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  Mercury - Bed Sediment and Pore Water  
   
     
 

 

Background

PhotographMercury is a pollutant of global significance. It is a naturally-occuring element that is mobilized by natural and anthropogenic means. At the regional and global scale, contamination occurs primarily through atmospheric deposition--reaching even remote areas of the world. The vast majority of environmental mercury contamination via natural and anthropogenic sources is inorganic mercury (Wiener qand others, 2002). Inorganic mercury, though toxic, is much less toxic than its organic form, methylmercury. Methylmercury is created in wetlands and bed sediments of aqueous systems primarily through the activity of certain types of bacteria. Methylation of the mercury atom enables the compound to move easily through biological membranes. In areas with sufficient methylmercury levels, this compound will accumulate in organisms more rapidly than it is excreted; this phenomenon is termed bioaccumulation. Bioaccumulation at the primary producer trophic level concentrates methylmercury to much higher levels than the surrounding surface water. These organisms are consumed by organisms at higher trophic levels, thereby concentrating the mercury; this phenomenon is termed biomagnification. It occurs all the way through to the highest trophic levels (Weiner and others, 2002). As of 2004, 44 states have listed fish advisories due to mercury contamination (USEPA, 2005).

PhotographThe health effects to humans, stemming from eating fish at these higher trophic levels are primarily neurologic (ATSDR, 1999). Risk of injury is highest to the young, either through direct injestion or through in vitro influence of a contaminated mother. The U.S. Environmental Protection Agency and the Food and Drug Administration recommend these populations limit their intake of fish and shellfish such as shrimp, canned light (non-albacore) tuna, salmon, pollock and catfish to 12 ounces per week (USEPA, 2005).

Studies of sediment and pore water are particularly important in understanding mercury cycling through biological compartments due to their role in methylmercury production. Pore water is the water located between sediment particles, and it therefore directly influences the microorganisms mediating mercury methylation. Its composition reflects an equilibrium with the surrounding sediment, and it impacts the bioavailability of toxins, such as mercury, to bacteria (Di Toro and others, 1991).

PhotographThe majority of mercury research done, thus far, has been conducted on mercury levels in lakes, reservoirs, and wetlands. Given the large volume of recreational fishing that occurs on streams however, it was apparent that more studies needed to be done (U.S. Fish and Wildlife Service and U.S. Census Bureau, 2002). The National Water-Quality Assessment Program (NAWQA) has undertaken two studies to look at mercury levels in streambed sediment: the Mercury Synoptic Study and the Mercury Topical Study. The more detailed of these studies, the Mercury Topical Study, included a concurrent investigation of pore water chemistry.

Mercury Synoptic Study

Click for location of sites sampled for the Mercury Synoptic and Mercury Topical studies in the Western Lake Michigan Drainages (WMIC) study unitThe goal of the Mercury Synoptic Study was to get a snapshot of the mercury levels in waterbodies (primarily streams) throughout the United States by conducting a one-time sampling of sediment, fish and water. Over 200 sites were sampled in 1998, 2002, 2004 or 2005. The Western Lake Michigan Drainages (WMIC) study unit, which occupies portions of Wisconsin and the Upper Peninsula of Michigan, contained 8 of these sites; these sites were sampled in the summer of 2002. All sediment samples were analyzed for a variety of constituents. No pore water samples were collected for this study.

A number of interesting findings relating to sediment have been reported from the 1998 data:

  • Methylmercury production is proportional to inorganic mercury levels in the sediment when levels are low. At high concentrations of total mercury, however, increases in concentration have less effect on methylmercury levels in the system (Krabbenhoft and others, 1999).
  • Basin land use was an important factor (Krabbenhoft and others, 1999):
    • Basins containing a high density of wetlands had higher methylmercury levels in sediment than those with fewer wetlands.
    • Basins that contained a mix of agricultural and forested land had the lowest total mercury levels in sediment, but the highest sediment levels of methylmercury (other land use categories considered were agriculture, reference, mining, and urban).
  • Geographically, sites on the eastern coast of the United States had the highest methylation efficiency (Krabbenhoft and others, 1999).
  • Bioaccumulation in fish showed significant correlation with methylmercury levels in sediment; no significant correlation was seen with levels of total mercury (Brumbaugh and others, 2001).

