The Bay is an estuary between Perdido Bay to the east and Weeks Bay to the west 
and has a watershed of about 44,700 acres (Alabama Water Watch, 2007). The mixing of 
freshwater and saltwater in Wolf Bay creates a diverse suite of hydrologic environments 
and rich ecosystems.  Four major tributary streams that flow into the bay include Wolf 
Creek, Sandy Creek, Miflin Creek, and Hammock Creek. The watershed is surrounded by 
developing urban centers (i.e., Foley and Elberta) that host large industrial and 
agricultural activities, all of which are potential point and non-point sources of pollution. 
The average annual precipitation in the study area is about 162.6 centimeters. There are 
no major rivers flowing across the watershed, which extends from south of I-10 to the 
Gulf of Mexico.  A fundamental characteristic of the Wolf Bay watershed is that overland 
flow during precipitation events is minimal and only a small percentage of precipitation 
is discharged to the surface streams. The majority of water infiltrates the subsurface 
aquifers immediately. However, the increasing impermeable land covers due to 
urbanization has reduced freshwater infiltration. Weeks Bay is located in southwestern 
Baldwin County off of Mobile Bay on the Gulf Coast coastal plain.  The largest surface 
stream in the Baldwin County (i.e., Fish River) discharges into the Weeks Bay, 
suggesting that the Weeks Bay may receive more freshwater inputs than the Wolf Bay. 
 
 7
Groundwater is the major source of water for municipal, irrigation, and industrial 
use in southern Baldwin County
 
(Chandler et al., 1996). The groundwater pumping rates 
increased six-fold from 7?10
6
 gpd to 4.2?10
7
 gpd from 1966 to 1995 due to an expansion 
of use in irrigation and the demands of growing population
 
(Robinson et al., 1996). Water 
is produced (up to 1,500 gpm) mainly from sand and gravel layers of Miocene-Holocene 
coastal plain aquifers (Chandler et al., 1996).   
 
The main aquifers, locally known as A2 and A3 (Figure 2), are composed of 
unconsolidated Holocene-Miocene sediments deposited in fluvial and shallow-marine 
environments. The details of groundwater migration, water-quality evolution along flow 
path, and residence time or renewal recharge rates in various parts of the watershed 
remain ambiguous.  The 
14
C and 
4
He groundwater ages (or residence time) were 
estimated to be in the range of 375-7500 years (i.e., the time since recharge) in the A3 
aquifer along flow path of 30 to 70 km (Carey et al., 2004). The groundwater residence 
time suggests that groundwater migrates from north to south at rates of about 1 to 15 m/yr 
and ultimately discharges into the Gulf of Mexico.  Regional water table slopes also 
indicate significant groundwater discharges southward into several coastal watersheds 
(e.g., Weeks Bay, Wolf Bay).  
 
 
 
 
 
 8
 
Figure 2: Hydrogeologic cross-section of southern Baldwin County, Alabama (modified 
from Chandler et al., 1985). 
 
 
 
 
 
 
 
 
  
 
 9
Hydrogeology of Southern Baldwin County 
 
Geologic units that crop out in the study area range in age from Tertiary to 
Quaternary (Figure 3) (Mooty, 1988). The Tertiary age sedimentary deposits are 
generally unconsolidated and the alluvial and terrace deposits of Quaternary age overlie 
the Tertiary age deposits in and adjacent to the floodplains of the larger streams and 
rivers, and along the coastal areas of the Gulf of Mexico (Mooty, 1988). 
 
The stratigraphy of southern Baldwin County as well as that of coastal and 
offshore Alabama consists of a relatively thick sequence of Jurassic to Holocene 
sedimentary rocks (Chandler et al., 1985). The middle Miocene to Holocene sedimentary 
rocks consist of interbedded sands, silts, gravels, and clays at relatively shallow depths, 
and host the freshwater aquifer zones in the Baldwin County area (Figure 4). These 
sediments thin towards the Gulf of Mexico and are part of three widely recognized 
geologic units defined by Reed (1971) as (1) the Miocene Series undifferentiated; (2) the 
Miocene-Quaternary Citronelle Formation; and (3) Quaternary alluvium, low terrace, and 
coastal deposits (Murgulet et al., 2008). The Miocene sediments are composed of white 
to light gray, fine to very coarse sands with some interbedded sandy, silty clay. The 
Pleistocene deposits have a greater abundance of interbedded sandy, silty clays as 
compared to the Miocene deposits. These deposits are overlain by sediments of Holocene 
age and consist of, white to pale-orange, fine- to coarse-grained sands, with some silt, 
clay, and shell hash (Chandler et al., 1996).   These sediments are underlain by 
undifferentiated Eocene and Oligocene clays, sands, and carbonates (Figure 4). 
 
Weeks Bay
Wolf Bay
Dauphin Island 
 
Figure 3: Geologic Map of the Wolf Bay area (modified from Mooty, 1988). 
 10
 
 
 
 
Figure 4: Generalized cross-section of southern Baldwin County (modified from Mooty, 
1988). 
C 
C
` 
 
 
 
 
 
 
 
 
 
 
 
 
 11
 12
Dauphin Island is located four miles off the southern end of Mobile County. It is a 
barrier island located between the Mississippi Sound and the Gulf of Mexico. The island 
is oval in shape on the east end, which is 1.4 miles wide and three miles long and narrows 
to 0.5 miles or less to the west extending approximately 12 miles. Dauphin Island is also 
located in the Coastal Lowlands subdivision of the southern Pine Hills District of the East 
Gulf Coast Plain section of the Coastal Plain province.  
 
Three hydrogeologic units underlie Dauphin Island; the Deep Sand Aquifer, the 
Shallow Sand Aquifer, and the Water-Table Aquifer (O?Donnell, 2002).  The Shallow 
Sand and Water Table aquifers are reportedly the only potential sources of fresh water on 
the island.  Most of the island?s surface lies very close to sea level and thus its freshwater 
resources are vey vulnerable to storm surge during major hurricanes.  
 
The Deep Sand Aquifer is designated as Miocene sediments present at a depth of 
500+ feet below sea level (O?Donnell, 2002). The deposit consists mainly of very fine to 
very coarse grain sub-angular to sub-rounded quartzose sand with shell fragments and 
traces of dark minerals with some clay and silt layers present.  
 
The Shallow Sand Aquifer is composed of Miocene sediments between 150 and 
500 feet below sea level and Pleistocene sediments between 50 and 150 feet below sea 
level and consists mainly of very fine to very coarse grain quartzose sand with some shell 
fragments, carbonized wood, silt and clay (O?Donnell, 2002). .  
 
 13
The Water-Table Aquifer, the top of which is visible at ground level on Dauphin 
Island, extends from ground level to the clay separating it from the Shallow Sand Aquifer 
(O?Donnell, 2002).  The aquifer consists of well to moderately sorted, medium to very 
fine grained quartz sand, lenses of dark brown humate, silt, limonite, and streaks of semi-
consolidated sands. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 14
METHODOLOGY 
 
Field Water Quality Data 
 
The water data for assessing spatial changes in hydrologic and chemical 
conditions in the Wolf Bay watershed were recorded using a multi-parameter TROLL 
9000. The lightweight, rugged TROLL 9000 is capable of monitoring up to 9 sensors 
simultaneously. Sampling was performed at 31 locations throughout the Wolf Bay 
(Figure 5).  Parameters measured in this study include temperature, pH, specific 
conductance, dissolved oxygen (DO), oxidation-reduction potential (ORP), and turbidity. 
Data were recorded at 1-meter depth intervals until bay bottom sediments were reached.   
 
Laboratory Geochemistry Data 
 
In addition to the in situ water chemistry data, seven water samples were collected 
from directly above the bay bottom sediments for laboratory geochemical (major ions and 
oxygen and hydrogen isotopes) analysis. A Van-Dorn sampler was used to collect the 
water, which was then placed in 250 mL bottles and placed on ice before being shipped 
to the laboratory. These samples were sent to ACTLABS for major ion and trace element 
analyses using Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Optical 
Emission Spectrometer (ICP-OES).  Anion concentrations were measured by ACTLABS 
using Dionex 2000 Ion Chromatograph (IC). Stable isotopes ratios (?
18
O and ?
 2
H) were 
determined using the standard CO
2
 equilibrium method at the National High Magnetic 
 15
Laboratory at Florida State University.  Results are reported in concentration units as 
permil deviations from the SMOW standard (Craig, 1961). Collectively, the results were 
used to assess the nature of mixing of surface water, groundwater, and seawater in Wolf 
Bay.   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
30? 22? 12? 
87? 31? 28? 
 
 
30? 16? 46? 
87? 37? 39? 
 
Figure 5: Sample location map in Wolf Bay, Alabama (aerial photos from 
alabamaview.org) 
 
 
 
 
 
 
 16
 17
Geophysical Survey 
 
Because of the limited knowledge of the freshwater/saltwater interface in the 
study area, a critical step in its comprehensive characterization should be to determine the 
salinity variation of groundwater.  A geophysical survey using a 48-channel automatic 
switching resistivity system (Advanced Geosciences Super-Sting R-1) was conducted at 
Dauphin Island and Orange Beach, Alabama, that constitute shallow freshwater/saltwater 
interface (e.g. Al-Jahar et al, 2007, Swarzennski et al, 2007). Because bulk resistivity is 
most sensitive to variations in pore fluid properties (freshwater, saltwater, contaminants) 
the resistivity method was chosen to delineate the freshwater/salt water interface (Barlow, 
2003). Two surveys were conducted. The first was on Dauphin Island, where shallow 
freshwater bearing wells were contaminated by Hurricane Katrina?s storm surge. The 
other survey was conducted at Gulf State Park near Orange Beach, Alabama, and was 
compared to results from the Dauphin Island survey. 
 
