Water Quality Impact of Decentralized Onsite Wastewater Treatment Systems: Case Study of Urbanizing Watersheds in Metropolitan Atlanta, Georgia.

Decentralized onsite wastewater treatment systems are an integral part of the wastewater infrastructure in the United States. Thus the effectiveness of these systems in removing contaminants from wastewater cannot be overemphasized. At the watershed scale, the impact of decentralized systems as a prominent source of fecal pollution to groundwater and surface waters have not been elucidated to allow for the inclusion of onsite wastewater treatment systems in watershed management programs. Water quality monitoring can provide the tools needed to understand the spatial and temporal dynamics of bacterial loads as impacted by onsite wastewater systems. Our study assessed the influence of decentralized wastewater treatment systems on the quality of water in streams in watersheds impacted by high or low density of septic systems. The seasonal and temporal distribution of fecal bacterial loads (in both water and sediment) was evaluated and the correlation with rainfall patterns and standard water quality parameters such as pH, dissolved oxygen, specific conductance and temperature were examined across 24 well characterized watersheds ranging in area from 0.07 to 3.4 square miles. The selected watersheds are in the Ocmulgee and Oconee River basins in the Southern Appalachians region – a sensitive ecological zone with 65% of rivers and streams in poor condition (NRSA Report 2013). Our data suggests that areas of high density septic systems significantly impact microbial water quality as indicated by higher E. coli and enterococci loads in stream water. Bacterial loading was also seasonally dependent, consistent with baseflow yield and septic system density. Ongoing work entails fecal source tracking and watershed modeling to complement routine indicator bacteria monitoring.