Lakes And Bogs Reveal Historical Mercury Trends In Maine

ORONO, Maine -- Rates of atmospheric mercury deposition in Maine appear to have reached a peak in the early to mid-1970s and to have declined significantly by 1982, according to a report by University of Maine geologists published in the December issue of the journal Water, Air and Soil Pollution (v. 100: 271-186, 1997).

Stephen A. Norton, Gordon C. Evans and Steve Kahl of the UMaine Department of Geological Sciences used archived cores from Sargent Mountain Pond and Big Heath in Acadia National Park on Mt. Desert Island to determine historical trends in mercury deposition from the atmosphere to the Maine landscape. The cores were collected in 1982 and 1983.

"Studies of other lake cores collected over a wide region in Maine between 1995 and 1997 indicate a similar history of atmospheric deposition. This picture strengthens the argument that the trends seen at Acadia National Park are representative for the region," says Norton. Those studies are being performed by graduate students Gordon (Chris) Evans, Richard Bindler and Ashley Smith.

In recent years, biologists have found some of the highest mercury concentrations in the world in freshwater fish and bald eagles along the Maine coast. While the general pathway from soils and water to animals is understood, the details are still fuzzy. Ongoing research by these authors working in cooperation with Terry Haines, UMaine professor of zoology, and his students is adding information about how changes in atmospheric deposition rates, water chemistry and water levels affects mercury accumulation in fish and birds.

The report concludes that by 1970, human activities appear to have been responsible for a seven-fold increase in the rate of mercury deposition over natural levels. This increase is clearly from the atmosphere because the area of Big Heath that was cored receives all of its nutrients and pollutants from the atmosphere.

A 40-fold increase was also found for lead over natural background values. These conclusions are based on the assumption that both elements remained immobile after they were deposited in the accumulating peat.

The annual fluxes of mercury and lead were determined by chemical analysis of different slices of the peat and lake sediment. The age of each slice was determined by analysis of a naturally occurring form of lead, which is also deposited from the atmosphere.

Because the cores had been kept in storage for up to 15 years, they were also analyzed for the possibility that mercury loss might have occurred after they were collected. The research team took samples from each core and subjected them to a variety of elevated temperatures for a week. Mercury levels were determined before and after that time. It was determined that significant losses in storage were unlikely.

"If mercury losses had occurred during storage, then the increase we calculated over natural background levels would have been even greater," Norton adds.

The work was supported by the UMaine Water Research Institute, the National Science Foundation and the National Park Service. Research on other lakes in Maine is continuing.