Groundwater Flow and Resident Time Modeling 
 
The 2-D numerical models of groundwater flow and groundwater residence time 
were performed using basin-scale flow model Basin2 (Bethke et al., 2003).  Hydraulic 
properties (e.g., hydraulic conductivity and storage capacity) of aquifers and aquifers 
geometry and extent were compiled from literature (Sakr, 1999, Alley et al., 2002) for the 
modeling projects. The groundwater model was fully integrated with groundwater 
 18
geochemical and isotope data. Calculated groundwater flow rates and residence time was 
calibrated against existing 
14
C and 
4
He isotope age data (Carey et al., 2004).   
 
Geographic Information Systems (G.I.S.) models 
 
 A GIS base map was created using four combined aerial digital orthoquadrangles 
(DOQs) of the Wolf Bay area (alabamaview.org). Surface water sampling locations were 
plotted using the GPS measurements from the field.  Spatial variations of water chemistry 
parameters within the bay were determined using 3-D Analyst in ArcGIS. The extent of 
Wolf Bay was clipped as a polygon and then interpolated into a surface map through the 
default kriging procedure to show the distribution throughout the bay at a 30m resolution. 
These maps were compared to find trends within the bay based on the differences in the 
field parameters.  
 
 
  
 
 
 
 
 
 
 
 19
 
 
WATER CHEMISTRY IN BALDWIN COUNTY AND DAUPHIN ISLAND 
 
Well Locations and Specifics 
 
              The wells located in the North Well Field owned by the Orange Beach Water 
System (Figure 6) are developed in the A2 and A3 aquifers (Table 1). Riviera Utilities 
wells in Foley (Figure 6) are constructed in the Citronelle Formation and upper sands of 
the Miocene undifferentiated deposits which is recognized as the A2 aquifer, where the 
sand layers of these units serve as the aquifer for these wells (Table 2). 
 
 
              Dauphin Island Water and Sewer Authority?s wells #10, #20, #30, #40, #50, #60, 
#70 and #80 produce from the Water-Table Aquifer, which consists of well to moderately  
sorted, medium to very fine grained quartz sand, lenses of dark brown humate, silt, 
limonite, and streaks of semi-consolidated sands (O?Donnell, 2002) (Table 5).  
 
 
 
 
 
 
 
 
 
 20
 
 
Table 1. Riviera Utilities, City of Foley Well Locations and Well Specifics.  
Riviera Utilities Well Locations (Foley, AL) 
Well 
I.D. Latitude / Longitude Well Depth (ft bls) Screened Intervals (ft bls) 
7 30.4047 / 87.6836 145 95-135 
8 30.4036 / 87.6836 152 105-130 / 135-145 
9 30.4080 / 87.6805 140 95-135 
10 30.3705 / 87.6869 238 155-195 
11 30.4038 / 87.6938 265 95-125 
12 30.4108 / 87.6827 300 185-210 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 21
 
 
 
Table 2. Orange Beach Water, Sewer, and Fire Protection Authority well location and 
well specifics.  
 
 
Orange Beach Well Locations (North Well Field) 
Well I.D. Latitude / Longitude Well Depth (ft bls) Screened Intervals (ft bls) 
Roscoe 
Well 30.3480 / 87.6508 326.4 
220.05-235.38 / 260.46-296.15 
 / 306.15-326.40 
Smith 
Well 30.3627 / 87.6580 352 
205.21-245.89 / 281.03-306.70 
/ 331.70-352.00 
Holasz 
Well 30.3466 / 87.6633 332.1 
179.63-199.98 / 203.98-234.68 
/ 260.68-332.10 
Roper 
Well 30.3552 / 87.6283 247.5 165.59-206.93 / 221.93-247.50 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6:  Distribution of municipal wells located in Foley and Orange Beach, Alabama.  
 
 
 
 
 
 
 
 22
 23
 
Groundwater Geochemistry of Baldwin County 
  
Groundwater chemistry data were collected from municipal wells managed by the 
Orange Beach Water, Sewer, and Fire Protection Authority, the City of Foley and Riviera 
Utilities, and from previous publications (Layne Geosciences, 1997; Goodwyn, Mills and 
Cawood, 2000) in the study area.  Figure 6 shows the locations of these wells which were 
installed in the aquifer zones of A2 and A3. The physio-chemical parameters and major 
ion concentrations of the analyzed groundwater samples are shown in Tables 3 and 4.  
According to the relative molar proportion of the dissolved ionic species, the 
groundwater in the A2 and A3 aquifers in the study area shows mixed nature (Ca-Mg-Na-
K and Cl-SO
4
-HCO
3
 types) (Figure 7). The groundwater is composed of varying values 
of alkalinity (range: 4.1 to 29.2; mean: 16.65), Na (range: 2.89 to 5 mg/l; mean: 3.95), Ca 
(range: 0.78 to 15.2 mg/l; mean: 7.99), Mg (range: 0.55 to 1.86 mg/l; mean: 1.21), Cl 
(range: 3.9 to 9.09 mg/l; mean: 6.5), SO
4
 (range: 3.73 to 12 mg/l; mean: 7.865), and 
variable pH (range: 4.2 to 8.58; mean: 6.39).  These groundwaters in general have low 
ion concentrations; their isotope ages and residence time (see sections below) correspond 
to young groundwater of meteoric origin.  
 
 
 
 
 
 24
 
Table 3. Groundwater major ion data from the Riviera Utilities, City of Foley Wells 
 
Riviera Utilities, City of Foley Production Wells Major Ions 
Analyte 
Symbol Ba Mg Ca Na Zn Mn Cl Fe 
SO
4 Alkalinity 
pH 
Unit 
Symbol mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L  
Wells 
7 & 9 <0.05 1.86 15.2 4.31 0.15 0.05 9.09 <0.05 3.73 27 8.58
Well 
10 <0.05 1.27 14 2.89 0.19 <0.01 5.48 <0.05 <0.05 29.2 8.07
 
 
 
 
 
        
 
 
 
        
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 4. Groundwater major ion data from the Orange Beach Water, Sewer, and Fire 
Protection Authority. 
 
 
Orange Beach North Well Field Major Ions 
Analyte 
Symbol SO Alkalinity pH Ba Mg Ca Na Zn Mn Cl Fe 
4
Unit 
Symbol mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L  
Smith 
Well 0.046 0.089 1.3 5 0.022 0.035 3.9 0.97 12 7.8 5.9 
Holasz 
Well 0.024 0.55 0.78 5 0.02 0.01 5.9 0.076 7.8 2.1 4.2 
 25
 
Roper 
Well             6  5.4 4.6 5.1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 26
 
 
 
 
 
Figure 7:  Piper diagram of drinking well water in southern Baldwin County.  
 
 
 
 
 
 
 
 
 
 
 
 
 27
 
Saltwater Intrusion in Southern Baldwin County  
 
Previous study (Murgulet and Tick, 2008) conducted in southern Baldwin County 
on the A1, A2, and A3 aquifers showed that groundwater near the coastal margin 
possesses poor water quality with relatively high total dissolved solids (TDS), salinity, 
and chloride concentrations (Figure 8).  
 
 In the A1 aquifer elevated levels of TDS, salinity, and chloride were observed. 
The average salinity concentrations for aquifer A1 were 1,153.9 mg/L with a maximum 
of 18,000 mg/L in close proximity to the coastline (Murgulet and Tick, 2008).  Chloride 
concentrations for the A1 aquifer were 486.2 mg/L with a maximum concentration of 
7,758.3 mg/L in the Gulf Shores area. Average TDS concentrations were 1,359.3 mg/L 
with a maximum concentration of 14,590 mg/L (Murgulet and Tick, 2008).  
 
The average salinity concentrations from the A2 aquifer was determined to be 
96.8 mg/L with a maximum concentration of 2,590 mg/L and the average chloride 
concentrations were 28.2 mg/L with a maximum concentration of 1,460 mg/L (Murgulet 
and Tick, 2008).  TDS concentrations averaged 146.8 mg/L with a maximum of 3,610 
mg/L.   
 
The groundwater samples obtained in the Murgulet and Tick (2008) study in the 
A3 aquifer exhibited relatively low concentrations of salinity and chloride compared to 
A1 and A2 aquifers. The average salinity concentration was 39.5 mg/L with a maximum  
 
 
 
Wolf Bay 
Foley
Orange Beach 
 
 
 
 
Aquifer A1 Cl 
Aquifer A1 TDS 
Aquifer A2 Cl 
Aquifer A2 TDS 
 
Figure 8:  Contours show the positions of the chloride (250 mg/L) and TDS (500 mg/L) 
levels equivalent to drinking water limits set by the EPA (data from (Murgulet and Tick, 
2008). These lines define the extent of saltwater intrusion occurring in the A1 and A2 
aquifers in southern Baldwin County.  
 
