DEVELOPMENT OF ALGORITHMS FOR MODELING ONSITE WASTEWATER SYSTEMS WITHIN SWAT
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Onsite wastewater systems (OWSs) are a significant source of nonpoint-source pollution to surface and groundwater in both rural and suburban settings. Methods to quantify their effect are therefore important. The mechanics of OWS biogeochemical processes are well studied. However, tools for their assessment, especially at the watershed scale, are limited. As part of this work, modeling capabilities were developed within the Soil Water Assessment Tool (SWAT) such that OWSs and their subsequent environmental impacts can be evaluated A case study was initiated on the Hoods Creek watershed in North Carolina to test the new SWAT algorithms. Included were: (1) field-scale simulations of groundwater quantity (water table height) and quality (N, P), (2) Monte Carlo evaluations of OWS service life to evaluate suggested calibration parameters, and (3) assessments of watershed-scale pollutant loadings within the model. Results were then analyzed at both the field and watershed scales. The model performed well in predicting both site groundwater table levels (R 2 = 0.82 and PBIAS = -0.8%) and NO3-N concentration in the groundwater (R 2 = 0.76, PBIAS = 2.5%). However, the performance for PO4-P simulations was less reliable due to difficulty in representing the mobility of soluble P in the soil. An advanced P algorithm is recommended to address the sophisticated physiochemical properties of soil particles and improve the model's performance. 2011 American Society of Agricultural and Biological Engineers ISSN 2151-0032.
