Skip to main content

The St. Louis River Mercury Total Maximum Daily Load (TMDL) will determine the mercury reductions needed for lakes and rivers in the St. Louis River watershed, to meet the water quality standard for mercury and support healthy consumption of fish. Mercury is toxic to humans; people can be exposed when eating fish pulled from waters with mercury contamination.

The MPCA is undertaking this TMDL study for many reasons, including the cultural and economic importance of fishing in the watershed and the exercise of tribal treaty rights.

The St. Louis River Watershed covers 2,926 square miles in northeast Minnesota, draining to Lake Superior. This project will also examine mercury contamination in the Cloquet River Watershed, which covers 794 square miles and joins the St. Louis River near Brookston.


The MPCA monitors the state’s major rivers and lakes to determine whether they meet water quality standards. The MPCA places waters that fail to meet water quality standards on the state’s Impaired Waters List. Impaired waters require a TMDL study, which determines how much of a specific pollutant a water body can receive and still meet the water quality standard.

  • Impaired waters list. Many waters in Minnesota are impaired for mercury; high levels of mercury in fish tissue fail to meet the state standard for aquatic consumption. Because the main source of mercury in fish is global air emissions that affect waters throughout the state, the MPCA developed the Minnesota statewide mercury TMDL to address mercury contamination. Its goal is to reduce mercury air emissions in Minnesota by 93 percent from 1990 levels.
  • Statewide mercury reduction plan. The statewide TMDL excluded some areas — such as the St. Louis River Watershed — where mercury levels in fish are so high that the 93 percent reduction won't be enough for the waters to meet the standard. The St. Louis River Watershed and the other exceptions will require their own mercury TMDLs.

Get involved

The SLR Mercury Public Forum for this project is open to any member of the public. The role of this group is to review and provide feedback on materials at key milestones and discuss the work at scheduled forums. To join this forum, simply sign up for the email updates below to receive meeting notices and project updates.

A Technical Advisory Team of scientists and government partners (tribal, federal, and state) provides technical, scientific, and policy expertise to the MPCA project team on developing the TMDL.

GovDelivery: St. Louis River Mercury TMDL MNPCA_434
Envelope and cursor

Stay connected

Sign up for updates on the St. Louis River Watershed mercury TMDL.

For more information