On-Site Wastewater Treatment Systems Effect on Watershed Nitrogen Load: SWAT Model Predictions.

The extent to which on-site wastewater treatment systems (OWTS) contribute to watershed-scale nitrogen loads is not well known. To answer this question, we used the Soil Water Assessment Tool (SWAT) model to simulate stream flow and nitrogen dynamics in a 44-km² watershed in metropolitan Atlanta with 66% low or medium density urban landuse. SWAT 2012 contains a new OWTS algorithm that simulates the effect of these systems on stream flow and nitrogen concentrations. Nitrogen added by lawn fertilizers and pastures amended with broiler litter was included. Averaged over the watershed, there were 86 OWTS per km². Model predictions were calibrated using measured daily stream flow and 47 nitrate concentrations (measured under baseflow conditions for the most part) from a USGS gage station from the beginning of 2003 to the end of 2006. The daily Nash-Sutcliffe (NS) coefficient indicated a good fit for stream flow (0.46). Nitrate concentrations were more difficult to predict (NS = -0.02) but were close the average concentration of about 1 mg/L. The calibrated model was run without OWTS to determine their contribution to the total N load at the watershed outlet. OWTS accounted for 37% of the total N load.