 
 
 
 
 
 28
 
 29
concentration of 136 mg/L and the average chloride concentration was 13.6 mg/L with a 
esults from the Murgulet and Tick (2008) study indicate that saltwater intrusion 
has occ
 
rea 
roundwater Geochemistry of Dauphin Island 
m Dauphin Island were received from the Dauphin Island Water and 
Sewer A
loride 
urface Water Chemistry of Wolf Bay and Weeks Bay 
Data from the surface water sampling in Wolf Bay are shown in Table 7, which 
include latitude and longitude, temperature, pH, ORP, conductivity, DO, and turbidity 
maximum concentration of 18.2 mg/L (Murgulet and Tick, 2008). TDS concentrations 
averaged 51.4 mg/L with a maximum of 190 mg/L north of the Intercoastal Waterway 
(Murgulet and Tick, 2008).  
 
R
urred in the A1 and A2 aquifers, as indicated by high levels of chloride and 
salinity in wells completed near the Gulf of Mexico. The front of saltwater in the A1
aquifer, as defined by the 250 mg/L chloride isochlor, already approaches the inland a
near Foley (Figure 8). 
 
G
  
Data fro
uthority. Dauphin Island currently has eight shallow wells in production. 
Chloride data was collected and Table 6 shows pre- and post- hurricane Katrina ch
levels in the wells. Table 7 shows the well locations and specifics of the shallow 
production wells. 
 
S
 
 30
from th
 to 
-
 
DO 
 
e July 2008 cruise survey.  Measurements were taken at different depths to 
delineate the spatial distribution of the bay?s water quality. The surface water near the 
Intercoastal Waterway (WB-1 through WB-10) is characterized by higher pH (7.75
7.93), higher temperature (29.05 to 31.72 ?C) and high conductivity (32,990 to 36,050 
?S/cm).  In contrast, surface waters near the input of Wolf Creek (WB-24 through WB
31) into the bay have relatively lower pH (7.55 to 7.74), lower temperature (27.47 to 
29.98 ?C) and lower conductivity (22,980 to 33,800 ?S/cm).  The surface water sampled
near the Wolf Creek mouth (where creek water mixes with bay water) has the lowest 
and ORP values (as shown in Table 7).   
  
 
 
 
 
 
 
 
 
 
 
 
 
 31
able 5. Chloride levels in selected Dauphin Island wells pre- and post- hurricane  
Katrina (2005). 
-03 Jul-06 Aug-06 Oct-06 May-07 
T
 
  Sep
WELL #10 40 320 380 360 320 
WELL #20   25 180 200 180 140 
WELL #30 20 300 320   180 
  Jul-92 Sep-0 Aug-0  Oct-07 
Dec-
07 7 7 May-07 
WELL #50 39 30 100 80 60 60 
WELL #60 46 30 100 120   80 
WELL #70  0 48 30 60 80 6 60 
WELL #80 26 30 50 100 80 80 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 32
Table 6. Dauphin Island Water and Sewer Authority well locations specifics. 
 
 
 
Dauphin Island Water and Sewer Authority Shallow Production Wells 
Well I.D. Latitude / Longitude Well Depth (ft bls) Screened Interval (ft bls) 
#10 30.25380 / 88.11003 30 18-28 
#20 30.25156 / 88.10153 32.5 20.5-30.5 
#30 30.25124 / 88.9686 34.5 22.5-32.5 
#40 30.25168 / 88.10746 33 21-31 
#50 30.24913 / 88.10757 40 23.65-33.65 
#60 30.249 24.75-34.75 40 / 88.10377 40 
#70 30.24767 / 88.09223 40 26.10-36.10 
#80 30.24676 / 88.09223 40 26.65-36.65 
 
 
 
 
 
 
 
 
 
 
 
 
 33
or comparison, surface water data collected from Wolf Bay (this study) and 
Weeks ater 
  
s of 
 
F
Bay (Monrreal, 2007) are shown in Tables 7 and 8. In Weeks Bay, the river w
and surface water near the river mouth are characterized by relatively low pH (5.99 to 
6.54), low temperature (27.80 to 31.65 ?C), and low conductivity (138 to 2017 ?S/cm).
The surface waters near the bay mouth however, have relatively high pH (7.8 to 8.75), 
high temperature (32.0 to 33.25 ?C), and high conductivity (3350 to 5706 ?S/cm). 
Conductivity of water in Weeks Bay is much lower than that in Wolf Bay. pH value
surface water in the upper Weeks Bay near the river mouth are also significantly lower. 
The surface water sampled near the river mouth where river water mixes with bay water 
has the lowest DO and ORP values generally. The major ion composition of surface 
water is shown in Table 9. 
 
 
 
 
 
 
 
 
 
 
 
 34
able 7.  Surface water chemistry from locations within Wolf Bay taken at surface level, 
collected in July, 2008. 
I.D. Latitude Longitude pH Conductivity ORP D.O.  Temperature Turbidity 
T
 
Sample 
WB-1 30.1805 87.08734 7.92 36050 161 5692 31.72 12.8
WB-2  30.1807 87.34482 7.93 35000 176 6644 31.2 11.2
WB-3 30.3015 87.58907 7.9 34220 71 7130 30.15 14.8
WB-4 30.3019 87.59952 7.8 32990 69 6972 30.27 13.4
WB-5 30.3049 87.59916 7.8 34360 26 7090 30.6 13.4
WB-6 30.3054 87.59403 7.79 34810 48 7149 31.65 10.1
WB-7 30.3058 87.58722 7 29.86 .81 35090 66 7151 10.3
WB-8 30.3057 87.58235 7.74 34530 73 7568 29.05 11
WB-9 30.3102 87.58242 7.8 34590 81 7095 30.75 9
WB-10 730.3102 87.58625 7.75 35190 86 7122 30.88 .5
WB-11 30.3098 87.59225 7.82 35030 103 6730 30.64 9.4
WB-12 30.3097 87.59503 7.75 34780 102 7025 30.21 8.7
WB-13 30.3147 87.59267 7.62 34840 121 7023 28.36 8.1
WB-14 30.3156 87.58917 7.65 34330 116 7110 29.54 7.7
WB-15 30.3166 87.58405 7.66 34120 125 6791 28.92 8.9
WB-16 30.3238 87.5858 7.5 34380 121 7228 27.27 6.2
WB-17 30.3239 8 77.58946 .67 32780 127 6329 30.43 7.2
WB-18 30.3233 87.59389 7.74 32680 123 7160 28.35 6.7
WB-19 30.3277 87.59825 7.73 32750 81 6977 29.67 7.7
WB-20 30.3295 87.59083 7.78 32910 119 6937 29.58 8.2
WB-21 30.3342 87.58184 7.76 34070 121 6203 29.73 7.3
WB-22 30.3345 87.5787 7.79 34060 131 6362 27.59 7.1
WB-23 30.3412 87.58103 7.75 33570 104 6928 27.41 8.1
WB-24 30.34 87.58235 7.74 33800 112 7060 28.16 9.7
WB-25 30.3376 87.58406 7.72 33790 -23 6766 29.44 13.14
WB-26 30.334 87.58916 7.61 33580 60 5587 27.47 3.2
WB-27 30.3334 87.59186 7.66 33080 94 6866 29.48 10.6
WB-28 30.3363 87.59828 7.58 29420 97 7777 29.81 7.2
WB-29 30.3379 87.59815 7.59 29200 106 6750 27.81 7.2
WB-30 30.3408 87.60319 7.6 23660 97 6772 29.72 9.6
WB-31 30.3451 87.60119 7.55 22980 114 7463 29.98 7.8
 
 
 
 35
able 8.  Surface water chemistry collected from locations within Weeks Bay and Fish 
iver taken at surface level in July 2005 (from Monrreal, 2007).  
Temperature Turbidity 
T
R
 
