Transport and deposition of mercury
Once mercury has been released to the natural environment it can be transported long
distances either through the air or via water courses. Mercury is volatile, meaning it can
evaporate from a lake or river, enter the air, and drift downwind where it may be
deposited.
Mercury released to streams and lakes through sewage plants and storm sewers often
contaminates the sediments of lakes and rivers on a local scale. But on a wider scale,
most of the mercury from human activities that enters lakes is first deposited (through the
atmosphere by rain, snow or attached to small dust particles) to the watersheds, and then
partially washed away to the aquatic ecosystems. Mercury also enters lakes directly from
natural geologic sources. In reservoirs that have been created by hydro dams, very high
levels of mercury are often found as a result of natural processes that become greatly
accelerated by the flooding of land and decomposition of vegetation.
Atmospheric emissions are a major concern with respect to mercury entering the
environment. Mercury emitted to the atmosphere, either directly or indirectly, may do
one of three things. It may fall-out near the point where it is emitted; this is referred to as
local deposition, normally considered up to 50 km from the source. The portion of the
mercury emitted from an incinerator, metal refining plant, or coal-plant that is deposited
locally can vary widely.
Another portion of the mercury is transported long distances before it is deposited at
some-point down-wind; known as regional deposition this is usually considered to be
within several 100 km. High levels of mercury in eastern North America, for instance,
are due in part to the down-wind deposition of mercury emissions from a large number of
major industrial sources in the American Midwest, the Ohio Valley and Ontario.
The final portion of atmospheric mercury enters what is known as the global atmospheric
pool. This refers to mercury that circles the globe for a year or more within the Earth’s
major weather systems. Mercury from the global pool may be transported for thousands
of kilometers before being deposited. In Canada, high levels of mercury deposition in the
Arctic are thought to be a result of mercury from the global pool together with regional
emissions from northern Europe and Asia.
One of the interesting and more complicated aspects of mercury is that it can be
deposited on plants and trees either in rain or as dry dust, or to a less extent enter plants
from soil, through the roots. Soils and plants can re-emit mercury to the atmosphere,
either through evapo-transpiration of the leaves, or on a much smaller scale, when trees
are burned in forest fires or fireplaces.