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Abstract
Septic systems, also known as onsite wastewater treatment systems (OWTS), are used widely across the southeast of U.S. As widely as these systems are used in the southeast and other parts of the U.S., their impact on microbial water quality has not been elucidated to allow for their proper management at the watershed level. The overall goal of this study was to isolate septic system impact at the watershed level by using multiple approaches including targeted monitoring of fecal indicator bacteria (FIB) and microbial source tracking (MST) markers as well as watershed scale modeling. Twenty four urbanizing watersheds impacted by a gradient of septic system density and land use characteristics were monitored over a three year period for water quality parameters including the FIB E. coli and enterococci, human-specific Bacteroidales genetic markers and standard water quality parameters (pH, dissolved oxygen, temperature, specific conductance). Septic system influence on fecal bacteria loads at the watershed level was also modeled with the soil and water assessment tool (SWAT). Correlation and multivariable regression analysis indicate that septic systems, specifically the density of septic systems and the proximity of septic systems to streams, were significant drivers of fecal pollution in urbanizing watersheds of metropolitan Atlanta. The influence of septic systems was seasonally dependent with the strongest impact observed in spring season. Analysis of the human-associated marker showed strong negative correlation (r = -0.64) to the proximity of septic systems to streams during the spring season. Additionally, the human marker was significantly higher in high density watersheds compared to low density areas overall. SWAT model results show septic system influence as a result of the proximity of septic systems to local streams, with the most significant influence observed when septic systems are less than 10 m from nearby streams. This study provides tools that can be used at the watershed level to understand the impact of septic systems on microbial water quality. The study findings can be used to support decisions regarding septic system management to protect water resources.