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An Overview
Sediments are loose particles of sand, clay, silt, and other
substances that settle at the bottom of a water body. They
come from eroding soil and from decomposing plants and animals.
Wind, water, and ice often carry these particles great distances.
Many of the sediments in our rivers, lakes, and oceans have
been contaminated by pollutants. Some of these pollutants,
such as the pesticide DDT and the industrial chemicals known
as polychlorinated biphenyls (PCBs), were released into the
environment long ago. The use of DDT and PCBs in the United
States was banned in the 1970s, but these chemicals persist
for many years.
Other contaminants enter our waters every day. Some flow
directly from industrial and municipal waste dischargers,
while others come from polluted runoff in urban and agricultural
areas. Still other contaminants are carried through the air,
landing in lakes and streams far from the factories and other
facilities that produced them. In cases like this, the sediment
may serve as a contaminant reservoir or source of contamination.
Experts believe that contaminated sediments are a widespread
and serious problem. Areas of concern are found on the Atlantic
and Pacific coasts, in the Gulf of Mexico and the Great Lakes,
and along inland waterways.
Contaminated sediments affect small creatures such as worms,
crustaceans, and insect larvae that inhabit the bottom of
a water body in what is known as the benthic environment.
Some kinds of toxic sediments kill benthic organisms, reducing
the food available to larger animals such as fish.
Some contaminants in the sediment are taken up by benthic
organisms in a process called bioaccumulation.
When larger animals feed on these contaminated organisms,
the toxins are taken into their bodies, moving up the food
chain in increasing concentrations in a process known as
biomagnification. As a result, fish and shellfish,
waterfowl, and freshwater and marine mammals, as well as benthic
organisms, are affected by contaminated sediments.
Species that cannot tolerate the toxic contaminants found
in some sediments simply die, reducing the variety of organisms,
also known as biodiversity, in the affected
environment. Animals that survive exposure to contaminated
sediments may develop serious health problems, including fin
rot, tumors, and reproductive effects.
When contaminants bioaccumulate in trout, salmon, ducks,
and other food sources, they pose a threat to human health.
In 1998, fish consumption advisories were issued for more
than 2,506 bodies of water in the United States. Possible
long-term effects of eating contaminated fish include cancer
and neurological defects.
Contaminated sediments do not always remain at the bottom
of a water body. Anything that stirs up the water, such as
a storm or a boat's propeller, can resuspend
some sediments. Resuspension may mean that all of the animals
in the water, and not just the bottom-dwelling organisms,
will be directly exposed to toxic contaminants.
Every year, approximately 300 million cubic yards of sediment
are dredged to deepen harbors and clear shipping lanes in
the United States. Roughly 3 - 12 million cubic yards of these
sediments are so contaminated they require special, and sometimes
costly, handling. If dredging to improve navigation cannot
be conducted because sediments are contaminated, the volume
of shipping on these waterways will decline.
No single government agency is completely responsible for
addressing the problem of contaminated sediments. A variety
of laws give federal, state, and tribal agencies authority
to address sediment quality issues. Private industry and the
public also have roles to play in contaminated sediment prevention.
Increasing public awareness of the problem is crucial to developing
an effective solution.
contaminated sediment home |
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locating contaminated sediments
species affected |
protecting sediments from contamination
management options |
preventing contaminated sediment
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