Statewide Mercury TMDL Information

Public Notice - 2020 Statewide Mercury TMDL Proposed Changes

The Michigan Department of Environment, Great Lakes, and Energy (EGLE) completed, and the United States Environmental Protection Agency (USEPA) approved, a statewide mercury Total Maximum Daily Load (TMDL) in 2018.  The TMDL addresses most inland waterbodies not supporting designated uses for fish consumption due to exceedances of the numeric mercury water column Water Quality Standard (WQS) and/or elevated mercury concentrations in fish tissue.  Mercury-impaired waterbodies not covered by the statewide mercury TMDL include the Great Lakes and connecting channels; contaminated legacy sites (i.e., Areas of Concern [AOC] and Superfund sites); and certain inland waterbodies where the level of pollutant reduction required to achieve the mercury WQS will be different than the mercury load that was calculated for the TMDL.  The Statewide Mercury TMDL and associated load reduction goals were developed to meet the target fish tissue concentration of 0.35 mg/kg based on the assumption that fish mercury concentrations will respond proportionally to reductions in atmospheric mercury loadings.

EGLE assesses fish tissue concentration data and mercury water column data on a biennial basis in accordance with our water quality monitoring strategy and our Integrated Report Assessment Methodology.  Waters determined to be mercury-impaired (above the 0.35 mg/kg fish tissue target or the water column target criteria of 1.3 ng/L) and below the 1.012 mg/kg fish tissue maximum would be considered addressed by the TMDL.  Waters determined to be mercury-impaired above the 1.012 mg/kg fish tissue maximum or above the ambient water column mercury concentration of 9.5 ng/L would not be considered as addressed by the TMDL.

EGLE’s intent was to modify the TMDL document by updating the list of waters covered by the TMDL (as defined in Appendix A) every other year, in conjunction with the submittal of Michigan’s biennial Integrated Report (IR), beginning with the 2020 version of the IR.  To that end, EGLE is presenting this proposed list of newly impaired water bodies to the public and the USEPA, as well as identifying some waterbodies that were erroneously included on the original list and therefore being removed from the TMDL.  Proposed mercury-impaired water bodies will be placed in the “Impaired, TMDL completed” Integrated Report assessment category (category 4a).  This updated list is based on additional water quality and fish contaminant monitoring data generated since the TMDL was finalized.  Once the USEPA approves the revised TMDL Appendix A, the newly proposed water bodies will be part of this statewide TMDL.  It is the EGLE’s intent that no new water bodies will be added to the Integrated Report assessment category “Impaired-TMDL needed” (category 5) for mercury-impaired waters.  

A cumulative list of all water bodies that were previously included in this TMDL can be found at EGLE’s TMDL web page (, click on “Michigan’s Statewide Mercury Total Maximum Daily Load” link).  This list will be updated to include newly identified waterbodies after the public notice period, as well as removing waterbodies as appropriate.

Statewide Mercury TMDL

Water bodies are impacted by atmospheric deposition of mercury throughout the state.  Mercury is a metal that occurs naturally in the environment.  Major uses of mercury in the United States include lighting, switches, instruments, the dental industry, laboratory uses, and other industrial applications.  Local and global anthropogenic activities such as mining, coal combustion, and industrial uses have release mercury in excess of pre-industrial period concentrations.

EGLE developed the statewide TMDL as required under Section 303(d) of the federal Clean Water Act (CWA).  The federal CWA requires a TMDL to be written for water bodies not meeting Michigan's water quality standards.  The purpose of the TMDL is to gather data, identify sources, and develop appropriate goals and reasonable assurance that will restore the designated uses to the water bodies.