Water quality and nonpoint sources in urban watersheds
Key findings from the first decade of NAWQA studies

Water resources in U.S. urban areas are impaired. ("Urban" refers primarily to residential and commercial development since World War II.) Many of the impairments are attributable to nonpoint sources, such as vehicle emissions and applications of pesticides and fertilizers around homes and in public areas.

Contaminants are widespread in urban streams, and biological communities are stressed

• Concentrations of total phosphorus exceeded the desired goal of the U.S. Environmental Protection Agency (USEPA) to control nuisance plant growth in more than 70 percent of urban streams. Excessive plant growth can lead to low levels of dissolved oxygen (hypoxia). Hypoxic conditions can be harmful to fish and other aquatic life.
  
  
• Insecticides are widespread and usually at higher concentrations than in agricultural areas. Two popular insecticides--diazinon and chlorpyrifos--are often found mixed together. Herbicides are in 99 percent of urban streams sampled; simazine and prometon commonly are found together.
  
  Pesticide concentrations rarely exceeded USEPA drinking-water standards, but every stream sampled exceeded at least one guideline for protecting aquatic life. Moreover, standards do not exist for many pesticides or for mixtures of pesticides
  
  
• Fecal coliform bacteria commonly exceeded recommended standards for water recreation.
  
  
• Biological communities are dominated by algae and aquatic invertebrates that can tolerate pollution, such as worms and midges, and omnivorous fish, such as catfish and largescale suckers.

Volatile organic compounds (VOCs) are widespread in shallow ground water in urban areas

• Most frequently detected are the solvents trichloroethene (TCE), tetrachloroethene (PCE), and methylene chloride; the gasoline additive methyl tert-butyl ether (MTBE); and chloroform, the solvent and disinfection by-product of water treatment.

Sediment and fish tissue in urban areas reflect past chemical use as well as current trends

• Banned insecticides such as DDT, chlordane, and dieldrin still persist in sediment and fish tissue. Concentrations exceeded guidelines to protect wildlife at more than 10 percent of urban sites. High concentrations of polychlorinated biphenyls (PCBs), which were used in insulation and as a lubricant but have not been sold since 1979, were also found in most fish tissue; guidelines were exceeded at nearly 70 percent of urban sites. Some states have issued fish-consumption advisories.
  
  
• Lead, mercury, cadmium, and zinc are elevated above background levels in sediment, most likely caused by emissions from industrial activities and motor vehicles. Lead concentrations have been decreasing, however, since being removed from gasoline in the 1970s.