Site-Specific Water Effect Ratios (WERs) for Chollas Creek Copper and Zinc Total Maximum Daily Loads (TMDLs)

The San Diego Water Board is responsible for the regulation, protection and administration of water quality. The Water Quality Control Plan for the San Diego Basin (Basin Plan) designates beneficial uses for water bodies in the San Diego Region and establishes water quality objectives and implementation plans to protect those beneficial uses.

Any change to the Basin Plan requires a Basin Plan amendment. All Basin Plan amendments are subject to a full public participation and hearing process prior to adoption by the San Diego Water Board. Basin Plan amendments also must be approved by the State Water Resources Control Board (State Water Board), Office of Administrative Law (OAL) and, in many cases, the US Environmental Protection Agency (USEPA).

The Basin Plan includes TMDLs for dissolved metals in Chollas Creek. These TMDLs originally went into effect on October 22, 2008. In accordance with the California Environmental Quality Act (CEQA), the San Diego Water Board prepared substitute environmental documents for the establishment of the TMDLs. Those documents contained significant environmental analysis and numerous findings related to the reasonably foreseeable methods of compliance, the impacts of the methods of compliance, feasible mitigation measures, and alternative means of compliance. Due to limited site-specific data during Chollas Creek Metals TMDL development, a default WER value of 1 was used to calculate numeric targets for metals. Since that time, data have been collected to update numeric target equations with site-specific WERs, which take into account the physical and chemical characteristics of the water in Chollas Creek. This approach is consistent with USEPA's water quality criteria for toxic pollutants in inland surface waters, enclosed bays, and estuaries (California Toxics Rule; 40 CFR 131.38). The update required a Basin Plan amendment.

On February 8, 2017, the San Diego Water Board adopted Resolution No. R9-2017-0015 amending the San Diego Basin Plan to incorporate site specific WERs into water quality objectives for toxic pollutants and Total Maximum Daily Loads for Copper and Zinc in Chollas Creek. This Basin Plan Amendment was approved by the State Water Board on September 17, 2019 and by the Office of Administrative Law on March 5, 2020. The United States Environmental Protection Agency (USEPA) approved the TMDL Basin Plan Amendment on March 26, 2020.


Documents associated with the adoption of this Basin Plan Amendment are available for review at the San Diego Water Board Office. To request a file review please contact the Regional Board receptionist at (619) 516-1990, or email

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