- 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
- 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
- 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
- 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 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
- 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
- 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.
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,