Predicting Nutrient Loadings and Fate and Transport of Nitrogen Derived from On-site Systems.

Abstract

The potential nitrogen loadings from on-site wastewater treatment systems (septic systems) to North Carolina's river basins have long been ignored. Yet the potential for these systems to have significant impact on surface water exists. This study assessed nitrogen inputs from septic systems on macro (large watershed) and micro (small watershed) scales. Cumulative potential nitrogen loadings from on-site systems to North Carolina's 17 river basins and 134 major sub-basins were estimated using census 1990 data. The soil and water assessment tool (SWAT) model was used to predict the fate and transport nitrogen derived from on-site systems as well as nitrogen exports to surface waters in small watersheds. The sensitivity of the SWAT model output to sources and resolutions of Digital Elevation Model (DEM) inputs on a micro scale was assessed for small watersheds.

A Geographic Information System (GIS) based area-driven normalization procedure was developed and implemented to estimate potential nitrogen loadings on the macro scale. Septic system density ranged from 5 to 20 systems/sq. km at a river basin level and 2 to 115 at a sub-basin level. Total potential nitrogen loading from on-site systems was approximately 14 million kg statewide.

A small watershed in the lower Coastal Plain was modeled using SWAT to quantify the nitrogen export to the watershed outlet on a micro scale. Over 95% of the septic system derived nitrogen was removed prior to stream discharge. Both the source and resolution of the DEM inputs affected the size of the watershed delineated, flow volume and predicted nitrogen export to the watershed outlet.