As droughts persist in the Southeastern US, the extent to which water use by on-site wastewater treatment systems (OWTS) represents consumptive use (defined here as water that does not return to streams) has been debated. The contribution of OWTS to nitrogen in streams is also unknown. To answer these questions, we used the Soil Water Assessment Tool (SWAT) model to simulate stream flow and nitrogen transport in a 44-km² watershed in metropolitan Atlanta with 66% low or medium density urban landuse. There were 162 OWTS/km². Model predictions were compared to measured stream flow and nitrogen concentrations from a USGS gage station during the period 2003-2010. Daily and monthly Nash-Sutcliffe coefficients indicated a good fit for the calibration period 2003-2006 (0.49 and 0.71) and validation period 2007-2010 (0.37 and 0.68) for stream flow. OWTS increased stream flow by 3.1% in the entire watershed and by 5.9 % in a high density subbasin. The effect of OWTS in increasing stream flow was greatest during years of low rainfall. Only 5.2% of the OWTS water use was consumptive (lost to ET and deep aquifer recharge). The nitrogen simulations are still being developed.