Answer: Trace essential elements such as fluorine, copper, selenium, molybdenum, and
others can be hazardous to living organisms if
present at high levels. Nonessential heavy metals such as arsenic, lead,
mercury, cadmium, chromium are usually toxic to organisms as much lower levels
than trace essential elements. Depending on the association that these
nonessential elements may form with natural geologic materials such as organic
matter, other elements or minerals, and adsorbers (such as clays), these elements
can range from being safe to being extremely toxic.
Because of growing public concern about the environmental contamination, it is
becoming increasingly important to better understand both the natural and human
processes that control the movement of elements at the Earth's surface. Elements
can be quite mobile in water, and the majority of our environmental problems are
ultimately associated with the contamination of surface and ground water.
When water comes into contact with rocks and soils, some of the minerals and
organic substances dissolve and enter the natural waters. Forests and grasslands
generally contribute only small amounts of these dissolved substances. However,
it is possible for an area to contain unusually high concentrations of minerals,
thereby depositing them to the waterways. For example, swamps and marshes often
produce acidic and colored water. Other areas that contributed natural
pollutants to water are those containing rocks with sulfide minerals,
particularly pyrite.
Inorganic substance are cycled naturally through our
environment at concentrations that usually do no adversely affect plants and
animals. However, the combination of some natural processes with human
activities can increase these substances to harmful or toxic levels. Therefore,
toxic substances may have both natural and human sources. Natural sources of
toxic substance include rocks, volcanoes, sediments, and soils. Human activities
that add toxic substances to the environment include smelting, manufacturing,
refining, chemical processing, fertilizer application, irrigation, and waste
disposal.
A large concentration of a substance commonly identifies a source of
pollution but may not necessarily indicate a problem. In addition to the
concentration, other characteristics of the substance must be considered. These
characteristics include the amount of the substance released, the rate of
release, its availability to organisms, and its residence time in a particular
ecosystem. (From USGS Circular 1105.) |
