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Summary-- Mercury in Bed Sediment and Aquatic Biota from NAWQA Study Units

Dennis A. Wentz--February 16, 2000

Data from 1991 NAWQA Study Unit summary reports and from a national pilot study of Hg in selected 1991, 1994, and 1997 NAWQA Study Units reveals the following major findings.

Sources and Significance

Atmospheric deposition is the primary source of Hg to most aquatic ecosystems in the eastern United States. The highest atmospheric deposition rates for total Hg are found along the northeastern coast of the United States from northern Virginia to southern Maine, but rates in the Ohio River Valley are only slightly lower (U.S. Environmental Protection Agency, 1997). Deposition rates are much lower west of the Mississippi River than to the east.

Only a small fraction of atmospheric Hg occurs as MeHg; however, inorganic Hg is readily methylated in the natural environment as a result of bacterially mediated sulfate reduction. MeHg is readily bioaccumulated and biomagnified, is the primary form of Hg in fish, and is a potent neurotoxin to humans and wildlife (fish, birds, and mammals).

Criteria, Standards, and Guidelines

Fish Consumption.

In their guidance to commercial processors of fish and fishery products, the U.S. Food and Drug Administration (FDA) (1998) has established a methyl mercury "action level" of 1 µg/g wet weight for the edible portions of all fin fish, crustaceans, and mollusks. States often use this value when deciding to issue consumption advisories or to close certain waters for commercial harvesting.

In 1998, Hg accounted for 77 percent of all fish consumption advisories issued in the United States and its territories (U.S. Environmental Protection Agency, 1999). The number of advisories has increased steadily since 1993 and was 8 percent higher in 1998 than in 1997. Advisories are now in effect in 40 States; 10 States--all in the northeast--have issued statewide advisories for their freshwater rivers and/or lakes. All of Lakes Superior and Michigan also have fish consumption advisories based on Hg.

Aquatic Life.

There are no well-established and nationally accepted guidelines for the protection of aquatic life from consumption of Hg.

Occurrence and Distribution in 1991 NAWQA Study Units

Information from 1991 Study Unit summary reports indicates a number of possible sources for elevated Hg concentrations in bed sediment and aquatic biota.

  • Atmospheric deposition is a likely source for elevated Hg concentrations in bed sediment of forested, agricultural, and mixed land-use basins in the ACFB and GAFL Study Units.

  • In the ACFB Study Unit, Hg concentrations in Asiatic clams were positively correlated with organic carbon concentrations in bed sediment and were highest in Coastal Plain basins where conditions favoring Hg methylation were prevalent (Couch, 1997). In the ALBE Study Unit, the highest Hg concentrations in fish were from Coastal Plain blackwater streams with low pH and high concentrations of DOC.

  • Concentrations of Hg and seven other trace elements increased with time in sediment cores from Amistad International Reservoir in the RIOG Study Unit. All eight trace elements are associated with the burning of fossil fuel, especially coal, suggesting that the trace-element source had an atmospheric component.

  • Hg concentrations in bed sediment were higher in basins draining urban areas than in basins draining agricultural, forested, or mixed land use in the ACFB, GAFL, HDSN, WILL, and WMIC Study Units.

  • Hg concentrations in bed sediment were elevated downstream from mined sulfide ore deposits in the NVBR, RIOG, and WILL Study Units. Cinnabar has been mined in the NVBR and WILL Study Units; geothermal discharges may also contribute Hg to bed sediment in the NVBR Study Unit.

  • A long-term industrial Hg source contaminated about 170 miles of the Shenandoah River and a tributary in the POTO Study Unit. A fish consumption advisory has been issued for this stretch of river.

A few 1991 Study Units reported that the FDA "action level" of 1 ppm was not exceeded in fish and/or Asiatic clams. This comparison is not easily made because the FDA action level has been set for edible tissue, whereas NAWQA Study Units typically submit whole fish (for small species, such as sculpin) or fish livers (for larger fish) for Hg analysis. Goldstein and others (1996) have provided regression relationships that may be useful in some situations for estimating Hg concentrations in muscle from those in whole fish or fish livers. The relationship between Hg concentrations in muscle and those in liver was significant for carp and marginally significant for channel catfish. Goldstein and others (1996) also showed that the relationship between Hg concentrations in muscle and in whole fish was significant and consistent among a variety of fish species.

National Hg Pilot Study

During the summer and fall of 1998, 21 NAWQA Study Units (2-1991 starts; 6-1994 starts; and 13-1997 starts) from across the Nation participated in a Hg pilot study sponsored jointly by the NAWQA and Toxics Substances Hydrology Programs (Krabbenhoft and others, 1999; Brumbaugh and others, in preparation). Total Hg and MeHg were analyzed in bed sediment and total Hg (assumed to be predominantly MeHg) was analyzed in fish muscle tissue.

