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Multi–Pollutant TMDL Implementation Plans for Los Angeles County — a Quantitative and Practical Approach

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The Los Angeles Regional Water Quality Control Board (Regional Board) and the U.S. Environmental Protection Agency Region IX (USEPA) have developed Total Maximum Daily Loads (TMDLs), which includes stormwater wasteload allocations (WLAs), for a number of pollutants assumed to originate from urban and stormwater runoff in some of the most populated and urbanized watersheds in the United States. These pollutants include indicator bacteria, nutrients, trash, metals, and organics. In response to the TMDLs, the County of Los Angeles developed multi-pollutant TMDL implementation plans for the unincorporated County areas of the Ballona Creek and Los Angeles River watersheds. The plans serve as a roadmap for implementing Best Management Practices (BMP), such as construction of new stormwater infrastructure and improvements to County-wide stormwater quality programs, over the coming decades to meet WLAs. The plans utilize a comprehensive and phased approach for BMP implementation to address current TMDLs established for waters within the watersheds, with consideration of future potential TMDLs.

To develop these plans, BMPs to treat stormwater and dry weather flows to reduce metals, nutrients, bacteria, and toxic pollutants were identified and selected. As part of this process, benefits of management activities were estimated, in terms of pollutant load reductions or improvement in water quality, to meet WLAs defined by the TMDLs. The process of BMP selection included an assessment of cost-effectiveness to provide assurance that the plans are practical and implementable. The plans also include integrated water resources approaches that consider BMPs that can address multiple pollutants and establish possible benefits and uses in the watersheds.

Qualitative and quantitative analyses were performed to evaluate the ability of BMPs to meet load reduction targets associated with WLAs. For most nonstructural BMPs, quantification of benefits in terms of pollutant load reductions are challenging and often require extensive survey and monitoring information to gage performance. For the purposes of these plans, a qualitative approach was used to evaluate the effectiveness and feasibility of the nonstructural BMPs. In addition, modeling analysis was performed to provide optimization of the most cost-effective combination and size of structural BMPs to meet wasteload allocations. The optimization results provided the foundation for BMP strategies recommended for phasing of TMDL implementation. Results established the recommended order and phasing for the structural BMPs and nonstructural BMPs. Most nonstructural BMPs were placed in implementation phases on the basis of the feasibility of implementation and wasteload reductions attained. The plans also provide the timing and planning-level costs for BMPs in the unincorporated County areas of the watersheds. These plans are meant to be iterative and adaptive to allow for modifications and improvements informed by ongoing continued study of the drainage system, extensive source investigations, emergence of and diagnosis of problem sources, and new technologies and methodologies for dry and wet weather treatment, and quantified benefits of BMPs through performance monitoring that continue to emerge.
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Keywords: Stormwater; TMDL; best management practices; modeling; watershed management

Document Type: Research Article

Publication date: 2011-01-01

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