Background

Atmospheric deposition [figure 1] is generally accepted as the principal mechanism of mercury introduction into aquatic ecosystems. New England is thought to have one of the highest deposition rates anywhere in the United States. Major sources of mercury deposition in New England are emissions from municipal waste incinerators, coal and oil boilers, and medical waste incinerators. Through environmental processes, mercury (Hg) in its inorganic form can be transformed into methyl mercury (MeHg) and accumulate in aquatic organisms. Urbanization is postulated to affect the production and accumulation rates of MeHg. The major route of human exposure to mercury is through the diet, more specifically from consumption of fish. MeHg is the most toxic and bioaccumulative form of mercury in the environment, and comprises greater than 95 percent of the mercury in fish. Elevated mercury concentrations in fish have resulted in the issuance of fish consumption advisories [figure 2] in all states of New England and throughout the Northeastern United States.

Figure credits:
Figure 1. Modeled/predicted mercury deposition rates for the contiguous 48 states (from Russ Bullock, NOAA/EPA)
Figure 2. Fish consumption advisories for mercury (Krabbenhoft and Rickert, 1995)

Purpose and study design

Results of a national assessment of mercury contamination of aquatic systems found that the New England Coastal Basins area had among the highest mercury concentrations and rates of mercury methylation in the nation (Krabbenhoft and others, 1999; Brumbaugh and others, 2001). Because of these findings, the New England Coastal Basins study team, in collaboration with the USGS Toxic Substances Hydrology Program, conducted a regional study of how total mercury (HgT) and MeHg in water and streambed sediments and HgT in fish varied in relation to the amount of urbanization in a watershed. The purposes of this study were to (1) determine whether gradients in HgT and MeHg contamination are evident along urban gradients; and (2) to evaluate whether Hg loading rates or ecosystem factors were more influential in generating regional Hg gradients in fish in New England streams.

Water and bed sediment were sampled during 1998 - 2000 from 55 stream sites, and fish tissue from a subset of 27 sites. Sites were selected to span a range from a major metropolitan area (Boston), where mercury loading is presumably greater, to comparatively remote regions of southern Maine, where known mercury emissions are far fewer and more distant. Most of the sites were chosen to be located in one ecoregion - the Northeastern Coastal Zone - to minimize the effect of natural features on study results. Sediment, water, and fish tissue samples were collected over a one week period during summer low flow conditions to show patterns of HgT and MeHg accumulation and partitioning relative to watershed conditions.

See Table 1 for site names and characterization.

Study Location Maps
click for full view Sediment and water sampling locations	click for full view Fish tissue sampling locations

General results

Concentrations of HgT ranged from 0.7 to 13.7 nanograms per liter (ng/L) in water and from 7.2 to 3,100 nanograms per gram (ng/g) dry weight in streambed sediment. HgT concentrations followed an urban gradient with highest concentrations in urban areas and lowest concentrations in rural areas in Maine and New Hampshire. Concentrations of MeHg ranged from 0.04 to 1.8 ng/L in water and from 0.3 to 15.6 ng/g dry weight in sediment. Concentrations of MeHg in water and sediment did not follow an urban gradient, but were positively correlated with concentrations of organic carbon. HgT concentrations in fish tissue ranged from 40 to 398 ng/g wet weight and were positively correlated with concentrations of MeHg in water and bed sediment. A positive relation was not observed between HgT concentrations in fish tissue and HgT concentrations in water and bed sediment. Methylation efficiency, as estimated by MeHg/HgT, ranged from 0.003 to 0.282 for sediment and water samples, with a median value of 0.071. Methylation efficiency was highest at sampling sites with low urbanization and high organic carbon concentrations. (Chalmers and Krabbenhoft, 2001)

Protocols used for the NECB Mercury study

Fitzgerald, W.F., and Watras, C.J., 1989, Mercury in surficial waters of rural Wisconsin lakes: Science of the Total Environment, v. 87/88, 277-291.

Olson, M.L., and DeWild, J.F., 1999, Low-level techniques for the collection and species-specific analysis of low levels of mercury in water, sediment, and biota, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical 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. 191-200.

Shelton, L.R. and Capel, P.D., 1994, Guidelines for collecting and processing samples of stream bed sediment for analysis of trace elements and organic contaminants for the National Water-Quality Assessment program, U.S. Geological Survey Open-File Report 94-458, 20 p.

References

Brumbaugh, W.G., Krabbenhoft, D.P., Helsel, D.R., Weiner, J.G., and Echols K.R., 2001, A national pilot study of mercury contamination of aquatic ecosystems along multiple gradients: Bioaccumulation in fish [PDF format, 1.8 MB]: U.S. Geological Survey Biological Science Report, BSR-2001-0009, 25 p.

Chalmers, A.T., and Krabbenhoft, D.P., 2001, Total and methyl mercury distribution in water, sediment, and fish tissue in New England streams, American Geophysical Union (AGU) Spring Meeting, May 29-June 2, 2001, Boston, Mass.

Krabbenhoft, D.P., Wiener, 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., U.S. Geological Survey Toxics Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999 - Volume 2 - Contaminants of Hydrologic Systems and Related Ecosystems: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, 13 p.

Krabbenhoft, D.P. and Rickert, D.A., 1995, Mercury Contamination of Aquatic Ecosystems: U.S. Geological Survey Fact Sheet 216-95, 4 p.

Related information

USGS Mercury Studies Team

Mercury in the Environment (USGS Fact Sheet, October 2000)

Fish consumption advisories (Mineral Resources Program, USGS)

New England Coastal Basins Mercury Deposition Network