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Water: Contaminated Sediments

Basic Information

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.

What are the contaminants?

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.

Where are contaminated sediments located?

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.

What species are affected?

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.

Possible long-term effects of eating contaminated fish include cancer and neurological defects.

Are these contaminents a threat to human health?

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.

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