Risk-Based TMDLs in Pathogen-Impaired Waters

A risk-management approach is proposed for estimating total maximum daily load (TMDL) load reductions in pathogen-impaired streams. The approach is based on reducing the observed alpha-percentile concentration to the alpha-percentile water-quality standard with a beta-percentile level of confidence. The required reduction is determined using an explicit policy decision of acceptable risk. A review of the methodologies currently being used to determine TMDL load reductions in eight southeastern states shows a lack of uniformity in TMDL methodology and the use of either unknown implicit or arbitrary explicit margins of safety. Application of the proposed approach to these cases demonstrates that the confidence of compliance of the adopted TMDL load reductions are mostly less than 50%. This indicates that the adopted TMDL load reductions would likely result in not meeting applicable water-quality standards.