Reasonable Assurance Analysis Approach for Compliance with the Santa Monica Bay Beaches Bacteria TMDL
The 2012 Los Angeles County Municipal Separate Storm Sewer System (MS4) Permit (Order No. R4-2012-0175) (Permit) became effective December 28, 2012. The Permit was created for the purpose of protecting the beneficial uses in the receiving waters in the Los Angeles region by ensuring that MS4s in the County of Los Angeles are not causing or contributing to exceedances of applicable water quality objectives. The Permit allows the permittees to customize their stormwater programs through the development and implementation of an Enhanced Watershed Management Program (EWMP) to achieve compliance with certain receiving water limitations and water quality based effluent limits (WQBELs). Agencies within the Santa Monica Bay (SMB) Watersheds agreed to collaborate on the development of four EWMPs separated by geographical area, extending from Malibu in the north to the Palos Verdes Peninsula in the south. The Permit requires that a Reasonable Assurance Analysis (RAA) be conducted for the water body-pollutant combinations (WBPCs) addressed by each EWMP.
The RAA involves the identification and evaluation of potential best management practice (BMP) implementation scenarios. The RAA must demonstrate achievement of Permit limits (including TMDL-based WQBELs) for each applicable WBPC.
The recommended RAA approach for the SMB Watersheds leverages the strengths of the publicly available, Permit-approved, GIS-based model that has already been developed for the region: the Structural BMP Prioritization and Analysis Tool (SBPAT). The decision to use SBPAT over other Permit-approved models (LSPC/WMMS and SUSTAIN) was based on the following factors: modeling of Santa Monica Bay hydrologic and watershed processes, the compliance metric expression associated with bacteria in Santa Monica Bay, availability of new open space water quality loading data, capability to conduct opportunity and constraints investigations, and characterization of water quality variability.
This presentation will address the complex challenges and opportunities associated with modeling fecal indicator bacteria in the SMB Watersheds, including model calibration, prediction of the wet weather exceedance-day based compliance metric (including establishment of target load reductions), quantification of non-structural BMP load reductions, identification of structural retrofit BMP opportunities and quantification of their load reductions, and demonstration of reasonable assurance (of achieving Permit limits). The methodology described could be applicable to other coastal watersheds facing similar compliance demonstration requirements. Initial results will be presented for specific subwatersheds and EWMP groups.