MODELING BACTERIA FATE AND TRANSPORT IN WATERSHEDS TO SUPPORT TMDLS

Abstract

Fecal contamination of surface waters is a critical water-quality issue, leading to human illnesses and deaths.Total Maximum Daily Loads (TMDLs), which set pollutant limits, are being developed to address fecal bacteria impairments.Watershed models are widely used to support TMDLs, although their use for simulating in-stream fecal bacteriaconcentrations is somewhat rudimentary. This article provides an overview of fecal microorganism fate and transport withinwatersheds, describes current watershed models used to simulate microbial transport, and presents case studiesdemonstrating model use. Bacterial modeling capabilities and limitations for setting TMDL limits are described for twowidely used watershed models (HSPF and SWAT) and for the load-duration method. Both HSPF and SWAT permit the userto discretize a watershed spatially and bacteria loads temporally. However, the options and flexibilities are limited. Themodels are also limited in their ability to describe bacterial life cycles and in their ability to adequately simulate bacteriaconcentrations during extreme climatic conditions. The load-duration method for developing TMDLs provides a goodrepresentation of overall water quality and needed water quality improvement, but intra-watershed contributions must bedetermined through supplemental sampling or through subsequent modeling that relates land use and hydrologic responseto bacterial concentrations. Identified research needs include improved bacteria source characterization procedures, datato support such procedures, and modeling advances including better representation of bacteria life cycles, inclusion of moreappropriate fate and transport processes, improved simulation of catastrophic conditions, and creation of a decision supporttool to aid users in selecting an appropriate model or method for TMDL development.