Sample 
I.D. Latitude Longitude pH Conductivity ORP D.O.  
WB1-S 30.4133 87.8255 6.47 909 540 5950 30.93 22.4
WB2-S 7 6.4130.4094 87.82658 942.6 609 5020 31.65 22.5
WB3-S 30.37686 87.8358 8.49 5700 533 6700 32.82 19.9
WB4-S 30.38289 87.8348 8.6 5522 467 7394 32.89 21.6
WB5-S 30.39041 87.8331 8.77 5174 493 8100 33.56 17.6
WB6-S 30.3981 8 87.83061 .43 2715 384 6360 33.02 23.3
WB7-S 30.40286 87.82869 6.28 300 488 4455 31.5 19.8
WB8-S 30.40080 87.82963 6.6 1311 518 5498 32.16 19.7
WB9-S 30.40722 87.82736 6.38 204 502 6233 32.17 22
WB10-S 30.38963 87.81616 7 3000 457 5038 32.0 29
WB11-S 30.39219 87.82005 8.0 32.66 29 3975 432 6330 8.2
WB12-S 30.39319 87.82469 8.62 32.60 24.14002 381 7360
WB13-S 30.39389 87.82908 8.66 5136 365 6110 33.17 19.8
WB14-S 30.39525 87.83678 8.3 2340 356 6658 32.66 18
WB15-S 30.39685 87.84175 8.06 1865 361 7150 32.69 31
WB16-S 30.39091 87.84086 8.41 19.92880 344 6800 33.06 
WB17-S 30.38669 87.84013 8.75 3350 327 7356 33.22 23.3
WB18-S 30.38302 87.83894 8.79 3255 324 7746 33.22 21
WB19-S 30.37844 87.83705 8.8 3436 330 7815 33.25 20.9
WB20-S 30.40122 87.83738 8.19 2108 290 7255 33.63 24.4
WB21-S 30.40672 87.83422 6.69 560 386 5369 32.72 26.2
WB22-S 30.41030 87.82988 7.03 1119 409 7150 31.99 29.9
FISH1-S 30.44500 87.80433 6.8 66.01 403 8851 31.4 9.9
FISH2-S 30.44272 87.80280 6 31.54 .63 65.31 482 8805 9.5
FISH3-S 30.44361 87.80728 6.57 63.97 514 9043 30.2 10.7
FISH4-S 30.44083 87.81160 6.9 71.4 455 9210 31.98 13.1
FISH5-S 30.43600 87.81261 6 6.98 9.26 447 9500 31.83 12.9
FISH6-S 30.43563 87.81891 6.82 76.5 454 9540 31.65 15
FISH7-S 30.43130 87.82372 6.83 98.3 443 9140 32.17 13.9
FISH8-S 30.42750 87.82855 6.61 105.2 444 8600 31.97 14.6
FISH9-S 30.42411 87.82477 6.7 123.3 441 8659 32.50 15.6
 36
 
Table 9. Major ion composition of surface water collected from Wolf Bay.  
K Mg Ca Na S Sr Cl Br SO4 
Analyte 
Symbol 
Unit Symbol /L    /L L L L mg mg/L mg/L mg/L mg ?g/ mg/ mg/ mg/L
Analysis 
Method 
ICP-
 OES
ICP-
 OES
ICP-
 OES
ICP-
 OES
ICP-
 OES
ICP-
 OES IC IC IC 
WB-10 306 00 0 0675 234 6830 536 3580 106 < 1 152
WB-14 282 625 215 6380 493 3320 11000 < 30 1520
WB-19 272 613 208 6130 489 3260 6800 26.1 949
WB-21 270 613 210 6150 483 3270 9750 < 110 390
WB-23 262 595 206 5950 468 3250 1 80700 37. 1510
WB-28 248 573 198 5800 455 3030 9870 35.9 1370
WB-31 267 608 208 6080 473 3230 10700 38.9 1490
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 37
DISSCUSSION  
 
Water chemistry is an important factor in the methylation and bioaccumulation of 
mercury.  Conditions are constantly changing especially in estuary environments and can 
affect the fate and biotransformation of Hg. Data collected from the previous studies 
conducted in Weeks Bay and this study in Wolf Bay reveal how the difference in the 
hydrodynamics of the bays affects water chemistry and mercury methylation.  Water 
samples were also collected for isotope and chemical analysis to investigate how water 
mixing and evaporation affects the water chemistry in the watershed.  From the collected 
field data, GIS-based computer models were created to help interpret the mixing of 
surface water and seawater and the degree of evaporation in the bays.  The Wolf Bay data 
were compared to the Weeks Bay data and analyzed to help obtain a better understanding 
of the water mixing within the bays and how that affects the methylation of mercury.  
 
Mercury Deposition and Precipitation 
 
Mercury deposition data from the Mercury Deposition Network (MDN) site AL02 
(Figure 9) (http://nadp.sws.uiuc.edu/mdn) were compared with USGS precipitation data 
from the same time periods reported in Monrreal (2007) to show possible correlations 
between mercury deposition and precipitation for the area near the Weeks Bay  and Wolf 
Bay watersheds. Figure 10 shows that total mercury wet deposition increases with the 
amount of weekly atmospheric precipitation.  
 
 
 
 
 
Figure 9:  Map of location of MDN sampling sites in the southeastern region of the 
United States (http://nadp.sws.uiuc.edu).  Data from sites AL02 were used for analysis of 
Hg deposition near Wolf and Weeks bays. 
 
 
 
 
 
 
 38
Date
H
g D
epo
siti
on (
ng/
m
2
)
0
200
400
600
800
1000
1200
1400
1/9/2007 2/13/2007 3/20/2007 4/24/2007 5/29/2007 7/3/2007 8/7/2007 9/11/2007 10/16/2007 11/20/2007 12/26/2007
-1
0
1
2
3
4
5
6
7
8
P
r
ecip
itat
ion
 (in
c
he
s)
 
Figure 10:  Fluctuations of total mercury wet deposition (collected by the Mercury 
Deposition Network, MDN) responding to rainfall in southwestern Alabama from 
January 2007 to January 2008. Rainfall data (vertical bars, in inches) were collected 
along the Fish River. The diagram shows increased mercury deposition during periods of 
higher precipitation, suggesting atmospheric deposition of mercury pollution.                             
 
 
 
 
 
 
 
 
 
 39
 40
The correlation of mercury wet deposition and precipitation data confirms that the likely 
source of mercury in Weeks Bay and Wolf Bay watersheds is atmospheric mercury 
deposition.  
 
Surface Water Chemistry  
  
Chemistry data of surface water demonstrates a mixing zone in Wolf Bay and 
Weeks Bay, resulting from seawater intrusion along the bottom of the bay.  Grouping of 
different geochemical characteristics (Figures 11-14) indicate that those waters have 
distinct chemical characteristics, such as pH, conductivity, D.O., and temperature. 
Seawater has higher temperatures, pH, and electrical conductivity. 
 
The chemistry data from the surface water samples show a mixing zone in Wolf 
Bay as a result of seawater intrusion along the lower portion of the bay.  The geochemical 
analysis shows elevated chloride
 
levels, which suggest that the bay has been 
contaminated by seawater.  The intrusion of seawater into the bay creates a front of high 
salinity, high pH water that is denser and able to wedge underneath the lighter, low-
conductive freshwater. Changes occur in water chemistry at the freshwater/saltwater 
interface and cause the bay to have pronounced stratifications due to the influx of 
seawater.  
 
 Each type of water has certain characteristics to fingerprint their origin such as 
pH, conductivity, D.O. and temperature. Mercury methylation favors waters with higher 
 41
temperatures, lower pH, and lower salinity (or electrical conductivity). Surface water near 
the Intercoastal Waterway has the highest pH, temperature, and conductivity values.  By 
comparing these parameters from Wolf Bay to Weeks Bay it may help to explain the 
spatial distribution of mercury methylation and its relation to the mixing of seawater, 
river water, and groundwater in the bays and the difference of the bays.   
 
By comparing the data collected in Wolf Bay there appears to be a transition 
between the waters entering Wolf Bay from fresh water sources and that of the saline 
water in the bay itself. Measurements of pH and conductivity taken along a north-south 
transect from the mouth of Wolf Creek to the Intercoastal Waterway show a trend of high 
conductive, high pH seawater entering the bay (Figures 15 and 16).  The data collected at 
lower depth have a higher pH and conductivity than those of the surface water.  This 
trend suggests that the higher pH and higher conductive, denser seawater is intruding 
along the bottom of the water column in the bay and mixing with the lower pH and lower 
conductive waters from the creeks flowing into Wolf Bay.  The pH, temperature and 
conductivity data collected at the surface and 1 meter depths in Wolf Bay also show the 
similar trend (Figures 17-20).  Contour gradients of conductivity, temperature, and pH 
are more pronounced in the upper bay which indicates that a saline wedge has formed by 
the mixing of saltwater and freshwater.  Waters at depth are more saline and warmer than 
those at the surface which is another indicator that dense seawater is intruding farther into 
Wolf Bay at depths below relatively fresher surface water.  The plots illustrate that warm 
dense seawater invades beneath cooler fresh waters from the creeks that feed Wolf Bay.  
The saltwater wedge formed within Wolf Bay is indicated by the presence of the 
 42
temperature and salinity stratifications. Previous studies in different watershed basins 
suggest that the highest mercury methylation primarily occur near the saline wedge, 
where lower pH water and low-salinity water are both present by mixing.  The 
conductivity values in Wolf Bay (ranging from 22,980 to 36,050 ?S/cm) appear to be 
much higher than those in Weeks Bay (ranging from 63.97 to 5700 ?S/cm). This high 
salinity implies a less favored condition for Hg methylation (Ullrich et al., 2001; Celo et 
al., 2005; and Monrreal, 2007). pH values show greater variations in Weeks Bay than 
those in Wolf Bay, probably reflecting stronger mixing and freshwater inputs in the 
Weeks bay watershed. 
 
The ORP and DO contour maps show areas of Wolf Bay that exhibit spatial 
variations in oxidized or reduced conditions (Figure 21).  Some bacteria, such as sulfate 
reducing bacteria (SRB), prefer anaerobic waters with low ORP values that may 
contribute to the methylation of mercury (King et al., 2002; Monrreal, 2007).  The lowest 
ORP values are located near the mouth of Wolf Creek, near the interface of fresh and 
brackish waters.  Water DO and ORP values in Wolf Bay are comparable to those of 
Weeks Bay.  However, in Weeks Bay low DO levels and reducing conditions prevail in 
the mixing zone near the mouth of Fish River.  The reducing conditions in Weeks Bay 
may indicate more intensive microbial activity, which is an important factor in the 
methylation of mercury.   
 
