The health of biological communities, including fish, can
be adversely affected by toxic substances in streambed sediment.
Streambed sediment is a habitat for biota, such as insects
and clams, which are consumed by fish.
Widespread historical use and environmental persistence of
many chemicals have resulted in their frequent detection in
streams throughout the Nation. There were downward trends
in some areas, but NAWQA studies found elevated levels of
many of these persistent contaminants in streambed sediment.
Once introduced into the aquatic environment, these contaminants
can ascend through the food
chain and bioaccumulate. As a result of bioaccumulation,
these contaminants are often present at higher concentrations
in fish tissue than in the sediment (see
NAWQA data show elevated concentrations of these chemicals:
- Organochlorine pesticides that are no longer used,
such as chlordane, dieldrin, and DDT -- Concentrations in
whole fish exceeded guidelines for the protection of fish-eating
wildlife at about 20 percent of all sampled sites.
- Polychlorinated biphenyls (PCBs), used until the
1970s for insulation in transformers and capacitors and
as a lubricant in gas pipeline systems -- Concentrations
exceeded guidelines for the protection of fish-eating wildlife
at more than 30 percent of all sites.
- Polycyclic aromatic hydrocarbons (PAHs), which
result from fossil fuel combustion and, therefore, are still
being introduced into the environment -- Concentrations
exceeded sediment-quality guidelines for the protection
of aquatic life at more than 40 percent of sites in urban
NAWQA information on organochlorine compounds and trace elements
in fish tissue is used by states
to establish and evaluate fish consumption advisories.
The U.S. Environmental Protection Agency has classified nine
trace elements (arsenic, cadmium, chromium, copper, lead,
mercury, nickel, selenium, and zinc) as "priority pollutants"
because they are toxic to aquatic organisms in low concentrations.