Modeling the Influence of Septic Systems on Fecal Bacteria Load in an Urbanizing Watershed in Georgia.

Effective management of fecal pollution at the watershed level is predicated on reliable identification of potential pollution sources. Modeling approaches have proved to be invaluable to efforts aimed at not just identifying pollution sources, but also understanding the complexities of non-point source pollution.  The soil and water assessment tool (SWAT) model have showed promise as a microbial fate and transport modeling tool for assessing the influence of non-point sources including septic systems on bacterial loads in rural and urban watersheds.  In this study, we used the SWAT model to examine the fate and transport of fecal bacteria in an urbanizing watershed impacted by high septic density, agricultural and wildlife activities. The watershed (45 km^-2 in area) was calibrated and validated for flow and bacteria using data from a USGS gage station. Model performance was good with a Nash Sutcliffe efficiency >0.67 for flow. Uncertainty analysis shows that sediment, bacteria and septic system parameters influence bacteria numbers in streams. These parameters include the channel cover factor (CH_COV), Linear parameter for calculating the channel sediment routing (SPCON), bacteria soil partitioning coefficient (BACTKDQ), bacteria partition coefficient (BACTKDDB), fecal coliform bacteria decay rate coefficient (COEFF_FECAL) and density of septic systems (SEP_DEN). The results suggest the influence of septic system density on flow and bacteria load in streams and provide the tools for assessment of septic impacts at the watershed level.