 
 
 
 
 
 
 
 
0
5000
10000
15000
20000
25000
30000
35000
40000
66.577.588.59
pH
C
o
nduc
t
i
v
i
t
y
 Wolf Bay
Weeks Bay
 
Figure 11:  Plot of pH vs. conductivity showing a general relationship in water chemistry 
parameters that demonstrate the differences in the Wolf Bay and Weeks Bay waters.  
 
 
 
 
 
 43
 
 
 
 
 
 
0
5000
10000
15000
20000
25000
30000
35000
40000
0 2000 4000 6000 8000 10000 12000
D.O.
C
ond
uc
t
i
v
i
t
y
 Wolf Bay 
Weeks Bay 
 
Figure 12:  Plot of conductivity vs. DO comparing the water chemistry parameters from 
locations within Wolf Bay and Weeks Bay. 
 
 
 
 
 
 
 44
 
 
 
 
 
 
 
6
6.5
7
7.5
8
8.5
9
29.00 30.00 31.00 32.00 33.00 34.00
Temperature
pH
Wolf Bay
Weeks Bay
 
Figure 13:  Plot of temperature vs. pH comparing the relationship in the mixing of the 
 45
water within Wolf Bay and Weeks Bay. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 46
ater chemistry within Wolf Bay and Weeks Bay. 
 
 
Figure 14: Plot of temperature vs. conductivity that compares the relationship between 
0
5000
10000
15000
20000
25000
30000
35000
40000
29.00 30.00 31.00 32.00 33.00 34.00
Temperature
C
ond
uc
t
i
v
i
t
y
Wolf Bay 
Weeks Bay 
w
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 47
Figure 15:  Plot of pH values at three different depths along a north-south transect from 
 
 
 
 
 
6.5
7
7.5
8
8.5
9
0
5
00
1
00
0
12
5
0
17
5
0
2
25
0
2
75
0
3
50
0
45
0
0
5
50
0
Distance (meters)
pH
the mouth of Wolf Creek to the Intercoastal Waterway. The higher pH water is near the 
Intercoastal Waterway and the invasion of seawater into Wolf Bay creates a saltwater 
front.  
 
 
 
 
Surface
1 m
2 m
Wolf Creek 
Intercoastal  
Waterway 
 
 
 
 48
 
Figure 16:  Plot of conductivity values at the surface along a north-south transect from 
e mouth of Wolf Creek to the Intercoastal Waterway.  The increase in conductivity 
y. 
 
 
22000
24000
26000
28000
30000
32000
34000
36000
38000
40000
42000
0
50
0
1
00
0
12
5
0
17
50
2
25
0
27
5
0
35
00
45
00
5
50
0
Distance (meters)
C
ondu
c
t
iv
it
y
 (
u
S
/
c
m
)
th
indicates a high salinity front which is created by the intrusion of seawater into Wolf Ba
 
 
 
 
 
 
Surface
1 m
2 m
Wolf Creek 
Intercoastal  
Waterway 
 
 
 
 
 
Figure 17:  Contour map of surface (top) and 1-meter depth (bottom) showing pH levels 
in Wolf Bay.  
 
 49
 
 
 
 
 
 
Figure 18:  Contour maps of surface (top) and 1-meter depth (bottom) showing 
temperature (?C) levels in Wolf Bay.  Like the pH readings, higher temperature water 
can be found closer to the Intercoastal Waterway. 
 
 
 
 50
 
 
 
 
 
 
Figure 19:  Contour map of surface (top) a -meter depth of conductivity levels (in 
?S/cm) in Wolf Bay.  Similar to temperature and pH, higher conductivity readings 
can be found near the Intercoastal Waterway. 
 
nd 1
 51
 
 
Figure 20:  Contour map of (surface) and 1-meter depth of DO levels.  Th highest 
DO readings are located at the mouth of W lf Creek. 
 
 
 
 
e 
o
 52
 
 
Figure 21: C  of ORP.  Lowest 
ORP zones are found at the mouth of Wolf Creek. 
 
 
 
ontour map of surface (top) and 1-meter depth (bottom)
 53
Figure 22 shows the geochemical characteristics of sampled waters in Wolf 
Bay and Weeks Bay using a piper diagram.  The surface waters in Wolf Bay and 
Weeks  
) 
 
ks Bay contains high amounts of Na and 
HCO
3
-
  which indicates sodium bicarbonate type of groundwater.  The Na-HCO
3
 type 
high alk
Bay both contain high amounts of Na and Cl, similar to the characteristics of
seawater.  The Wolf Bay surface water has the same SO
4
/Cl ratios (average 0.052
with respect to that of seawater (~0.052), suggesting that dissolved SO
4
 in Wolf Bay 
has not been affected by bacterial sulfate reduction, a key process for initializing Hg
methylation. By contrast, very low SO
4
/Cl ratios are found in Weeks Bay where Hg 
methylation has occurred (Monrreal, 2007). 
 
The groundwater analyzed from Wee
alinity of the groundwater in Weeks Bay is most likely a result of the 
combination of dissolution of calcite and ion exchange (Marimuthu, 2005; Penny et 
al., 2005; and Monrreal, 2007).   
 
 
 
 
 
 
 
 
 
 54
 
 
 
 
Figure 22:  Piper diagram showing surface and groundwater compositions compared 
to that of seawater.  Surface waters resemble those of seawater (i.e., Na-Cl types).  
 
 
 
 
 55
 
Major ion analyses can also aid in providing more information on the physical 
ixing and biogeochemical reactions that take place in the bays.  Graphs were plotted 
to evalu
e 
een 
seawater and freshwater in Weeks Bay, while more non-conservative mixing can be 
found i ions 
m
ate the mixing of the waters and compared to each other (Figures 23-26).  In 
the plots, chloride, a conservative (non-reacting) species, is plotted on the x axis.  The 
other species of interest, which may or may not be conservative, is plotted on the y 
axis.  A straight line was drawn between the seawater and freshwater end-members 
and the behavior was based on the proximity of the data points to the line. If those 
data points lie close to or on the mixing line, that indicated the dissolved species 
exhibit a more conservative behavior. Data points that deviate significantly from th
conservative mixing line are considered non-conservative.  The species enrichment or 
depletion is in solution may be dependent upon biogeochemical processes such as 
mineral dissolution or precipitation, ion-exchange, or microbial processes.  
 
In all of the graphs, linear trends reveal the conservative mixing betw
n Wolf Bay.  Wolf Bay waters in general have higher major ion concentrat
with respect to those in Weeks bay because of less freshwater inputs.  The results of 
the graphical analyses indicate Na
+ +2
, Ca , Mg
+2
 generally exhibit conservative 
behavior during mixing in Weeks Bay and less conservative behavior in Wolf Bay. 
 
 
 
 56
 
 
0
5
10
15
20
25
30
35
40
45
4000 5000 6000 7000 8000 9000 10000 11000 12000
Chloride (ppm)
B
r
om
i
de
 (
ppm
)
Wolf Bay 
 
Figure 23:  Plot of chloride vs. bromide showing a more linear pattern between the 
water in Weeks Bay and less in Wolf Bay.  
 
 
 
 
 
 
 
 
 
Weeks Bay 
 
Weeks Bay 
Wolf Bay 
 57
 
 
 
0
50
100
150
200
250
300
4000 5000 6000 7000 8000 9000 10000 11000 12000
Chloride (ppm)
C
a
lc
iu
m
 (
p
p
m
)
 
Weeks Bay 
Wolf Bay 
 
Figure 24: Plot of chloride vs. calcium showing similar patterns between waters.  
 
 
 
 
 
 
 
 
 58
 
 
 
0
100
200
300
400
500
600
700
800
900
4000 5000 6000 7000 8000 9000 10000 11000 12000
Chloride (ppm)
M
a
gne
s
i
um
 (
ppm
)
 
Weeks Bay 
Wolf Bay 
 
Figure 25:  Plot of chloride vs. magnesium with a similar pattern between Weeks and 
Wolf Bay.  
 
 
 
 
 
 
 
 
 59
 
 
 
 
Figure 26: Plot of chloride vs. sodium showing the linear pattern of waters within 
Weeks Bay and the waters of Wolf Bay.  
 
0
1000
2000
3000
4000
5000
6000
7000
8000
4000 5000 6000 7000 8000 9000 10000 11000 12000
Chloride (ppm)
S
o
diu
m
 (
ppm
)
 
Weeks Bay 
Wolf Bay 
 
 
 
 
 
 
 
 60
 
 
 
0
200
400
600
800
1000
1200
1400
1600
5000 6000 7000 8000 9000 10000 11000 12000
Chloride (ppm)
SO
4
 (p
p
m
)
 Wolf Bay 
Weeks Bay 
 
Figure 27: Plot of chloride vs. SO
4
 showing the linear pattern of waters within Weeks 
and Wolf Bay. 
 
