National Water-Quality Assessment (NAWQA) Program
Updated: 26 August 2009
Mercury is one of the most serious contaminants threatening our Nation’s waters because it is a potent neurotoxin in fish, wildlife, and humans. It is a global pollutant that ultimately makes its way into every aquatic ecosystem through one of two routes: Point-source discharges or atmospheric deposition. Atmospheric deposition is the primary source of mercury to most aquatic ecosystems. According to the U.S. Environmental Protection Agency (USEPA), emissions from coal-fired power plants are the largest source of mercury to the atmosphere.
Mercury is deposited from the atmosphere primarily as inorganic mercury. Methylation—the conversion of inorganic mercury to organic methylmercury—is the most important step in the mercury cycle because it greatly increases toxicity and potential for accumulation in aquatic biota.
As methylmercury is formed in an ecosystem, some portion of it is transferred to the water, and some portion of methylmercury in water is taken up or bioaccumulated by the base of the aquatic food web, such as in algae. Animals higher up in the food web accumulate mercury from their food. Methylmercury in aquatic food webs increases at every trophic level (biomagnifies) to reach highest levels at the top of the food web, such as in top-predator fish (fish that eat mostly other fish). Methylmercury levels in top-predator fish are typically more than one million times higher than methylmercury levels in the water that the fish inhabit, and nearly all of the mercury found in fish tissue is methylmercury.
Some fish may be unsafe to eat because of high mercury levels; however, fish are an important part of a healthy diet, so the best thing people can do is to become informed. Visit websites of the USEPA, FDA, and your State’s health agency to find out which fish from which waterbodies in your area are safe or not safe to eat. The USEPA and FDA provide guidelines so that the public can make informed decisions about which fish species are safe to eat—these agencies recommend including fish as part of a healthy diet but urge us to choose kinds (species) of fish that are lowest in mercury. They also let us know which fish to minimize or avoid. The USEPA has established a criterion of 0.3 part per million (mg/kg, wet weight) for methylmercury in fish for the protection of people who consume average amounts of fish.
The USEPA has established a web site with information on fish-consumption advisories associated with mercury contamination. This site links to both Federal and State advisories. State agencies typically produce more locally tailored information, including maps, lists of fish species collected from specific water bodies, average concentrations of mercury, and specific consumption guidelines. Fish consumption advice is often tailored to sensitive populations (children and women of child-bearing age) and non-sensitive populations.
The purpose of this study was to determine the geographic and geochemical characteristics of stream basins that relate to fish-mercury levels in streams. The study involved a one-time sampling of streams across the U.S. for mercury in fish, water, and streambed sediment. Nearly all the mercury in fish is in the methylmercury form, so it is important to assess methylmercury in natural waters to understand fish-mercury levels. Methylmercury and other chemical characteristics were measured in water and streambed sediment.
The streams were selected to represent a range of stream ecosystem types across a large geographic range. We targeted streams in watersheds that were agricultural, urbanized, undeveloped (forested, grassland, shrubland, and wetland land cover), and mined (for gold and mercury). The streams span a range in environmental conditions.
The detection of mercury in every fish sample was not a surprise because mercury can be transported long distances in the atmosphere, and the science is such that we are now very good at detecting low levels of mercury. The fact that a quarter of fish samples were above the USEPA mercury criterion was also not surprising because 48 of 50 States have fish consumption advisories. This means that 48 out of 50 States have at least one commonly consumed fish species that exceeds the 0.3 ppm USEPA criterion for the protection of people who eat average amounts of fish. Mercury is currently the second leading cause of impaired waters in the United States, accounting for over 9,000 impaired waters (as of 26 August 2009). So the findings were not a surprise, but emphasize widespread occurrence of methylmercury contamination of aquatic ecosystems.
Atmospheric deposition (rain, snow, dry particles) is the main source of mercury to most of the streams in our study. Of the 291 streams sampled for fish, 59 were potentially affected by historical mining of mercury or gold. Stream basins that were designated as “mined” were treated separately for the purposes of our data analyses; however, this distinction was made only for data analyses in our report and does not necessarily imply impacts of mining in these basins. The main source to most aquatic environments in the US is from atmospheric deposition. See the following questions “Has mercury always been in fish? Is atmospheric mercury natural?”
Although there has always been some mercury in the atmosphere from natural sources (volcanoes and degassing of elemental mercury from the oceans), human activities have increased the amount of mercury emitted to, and deposited from the atmosphere. Anthropogenic (human-caused) sources of mercury to the atmosphere are largely from combustion of materials that contain mercury, with coal-combustion (electric utility boilers and commercial/industrial boilers) being the largest source in the U.S., according to the 1997 EPA Report to Congress.
In 2007, an international panel of experts concluded, “remote sites in both the Northern and Southern hemispheres demonstrates about a threefold (2X–4X) increase in Hg [mercury] deposition since preindustrial times” (Lindberg and others, 2007). In the most remote parts of North America, for example lakes in Glacier Bay, Alaska, current rates of atmospheric mercury deposition are about double what was observed in pre-industrial times (Engstrom and Swain, 1997). In the continental U.S., proximity to more mercury emission sources has resulted even larger increases—typically about three to four times pre-industrial rates (Lorey and Driscoll, 1999; Swain and others, 1992; Van Metre and Fuller, 2009).
