Modeling the influence of septic systems on fecal bacteria load in a suburban watershed in Georgia (GWRC 2017)
Citation:
Sowah, R., M. Habteselassie, D. Radcliffe, AND M. Molina. Modeling the influence of septic systems on fecal bacteria load in a suburban watershed in Georgia (GWRC 2017). In Proceedings, 2017 Georgia Water Resources Conference, Athens, GA, April 19 - 20, 2017. Georgia Institute of Technology, Atlanta, GA, 1-5, (2017).
Impact/Purpose:
Recent studies by the present authors suggest the impact of septic systems on microbial water quality of streams (Sowah et al., 2014; Sowah et al., 2016). These studies show that FIB loads in streams can be partly explained by the density of septic systems and the average distance of septic systems to streams in the watersheds. Understanding septic system impacts at the watershed level would require multiple approaches including watershed scale modeling. Watershed scale models such as SWAT have showed promise as a predictive tool for studying the impacts of septic systems on water quality and quantity (Oliver et al., 2014b; McCray et al., 2009). The goal of this study was to evaluate the water quality impact of septic systems on bacterial loads using the SWAT watershed scale model.
Description:
Watershed scale models such as the soil and water assessment tool (SWAT) are promising tools for studying the impacts of septic systems on water quality and quantity. In this study, SWAT was used to assess the influence of septic systems on bacterial loads in a suburban watershed. The model was first calibrated and validated for flow in the Big Haynes Creek watershed located in Gwinnett County, GA. The model performed satisfactorily for flow predictions at the watershed level with Nash Sutcliffe Efficiency (NSE) values of 0.67 and 0.70 for calibration and validation periods, respectively. Flow predictions in the study area shows that on average septic systems contributed approximately 6% to the total water yield annually. This observation is significant and contradicts suggestions that septic systems are 100% consumptive use. Although NSE values for bacterial predictions were low in this study, the results in general compares with previous SWAT bacterial studies and reflects the uncertainties involved in bacterial modeling at the watershed scale. Model results suggest that failing septic systems were not a significant source of fecal bacteria in streams. In contrast, bacterial output was sensitive to instream deposition from livestock including cattle in the watershed. Further assessment of alternative pathways for effluent transport from failing septic systems into streams is necessary to fully understand septic influence at the watershed level