 
 
 
 
 
 
 
 61
Interestingly, sulfate exhibits non-conservative depletion in Weeks Bay (about 
0%) and more conservative in Wolf Bay (Figure 27).  Bacterial sulfate reduction in 
Weeks 
ry in the areas 
 of freshwater tributaries according to previous studies.  In the 
previou
 
noit 
t 
 
 
 
1
Bay may be the reason for this depletion but more microbiology research is 
needed to verify the biogeochemical reactions that remove sulfate.   
  
Estuary environments show the highest levels of methylmercu
near the mouths
s studies, the upper estuaries of freshwater/saltwater mixing zone contained 
low DO levels, low pH, and low salinity (Baeyens, 1998; Benoit, 1998; and 
Leermakers, et al., 2001).  These conditions are ideal for sulfate-reducing bacteria to
exist which have been shown to play a part in the methylation of mercury (Be
2001; King, 2002; and Monrreal, 2007).  These same desired geochemical conditions 
may exist within Weeks Bay near the mouth of the Fish River (Monrreal, 2007), bu
not in Wolf Bay where water salinity is too high.     
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 62
Table 10.  Oxygen and hydrogen isotope composition of groundwater, bay water, and 
ver water in Wolf and Weeks Bay watersheds.   
Name Source 
O, /
00
  
(SMOW) 
?D, /
00
   
(SMOW) 
ri
 
 
Sample Water ?
18 0 0
9 Transition Zone -2.1 -14.9 
011A 
011B 
W  
W
14 Weeks Bay -1.5 -10.1 
W  
W  
W  
W  
Tra
Tra
Sea Water 
eeks Bay
eeks Bay
-1.2 
-1 
-7.5 
-9.6 
18 Fish River -2.3 -11.9 
23 Fish River -2.2 
-
-15.5 
27 Fish River 2.7 -16.5 
28 Weeks Bay -2.3 -15.6 
33 Weeks Bay -1.8 -9.4 
38 Weeks Bay -1.6 -8.1 
41 Weeks Bay -1.6 -6.3 
41 Weeks Bay -1.5 -4.3 
W13 Groundwater -4.4 -23.1 
W14 Groundwater -4.1 -26.6 
W15 Groundwater -2.7 -17.3 
W16 Groundwater -4.4 -26.9 
-1.7 -6.7 WC-1 Wolf Creek  
-1.5 -9.7 WC-2 Wolf Creek  
WB10 Wolf Bay 0.3 -0.2 
WB12 Wolf Bay 0.3 -0.3 
WB14 Wolf Bay -1.6 -0.3 
WB16 Wolf Bay -1.6 -0.3 
WB19 Wolf Bay -0.4 -0.4 
WB21 Wolf Bay 0.8 -0.3 
WB23 Wolf Bay -1.1 -0.4 
WB26 Wolf Bay 1.8 -0.4 
WB28 Wolf Bay -0.5 -0.6 
WB30 nsition Zone -0.6 -0.5 
WB31 nsition Zone -0.9 -0.5 
  0 0 
 
 
 
 
 
 
 63
Stable Isotope Signatures of Stream Water, Bay Water, and Groundwater 
 and 
emonstrates the role that water mixing (i.e., with groundwater and seawater) and 
evapora
r 
n 
 
 
r? 
 by 
ulf 
 
Stable isotope analyses provides more details about the mixing of waters
d
tion play in influencing the chemistry of Wolf Bay and Weeks Bay surface 
water.  Comparing oxygen and hydrogen isotopes of surface water and groundwate
sampled sites along with seawater signature as well as the local meteoric water line 
shows how mixing and evaporation affect water chemistry in both bays (Figure 28).  
Deuterium (?D) and oxygen (?
18
O) isotope ratios of groundwater and surface water i
Wolf and Weeks Bay are plotted along with seawater and evaporation trajectory of 
the local meteoric water line (LMWL). Evaporation preferentially lifts lighter 
16
O and 
1
H isotopes from water to atmosphere. As evaporation occurs in surface waters, the 
remaining waters become enriched in heavy isotopic (
18 2
O and H) composition. 
Seawater is enriched with 
18 16
O and O with respect to surface meteoric water. Stable
isotope signatures of groundwater fall close to the local meteoric water line, 
indicating very little to no evaporation or mixing prior to infiltration from pore water 
in the unsaturated zone. In contrast, stable isotope profiles of river water show
enrichment of 
18 2
O and H, indicating they undergo greater evaporation than 
groundwater.  The Weeks Bay water represents a mixture of three ?end-membe
waters: one of seawater, one of river water, and one of groundwater impacted
variations in evaporation rates. Wolf Bay water plots closer to the seawater which 
represents greater evaporation rates and/or more influence of seawater from the G
of Mexico.  Isotopic signatures of Wolf Bay water also indicate less input from 
 64
surface (meteoric) with respect to Weeks Bay, which is consistent with its high 
electrical conductivity and salinity nature.  The nature and extent of hydrologic 
mixing and evaporation strongly influence water quality in both bays, which has
profound impacts on the potential of Hg methylation in bays.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 65
Seawater
mixing
evaporation
 
Figure 28:  Plot of evaporation trajectory of local meteoric water line (LMWL) and 
seawater mixing trend, shown using deuterium (?D) and oxygen (?
18
O) isotope ratios 
of groundwater and surface water in Wolf Bay. The Weeks Bay water represents a 
mixture of three ?end-member? waters: one of seawater, one of river water, and one 
of groundwater impacted by variations in evaporation rates. Wolf Bay water plots 
closer to the seawater which represents greater evaporation rates and/or more 
influence from the Gulf of Mexico. 
 
 
 
 
 
 
 
 
 
 
 
 
 66
SUBSURFACE GEOPHYSICAL SURVEY 
 
Electrical Resistivity  
 
Electrical resistivity surveys were performed on Dauphin Island at the Isle 
Dauphin golf course (Figure 29) and at Gulf State Park, located near Orange Beach 
(Figure 30). A 48-channel AGI SuperSting? Automated Resistivity Meter was used 
for these surveys.  A 2-D dipole-dipole array was used, with an electrode spacing of 
three meters along the survey transect. A roll-along technique was used to extend 
each transect to 450 meters at Dauphin Island and 450 meters at Gulf State Park. The 
transects were geo-referenced using a sub-meter accuracy Trimble GPS. 
  
Field data were processed using the EarthImager2D resistivity processing 
software.  Results were interpreted to estimate the depth to the saltwater interface or 
areas of saltwater intrusion. The measured apparent resistivity, calculated apparent 
resistivity, and the calculated true resistivity along the transect are shown in Figure 31 
for Dauphin Island and Figure 32 for Orange Beach.   
 
Resistivity of water may vary from 0.2 to over 1000 ? m depending on its 
ionic concentration and the amount of dissolved solids, and average seawater has a 
resistivity of 0.2 ? m (Nowroozi et al., 1999).  Resistivity of natural water and 
sediments without clay may vary from 1 to 100 ? m while the resistivity of a layer 
saturated by saline water and some dissolved solids is in the range of 8 to 50 ? m (De 
 67
Breuk and De Moor, 1969, Sabet, 1975, Goodell, 1986, Flanzenbaum, 1986, Zohdy et 
al., 1993, Nowroozi et al., 1999).  
 
In the Dauphin Island profile (Figure 31) resistivity ranged from 0 to over 
6000 ? m.  In the shallow subsurface the data yielded a wide range of resistivity 
readings. The higher readings probably reflect unsaturated unconsolidated sediments. 
The lower readings, especially in the 6-to 20-meter depth range, may indicate 
possible saltwater encroachment (indicated by arrows in Figure 31). These anomalies 
of low resistance may be the result of possible saltwater contamination within the 
shallow groundwater that percolated from the surface after the hurricane Katrina 
storm surge. The true freshwater-saltwater interface was not located. In order to 
detect the interface using this method, a longer transect will need to be performed to 
achieve a greater depth in the survey. These results imply that the depth of the 
saltwater wedge is greater than 50 meters. 
 
In the Orange Beach profile (Figure 32) resistivity ranged from 223 to 4444 ? 
m.  These readings appear to indicate the high resistance of unsaturated layers. The 
survey was conducted on a golf course with multiple groundwater wells that provide 
irrigation to the course and it was also conducted during a drought. With the 
groundwater withdrawal from the wells and little to no recharge, groundwater may 
have been at a greater depth. The depth of the freshwater-saltwater interface was 
greater than anticipated therefore further studies should be conducted employing a 
 68
longer transect in order to increase the depth of coverage and possible detection of the 
freshwater-saltwater interface.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 69
 
 
1 km
 
 
Figure 29: Location of geophysical survey at Dauphin Island, Alabama.  
 
 
 
 
 
 70
 
 
 
1 km
 
 
Figure 30: Location map of geophysical survey at Orange Beach, Alabama.  
 
 
 
 
 71
 
 
 
 
 
0 50 100 150 200 250 300 350 400 450 
Dauphin Island Electrical Resistivity Survey
Ohm-m 
S N 
 
 
Figure 31:  Results from electrical resistivity survey at Dauphin Island. The transect 
used an electrode spacing of 3 meters. Arrows are pointing to the low resistive 
anomalies in the survey.  The error limit of the data was 2.72 %, which is in the 
acceptable range as stated in Advanced Geosciences Inc. (2002).  The true resistivity 
was calculated using EarthImager 2D. (Profile location is located in Figure 29).  
 