There are numerous human-influenced disturbances which result in greater conversion of inorganic mercury to methylmercury. These include reservoir construction (Bodaly and others, 1997) and sulfate loading (a widespread atmospheric pollutant, and occasionally associated with land use) (Drevnick and others, 2007; Gilmour and others, 1998; Jeremiason and others, 2006). Such disturbances can increase the amount of methylmercury in aquatic ecosystems, including fish that inhabit those ecosystems, even without a change in mercury loading.
mineralization occurs in very limited areas (see chapter “Environmental Impact
of Mercury Mines in the Coast Ranges,
High levels of
mercury in fish, and methylmercury in stream water, were found in two general
settings: (1) Parts of the eastern
Mercury levels in fish were related to the concentration of methylmercury in stream water; the density or abundance of evergreen forest and wooded wetland in a stream’s watershed; increasing dissolved organic carbon concentration; and decreasing pH. The positive correlation with wetland abundance was expected because researchers have long known that these landscapes are particularly conducive to conversion of inorganic mercury to methylmercury.
Decisions by the USEPA regarding a new national mercury emissions regulation will be significantly aided by the improved scientific understanding provided by this study of how mercury sources, watershed cycling, and stream-based food webs interact. Previous to this study, a very limited number of studies had delved into the details of what controls mercury contamination levels in stream ecosystems.
Section 112 of the 1990 Clean Air Act Amendments (CAAA) identify seven priority air pollutants, of which mercury is one, and require the USEPA to identify the sources of 90% of each pollutant and subject these sources to maximum achievable control technologies. Current considerations for mercury by the USEPA are specific to coal and oil fired electric utilities. Our study results relate to CAAA Section112(d)(2), which specify that "any non-air quality health and environmental impacts" can be considered before making a determination on standards for new or existing sources. Our study results provide many new insights into factors regulating mercury contamination levels of stream-based food webs, including the importance of methylmercury sources within watersheds. Finally, the results from this study support the notion that not all locations are equal in terms of how they respond to mercury loads and changes to mercury loads. It is important for decision makers to realize that different watersheds, and often different areas within the same watershed, may respond differently to changes in atmospheric mercury loads.
Lead author, Barbara Scudder (firstname.lastname@example.org); 608-821-3832
Coauthor, Mercury in Stream Ecosystems study coordinator, Mark Brigham (email@example.com); 763-783-3274
Coauthor, Lia Chasar (firstname.lastname@example.org); 850-553-3649.
Coauthor, David Krabbenhoft (email@example.com); 608-821-3843
Links to the interpretive and data reports, and to related journal papers and methods reports, are at: http://water.usgs.gov/nawqa/mercury/pubs/
Bodaly, R.A., St. Louis, V.L., Paterson, M.J., Fudge, R.J.P., Hall, B.D., Rosenberg, D.M., and Rudd, J.W.M., 1997, Bioaccumulation of mercury in the aquatic food chain in newly flooded areas, in Sigel, A., and Sigel, H., eds., Metal ions in biological systems: Mercury and its effects on environment and biology: New York, Marcel Decker, Inc., p. 259-287.
Drevnick, P.E., Canfield, D.E., Gorski, P.R., Shinneman, A.L.C., Engstrom, D.E., Muir, D.C.G., Smith, G.R., Garrison, P.J., Cleckner, L.B., Hurley, J.P., Noble, R.B., Otter, R.R., and Oris, J.T., 2007, Deposition and cycling of sulfur controls mercury accumulation in Isle Royale fish: Environmental Science and Technology, v. 41, p. 7266-7272.
Engstrom, D.R., and Swain, E.B., 1997, Recent declines in atmospheric mercury deposition in the Upper Midwest: Environmental Science and Technology, v. 31, no. 4, p. 960-967.
Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., and Nater, E.A., 1998, The case for atmospheric mercury contamination in remote areas: Environmental Science and Technology, v. 32, no. 1, p. 1-7.
Gilmour, C.C., Riedel, G.S., Ederington, M.C., Bell, J.T., Benoit, J.M., Gill, G.A., and Stordal, M.C., 1998, Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades: Biogeochemistry, v. 40, p. 327-345.
Jeremiason, J.D., Engstrom, D.R., Swain, E.B., Nater, E.A., Johnson, B.M., Almendinger, J.E., Monson, B.A., and Kolka, R.K., 2006, Sulfate addition increases methylmercury production in an experimental wetland: Environmental Science and Technology, v. 40, p. 3800-3806.
Lindberg, S., Bullock, R., Ebinghaus, R., Engstrom, D., Feng, X., Fitzgerald, W., Pirrone, N., Prestbo, E., and Seigneur, C., 2007, A synthesis of progress and uncertainties in attributing the sources of mercury in deposition: Ambio, v. 36, no. 1, p. 19-32.
Lorey, P., and Driscoll, C.T., 1999, Historical trends of mercury deposition in Adirondack lakes: Environmental Science and Technology, v. 33, no. 5, p. 718-722.
Swain, E.B., Engstrom, D.R., Brigham, M.E., Henning, T.A., and Brezonik, P.L., 1992, Increasing rates of atmospheric mercury deposition in midcontinental North America: Science, v. 257, p. 784-787.
Van Metre, P.C., and Fuller, C.C., 2009, Dual-Core Mass-Balance Approach for Evaluating Mercury and 210Pb Atmospheric Fallout and Focusing to Lakes: Environmental Science and Technology, v. 43, no. 1, p. 26-32.
Wiener, J.G., Knights, B.C., Sandheinrich, M.B., Jeremiason, J.D., Brigham, M.E., Engstrom, D.R., Woodruff, L.G., Cannon, W.F., and Balogh, S.J., 2006, Mercury in soils, lakes, and fish in Voyageurs National Park (Minnesota)—Importance of atmospheric deposition and ecosystem factors: Environmental Science and Technology, v. 40, p. 6261-6268.