Total Maximum Daily Load process

The Total Maximum Daily Load (TMDL) — or water quality improvement project — process is a science-based approach to cleaning up polluted water so that it meets state water quality standards. A TMDL is a numerical value that represents the highest amount of a pollutant a surface water body can receive and still meet the standards.

The federal Clean Water Act requires states to develop a TMDL plan for each water body on the state's polluted waters list, also known as the 303(d) list. The TMDL process is just one strategy used to clean up polluted waters.

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TMDL process in Washington

The process starts after we complete an assessment of data that we collect about the state’s water bodies. We put assessed water bodies into categories based on the level of impairment. Those not meeting water quality standards go into the impaired category, known as the 303(d) list, named for the section of the federal Clean Water Act that establishes this overall process. The 303(d) list shows us which pollutants are causing impairment in a water body.

We use the 303(d) list to determine what water quality improvements are most needed. Based on these needs, we develop water quality improvement projects. We use the TMDL process for projects where we determine that it will be the most effective tool. We work with local governments, citizens, and other interested parties to identify pollution sources within the watershed and determine what needs to change in order to reduce or eliminate that pollution.

Key parts of the TMDL process

Each TMDL project is unique, but there are essential elements to the process.

We begin with a study to determine what the TMDL is for the water body. A Total Maximum Daily Load is a numerical value that represents the highest amount of pollutant a surface water body can receive and still meet water quality standards.

The study includes monitoring, followed by analysis. The monitoring helps identify sources and amounts of pollutants causing the water quality problem. The technical analysis determines the pollution reductions each source must make to protect the water. Any amount of pollution over the TMDL level needs to be reduced or eliminated to achieve clean water. We develop the following measures based on the results of the study:

  • Loading capacity for the pollutant. The loading capacity is the sum total of all of the pollutant loading the water body can absorb without violating water quality standards.
  • Load allocations (LAs) for runoff pollution (nonpoint) sources. The LA quantifies how much of the pollutant(s) can be discharged from nonpoint sources, along with the other pollutant 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. *Load allocations and wasteload allocations may be based on surrogate measures
  • A margin of safety. An allowance so that surface water quality 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 effective in the future.
  • Consideration of seasonal variation of flows and contaminant concentrations. This ensures that water quality 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, community organizations, and other interested parties.
  • An administrative record.
  • An estimate of when the water body will meet water quality standards.


We submit the TMDL report to the Environmental Protection Agency (EPA) for approval once we have reviewed it through a public comment period. From there, we make changes and adjustments, as needed. We submit a final report to the EPA that includes the TMDL, project plan, and implementation plan.

After EPA approves our report, we implement the plan. During that process, monitoring shows us how well water quality is improving. If the water body health is not improving as expected, we make adjustments in the process, where needed.

When the water body meets water quality standards, its assessment status is changed to Category 1: Meets tested standards for clean waters. We continue periodic monitoring to ensure that the water body maintains state standards.

Dispute resolution

The TMDL process includes getting feedback from watershed residents, local governments, and other stakeholder groups. If parties disagree, our regional TMDL lead and Water Quality Program regional office supervisor work with them to help resolve their differences. If needed, the local entity or citizen can petition for dispute resolution, using the procedures in Water Quality Program Policy 1-25. We provide these processes to help solve or negotiate technical, procedural, or other disagreements regarding the TMDL project.

Public involvement

TMDL is a public process. That means we work to keep people informed and to consider their concerns and ideas. We coordinate with local and tribal governments and we incorporate environmental justice practices to make participation as open and accessible as possible. Community members and organizations can participate in TMDL project development and implementation in a number of ways. We strive to include local input as we plan and carry out TMDL projects.

Annual webinar

We host a yearly webinar to share where we are conducting water cleanup plans, provide updates on ongoing work, and to receive feedback on our prioritization of TMDL work. This provides opportunities for communities and partners to help us as we establish and carry out our water cleanup priorities.

We held the annual webinar on Oct. 19, 2023, you can view slides. The public comment period for TMDL prioritization is now closed, and we are reviewing comments and working on responses. The feedback we receive will be considered as we make progress on current work and consider potential future plans.