Comparisons of Methods to Calculate TMDLs for E. Coli: Load Duration Curves, Mass Balance, and HSPF

Total maximum daily load (TMDL) studies determine the maximum amount of a pollutant that a body of water can assimilate while still meeting water quality standards. TMDL studies generally follow a standard procedure—identification of sources, data gathering and monitoring if needed, and use of a water quality model to determine the TMDL. This paper presents findings of a study being conducted on Buffalo Bayou, an urban bayou in Houston, Texas. Identified potential sources of bacteria include point sources — such as wastewater effluent discharges, sanitary sewer overflows, and dry/wet weather municipal storm sewer system (MS4) discharges — and nonpoint sources such as, wildlife, stream sediment, and failing septic tanks. This study has developed three separate methods of identifying TMDL end points: load duration curves (LDCs), a mass balance method using a tool called Bacteria Loading Estimator Spreadsheet Tool (BLEST), and a spatially and temporally variable Hydrologic Simulation Program — Fortran (HSPF) model.