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Water Quality Program

Frequently Asked Questions about TMDL Surrogates for Stormwater

Mouth of the Wenatchee River as it enters the Columbia River.  Photo courtesy of Bryan Neet, Dept. of Ecology.

On January 3, 2013, the United States District Court for the Eastern District of Virginia issued its decision in Virginia Department of Transportation, et al. v. United States Environmental Protection Agency, vacating a total maximum daily load (TMDL) established by EPA for Accotink Creek in Virginia.  That decision raised some questions as to how it may affect development of stormwater-based water cleanup plans (also known as total maximum daily loads, or TMDLs) in Washington State.

A TMDL must include pollutant reduction targets for stormwater sources. This target, or wasteload allocation, specifies the amount of the pollutant(s) that must be removed from the stormwater discharge, or sets targets for the reduction of high stormwater flows that cause pollution.

Polluted stormwater runoff is generally not treated and transports toxic chemicals, nutrients, and bacteria to our lakes, rivers, and Puget Sound when it rains.

A wasteload allocation can include surrogate measures to provide meaningful pollution reduction targets that can be more helpful to a municipal stormwater permittee than a direct pollutant target alone.

Use of stormwater surrogates is a new, key tool in innovative, modern TMDLs.

Q: What are surrogate measures and how are they used in TMDLs?

A: Surrogate measures are used for TMDL allocations when the direct pollutant is too expensive or difficult to measure and there is a direct correlation between the surrogate and direct measures.

Surrogate measures are either indirect pollutant targets (e.g. measuring total suspended solids [TSS] as an indication of the concentration of copper or mercury) or as an “other appropriate measure” (e.g. an effective shade target to shade and cool a stream).

Surrogate measures are also used to set a target for implementation activities, such as how much stream shade is needed to reduce solar radiation that heats rivers or the percent impervious area target within a stormwater permit boundary to reduce stormwater flow that can cause pollution.

Q: Can the Washington State Department of Ecology establish a wasteload allocation that uses stormwater flow as a surrogate measure to address biological impairments as defined in the surface water quality standards?

A: The Report of the Federal Advisory Committee on the Total Maximum Daily Load (TMDL) Program (EPA #100-R-98-006, 1998) includes the following guidance on the use of surrogate measures for TMDL development:

When the impairment is tied to a pollutant for which a numeric criterion is not possible, or where the impairment is identified but cannot be attributed to a single traditional “pollutant,” the state should try to identify another (surrogate) environmental indicator that can be used to develop a quantified TMDL, using numeric analytical techniques where they are available, and best professional judgment (BPJ) where they are not. The use of BPJ does not imply lack of rigor; it should make use of the “best” scientific information available, and should be conducted by “professionals.” When BPJ is used, care should be taken to document all assumptions, and BPJ-based decisions should be clearly explained to the public at the earliest possible stage.

The criterion must be designed to meet water quality standards, including the waterbody’s designated uses. If they are used, surrogate environmental indicators should be clearly related to the water quality standard that the TMDL is designed to achieve.

Use of a surrogate environmental parameter should require additional post-implementation verification that attainment of the surrogate parameter results in elimination of the impairment. If not, a procedure should be in place to modify the surrogate parameter or to select a different or additional surrogate parameter and to impose additional remedial measures to eliminate the impairment."

The municipal stormwater general permit includes requirements (without needing a TMDL wasteload allocation) for permittees to regulate their stormwater flow to the receiving water body.  The linkage between the impaired designated use and stormwater flow, established in a TMDL, should ensure that reducing stormwater flow will result in attainment of water quality standards.

Q: Does the Accotink decision affect Ecology’s authority to establish wasteload allocations using stormwater flow as a surrogate?

A: No. The Accotink decision does not apply to the use of stormwater flow surrogates by Ecology for TMDL pollutant allocations when following guidance established by the EPA and as authorized by state and federal laws and rules. Ecology has successfully used surrogate measures for TMDLs in the past and will continue to use them where appropriate to establish meaningful and achievable water cleanup targets.

Q: What is Ecology’s authority for establishing TMDLs using surrogates for pollution problems for which there are no numeric criteria (only narrative criteria) to apply to the receiving water body?

A: When a TMDL is developed for a pollutant for which there are no numeric criteria, the TMDL must make a narrative link between the surrogate measure and the designated use that is impaired. The TMDL must also demonstrate a link between the surrogate measure and the pollution source, and how the wasteload allocation and surrogate measure will restore and protect the river or lake’s designated uses.


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Last updated June 2013