Mercury is a persistent pollutant that has adverse effects on the neurological development of humans, fish and wildlife. Of particular concern is mercury exposure to the developing fetus or children from subsistence fishing populations. While these concerns are heightened in areas that exhibit substantial contamination from mercury use, it is important to note that remote areas that are free from point sources can also have elevated levels of mercury in fish. This is largely because of two mechanisms: (1) mercury emitted to the atmosphere can travel great distances before depositing; and, (2) microorganisms in the local environment can efficiently convert inorganic mercury into methylmercury – the form that bioaccumulates up the food web.
A long-term problem
Mercury is truly a global issue that has confronted resource managers and regulators for decades. In the 1980s, researchers could not reliably measure the concentration of mercury in environmental samples due to the very low levels (parts per trillion) generally observed. This made it difficult to consider remediation strategies – even into the start of the 21st century. Improvements in speciation and analytical techniques have steadily progressed since then, along with the reliability of the data produced. In recent years, scientific understanding of possible solutions has steadily emerged, and today it is possible to identify and track the sources of mercury through an ecosystem.
Sources of mercury
Mercury has many sources, including some that are dispersed globally. In freshwater systems, besides naturally occurring mercury, the dominant sources of mercury are atmospheric deposition, watershed inputs, and industrial point-source discharges. Research has shown that human-induced sources contribute more mercury to the environment than natural sources.
Natural sources of mercury
Natural sources of atmospheric mercury include volcanoes, metalliferous geologic deposits, and volatilization from the ocean. Although all rocks, sediments, water, and soils naturally contain small amounts of mercury, scientists have found locations where with mercury enrichment due to geologic placement. In the United States, these are most often observed in California and Alaska.
Atmospheric mercury, the most commonly distributed source of mercury in the U.S., includes emissions from coal combustion, chlorine alkali processing, waste incineration, and metal processing. Once in the atmosphere, mercury is widely disseminated and can circulate for years, accounting for its global distribution.
Atmospherically-deposited mercury to terrestrial settings eventually accumulate in soils. Most of this soil-bound mercury is associated with organic carbon (e.g, soil humus) and is often transported with eroded soil particles, like those eroded during a storm. Runoff can carry watershed mercury downstream to receiving rivers, lakes, and wetlands.
Man-related (point-source) discharges
Man-related releases of mercury can contribute locally concentrated amounts of mercury into receiving water bodies. These point-source discharges include a wide variety of processing, including industrial or mining discharges, chlor alkali plant, metal processing, medical and other wastes, and mercury and gold mining. In the past, man-related sources mercury contributed a much larger amount of the total releases in the United States. However, since recognition of these sources became more clearly understood and environmental polices lead to significant reductions (and in some cases elimination) of some of these source categories, and it is only in developing countries where these sources remain prominent.