Modeling Bacteria Fate and Transport in Watersheds to Support TMDLs

An overview of the sources, fate, and transport of bacteria in the environment is presented. The models currently used to develop bacterial impairment TMDLs is discussed. Bacterial modeling capabilities and limitations for setting TMDL limits are described for two widely used watershed models (HSPF and SWAT) and for the load-duration method. Both HSPF and SWAT permit the user to discretize a watershed spatially and bacteria loads temporally. The models are also limited in their ability to describe bacterial life cycles and in their ability to adequately simulate bacterial concentrations during extreme climatic conditions. The load-duration method for developing TMDLs provides a good representation of overall water quality and needed water quality improvement, but intra-watershed contributions must be determined through supplemental sampling or through subsequent modeling that relates land use and hydrologic response to bacterial concentrations.