 
 
 
 
 
 
 
 72
 
 
 
 
0 50 100 150 200 250 300 350 400 450 
Ohm-m 
Orange Beach Electrical Resistivity Survey
S 
N 
 
 
Figure 32: Results from electrical resistivity survey at Orange Beach. The transect 
used an electrode spacing of 3 meters. The error limit of the data was 2.74 %, which 
is in the acceptable range as stated in Advanced Geosciences Inc. (2002).  The true 
resistivity was calculated using EarthImager 2D. (Profile location is located in Figure 
30). 
 
 
 
 
 
 
 
 
 
 
 
 
 73
HYDROLOGIC MODELS 
 
 Surface Flow of Wolf Creek and Fish River  
 
To understand and compare the influx of freshwater into Wolf Bay and Weeks 
Bay, data collected by the United States Geological Survey (U.S.G.S.) was obtained 
to compare the surface water flow into the bays (Figure 33). The USGS WaterWatch 
website (http://waterwatch.usgs.gov/) provides real-time, short-term (hourly) changes 
in gaged rivers and streams. Figure 33 shows stream discharge (ft
3
/sec) computed at 
two USGS gage stations in 2010 at Wolf Creek (station ID 02378170), a major 
tributary stream of Wolf Bay, and Fish River (station ID 02378500), a major tributary 
of Weeks Bay.  The computed USGS data show that stream discharges are much 
higher (?100 ft
3
/sec) in Fish River than those of Wolf Creek (around 10 ft
3
/sec). The 
high freshwater inflow allows a low-salinity water body to lie on the top of the 
invading salinity seawater to form a wedge interface (Figure 15).  The mixing of 
warm, acidic, and low-salinity waters in the upper Weeks Bay (near the mouth of the 
Fish River) may provide favorable conditions for Hg methylation, as observed in the 
field.  In Wolf Bay, by contrast, less freshwater inflow results in high-salinity of 
water throughout the bay, which in turns prevents key microbial processes that 
initialize Hg methylation and bioaccumulation. The result suggests that Hg 
biotransformation is strongly influenced by hydrochemical conditions (i.e., mixing of 
freshwater and saltwater) in coastal watersheds. 
 
 
 
 74
      
             
0
20
40
60
80
100
120
140
160
3/
27
/
2
01
0
3/
29
/
20
1
0
3
/
3
1/
20
1
0
4
/
2/
20
1
0
4/
4/
20
1
0
4/
6/
2
0
10
4
/
8
/
2
0
10
4
/
1
0/
20
1
0
4
/
1
2/
2
01
0
4/
14
/
2
01
0
4/
16
/
2
0
10
4/
1
8/
20
1
0
4
/
2
0/
20
1
0
4
/
22
/
2
01
0
4/
24
/
2
0
10
4/
26
/
20
1
0
D
i
s
h
a
r
g
e
 (
c
u
b
ic
 f
e
e
t
 p
e
r
 s
e
c
o
n
d
)
 
 
 
 
 
0
2
4
6
8
10
12
14
16
3/
2
7
/
2
01
0
3
/2
9
/
2
01
0
3/
3
1
/2
0
1
0
4/
2
/
2
0
10
4/
4/
2
0
10
4
/
6
/
2
0
1
0
4
/
8
/
2
0
10
4/
1
0/
20
1
0
4
/
13
/
2
0
1
0
4
/
15
/
2
0
1
0
4
/
17
/
2
0
10
4
/1
9
/
2
01
0
4
/
21
/
2
01
0
4
/
24
/
2
01
0
D
i
s
ch
arg
e
 (
c
u
b
i
c
 f
e
et
 
p
er 
se
co
n
d
)
 
 
 
Figure 33: Hydrographs of Fish River (top) and Wolf Creek (bottom) showing 
discharge in cubic feet per second. (Data collected by U.S.G.S.) 
 75
Regional Groundwater Flow and Groundwater Residence Time 
 
Figure 34 shows the measured water table elevations (Chandler, 1985) and 
simulated groundwater flow patterns in southern Baldwin County.  Groundwater flow 
directions were drawn using SURFER software. Groundwater recharges at high 
elevations in the northern part of the study area near Loxley and migrates in the 
general direction of south toward the coastal areas.  Moderate hydraulic gradients (on 
the order of a few m/km) exist in shallow aquifers in the study area.  Although 
hydrologic properties (e.g., permeability) of the aquifers remain poorly unknown, 
available groundwater radioactive isotope data suggest moderate groundwater 
discharge rates of a few meters per year (see section below). 
  
A regional basin hydrology program Basin2 was used to simulate groundwater 
flow direction and also to calculate groundwater residence time by using the transport 
and decay of 
36
Cl through the aquifers.  The program uses a finite-difference grid 
which consists of nodal blocks that are arranged into columns and rows and cover a 
two-dimensional basin cross-sectional area.  Each nodal point contains the properties 
of each block.  Basin2 calculates a number of variables such as temperature, pressure, 
solute (Cl) concentration, and isotopic compositions at each nodal point.  The 
software is able to model the hydraulic characteristics and isotope transport capability 
of the aquifers. 
   
 76
            Basin2 requires the construction of a cross-section that represents the 
stratigraphy and lithology of the basin in order to simulate groundwater flow.  The 
data for the cross-section was entered in Basin2 in a column format which consisted 
of thickness and lithologic composition (see attached CD).  Thickness and 
composition of each unit was collected from previous stratigraphic data (Mooty, 
1988). Basin2 uses three default lithologies (sandstone, shale, and carbonate) that can 
be entered into the model.  
 
Groundwater flow and residence time in Baldwin County was modeled in 
two-dimension using a basin-scale groundwater flow model Basin2. The Basin2 
model calculates groundwater flow resulting from density variation, sediment 
compaction, topographic relief, and the transport of heat in the basin strata. The 
simulation considers the regional flow in response to topographic relief and water 
table variations cross southern Baldwin County from Bay Minette to Orange Beach. 
Subsurface data from Chandler et al. (1985) were used to reconstruct the 
hydrostratigraphy in the cross section (Figure 35).     
 
 
 
 
 
 
 
 77
 
 
 
 
45 
25 
0 
Contour Interval (m) 
 
 
Figure 34:  Equipotential map of southern Baldwin County, Alabama. Groundwater 
flows to the south towards the Gulf of Mexico. Also, notice how groundwater is 
flowing into Weeks and Wolf Bay.  
 
 
 
 
 
 
 
 78
 
The hydrostratigraphy units used in the model include sand, clay, and 
carbonate.  Permeability of sand, clay, and carbonate are set to 10, 10
-4
, and 10
-2
 
darcy, respectively.  The permeability of the sandy aquifers (Aquifer zones A1, A2, 
and A3) is adjusted to reflect the transport characteristics of these aquifers and the 
short groundwater residence time (Figure 35).  The variations (and uncertainty) on 
permeability of clay and carbonate have little effects on groundwater flow in sandy 
aquifers. The sides of the basin remain open to groundwater flow.  The bottom of the 
cross-section is set to be a no-flow boundary to reflect the low-permeability pre-
Oligocene basement rocks. The input file for simulation is shown in Appendix A.  
 
Simulation shows that groundwater migrates in the general direction to the 
south towards the Gulf of Mexico in all three major aquifer zones (Figure 35). The 
model also predicts that local groundwater flow system may exist in the shallow 
aquifer (i.e., Aquifer A2) where pronounced local topographic relief exists and causes 
undulations in the water table.  Such local and shallow groundwater flow system has 
recharge areas near a local topographic high spot and a discharge area at the adjacent 
topographic low.  The model predicted flow velocities are on the order of a few to 
tens of meters per year. The extension of the local flow system is limited to shallow 
depths (on the order of a few hundreds of meters) and the deeper A3 aquifer is 
dominated by regional flow system with more consistent north-to-south flow 
directions.  It is expected that groundwater residence time of deeper regional flow is 
much longer than those of shallow local flow systems. 
 79
 
In Aquifer A1, the velocity arrows show that the groundwater is discharging 
to the surface, near the shore of the Gulf of Mexico. Groundwater in the Aquifer A2 
flows to the south and then flow upwards into the A1 aquifer around the discharge 
area. Groundwater flow in the aquifer seems to be going either upward or downward 
where it encounters the clay unit. 
 
The A3 aquifer, the largest of the three aquifers, is dominated by a regional 
flow that is directly south and appears to discharge into the Gulf of Mexico. The 
Pensacola Clay which is located at the southern end of the base of the study area has 
an upward flow where it is narrowly overlying the carbonate sequence and a more 
southerly flow through the thicker part of the unit. The northern part of the study area 
at the base lies the carbonate unit which pinches out in the Pensacola Clay.  The flow 
through the carbonate has a direct southern flow.  
 
  The flow is driven by hydraulic gradients caused by topography and 
discharges into the Gulf of Mexico.  The A2 aquifer is an active flow system that has 
a calculated recharge rate of 40+/-16 cm/yr (from Dowling et al., 2004). The A3 
aquifer has a calculated horizontal velocity of 13+/-5 m/yr which suggests that 
substantial ground water discharge to the ocean is occurring in this aquifer. The flow 
rates with 
14 4
C and He groundwater ages (or residence time) of 375-7500 years (i.e., 
the time since recharge) along flow path of 30 to 70 km were calculated by Carey et 
al. (2004). 
 
