Total Maximum Daily Loads

A Total Maximum Daily Load, or TMDL, is a water quality restoration goal that identifies the maximum amount of a specific pollutant that may be present and still allow a waterbody to remain functional and healthy.

Each TMDL addresses a particular waterbody segment and associated water quality problem, such as excessive levels of nutrients (nitrogen and phosphorus), depleted oxygen levels, high bacteria counts or metals.

When identifying priority waters for TMDL development, the department holds workshops across the state to obtain feedback from the public on the agency’s overall approach, the criteria used to set the priorities and the resulting TMDL development plan.

The department scientifically derives the maximum allowable amount of a pollutant by studying the waterbody segment, examining existing data, conducting intensive field surveys and employing mathematical models that reflect water quality dynamics.

Watershed models, used to simulate rainfall runoff volumes and pollutant loads, can be linked to surface water hydrodynamic models, used to simulate water movement, and water quality models. Analyzed together, the models can predict the impact of pollutants in an aquatic system.

The TMDL process is developed with stakeholder and public input. The department holds public workshops within the region of each affected waterbody. Stakeholder feedback is evaluated and taken into consideration as TMDLs are adjusted and finalized.

TMDLs are adopted by Secretarial rule and represent the foundation for the development of site-specific restoration plans (Basin Management Action Plans).

For an online map including adopted TMDLs and those under development, click here.

For more about the TMDL program, including the two-year TMDL development plan, click here.

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