Modeling the Effect of Failing on-Site Wastewater Treatment Systems on Watershed Nitrate Yield with SWAT.

Onsite Wastewater Treatment Systems (OWTS) can be a source of nitrogen pollution of surface and groundwater as a result of failing or high density systems. In Georgia 1.7% OWTS failure was reported by EPA (2002). However, the contribution of failing OWTS to nitrogen load at the watershed-scale is unknown. We used the Soil Water Assessment Tool (SWAT) model to simulate nitrogen load to a stream in three scenarios; no OWTS (just added nitrogen fertilizers), OWTS with no failing systems, and OWTS with failing systems. A 44 km² watershed with about 67% residential land use was selected in metropolitan Atlanta, Georgia. The precipitation and temperature data for this area was obtained through the PRISM (Parameter-elevation Regression on Independent Slopes Model) climate and weather system. In SWAT 2012 a biozone algorithm model was integrated to simulate the effect of OWTS on nutrients load. In this model an active system becomes a failing system as a result of biozone clogging or hydraulic failure which causes surface ponding of OWTS effluent in a drainfield. For simulating failing OWTS, it was assumed that installed OWTS in soil hydrologic group D (about 1% in the studied watershed) are more susceptible to failure. The model was run from 1998 to 2007 with 4 years warm up and a daily time step. Failing OWTS caused nitrate yield to the stream from surface runoff to increase by 2.3% and from groundwater to decrease by less than 1% in comparison with all active OWTS. The calibrated model for stream flow showed a good fit with a daily Nash-Sutcliffe coefficient of 0.63 and 0.65 for active and failing OWTS, respectively.