Toxic Substances Hydrology Program
Clues to High Mercury Along the Gulf Coast
The unique atmospheric conditions along the coast of the Gulf of Mexico in the United States enhance the atmospheric deposition of mercury. The relatively high deposition rates and the region's high rainfall amounts and abundant wetlands lead to many locales that are conducive to generating methylmercury. Methylmercury is the most toxic form of mercury and the form that most readily enters food webs. These are the findings of a team of U.S. Geological Survey (USGS) scientists and their colleagues‡.
While inland freshwater wetlands are widely acknowledged to be important landscapes for the generation of methylmercury, little is known about the fate of methylmercury in coastal environments. Scientists do know that the northern Gulf Coast region has some of the highest rates of atmospheric deposition of mercury in North America.
Measuring Atmospheric Mercury Deposition
Using the USGS Mobile Atmospheric Mercury Laboratory, USGS scientists and their colleagues observed strong daily cycles in the formation of reactive gaseous mercury in the near coastal atmosphere. Reactive gaseous mercury is the dominant form of atmospheric mercury in atmospheric fallout and is most accessible to biological processes, including those that generate methylmercury. Observed reactive gaseous mercury concentrations were found to correlate with daily cycles in sunlight intensity and the concentrations of ozone. These two factors are indicative of oxidizing conditions in the atmosphere that could allow the transformation of more inert forms of mercury to reactive gaseous mercury. Peak reactive gaseous mercury concentrations (approximately 90 picograms per cubic meter of air [pg/m3]) were well above background concentrations measured in non-coastal areas (less than 25 pg/m3). The scientists hypothesize that the high rates of reactive gaseous mercury formation and the subsequent deposition or incorporation into raindrops, combined with the Gulf Coast region's high annual rainfall amounts are likely contributing factors to the region's overall high mercury deposition rates.
Fate of Mercury in Coastal Wetlands
Another team of USGS scientists and their university colleagues studied the occurrence of methylmercury in black-water swamps, marine marshes, and other wetlands in and around Lake Pontchartrain, Louisiana. They found that the water in wetland sediments (pore water) had elevated concentrations of methylmercury relative to river sediment pore water and to water in rivers and other open water areas, thus indicating that wetland sediments in coastal environments are a key location for methylmercury production. These results suggest that marine and brackish wetlands along the Gulf Coast are potential sources of methylmercury to estuarine food webs in the Gulf of Mexico region—food webs that include marine fish that humans consume.
These two studies can help water-resources managers, environmental regulators, and the scientific community understand why mercury deposition is elevated in these coastal settings and how methylmercury is produced. Funding for these studies was provided by the USGS Toxic Substances Hydrology Program, the U.S. Department of the Interior Landscapes Program, the USGS Mendenhall Research Fellowship Program, the USGS Hydrologic Research and Development Program, and the USGS Hydrologic Networks and Analysis Program.
Engle, M.A., Tate, M.T., Krabbenhoft, D.P., Kolker, A., Olson, M.L., Edgerton, E.S., DeWild, J.F., and McPherson, A.K., 2008, Characterization and cycling of atmospheric mercury along the central US Gulf Coast: Applied Geochemistry, v. 23, no. 3, p. 419-437, doi:10.1016/j.apgeochem.2007.12.024.
Hall, B.D., Aiken, G.R., Krabbenhoft, D.P., Marvin-DiPasquale, M., and Swarzenski, C.M., 2008, Wetlands as principal zones of methylmercury production in southern Louisiana and the Gulf of Mexico region: Environmental Pollution, v. 154, no. 1, p. 124-134, doi:10.1016/j.envpol.2007.12.017.