Reducing sulfate could help solve the mercury problem, say two reports

Photo: Pike pursuing yellow perch

June 30, 2024:  For decades, Americans have been warned of the health risks of eating fish from waters tainted with mercury.  Mercury tends to bioaccumulate, that is, it moves up the food chain. The higher concentrations in larger fish pose particular dangers to humans and also to the animals and birds that eat those fish.

The Minnesota Pollution Control Agency has long noted that fact, and also the role of methylation in the process. In its 2007 Minnesota Statewide Mercury Total Maximum Daily Load report, the MPCA noted that “nearly all the mercury that accumulates in fish tissue is methylmercury. Inorganic [elemental] mercury, which is less efficiently absorbed and more readily eliminated from the body than methylmercury, does not tend to bioaccumulate” (p. 2). 

That report also pointed to a link between sulfate levels and mercury in waterways.  Wherever sulfate levels are high, the bacteria that accompany sulfates magnify the methylation process: “Sulfate-reducing bacteria (SRB) have been shown to be responsible for most of the transformation of deposited mercury into methylmercury” (pp. 30-31).  “Deposited mercury” refers to elemental mercury that is cycled from the atmosphere into water and land.

This June in northern Minnesota, those two passages were cited in a new report issued by the White Iron Chain of Lakes Association, representing a series of lakes at the south edge of the Boundary Waters.  The MPCA’s findings are newly relevant to WICOLA, since Birch Lake, the largest in the chain, was listed as impaired for sulfate for the first time this spring.  Birch Lake has two inflows that carry discharge from taconite mines.  Its waters flow north through four other lakes before entering into the Boundary Waters Canoe Area Wilderness.

For over 30 years, WICOLA has gathered and tested water samples from throughout its lakes and submitted the data to the MPCA, issuing its own report every 5 years. With the listing of Birch Lake, the 2023 Water Quality Report includes a new discussion of mercury and sulfate, citing the MPCA report on pages 33 and 40.

Sulfate-reducing bacteria proliferate naturally wherever sulfate is found. So one key to solving the mercury problem in northern Minnesota is to clean up the sulfate at the point of discharge from mining operations.

That’s where the Clearwater Biologic system comes in. Installed at the point of discharge, it’s a closed system that uses SRB in a controlled way. When the bacteria are put to work remediating the sulfate at the source, the effect is to minimize the naturally occurring SRB downstream in lakes and rivers. That means less methylation of mercury and less bioaccumulation.  Eating local fish could make a comeback.

The MPCA’s 2007 Minnesota Statewide Mercury Total Maximum Daily Load report can be found here as a PDF: see “Mercury Cycle” (section 2.1, p. 2) and “Interaction between Sulfate and Mercury Methylation” (section 5.4, pp. 30-31). WICOLA’s 2023 Water Quality Report can be found on the WICOLA website. See the sections on mercury (p. 33) and sulfate (pp. 44-45).

Image from MPCA’s Minnesota Statewide TMDL report, captioned “A link in the aquatic food chain: a walleye pursuing a yellow perch.”