Total Maximum Daily Loads (TMDLs)

WDNR addresses polluted or impaired waters (on the Impaired Waters List hyperlink) by using an analysis known as a Total Maximum Daily Load or TMDL. TMDLs are required on impaired waterbodies by the federal Clean Water Act.

A TMDL [PDF 445KB] is the amount of a pollutant a waterbody can receive and still meet water quality standards. A TMDL establishes the amount of pollutant reduction needed from each source to meet water quality goals.

Three components make up a TMDL: WLA + LA + MOS:

1. The wasteload allocation (WLA) is total allowable pollutant load from all point sources (POTWs, industrial wastewater, CAFOs, MS4s).
2. The load allocation (LA) the allowable pollutant load from nonpoint sources (agricultural, residential runoff, etc.).
3. The margin of safety (MOS) accounts for uncertainty in the modeling and calculating the WLAs and LAs. Another part of the "formula" may be reserve capacity, accounting for future growth within the watershed.

To determine the TMDL, targets or numeric goals are set to meet water quality standards and can be based on water quality criteria (if available), best local data and/or reference sites. Once targets are set for a waterbody, they are used to calculate the TMDL (stream flow x target concentration = load). Once the TMDL is established for an impaired waterbody, the pollutant reduction needed from each source can be determined:

Existing pollutant loads - the target loads (TMDL) = load reduction needed

The load reduction is divided between each point and nonpoint source (allocations). Federal and state regulations and programs require implementation of TMDLs to meet water quality standards.

Listed below are TMDLs that have been approved by the US EPA:

• Approved TMDLs

For questions and comments about the material contained in this site, contact:
Nicole Richmond regarding TMDL Development or
Corinne Billings for TMDL Implementation concerns and questions.