Historical Background of Mercury in the Environment
Mr. Charles Moore
South Carolina Department of Natural Resources

Mercury is a basic chemical element of our solar system. There is a fixed amount on earth that cannot be created or destroyed. Mercury cycles through the earth's biosphere; including the atmosphere, surface waters, aquatic sediments, soils, as well as all plant and animal life. Mercury emissions into the environment can be characterized by three sources: the natural release and cycling of geologically bound mercury, anthropogenic releases, and the re-emission of mercury to the atmosphere from that previously deposited. EPA estimates 50 to 75 percent of the mercury released annually comes from human activities. Of approximately 200,000 tons of mercury emitted to the atmosphere since 1890, about 95 percent resides in terrestrial soils, 3 percent in ocean surface waters and 2 percent in the atmosphere. Mercury is a known toxicant, affecting growth, reproductive success, and development of both plant and animal life. It is a neurotoxin that bioaccumulates through the food chain with its primary pathway to humans being through the consumption of fish.

The natural global bio-geochemical cycling of mercury involves the degassing of mercury from soils and surface waters, atmospheric transport, and the deposition of mercury back to the land and open water. It may then be either re-volatized into the atmosphere or converted to insoluble mercury sulfide that is absorbed to the soil, or bio-converted into more volatile or soluble forms that re-enter the atmosphere or are bioaccumulated in aquatic and terrestrial food chains.

Mercury occurs in three oxidation states. Metallic or elementary mercury has no charge and quickly vaporizes from its liquid form. Over 50% to 95% of the mercury found in the atmosphere is gaseous mercury (HgO) that has a residence time in the atmosphere of between 6 days and 2 years. During this time it is transported great distances, circulating globally. Elementary mercury is not very soluble and atmospheric water (rain and snow) does not serve as a significant means of transfer. Elemental mercury in the atmosphere is oxidized by ozone, hydrogen peroxide, hypochlorite or organoperoxide compounds.

Reactive gaseous mercury (mercuric, with a double electric charge, and mercurous, with a single positive charge) occur at much lower levels than the elementary form representing approximately 3% of the total gaseous mercury in the air. These forms are water-soluble and are removed from the air by gravity (dry deposition), and by rain, snow, dew and humidity (wet deposition). They have an atmospheric residence time of hours to days. Mercury adsorbed onto organic and inorganic microparticulates, may range from less than one percent to 40% of the total ambient mercury level in industrialized areas. Particulate forms are effectively removed by rain and have a relatively short residence time in the atmosphere.

Man's activities that release mercury into the environment are a complex combination of (a) activities that directly emit or inject mercury into the air, soil or water and (b) industrial utilization in products that eventually may be returned to the environment though landfills, combustion, or other means. Estimates of the annual total global input to the atmosphere from all sources including natural, anthropogenic, and oceanic emissions is about 5,500 - 6,000 tons with US sources estimated to have contributed about 3 percent. Mercury today is utilized in the electrical industry (switches, thermostats, batteries etc.), dentistry (dental amalgams, which are 50% mercury), medicinal products including antiseptics (mercurochrome), laxatives, worming medications, teething powers, pharmaceutical preservative products (thimerosal), a red tattoo dye, measuring devices, (thermometers), numerous industrial processes (the production of chlorine and caustic soda), in nuclear reactors, as an anti-fungal agent (wood processing), a solvent for reactive and precious metal, a coloring agent for paint as well as numerous other uses.

Global production of mercury, primarily from cinnabar (mercuric sulfide) mines, has declined 38% from 5,356 tons in 1990 to 3,337 tons in 1996. U.S. mercury production has declined from more than 2,000 metric tons per year in the 1970's, to less than 500 metric tons in 1996, most resulting from secondary sources and industrial recovery. Although the domestic use of mercury has shown a downward trend since the early 1970s mercury imports (277 metric tons in 1995) have escalated in recent years as a result of the suspension of mercury sales from the National Defense Stockpile in 1994, which had formerly been a major supplier of mercury to the domestic market.

Of the estimated 158 tons of mercury emitted annually into the atmosphere by human activities in the United States, approximately 87 percent is from combustion point sources, 10 percent from manufacturing, and 3 percent from all other sources. Of this total, about one-third (52 tons) is deposited within the lower 48 states and two-thirds (107 tons) is transported outside of U.S. borders. An additional 35 tons is deposited within US borders from the global reservoir for a total annual mercury deposition of 87 tons. Four specific source categories (all high temperature waste combustion of fossil fuel processes) account for approximately 80 percent of total mercury emissions in the U.S.: coal-fired utility boilers (33 percent), municipal combustion (19 percent), commercial /industrial boilers (18 percent) and medical waste incinerators (10 percent). In 1994, electric power plants built during the 1940's to 1970's emitted an estimated total of 91,422 pounds of mercury. The vast majority (95%) came from coal-burning plants, and most of that was from plants built prior to 1977 (77%).

According to EPA documents the amount of mercury in the atmosphere is estimated to have increased by 200 % to 500 % since the beginning of the industrial revolution. Others report that there is 3 to 6 times more mercury today vs. pre-industrial times in Atlantic Ocean water, Atlantic bird feathers, peat bogs, soils and lake sediments. Whereas mercury deposition rates have decreased in the vicinity of some localized sources in the western United States during the 1990s, measurements continue to increase in remote sites in northern Canada and Alaska indicating that the global atmospheric burden is continuing to increase.

The production and utilization of mercury are decreasing both on a worldwide and national level. However, based on past mercury releases, it may take decades and perhaps longer, before we observe measurable declines in the environment and affected biological systems. Increasing background levels of mercury increase the potential impact of emissions from local point sources to affected areas.

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