Onsite Wastewater Treatments Systems (OWTS) are widely used for wastewater treatment in the Southeast, with higher percent use (35%) than the national average (25%). However, their impact on the water resource in this region is poorly understood. This poster will present results from a study that is examining the impact of OWTS on water quality in urbanizing watersheds of Ocmulgee and Oconee River basins in Georgia. Our study area includes 24 well characterized watersheds with high or low density OWTS in the Metropolitan Atlanta area. Streams in the watersheds have been monitored since 2011 periodically under base- and storm-flow conditions and tested for water quality indicators. While the streams had similar pH, dissolved oxygen and temperature, other water quality parameters such as specific conductance, chloride, E. coli and enterococci stream loads were higher in watersheds with high than low density of OWTS during the relatively dry season of the year. There seems to be a strong and positive correlation between OWTS density and fecal indicator bacteria (FIB) concentrations above 100 OWTS per square km. FIB were 10 to 100 fold higher under storm than base-flow conditions in some streams of both groups of watersheds, potentially indicating the presence of other sources of contamination apart from OWTS. Overall, OWTS showed seasonal and temporal variations in their impact on water quality in the monitored watersheds.