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 table.)

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 areas.
  

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.