AERA | Mercury guidance

Minnesota’s surface waters are impaired by unacceptable levels of mercury, virtually all of which comes from the air. Releases to the air can be unavoidable. MPCA’s goal is to minimize increases in mercury emissions associated with new or expanding facilities.

Project proposers emitting mercury should refer to MPCA’s guidelines on identifying mercury inputs and environmental releases. Mercury sources of particular concern include taconite production, secondary metal processors, fuel combustion in electricity generating stations and industrial boilers (except when burning only natural gas), and sewage sludge, municipal or other incineration. The Hg-01 form is available for project proposers to demonstrate that their mercury emissions have been accurately quantified and the project’s mercury emissions have been minimized through an effort to identify cost-effective emission reduction alternatives.  

These sources may also be asked to address the potential for adverse impacts from their facility to people who may consume fish from fishable water bodies in the vicinity of their facility. Risks from inhaling mercury emitted to the air from a facility are quantitatively evaluated in the AERA process. However, risks from ingesting mercury-contaminated fish are not easily assessed because the magnitude of the risk depends on the presence and highly variable nature of surrounding water bodies, the chemical form of mercury emissions, and the fate and transport of mercury in the environment. The MPCA has developed a model that eliminates much of the uncertainty inherent in mechanistic models due to this variability. The MPCA model, discussed below, relies on measured concentrations of mercury in Minnesota fish and a measured ambient background mercury deposition velocity to assess potential risks from eating mercury-contaminated fish.

MPCA mercury risk estimation method for the fish consumption pathway

The MPCA mercury risk estimation method (MMREM) is not a mechanistic model of mercury methylation and bioaccumulation, but rather combines empirical fish contamination data with the premise that mercury concentrations in fish will achieve a steady state in relation to atmospheric mercury deposition (USEPA 2001). The MMREM assesses the incremental mercury risk associated with eating fish from water bodies near permitted or potentially permitted sources. The MMREM can be used to estimate the noncancer oral hazard quotients associated with fish tissue consumption based on increases in mercury deposition due to a given project.


A brief outline of the methodology for estimating incremental fish tissue mercury concentrations and hazard quotients associated with the project emissions.

  1. Characterize mercury air concentration(s) from proposed project
  2. Select one or more water bodies for evaluation
  3. Delineate the watershed
  4. Estimate incremental mercury mass loading to water body due to ambient mercury in the atmosphere.
  5. Estimate incremental mercury mass deposited to each evaluated water body and its watershed due to proposed project.
  6. Estimate the percent increase in mercury loading in water bodies from project
  7. Estimate fish fillet methylmercury concentration from project emissions
  8. Estimate the incremental methylmercury exposure for the fisher scenario
  9. Estimate the incremental noncancer hazard quotient

Detailed guidance and a spreadsheet developed by MPCA representing both pre- and post-project fish impacts are available to calculate an incremental hazard quotient from eating mercury-contaminated fish. The January 2009 version of the MPCA’s Mercury Risk Estimation Model (MMREM) has been updated to reflect comments from independent reviewers. The fraction of the terrestrial watershed reaching the water body increased from 10% to 26%.

The MPCA has a mercury website that contains general information on what mercury is, where mercury comes from, and what we are doing about the mercury problem in our state.


For additional information on mercury in the environment or specific mercury questions on the AERA process, please contact: