The bottom of streams, rivers, lakes, ponds, mudflats, and even oceans is made up of materials that were deposited there by the natural forces of currents (a constant flow of water in a predominant direction), gravity (attraction between two masses), and flows of incoming streams and rivers. This material, consisting of soil, pebbles, silt, clay and other material, is known as sediment. Sedimentation (the deposit of sediments) becomes a problem if it is contaminated by toxic (poisonous) chemicals or harmful microorganisms. Just as soil and other material is carried to the bottom of water bodies, harmful chemicals or organisms can collect on the sediments.
The problem of sediment contamination is increasing in many areas throughout the world. The United States Environmental Protection Agency (EPA) conducted a survey across the country in 1998 in which they found hundreds of contaminated sites. Many of these were located in coastal areas, which are rich habitats for plant and animal life. According to the EPA, every major harbor in the United States has some degree of contamination in the local sediment.
Consequences of sediment contamination: bioaccumulation
Sediment is often a rich source of food for the living creatures in fresh or salt water. For example, rivers deposit large amounts of sediment into deltas, the point where the river enters the sea. When the tide goes out, the sediment is uncovered. This muddy region can be home to clams, which in turn become food for animals such as seals.
Toxic materials in the sediment can be taken in by small creatures such as mussels or clams. When many of these smaller animals are eaten by a larger animal (such a seal), the toxins become more concentrated in the larger creature. This pattern can be repeated as predator organisms eat the usually smaller prey organisms (a relationship that is called the food web or food chain), and the concentration of the harmful chemical becomes greater. The increasing concentration of harmful substances accumulated through the food web is known as bioaccumulation. Bioaccumulation of toxic substances can cause illness, birth defects, or death to affected organisms, including humans.
In the 1950s, people in Japan whose diet contained several servings of tuna per week suffered nerve and brain damage from eating tuna that contained high levels of the metal mercury. In 2004 the U.S. Federal Drug Administration published new recommendations about including tuna and other fish in the diets of young children and women who are of childbearing age. The recommendations suggest that persons in these age groups should limit light tuna to two six-ounce (170 grams) servings per week, and that canned albacore tuna and fresh tuna be limited to one six-ounce serving per week.
Other examples of sediment contamination
By the 1960s pesticides (chemicals designed to kill or harm insects and pests) became more popular and were used in agriculture, on recreational areas such as golf courses, and neighborhood lawns. Correspondingly, pesticides began to appear in water and sediment.
PCB Effects on Bird Populations
Polychlorinated biphenyls (PCBs) were plastic materials made in the United States beginning in 1929 and ending in 1977, although some other countries continue to use PCBs. The persistence of PCBs in sediment and in sediment-dwelling creatures is well documented. The effects that these compounds have on birds higher up in the food chain are a concern to scientists, but are less clear.
Studies have shown that female birds that contain higher than normal level of PCBs lay eggs that have thinner and more fragile shells. When the mother bird sits on the eggs to keep them warm, the eggs can break, killing the developing chick. As well, PCBs appear to cause malfunctioning in the structure of the genetic material. Defects that have occurred in offspring of female birds that were exposed to PCB include beaks that cross (making it hard to feed), extra toes, malformed feet, and liver disease. Together, these various defects make survival of the bird species more difficult. Some species affected in the past or currently affected by PCB contamination include the bald eagle, some raptors, some owls, and the kestrel.
Polycyclic aromatic hydrocarbons (PAH) are harmful chemicals produced when coal, oil and gas, garbage, or other substances such as tobacco or charbroiled meat are burned. Many PAHs are caused by natural events such as forest fires and volcanoes, but most of the PAH particles in the air come from the exhaust of automobiles. PAH compounds have a structure that is difficult to break apart in water. As PAH particles come into contact with surface water, either from the air or through industrial or municipal (community) wastewater, their solid structure causes them to sink and stick to sediment particles. PAH particles can also move through soil layers to contaminate groundwater (freshwater in rock and soil layers beneath Earth's surface)
Many heavy metals (metallic elements such as mercury and lead) can cause illness even if present in humans at low levels. Heavy metals including lead, cadmium, cobalt, nickel, and mercury, which can all gather in sediment, can alter blood cell formation. As well, heavy metals can damage the liver, kidneys, blood circulation, nerves, and may also be a trigger of cancer. Heavy metals have been found in sediments downstream from many of the world's major cities, such as the Limmat River downstream from Zürich, Switzerland, and the Pearl River Delta between Hong Kong and Macao, China. Industrial wastes, sewage, litter, marine boat traffic, and runoff from mines are all potential sources for heavy metal contamination of sediment.
Historical sediment contamination
Because chemicals can stay in sediment for decades or longer, sediment contamination is sometimes due to activities that were ended years ago. For example, the pesticides chlordane and DDT were recognized as a threat to the environment and were banned from use in North America in the 1960s. Yet these chemicals are still recovered in small numbers from some sediments.
The knowledge that sediment contamination can be a long-term problem makes the effort to reduce sediment contamination challenging for scientists. Even though the levels of some toxic compounds are likely to remain high in sediment for some years to come, the outlook is promising. The restricted use of unhealthy pesticides and other chemicals should eventually reduce their levels in the sediment.
Brian Hoyle, Ph.D.
For More Information
Postel, Sandra, and Brian Richter. Rivers for Life: Managing Water for People and Nature. Washington, DC: Island Press, 2003.
U.S. Department of Health and Human Services and U.S. Environmental Protection Agency. "EPA-823-R-04-005: What You Need to Know About Mercury in Fish and Shellfish." U.S. Food and Drug Administration Center for Food Safety and Applied Nutrition.http://www.cfsan.fda.gov/~dms/admehg3.html (accessed on September 1, 2004).