The impact of on-site wastewater treatment systems (OWTSs) on the N load and baseflow in streams of urbanizing watersheds was investigated. Synoptic samples and baseflow measurements of streams affected by low (LDS) and high density (HDS) OWTSs were taken four times from 2011 to 2012. Results suggested an increase in baseflow in HDS watersheds which may off-set effects of development and maintain baseflow under drought conditions but also indicated a positive correlation between nitrate-N concentrations and OWTS density within the watershed. The Soil and Water Assessment Tool (SWAT) was calibrated to predict stream discharge in a gauged watershed of Gwinnett County, Georgia and used to quantify the influence of OWTSs on water quantity. Analysis showed a 5.9% increase in water yield due to the addition of OWTSs. Results provided data that may be used to inform users and watershed planners about the influence of OWTSs on water quality and quantity.