Statewide mercury TMDL

Our Plan to reduce mercury releases by 2025 is based on Minnesota's statewide mercury total maximum daily load (TMDL) study, which was approved by the Environmental Protection Agency in 2007. The plan includes input and recommendations from a broad range of stakeholders.

The impaired waters TMDL process consists of data collection and assessment, listing those waters not meeting water quality standards, developing a pollution reduction plan, implementing the plan, and then evaluating the implementation by collecting more data.

Statewide mercury TMDL

There is a strong connection between the Minnesota Department of Health's fish consumption advisory and lakes, rivers, and other bodies of water that have excess mercury. When the advisory is more restrictive than one meal per week, the water body is impaired. Fish in northeastern Minnesota have significantly higher mercury concentrations than fish in the rest of the state.

Atmospheric deposition of mercury is uniform across the state and supplies more than 99.5% of the mercury getting into fish. Agency research shows that 70% of current mercury deposition in Minnesota comes from people sources and 30% from natural sources, such as volcanoes. Ninety percent comes from outside the state and falls to the federal government to address. Minnesota has no known natural sources that emit mercury directly to the atmosphere.

The long-term goal of the mercury TMDL is for the fish to meet water quality standards, which will require a 93% reduction in state air emissions from 1990 levels. Regulated sources of mercury will be required to emit less than 1% of the total load to the state. All but the smallest dischargers must develop mercury minimization plans.

TMDL revisions

Every two years, the MPCA updates the approved statewide mercury TMDL. Mercury impairments are mainly for excess mercury in fish tissue but can also be for excess mercury concentrations in water columns, which are listed in Appendix A of the Impaired Waters List. Fish tissue impairments in Appendix A have mercury concentrations greater than 0.2 mg/kg and equal to or less than 0.572 mg/kg. Greater concentrations are not assigned to the mercury TMDL and result in inclusion in the TMDL list. The mercury TMDL documentation and reduction goals are not being changed with Appendix A revisions.

EPA approval


Miranda Nichols, 651-757-2614, 800-657-3864