Cost-Effective TMDL Implementation Planning and Hydrology as a Critical Decision-Aiding Tool

An innovative, risk-based approach to implementation planning for Bacterial TMDL compliance was developed by regulated agencies discharging to the Santa Monica Bay in Los Angeles County, California. This work was in support of the development of the Bacterial Implementation Plan, which uses an integrated water resources management approach, addressing multiple pollutants, beneficial use opportunities, and multiple agencies in its overall solution. The purpose of this analysis was twofold: first, to estimate the capture volumes of stormwater runoff that must be managed to meet the TMDL requirements; and second, to assist the agencies and stakeholders with the establishment of levels of confidence and risk in estimating stormwater runoff volumes and evaluating potential Implementation Plan alternatives. An ancillary, yet significant benefit of the method is that it allows for a collaborative and informed approach toward decision-making; this approach has been well-received by regulatory agencies and environmental groups. Many of the beaches along Santa Monica Bay were listed on California's 1998 section 303(d) list due to impairments for coliform or for beach closures associated with bacteria. The beaches appeared on the 303(d) list because the elevated bacteria and beach closures prevented full support of the beaches designated use for water contact recreation (REC-1). The California Regional Water Quality Control Board (Regional Board) implemented bacteria objectives using a "reference system/anti-degradation approach" which recognizes that there are natural sources of bacteria that may cause or contribute to exceedances of the single sample objectives. The Santa Monica Bay Wet Weather Bacteria TMDL (for Jurisdictions 2 and 3, which includes the County of Los Angeles, Cities of Los Angeles, Santa Monica, and El Segundo, and Caltrans), stipulates both rolling 30-day geometric mean and single sample limits, and provides for an "allowable number of wet weather exceedance days" (daily sampling). In the subject jurisdiction, 17 exceedance days are allowed. Preliminary sizing was estimated based on a review, analysis and ranking of 50 years of daily precipitation data, and the conversion of target precipitation rates to storage/treatment volumes. Associated with this target volume is a historically-based expectation of exceedance. To address hydrologic patterns, particularly inter-event storm patterns, intensities, and durations, a 50-year continuous hydrologic model was developed. This model simulates the effectiveness of the preliminary sizing approaches and offers insights to where water quality treatment approaches can be optimized. In addition the model provides for an evaluation of potential exceedance/violation so that an assessment of the criticality of such violations can be made.