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Water Quality Program

TMDL Surrogates for Focused Stormwater Improvements

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

Under our Clean Water Act authorities, Washington State uses permits to regulate stormwater runoff from municipal storm sewers, construction sites and industrial facilities. In addition, we have authority to develop collaborative plans to clean up our polluted waters using Total Maximum Daily Load (TMDL) studies, which are one type of water quality improvement project.

Washington has a federal mandate to clean up our state’s polluted waters. It is the state’s responsibility to write TMDLs and permits that protect and restore water quality. It is important to address the higher stormwater flows, as well as the pollutants that are carried in it, as a result of urbanization and human activities.

TMDL wasteload allocations for stormwater

Pollutants like bacteria, heavy metals, and toxics are found in stormwater. When these pollutants build up in the environment we call that pollution. Pollution can also be caused by high volumes of stormwater that can scour and erode streams. A TMDL must include pollutant reduction targets for stormwater sources. This target, or wasteload allocation, specifies the amount of the pollutants that must be removed from the stormwater discharge, or sets targets for the reduction of high stormwater flows that cause pollution.

A traditional TMDL addresses separate types of pollutants, such as temperature or dissolved oxygen. However, when we have trouble finding or measuring a pollutant, we use “surrogates” as tools to reduce pollution. Using surrogates can also be more cost effective than measuring a suite of pollutants that are expensive to analyze.

Stormwater flow (polluted runoff) is a surrogate (a substitute) we now include, with other surrogate measures, when appropriate in TMDLs to tackle a broad spectrum of pollution efficiently. A benefit of managing stormwater flows in TMDLs is that it helps reduce a wide array of pollution rather than one type of pollutant. Washington strives to continuously improve the way we protect and clean up our waters. Use of stormwater surrogates is a new, key tool to address multiple pollutants in meaningful ways to clean up our dirty waters.

What Actions Reduce Stormwater Flow?

Stormwater retrofit projects correct deficiencies in older, existing stormwater infrastructure in urban areas. The projects can reduce pollutants in stormwater, reduce pollution from high stormwater flows, or both. Many retrofit projects use low-impact development techniques.

Low impact development is a construction technique that uses vegetation, healthy soils, porous pavement (see example video here), and other tools to treat stormwater and keep it from running off a site and carrying pollution downstream.

Public education, smart land use planning, and green infrastructure development are also parts of the solution. Green infrastructure includes stormwater retrofit projects and low impact development.

Frequently asked questions about TMDL Surrogates for Stormwater

Water Quality success story: Shoreline’s Aurora Corridor Improvement


Last updated June 2013