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QUANTIFYING NITROGEN EXPORTS FROM ON-SITE SYSTEMS TO SURFACE WATERS WITHIN SMALL WATERSHEDS USING THE SWAT MODEL

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Watershed Management to Meet Water Quality Standards and Emerging TMDL (Total Maximum Daily Load) Proceedings of the Third Conference 5-9 March 2005 (Atlanta, Georgia USA) Publication Date 5 March 2005  701P0105.(doi:10.13031/2013.18098)
Authors:   S. Pradhan, M. T. Hoover, H. A. Devine, J. G. Arnold3 and M. Di Luzio
Keywords:   SWAT model, TMDL, on-site systems, nitrogen export, and watershed

The impact of on-site systems (septic systems) on the quality of groundwater and surface water is relatively unknown. However, on-site systems can potentially be a substantial contributor of nitrogen to surface waters. Quantifying nitrogen export from septic systems to adjacent surface waters is a challenging task due to limitations of existing small watershed scale hydrologic models for tracking the fate and transport of nitrogen derived from on-site systems. The SWAT (Soil and Water Assessment Tool) model is frequently used in TMDL assessments. However, it has not been modified to track the fate and transport of septic system derived nitrogen. Therefore, the SWAT model was calibrated in a small watershed (179 hectares) where nitrogen exports from septic systems to surface waters were known from previous field research. Considering the small size of the watershed, high resolution DEM derived from LIDAR (Light Detection and Ranging) data was used to represent detailed topography of the watershed. The percentage of nitrogen exported to the sub-basins outlets (sub-basin 4 and 7) calculated from the model simulation matched with field data quite well. The calibrated model was used throughout the entire Hoods Creek watershed to predict the effect of development and of increasing housing density upon cumulative nitrogen exports from on-site systems and its delivery to surface water at the watershed outlet. For current development, over 95% of the estimated yearly input of 2,576 kg of septic systems derived nitrogen was removed within the watershed prior to its discharge into the Trent River at the watershed outlet. However, potential increases in development density would lead to substantial exports of nitrogen to the surface water. This study provides initial information about the use of SWAT model to quantify fate and transport of nitrogen derived from septic systems in small watersheds.

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