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Mercury in Lake Superior National Parks

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Overview of Lake Superior study
Accomplishments and Description of Field Work
Key Findings
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USGS geologists sampling soil, Voyageurs National Park
Geologists Connie Dicken, Laurel Woodruff, and Shana Pimley (l. to r.) preparing to sample soil, Voyageurs National Park.

Mercury as an environmental contaminant is a concern in the Lake Superior region including some National Parks where there are high levels of mercury in game fish in many Park lakes. This contamination can create health problems both for humans and wildlife that consume fish. A critical question for land and resource managers and regulators is whether this contamination is from local sources, including anthropogenic sources, vs. regional or global mercury emissions.
Our research in Isle Royale National Park, Michigan, and Voyageurs National Park, Minnesota, and in nearby Superior and Chequamegon National Forests, is helping to define the factors that control mercury accumulations in forests, and the relationship between mercury content in forest soils and mercury contamination of aquatic ecosystems.
In particular we have identified forest fires as a principal factor in the mercury cycle because of the ability of fire to volatilize mercury sequestered in forest floor organic material.


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Prescribed burn area in Superior National Park 2 days after fire was extinguished.
USGS Geologist Connie Dicken examining prescribed burn area in Superior National Forest two days after the fire was extinguished. A soil survey was conducted prior to the fire in 2000, and soil compositions are being monitored periodically since the fire. This relatively severe fire consumed much of the forest floor organic layer and released the contained mercury to the atmosphere.

Field Work:
Working in conjunction with the National Park Service and the US Forest Service, we have been collecting soil samples in several studies areas in the Lake Superior region. Typically we collect three soil samples from a single site: O-, A-, and C-horizon soils. On Isle Royale we also collected paired lichen samples from soil sample sites. Samples are air dried and submitted for geochemical analysis to the USGS.
To study the effects of fire on soil geochemistry, we have sampled across the 1936 burn areas in Voyageurs and Isle Royale National Parks. The fires of 1936 were particularly severe in the Upper Midwest, killing large stands of timber and in many areas burning soils down to bare bedrock.
At Snowbank Lake in the Superior Forest, we sampled before a prescribed burn in 2000 and resampled the same sites 4 times in the following 3 years.

We have completed studies of mercury distribution in soils in Isle Royale National Park, Michigan. Geochemical data on mercury and a number of other elements and media are available online (Isle Royale NP data), and final reports are in preparation.
An initial phase of a study on bedrock and soil geochemistry in Voyageurs National Park, Minnesota, is complete. Geochemical data are available online (Voyageurs NP data). Our results in Voyageurs National Park are being integrated into a multidisciplinary study of mercury in Park watersheds to define factors that control the transport of mercury to lakes, methlyation of mercury, and its concentration in the aquatic food web.
Data on water chemistry and mercury contents of age-1 perch and northern pike were collected by the staffs of:
  • the USGS Water Resources District Office in Mounds View, Minnesota;
  • Upper Midwest Science Center, La Crosse, Wisconsin; and
  • the Minnesota Pollution Control Agency.
  • For 3 years we have been monitoring post-fire response in soils in a prescribed burn area in Superior National Forest near Snowbank Lake, Minnesota. Preliminary results are presented in a number of abstracts listed below (products section).


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    Key Findings:
    Our studies in Voyageurs and Isle Royale National Parks have shown:

    These findings, when combined, show that fire history plays an important and previously under-appreciated role in the mercury cycle in northern forests. Forest fire burning through a watershed decreases the mercury load contributed to the aquatic system in response to short- (during the fire) and long-term (years following fire) mercury losses. The decrease in mercury load apparently can persist for decades,even as the mercury content of soils gradually increases due to continued atmospheric input. Although many other factors must be considered, fires may lessen the possibility of severe mercury contamination in fish by cleansing a watershed of mercury that had been sequestered at the forest floor by decades of accumulation.

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    Cannon, W.F., and Woodruff, L.G., 2003, The geochemical landscape of northwestern Wisconsin and adjacent parts of northern Michigan and Minnesota (geochemical data files): U.S. Geological Survey Open-File report 03- 259, onine only, version 1.0.

    Cannon, W.F. and Woodruff, L.G., 2000, Some factors that control the distribution of mercury in surficial materials in Isle Royale National Park- earth, wind, and fire (abs): Institute on Lake Superior Geology, 46th Annual Meeting, Proceedings Volume 46, p. 8-9

    Cannon, W.F., Dean, W.E., and Bullock, J.H., 2001, Eleven thousand years of mercury deposition in Elk Lake, Minnesota: effects of climate and vegetation change on the mercury cycle (abs): 2001, Geological Society of America, 2001 Annual Meeting, Abstracts with Program, v. 33, p. 186.

    Cannon, W.F., Dean, W.E., and Bullock, J.H., 2001, Eolian deposition of mercury in Elk Lake, Minnesota - the impact of Holocene climate fluctuations on the mercury cycle (abs): International Association of Great Lakes Research, 45th Conference, p. 19.

    Woodruff, L.G. and Cannon, W.F., 2002, Impact of fire on the forest floor and mineral soils, Snowbank Lake, Minnesota (abs): Institute on Lake Superior Geology, 48th Annual Meeting, Proceedings Volume 48, p. 54-55.

    Woodruff, L.G., Cannon, W.F., Dicken, C.L., Bennett, J.P., and Nicholson, S.W., 2003, Bedrock, Soil, and Lichen Geochemistry from Isle Royale National Park, Michigan: U.S. Geological Survey Open-File Report 03-276, Version 1.0 .

    Woodruff, L.G., Cannon, W.F., Dicken, C.L., and Pimley, S., 2002, Bedrock and Soil Geochemistry from Voyageurs National Park, Minnesota: U.S. Geological Survey Open-File Report 02-196, Online Only, Version 1.0.

    Woodruff, L.G., and Cannon, W.F., 2001, The effect of fire on mercury and carbon in forest soils: results from northern Michigan and Minnesota (abs): Geological Society of America, 2001 Annual Meeting, Abstracts with Program, v. 33, p. 186.

    Woodruff, L.W., Harden, J.W., Cannon, W.F., and Gough, L.P., 2001, Mercury loss from the forest floor during wildfire (abs): EOS Transactions of American Geophysical Union, 82(47), Fall Meeting Supplement, Abstract B32B-0017.

    Woodruff, L.W., Cannon, W.F., and Dicken, C.L., Impact of fire on the geochemistry of forest soils: poster available on line at http://firescience.cr.usgs.gov/html/woodruff_abs02.html .

    USGS geologists sampling soils in Isle Royale National Park
    USGS Geologists William Cannon and Connie Dicken sampling soils in Isle Royale National Park. Samples are carefully collected according to soil horizon to determine soil mass per unit area.


    William F. Cannon, Geologist
    U.S. Geological Survey
    Eastern Mineral Resources
    954 National Center
    Reston, VA 20192

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