Bacteria TMDLs: An Implementation Planning Effort Predicting Costs, Benefits, And Uncertainties During Development Of A Comprehensive Load Reduction Plan For Compliance With San Diego County Bacteria Tmdls

As part of compliance activities that are responsive to the Total Maximum Daily Load (TMDL) for Fecal Indicator Bacteria, local agencies in two San Diego area watersheds prepared Comprehensive Load Reduction Plans (CLRP). The objective of the CLRPs was to identify activities that, if implemented, could conform to stipulated waste load allocations. The CLRPs address multiple impairments and pollutants of concern which allows for implementation of activities over a 20-year compliance schedule.In order to identify and schedule the activities expected to support TMDL compliance efforts, the plans describe and interpret (and numerically model) interim and ultimate regulatory requirements; specifically the metrics upon which compliance is anticipated to be determined. The implementation strategies include a suite of nonstructural and structural (source and treatment control) BMPs, and in this context examine allowable annual exceedance frequencies and allowable exceedance days, beyond which discharges exceeding fecal indicator bacteria criteria constitute violations.To predict effectiveness and uncertainties of implementation activities, a robust wet weather computer model was developed with the ability to simulate hydrologic and pollutant loadings (with locally and regionally developed data). The wet weather model adopted was the San Luis Rey Structural BMP Prioritization and Analysis Tool (SBPAT), a GIS-based and EPA SWMM-based water quality model modified to reflect local Event Mean Concentrations, local runoff characteristics, and current BMP performance and effluent information from the International BMP Database.Using the water quality model and TMDL compliance requirements, estimates were made of the bacteria load reductions predicted to achieve compliance, given the aforementioned exceedance day criteria.