We have heard from our parents not to play with the shiny little balls spilled from broken thermometers. We have heard rumors of health problems related to dental fillings. And we commonly hear warnings in the news not to eat mercury-contaminated fish. Why is mercury such a problem?
Mercury has always been a part of the environment in low concentrations. A trace element that can be found in bedrock all around the globe, mercury is also present in soil and plants. Although fire, flooding, and volcanic eruptions can release some mercury from these places into air and water, it is human activities that have created the current problem. Mining and manufacturing of mercury and, more importantly, the burning of fossil fuels, have released mercury from its relatively safe storage in the earth.
Today in New Hampshire and Vermont, 47 percent of the mercury in our air comes from local sources - primarily from burning home heating oil, wood, other fossil fuels, and municipal trash. The coal-burning industry in the Midwest accounts for another 30 percent. The remaining 23 percent comes from global sources.
Most of the Northeast is forested, and hence much of the mercury in the air around us is absorbed by leaves on trees and drops with them to the ground in the fall. Having an affinity for carbon, mercury atoms then attach themselves firmly to organic carbon molecules in the soil. In the forest, this is a fairly stable environment.
But what happens when there are heavy rains or floods? Jamie Shanley, of the U.S. Geological Survey in Vermont, has done research documenting annual flushes of mercury from the soil with snowmelt and major rain events. Soil erosion can also deposit mercury directly into rivers and streams, Shanley says, and efforts to reduce soil erosion on logging jobs, farms, and construction sites are key to keeping mercury bound up in the soil. Although each flush removes only a small amount of the total mercury present in the soil, it becomes significant if the mercury arrives in a waterbody that is low in oxygen and becomes methylated.
Methylation, or the formation of methyl mercury, takes place in anaerobic (oxygen-deprived) conditions, such as in a shallow wetland or swamp. Once formed, methyl mercury can move with soil water into nearby streams and lakes. Neil Kamman, of Vermont’s Department of Environmental Conservation, explains that repeated floodings continue to release methyl mercury into the water as anaerobic conditions are re-created each time a flooding occurs. These repeated floodings are most common as water levels change behind reservoir dams but also occur less frequently in beaver wetlands.
Arriving in a lake or pond, methyl mercury is absorbed initially into algae. The algae are then eaten by plankton and small fish low in the food chain, which are, in turn, eaten by larger fish. A big fish eating a small fish will absorb 95 percent of the mercury contained in the smaller fish. It is easy to see that, as you move up the food chain to bigger and bigger fish, mercury levels become greatly concentrated. The more levels in the food chain, the more mercury accumulates in the top level. The larger and longer-lived the fish, the more mercury it is likely to contain.
Methyl mercury has the potential to cause nervous system dysfunctions in humans. Though experts debate the mechanism and severity of mercury poisoning, all agree on at least a few points: fish do contain methyl mercury that can be absorbed when eaten; children and fetuses with still-developing nervous systems are the most susceptible; and populations where fish are a staple food item are at the greatest risk.
Stricter air pollution laws have reduced mercury emissions since the 1980s, but the many years of accumulated mercury in the soil remain a concern. Even if we stopped all mercury pollution today, mercury would remain a contaminant in our environment for a long time to come. With caution, however, the problem can be managed through careful land management practices.
To further reduce new mercury emissions into the environment, some obvious solutions include cleaning up smokestack emissions, improving mercury waste recycling, and phasing out mercury-added products like the old-fashioned thermometer. Current recommendations for fish consumption are available to help us manage our own personal health – try the Environmental Protection Agency website at www.epa.gov/mercury/index.html. Above all, continued research is necessary to answer the many remaining questions about how mercury moves in the environment.