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Water Quality Program

Total Maximum Daily Load

What is a "total maximum daily load"?

A total maximum daily load (TMDL) is a numerical value representing the highest amount of pollutant a surface water body can receive and still meet water quality standards. Any amount of pollution over the TMDL level needs to be reduced or eliminated to achieve clean water. The Clean Water Act requires that states develop a TMDL for each of the water bodies on the state's 303(d) list. The state sends the finalized TMDL to EPA as part of a water quality improvement report (WQIR) for approval.

TMDL process in Washington State

In general, the TMDL strategy is a common-sense, science-based approach to cleaning up polluted water so that it meets the state water quality standards. Washington State's TMDL process starts with identifying pollution sources within a watershed and determining what needs to change so that pollution is reduced or eliminated.

Pollution sources are broken down into two categories. The first is nonpoint pollution, where the source runs directly off the land into the water. The allowable discharge from all the nonpoint sources is called the load allocation. The second category is point source pollution, which typically flows out of a pipe and is regulated by an NPDES permit. The allowable discharge from a permitted point source is called a wasteload allocation. The water quality improvement report (WQIR) then sets out the actions required for each point source and land use in the project area to ensure TMDL allocations are met.

In short, our TMDL approach:

  • Assigns load allocations to nonpoint pollution sources.
  • Assigns wasteload allocations to point sources which may require more stringent NPDES permit limits.
  • Uses surrogate measures for pollutant allocations when it is connected to the direct pollutant and the surrogate is more efficient and cost-effective to measure.
  • Designates suites of best management practices (BMPs) for various land-use categories.
  • Details the actions needed to attain standards and return waters to good health.

Elements of a TMDL

Each TMDL project is unique, but there are essential elements to the process. As long as these elements are included, the TMDL project should result in a water quality improvement plan that is complete, acceptable to the public, and approvable by EPA. These elements include:

  • An initial study of the water quality problems. This includes a monitoring study identifying the sources and amounts of pollutants causing the water quality problem, and a technical analysis to determine how much pollution sources must be reduced to protect the water.
  • Public involvement. Public involvement, along with coordination with tribal governments and consideration of environmental justice issues (as appropriate), is important at all key decision steps of the process.
  • Loading capacity for the pollutant. This is the sum total of all of the pollutant loading the water body can absorb without violating water quality standards.
  • Load allocations (LAs) for nonpoint sources. The LA quantifies how much of the pollutant(s) can be discharged from nonpoint sources, along with the other sources, and have the water body still meet water quality standards.
  • Wasteload allocations (WLAs) for point sources. The WLA quantifies how much of the pollutant(s) can be discharged from point sources, along with other sources, and have the water body still meet water quality standards.
  • A margin of safety. An allowance so that surface water quality (WQ) standards will be met under the worst conditions likely to be experienced.
  • A reserve capacity. This factor estimates the effect of population growth and future land uses on pollutant WLAs and LAs so they will continue to be adequate in the future.
  • Consideration of seasonal variation of flows and contaminant concentrations. This ensures that WQ standards are met during all seasons of the year.
  • An implementation plan. A detailed plan to prevent, reduce, or clean up excess pollution.
  • A follow-up monitoring plan. To demonstrate the success of pollution controls contained in the implementation plan or the need for additional action.
  • Reasonable assurances. Assurances for the success of the implementation plan based on the involvement of local governments, tribes (as appropriate), participation of local citizens and special interest groups, etc.
  • An administrative record.
  • An estimate of when the waterbody will meet WQ standards.

Dispute Resolution

The TMDL process includes getting feedback from watershed residents, local governments, and other stakeholder groups. Ecology makes every effort to include local input regarding the development of a specific TMDL project. If the parties disagree, they should first try to resolve differences by working with the regional TMDL lead and Water Quality Program regional office supervisor. Should these options not settle the issue, then the local entity or citizen can use the procedures in WQP Policy 1-25 to invoke a formal request for dispute resolution. Causes for dispute could include a technical or procedural disagreement of the TMDL project.

If the local entity or resident does not first attempt to resolve the issues with the regional Water Quality Program office, they will be referred to the regional office before a dispute resolution process can be initiated.


Once the water quality improvement plan is approved by EPA, the next step is to put the TMDL implementation plan into action. Community members and organizations can participate in TMDL projects in a number of ways.

Find TMDL projects in your area

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Last updated June 2013