Literature review: Literature Review-boizone.pdf

Research scientists: Santhi Chnnasamy¹, Jaehak Jeong¹, Raghavan Srinivasan²

1: Texas AgriLife Research
2: Texas A&M University

Use of on-site septic tank system is increasing every year by 33% with new residential development. They are considered to cause significant non-point source pollution to water bodies especially in rural areas and suburban areas. Quantifying their effects on water quality is important (McCray et al., 2005). On-site septic tank systems discharge septic tank effluent to the soil wherein percolation through an unsaturated zone provides treatment of many pollutants. Remaining constituents are eventually transported via ground water into surface waters. Watershed scale modeling tool can serve as a useful tool for tracking the fate of on-site septic tank systems discharged through soil to the receiving waters of a river basin. Studies conducted at Colorado School of Mines have indicated that a biozone is developed in the soil matrix due to delivery of septic tank effluent (McCray et al., 2005; Heatwole et al., 2007). This biozone has impact on hydrologic properties and transport and discharge of pollutants to receiving watersheds. Currently algorithms are being developed within SWAT model to represent processes taking place in biozone. It will describe the nutrient movement through subsurface soil zones to ground water and surface water and test the impacts of effluents delivered from conventional, advanced and failing septic tank systems on downstream water quality in a large watershed. A user-friendly GIS-interface will be developed so that it can reach the broad audience throughout the United States and other parts of the world. Once the capability is developed, it is expected to have significant impacts in terms of predicting water quality of the receiving waters due to the combined effect of all point and nonpoint source loads, including septic tanks. It will be useful for TMDL analysis and cost and benefit analysis of onsite-systems versus centralized systems.

Task 1.  Conduct literature review on biozone processes at OWS, fate and transport of nutrients from OWS through subsurface soil zones, and water quality impacts of OWS on ground water and surface water systems.  

Task 2.  Develop biozone algorithm that accounts for the growth and build up of bacteria biomass at the infiltrative surface, the decay of biomass and transformation into plaque, the impacts on field capacity and hydraulic conductivity, the transformation of nitrogen and phosphorus constituents within the biozone layer, and the transport of these constituents from the biozone to underlying soil strata. 

 
Task 3.  Incorporate biozone algorithm into the 2005 SWAT program by developing the necessary links with nutrient, soil moisture, ground water and surface water subroutines currently in the model. 

Task 4.  Conduct database search throughout the United States to identify a perspective watershed that would be best suited for model testing and calibration of the proposed biozone algorithm.  Compile and organize all supporting GIS data layers required for watershed modeling using AVSWAT-X for the selected test watershed. Construct test watershed project within AVSWAT-X.  Prepare report that summarizes the steps taken to select the test watershed, describes the watershed history and background, and outlines model construction and set up within AVSWAT-X.   

Task 5.  Employ automated and manual approaches within SWAT to calibrate the hydrologic components of the test watershed constructed in Task 4.  Calibrate the revised version of SWAT 2005 developed in Task 3 to simulate the fate and transport of N and P on the selected test watershed.  Refine or modify the biozone algorithm or the links with soil moisture or nutrient cycling subroutines within SWAT as necessary to best represent known conditions on the test watershed.  Assist DEQ in validating revised version of SWAT 2005 on watersheds within Montana.  Prepare report that documents findings from model simulations with the revised version of SWAT.

Task 6.   Develop GIS interface within AVSWAT-X that provides practitioners with an ArcView formatted approach to assessing OWS on nitrogen and phosphorus. 

 
Task 7.  Assemble documentation that describes biozone algorithm development, the associated biozone algorithm links within the SWAT 2005 program, and the GIS graphical user interface within AVSWAT-X, following the same format as the existing 2005 Soil and Water Assessment Tool Theoretical Documentation Manual, 2000 ArcView Interface Manual, and 2005 Input/Output File User’s Manual developed by USDA ARS