 80
Comparing the models produced by Basin2 to previous studies conducted in 
the A3 aquifer, it seems that the calculated residence time is comparable to the 
isotope ages suggested by Dowling et al. (2004) and Carey et al. (2004).  Carey and 
others (2004) (Figure 36) estimated that the groundwater ages range from 730 years 
near the top of the aquifer, below the aquitard, to 6630 years found in the deeper A3 
aquifer near the coast above the seawater encroachment line.  The average age of 
waters sampled and analyzed in Carey?s model is 3033 years. As expected the deeper 
units have the oldest groundwater and the recharge area at the northern end of the 
model of all aquifers have younger waters in the Basin2 model (Figure 37). 
According to Carey et al. (2004), high 
14
C level near the recharge area represents the 
youngest groundwater with strong atmospheric signature; groundwater ages as it 
migrates away from the recharge area (Figure 38). The ages of water in the Basin2 
model appear to be in the less than 3000 - 6000 years range, with the older waters 
discharging into the Gulf of Mexico and younger waters closer to the recharge areas.  
             
 
 
 
 
 
 
 
 
 
 
 
 
 81
 
 
South North
Wolf Bay 
 
 
Figure 35: Calculated regional groundwater flow from north to south in Baldwin 
County. Arrows indicate groundwater flow direction in the aquifers. 
 
Orange Beach 
Bay Minette 
Aquifer A1 
Aquifer A2
Aquifer A3
Pensacola Clay Carbonates
 82
 
 
 
Figure 36: Cross-section of A2 and A3 aquifers in southern Baldwin County with 
circles representing the location and depth of samples with the corresponding ages of 
water from Carey et al. (2004) and Dowling et al. (2004).  
 
 
 
 83
 
 
 
Figure 37: Basin2 model showing calculated residence time (color map) in southern 
Baldwin County (see Figure 35 for plan view reference). 
 
 
 
 
 
 
 
 84
 
 
Figure 38: Plot of hydrologic residence time in the A3 aquifer vs. distance. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 85
CONCLUSIONS 
 
 
The mixing that is taking place within the Weeks Bay and Wolf Bay 
watersheds strongly impacts the water chemistry and also the methylation of mercury 
especially in Weeks Bay.  The mixing of seawater, freshwater, and groundwater in 
Weeks Bay provides desired geochemical conditions (i.e. warm, low pH and low 
salinity) that are necessary for methylation. Surface water in both bays are Na-Cl 
type. Wolf Bay has a higher salinity (closer to seawater) that prohibits the favorable 
conditions to promote methylation of mercury. The oxygen and hydrogen isotope data 
suggest that the chemistry and quality of surface waters of river, creeks, and bays are 
affected by evaporation, meteoric recharge, groundwater discharge, and mixing with 
seawater.  The stable isotope signatures of groundwater fall close to the LMWL, 
which indicate minimum evaporation prior to surface discharge.  Fish River water 
and Weeks Bay water show enrichment of 
18
O and 
2
H relative to groundwater, 
indicating that they undergo greater evaporation or mixing with isotopically heavier 
seawater. Wolf Bay water is enriched in heavy 
18 2
O and H with respect to those in 
Weeks Bay, this and its higher salinity reflects less freshwater inputs and greater 
proportion of seawater mixed in the bay. Geochemical analysis suggests that, most 
major ions (Na
+ 2+
, Ca , Mg
2+ - -2
, Br , etc.), with the exception of SO
4
, exhibit 
conservative behavior during water mixing in Weeks Bay but most exhibit a more 
non-conservative behavior in Wolf Bay.  Along with physical mixing and 
evaporation, biochemical processes such as sulfate reduction due to bacteria may be 
at work in the Weeks Bay watershed as indicated by non-conservative depletion of 
 86
-2
SO
4
.  It is known that bacterial sulfate reduction is the critical first step for Hg 
methylation and bioaccumulation.   
 
The intrusion of seawater into Wolf Bay and Weeks Bay creates a front of 
high salinity, high pH water that penetrates below low pH, low-conductivity 
freshwater.  Along this wedge interface, water at different depths exhibit difference in 
water chemistry. The higher discharge of low pH, low-salinity surface water (i.e., 
Fish River) into Weeks Bay may contribute vital conditions that promote the 
methylation of mercury. By contrast, salinity in Wolf Bay may be too high for the 
methylation to take place.  The most rapid changes in water chemistry occur at the 
mouth of Wolf Creek flowing into Wolf Bay and in Weeks Bay at the mouth of the 
Fish River.  The mixing of several waters of different chemical characteristics creates 
a favorable environment (i.e., the presence of higher temperature, low pH, and low-
salinity waters) where methylation may occur and cause the spread of mercury 
contamination throughout the watershed via bioaccumulation in Weeks Bay but not 
Wolf Bay.  Low DO and ORP values observed in this mixing zone suggest active 
microbial processes that are an important factor in initializing Hg methylation in 
Weeks Bay near the mouth of the Fish River, but Wolf Bay has higher DO values in 
this mixing zone. Our results imply that Hg biotransformation is strongly influenced 
by hydrochemical conditions in coastal watersheds. 
 
 
 87
The most likely source for mercury contamination found in the southern 
Baldwin County watersheds is from atmospheric deposition.  By examining the 
precipitation data from the USGS and mercury deposition data from the MDN show 
that an increase in precipitation results in an increase mercury deposition in the 
watersheds. 
 
The geophysical surveys conducted on Dauphin Island and Orange Beach 
yielded mixed results.  On Dauphin Island the electrical resistivity survey had 
readings that ranged from highly resistive (unsaturated layers) to low resistive, which 
possibly indicating saltwater in the shallow subsurface. The results from the survey 
conducted in Orange Beach indicated that the shallow subsurface is comprised of 
highly resistant unsaturated layers and not likely to be contaminated by shallow 
saltwater. The freshwater-saltwater interface in this area turned out to be deeper than 
anticpated.  A longer survey length would be needed in order to locate the depth of 
the freshwater-saltwater interface. 
 
Regional scale groundwater flow models were developed in a cross section 
extending from the northern recharge areas (near Bay Minette) to the Gulf Coast.  
The model predicted two flow regimes in major aquifer zones.  A local flow regime is 
present in Aquifer A2 due to local variations in topography and water table 
undulations.  In the deeper Aquifer A3, a regional flow regime dominates in which 
flow directions are more consistent (i.e., from north to south) and controlled by the 
net topographic slope.  Groundwater discharges southwards into the coastal estuaries 
 88
(e.g., Wolf and Weeks bays) and Gulf of Mexico. Calculated groundwater flow 
velocities in major aquifers range from a few to tens of meters per year.   The model 
calculated that groundwater residence time of major aquifers ranges from 0 near the 
recharge area to about 7000 years near the Gulf Coast along a 70 km flow path.  The 
calculated groundwater residence time is consistent with 
14 4
C and He ages measured 
by Carey et al. (2004).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 89
REFERENCES 
 
 
Advanced Geosciences Inc., 2002, Instruction manual for EarthImager 2D, version 
2.2.0, resistivity and IP inversion software: Austin, Texas, 134p.  
  
Alabama Water Watch, 2007, Citizen Volunteer Water Monitoring on Alabama?s 
Coast - Wolf Bay. 
 
Alley, W.M., 1999, Sustainability of Ground-Water Resources, USGS Circular 1186, 
79p. 
 
Alley, W.M., Healy, R.W., LaBaugh, J.W., Reilly, T.E., 2002, Flow and storage in  
       groundwater systems, Science Vol 296, number 5575, 1985-1990 pp. 
 
Al-Jahar, M., Davidson, S., Linhoff, B., Rempe, D., Sharp, J., 2007, Variations in the  
      freshwater/saltwater System on North Padre Island, Texas: free convection or   
     Ghyben-herzberg lens? Paper No. 174-21. 
 
Barlow, P.M., 2003, Groundwater in freshwater/saltwater environments of the 
Atlantic Coast, United States Geological Survey Circular 1262.  
 
Baeyens, W., C. Meuleman, B. Muhaya, and M. Leermakers, 1998, Behaviour and  
       Speciation of Mercury in the Scheldt Estuary (Water, Sediments and Benthic  
       Organisms): Hydrobiologia, v. 366, p. 63-79. 
 
Bethke, C.M., Lee, M.-K., and Park, J., 2003, A Guide to Use Basin2 Software 
Package, Release 5.0, 
http://www.geology.uiuc.edu/~bethke/pdf/Basin2UsersGuide.pdf , 20 p. 
 
Carey, A.E., Dowling, C.B., and Poreda, R.J., 2004, Alabama Gulf Coast 
groundwaters: 4He and 14C as groundwater dating tools, Geology, 32, p. 289-
292. 
 
Celo, V., D. R. S. Lean, and S. L. Scott, 2006, Abiotic Methylation of Mercury in the  
         Aquatic Environment: Science of the Total Environment, v. 368, p. 126-137.  
 
Chandler, R.V., Gillett, B., and DeJarnette, S., 1996, Hydrogeologic and water-use 
data for southern Baldwin County, Alabama, Geological Survey of Alabama 
Circular 199, 124 pp. 
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