Hydraulic Management of SDI Wastewater Dispersal in an Alabama Black Belt Soil
Type of Degreedissertation
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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 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.