For the latest marine science and other ocean news, subscribe to Ocean Update, Marine Science Review and our other free newsletters.

Contaminated Sediments

The Issue

• The muds that lie at the bottom of estuaries and other coastal waters, as well as lakes and rivers, often are contaminated with toxic chemicals of human origin-including toxic metals, oil, and synthetic organic chemicals. Nearly all contaminated sediments contain a complex soup of toxic substances, though the relative importance of particular contaminants varies from location to location.
• Contaminants accumulate, sometimes to very high concentrations, in animals living in or upon sediments and animals that then prey upon them.
• Toxic contaminants lead to a severe reduction in the diversity of bottom dwelling organisms that live in affected estuaries or coastal regions. And adverse effects can spread, via the food chain, to fish, birds, and mammals that feed on contaminated sea life.
• Those species that persist despite contamination may be subject to chronic ailments including diseases, deformities, and reproductive maladies. Because they often contain high concentrations of toxic chemicals in their tissues, these organisms become a threat to human health when eaten.
• The contamination of marine sediments becomes an important political issue when ports are dredged and contaminated dredged materials have to be dumped someplace, and it becomes a human health issue when fisheries are affected due to contaminated or diseased fish.

The Causes

• The major sediment contaminants are synthetic organic chemicals (e.g., PCB's, chlorinated pesticides) and toxic metals (e.g., Mercury, Cadmium)-the products, byproducts and wastes of industry.
• Contamination from human activities on land 15 washed down rivers and transported through the air into estuaries and the ocean. A smaller portion of contamination results directly from activities at sea, such as shipping, offshore oil and gas exploration and seabed mining.
• The land sources are varied and may be categorized as discrete (or point) sources and diffuse (or non-point) sources. Industrial and sewage treatment pipelines are classic examples of point sources, while agricultural and urban runoff are the major non-point sources.
• Municipal discharges through sewage treatment plants and combined sewer overflows include an array of toxic contaminants, from households, industries linked to the sewage treatment facilities, and storm water runoff from city streets.
• Pesticides drain off farmlands; toxic substances wash from roadways, residential and commercial areas, construction sites, and marinas and shipyards; chemical emissions from industrial processes and the combustion of fuel coal and wastes produce particuates that drift in the atmosphere and fall onto the ocean; and toxic metals wash out from mine sites.

The Context

• The contamination in bottom sediments may have been there a long time-the remnants of bygone industries, spills, dumping activities, shipping, etc. Or they may be recent and ongoing.
• Because of the physical and chemical nature of some sediments, many pollutants will cling to them and accumulate to concentrations much higher than in the overlying waters. From there they seep out into the water as it sloshes over the bottom. These contaminants may also be ingested or absorbed by animals feeding in or contacting the mud.
• As an individual animal consumes or absorbs contaminants from its food and surroundings. the contaminants are often stored in body tissue and concentrate to higher and higher levels over time-a phenomenon called bioaccumulation. When the toxic substances enter the living part of the ecosystem, they become ever more concentrated as they are passed along the predatory food chain-a phenomenon called biomagnification.
• The EPA and the Army Corps of Engineers have developed guidelines to determine whether dredged sediments are too contaminated to be dumped in another place in the marine environment. However, in practice, most coastal dredged material is permitted for dumping in marine waters even when there is concern about contamination.
• The EPA has recently produced a report on sediment quality "The National Sediment Quality Survey," which highlights the extent of the contaminated sediments problem in this country. This could be used as a screening device for identifying sediments that need careful analysis before allowing them to be dredged and dumped. However, there are no sediment quality criteria and standards similar to the well established, legally binding EPA Water Quality Standards.
• There are sediment decontamination methods available or in the research stage. These include incineration, chemical detoxification, and biological decontamination. None have been routinely applied to sediments inplace or to dredged materials; and some may themselves be problematic. One management technique often promoted is capping-the covering up of contaminated sediments with dean sand or day in place or at dump sites; however some experts question its long-term effectiveness.
• The EPA and NOAA are working together on better implementation of non-point source controls, which could reduce the future contamination of sediments in coastal waters off the mouths of rivers.

Further Reading

Long, E.R., D.D. MacDonald, S.L. Smith, and F.D. Calder. 1995. Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments. Environmental Management 19 81-97.

US. Environmental Protection Agency. 1996. The National Sediment Quality Survey A Report to Congress on the Extent and Severity of Sediment Contamination in Surface Waters of the United States. EPA-823-D-96-002. Office of Science and Technology, Washington, DC.