Dyken Pond: Mercury in Fish/Power Plants: Mercury in Coal-Part II
Dyken Pond: Mercury in Fish/Power Plants: Mercury in Coal-Part II
by John Leahy (Eastwick Press, July 21, 2006)
Part I of this series discussed the fish consumption advisory that was released by the NYS Department of Health (NYSDOH) for Dyken Pond due to the elevated levels of mercury in largemouth bass. NYSDOH recommends eating no more than one meal per month of largemouth bass taken at Dyken Pond due to elevated mercury levels. Women of child bearing age and children are urged to refrain completely from eating any largemouth bass from Dyken Pond. Part II will now attempt to answer the question – where does the mercury come from?
Mercury is an element that occurs everywhere in nature and is found as a component in many rock types. It is released into the environment through natural processes like volcanoes and erosion and through human activities such as fuel combustion, mining, metallurgy, and various other industrial processes. Man’s activities, particularly the combustion of coal, are increasing the rate at which mercury is released into the environment and increasing the potential hazards associated with mercury. According to the US Geological Survey (USGS), studies of sediment cores indicate that mercury concentrations in sediments deposited since the industrial revolution tend to be three to five times greater than mercury concentrations in older sediment layers. New York State Department of Environmental Conservation (DEC) Spokesperson Kimberly Chupa said that mercury deposition generally comes from the burning of coal to create electricity. According to US Environmental Protection Agency (EPA) data, about 43% of national mercury air emissions now come from coal fired power plants. Industrial boilers and gold mining both contribute an additional 10% each, incineration of hazardous waste and chlorine production contribute about 5% each, and other processes contribute smaller amounts. EPA data also shows that government efforts have reduced mercury air emissions by 45% between 1990 and 1999, mostly by removing mercury from municipal and medical waste incinerators.
The most important feature in the release of mercury air emissions is the mercury content of a fuel. Mercury is found as a small impurity in coal and content varies with different coal types. USGS data shows an average mercury content for US coals to be 0.17 ppm (part per million). Studies like one performed by the Indiana Geological Survey, demonstrate that mercury has a high affinity for sulfur molecules, resulting in higher sulfur containing lignite and subbituminous coal typically having higher mercury content as well. Other fuels burned in power plants like natural gas have much lower mercury contents and correspondingly lower mercury air emissions.
According to EPA, about 50 tons of mercury are emitted into the atmosphere by coal combustion in power plants each year. After combustion, mercury can remain suspended in the atmosphere and travel great distances and thus be transported all around the globe. Mercury deposition is found as far away as the arctic and it is very possible that mercury deposited in New York State could have originated from coal combustion in mid west power plants.
When mercury precipitates out of the air it is deposited on land or in water bodies where it accumulates in bottom sediments. In the water sediments the mercury is acted on by bacteria and converted to a highly toxic form known as methylmercury. Methylmercury, an organic form of mercury, is the form in which human beings are usually exposed to mercury. Fish absorb methylmercury that is dissolved in water as it passes through their gills or by consuming sediments as bottom feeders. The methylmercury accumulates in fish flesh and concentrates as it moves up the food chain. With concentrations of mercury in sediments typically being on such a small scale and problematic to measure, the indicator usually relied upon to show the presence of mercury is the quantity found in fish.
According to the USGS, studies have shown that the methylation process is accelerated in conditions of high acidity and dissolved organic carbon content. This compounds the impact of the combined water quality problems and creates a synergistic effect in which the total impact is greater than the sum of the individual contamination problems. In addition, since methylmercury can be broken down by the ultraviolet rays in sunlight, methylmercury is most likely to concentrate in bodies of water where the rate of methylation is greater than the reverse process of detoxifying. The conditions of acidity frequently found in bodies of water in the northeast and north central US are therefore more likely to foster the development and accumulation of methylmercury than regions that feature less acidic water.
What steps are federal and state governments taking to control the spread and accumulation of mercury in our environment and to protect the population from health hazards associated with mercury? Most of these steps focus on issuing fish consumption advisories when deemed appropriate, reducing air emissions of mercury, and removing mercury from industrial processes. Part III of this series will look at these regulatory efforts and the some of the criticism that they have generated.