The Impact of On-Site Wastewater Treatment Systems on the Nitrogen Load and Baseflow in Urbanizing Watersheds of Metropolitan Atlanta, Georgia

On-site wastewater treatment systems (OWTSs) are widely used in the Southeastern United States for municipal wastewater treatment. As urban and suburban populations increase, the use of OWTSs is expected to further increase. This region heavily depends on surface waters for its water supply, therefore, the impact of OWTSs on surface water quality and quantity must be investigated. Conventional OWTSs can be potential sources of N pollution for groundwater and streams that can cause human health concerns and stimulate algal growth resulting in eutrophication. The overall goal of this project is to determine the impact of OWTSs on the N load and baseflow in urbanizing watersheds of Ocmulgee and Oconee River basins in Georgia. This paper presents preliminary results of the differences in the N load and baseflow as well as other water quality indicators such as electrical conductivity (EC) and chloride (Cl) in streams of watersheds impacted by high (HD) and low density (LD) OWTSs. Synoptic samples and discharge measurements of 24 watersheds were taken 3 times per year in fall, spring, and summer under baseflow conditions. EC and Cl concentrations were significantly higher in HD OWTS watersheds for all three sampling events. N concentrations were not statistically different between HD and LD watersheds for all three sampling events. Baseflow measurements in the fall and spring were not statistically different between HD and LD watersheds, but summer measurements were significantly higher in the HD watersheds. The results indicate the presence of OWTS effluent in streams of watersheds with HD OWTSs, while N analysis indicates a reduction in concentration through dilution and denitrification. However, increased baseflow in watersheds impacted by HD OWTSs results in an increase in total N load. Further analysis is needed to accurately determine and quantify the impact of OWTSs on water quality and quantity at the watershed-scale.