Sources of Mercury
Natural sources of mercury include volcanoes, forest fires,
cannabar (ore) and fossil fuels such as coal and petroleum.
Levels of mercury in the environment are increasing due to discharge
from hydroelectric, mining, pulp, and paper industries. Incineration
of municipal and medical waste and emissions from coal-using power
plants also contribute to high levels of mercury.
Mercury released from ongoing human activity in
the U.S. can be separated into four broad categories. The first
category is “area sources”. Landfills, dental preparations,
and laboratory use are defined as area sources. The second category
is combustion processes. These include coal-fired power generation,
medical waste incinerators, and municipal waste combustors. The
third category is the manufacture of metals, alkali, and cement.
Other industrial processes fall into the fourth category.
Coal Burning Power Plant
In the past, mining was a substantial source of mercury
in some areas. For example, the hydraulic placer-gold mines
of the Sierra Nevadas released several thousand tons of
mercury to the environment from the 1860s to the early 1900s.
The U.S Geological Survey (USGS) believes that high levels
of mercury in fish, amphibians, and invertebrates downstream
of hydraulic mines are a result of historic mercury use.
Power plants are now the largest anthropogenic source of Mercury
in the United States. To address this concern, new legislation
has been proposed to cut emissions of pollutants from power plants.
However, different parties disagree on how mercury should be regulated.
In December 2004, the Environmental Protection Agency will reach
its deadline for setting maximum achievable control technology
(MACT) provisions for mercury under the Clean Air Act.
Mercury in the air is deposited into the water. Bacteria in lake,
stream, and ocean sediments then convert elemental mercury into
organic mercury compounds such as methylmercury.
Mercury is able to travel long distances in the air. There is
a global reservoir of airborne mercury circulating worldwide at
any one time. Both natural and anthropogenic emissions contribute
to the global mercury reservoir. It is estimated that the annual
global input of mercury into the reservoir is 4,900 tons.
In 1995, it was estimated that forty percent (32 metric tons
(t)) of mercury deposited form the air onto U.S. water and soil
came from the global mercury reservoir. The other sixty percent
came from anthropogenic sources in the U.S. There is uncertainty
at this time as to how long some forms of mercury persist in the
The “recycling” of anthropogenic mercury also raises
levels of mercury in the environment. Recycling takes places when
mercury in water volatizes and contributes to the increase of
atmospheric mercury concentrations.