  • MeHg concentration in bed sediment correlated positively and most strongly with the concentration of acid volatile sulfide (AVS) and with the percentage of wetland in the basin. MeHg in bed sediment also correlated positively with loss on ignition (LOI, a surrogate for organic carbon) and dissolved organic carbon (DOC), and negatively with surface water pH. High DOC concentrations and low pH generally are indicative of environments with high metal mobility.

  • When normalized to LOI, MeHg production (as estimated from MeHg concentrations) appears to be proportional to total Hg concentration in bed sediment at total Hg concentrations less than about 1 µg/g).

  • Normalization of total Hg concentrations in bed sediment by LOI and by atmospheric Hg deposition rates provided an index of atmospheric Hg accumulation (AHA) and allowed basins with important nondepositional sources to be identified. The COOK, GRSL, NROK, NVBR, SACR, and UCOL Study Units showed signatures explainable by mining of sulfide ores; the OAHU Study Unit exhibited a signature that probably was influenced by Hg in volcanic emissions; and the YELL Study Unit yielded a signature that may have been elevated by Hg from hot springs. Of the study units sampled, the SOFL Study Unit (lowest AHA) was most representative of areas with predominantly atmospheric sources.

  • Mixed agricultural/forested basins showed the highest Hg methylation efficiencies (as measured by MeHg/total Hg ratios) in bed sediment and water. Mined basins showed the lowest methylation efficiencies, and background/reference, agricultural, and urban basins were intermediate.

  • For all fish species, Hg concentrations in fish were most strongly correlated with MeHg concentrations in water. The highest correlations were found for Hg concentrations in fish that had been normalized to age or length. Hg concentrations in fish were less strongly correlated with MeHg concentrations in bed sediment, and were poorly correlated with total Hg concentrations in either water or bed sediment.

  • Fish with Hg concentrations above 0.5 µg/g wet weight generally were from coastal or lowland Study Units (ACAD, LINJ, MOBL, SANT, SOFL) where relatively high percentages of wetlands may enhance methylation rates. Exceptions include the NVBR, SACR, and YELL Study Units, which have important nondepositional Hg sources.

  • In ranking basins by Hg bioaccumulation in fish, normalization of Hg concentrations in fish to either length or age produced similar results to those obtained with non-normalized concentrations; however, normalization by weight sometimes produced anomalous results.

  • The highest Hg concentrations in fish were found in mixed agricultural/forested basins, followed by basins impacted by mining. Urban basins exhibited the lowest Hg concentrations in fish; agricultural and background/reference basins were intermediate.

References

Brumbaugh, W.G., Krabbenhoft, D.P., Helsel, D.R., and Weiner, J.G., in preparation, A national pilot study of mercury contamination of aquatic ecosystems along multiple gradients—Part 2: Bioaccumulation in fishes.

Couch, C.A., 1997, Environmental distribution of mercury related to land use and physicochemical setting in watersheds of the Apalachicola-Chattahoochee-Flint River Basin: in Hatcher, K.J., ed., Proceedings of the 1997 Georgia Water Resources Conference, University of Georgia, Athens, p. 33-39.

Goldstein, R.M., Brigham, M.E., and Stauffer, J.C., 1996, Comparison of mercury concentrations in liver, muscle, whole bodies, and composites of fish from the Red River of the North: Canadian Journal of Fisheries and Aquatic Sciences, V. 53, p. 244-252.

Krabbenhoft, D.P., Weiner, J.G., Brumbaugh, W.G., Olson, M.L., DeWild, J.F., and Sabin, T.J., 1999, A national pilot study of mercury contamination of aquatic ecosystems along multiple gradients, in Morganwalp, D.W., and Buxton, H.T., eds., Proceedings of the 7th national Toxics Substances Hydrology Program meeting, Charleston, South Carolina, March 8-12, 1999, Volume 2, Contamination of hydrologic systems and related ecosystems: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, p. 147-160.

U.S. Environmental Protection Agency, 1997, Mercury study report to Congress: U.S. Environmental Protection Agency report EPA-452-97-003-010, 1980 p.

U.S. Environmental Protection Agency, 1999, Update—National listing of fish and wildlife advisories: U.S. Environmental Protection Agency report EPA-823-F-99-005, 6 p. (pdf version available at http://www.epa.gov/OST/fish/ )

U.S. Food and Drug Administration, 1998, Fish and fishery products hazards and controls guide, 2d ed.: U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, 276 p. (html and pdf versions available at URL http://vm.cfsan.fda.gov/~dms/haccp-2.html )

VanMetre, P.C., Mahler, B.J., and Callender, Edward, 1997, Water-quality trends in the Rio Grande/Rio Bravo Basin using sediment cores from reservoirs: U.S. Geological Survey, Fact Sheet FS-221-96, 8 p.

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Last modified: 18Feb2000