All sampling for the Mercury Synoptic Study has been completed. Results from the remaining two cycles (2002 and 2004/2005) are currently being reviewed, and, once completed, will be combined with the 1998 data. Once combined, these clean datasets will be analyzed, and the results summarized. Data is still being loaded into the database; available data can be obtained by the public. Reports summarizing the results will be available to the public upon publication.

Mercury Topical Study

Click for location of sites sampled for the mercury topical studyIn contrast with the Mercury Synoptic Study, the Mercury Topical study had fewer, but more intensely sampled sites. This study focused on 8 sites in three geographically-separated study units: WMIC with 3 sites, the Georgia-Florida Coastal Plain Drainages (GAFL) with 3 sites, and the Willamette Basin (WILL) with 2 sites.

Sampling for the Mercury Topical Study consisted not only of sediment, fish and surface water, but included pore water, algae, and benthic macroinvertebrates. Sediment samples were collected concurrently with pore water samples on a quarterly basis from 2003 through 2004. Collection efforts were designed to examine the temporal and spatial trends within and between streams. Samples were analyzed for a variety of constituents.

The study described here refers to the first cycle of the Mercury Topical Study. Additional study units have been selected for a second cycle. The sampling for this first cycle has been completed, and data is being checked for accuracy. Clean datasets are currently undergoing analysis. Data is being loaded into the database; available data can be obtained by the public. Reports summarizing the results will be available to the public upon publication.

Table 1.  List of general constituents analyzed for in sediment and pore water samples taken in conjunction with the Mercury Synoptic and Mercury Topical studies.

Media

Study

Constituents sampled

Sediment

 

 

 

Mercury Synoptic and Mercury Topical

Total mercury

 

Mercury Synoptic and Mercury Topical

Methylmercury

 

Mercury Synoptic and Mercury Topical

Acid-volatile sulfide

 

Mercury Synoptic and Mercury Topical

Stable carbon and nitrogen isotopes

 

Mercury Synoptic and Mercury Topical

Particle size

 

Mercury Topical

Methylation rate potentials

 

Mercury Topical

Demethylation rate potentials

Pore water

 

 

 

Mercury Topical

Total mercury

 

Mercury Topical

Methylmercury

 

Mercury Topical

Dissolved organic carbon

 

Mercury Topical

Sulfide

 

Mercury Topical

Nutrients

 

Mercury Topical

Anions

 

Other Items:

References:

Agency for Toxic Substances and Disease Registry (ATSDR), 1999, Toxicological profile for mercury: Atlanta, GA, U.S. Department of Health and Human Services, Public Health Service.

Di Toro, D.M., Zarba, C.S., Hansen, D.J., Berry, W.J., Swartz, R.C., Cowan, C.E., Pavlou, S.P., Allen, H.E., Thomas, N.A., and Paquin, P.R., 1991, Technical basis for establishing sediment quality criteria for nonionic chemicals using equilibrium partitioning: Environmental Toxicology and Chemistry, v. 10, p. 1541-1583.

Brumbaugh, W.G., Krabbenhoft, D.P., Helsel, D.R., Wiener, J.G., and Echols, K.R., 2001, A National Pilot Study of Mercury Contamination of Aquatic Ecosystems Along Multiple Gradients-Bioaccumulation in Fish: Biological Sciences Report 2001-0009.

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- U.S. Geological Survey Toxic Substances Hydrology Program Proceedings of the technical meeting, Charleston, S.C., March 8-12, 1999. U.S.G.S. Water-Resources Investigations Report 99-4018B, Vol 2, p 147-160.

U.S. Environmental Protection Agency (USEPA), 2005, 2004 National listing of fish advisories: Office of Water, EPA-823-F05-004, 6 p.

U.S. Fish and Wildlife Service and U.S. Census Bureau, 2002, 2001 National survey of fishing, hunting, and wildlife-associated recreation, [variable pagination].

Wiener, J.G., Krabbenhoft, D.P., Heinz, G.H., and Scheuhammer, A.M., 2002, Ecotoxicology of Mercury, in Hoffman, D.J., and others, eds., Handbook of ecotoxicology (2d ed.): Boca Raton, Fla., Lewis Publishers, p. 409-463.

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