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

Applications of fertilizers, manure, and pesticides have degraded the quality of streams and shallow ground water in agricultural areas, which make up more than 50 percent of the continental United States.

Nutrients (nitrogen and phosphorus) are often elevated in agricultural areas

• Nitrate concentrations exceeded the U.S. Environmental Protection Agency (USEPA) drinking water standard in about 20 percent of shallow wells sampled under farmland. High levels of nitrate in drinking water can be harmful to humans, particularly infants.
  
  Areas that are most vulnerable to nitrate contamination include the Central Valley of California and parts of the Pacific Northwest, the Great Plains, and the Mid-Atlantic region.
  
  
• Average annual concentrations of phosphorus exceeded the USEPA desired goal for preventing nuisance plant growth in nearly 80 percent of streams sampled.
  
  Excessive plant growth in streams can lead to low levels of dissolved oxygen, which can harm fish and other aquatic life.

Pesticides, especially herbicides, are widespread in agricultural areas, and commonly occur in mixtures

• At least 1 pesticide was found in more than 95 percent of stream samples; about two-thirds of the samples contained 5 or more pesticides. Pesticides were in more than 60 percent of shallow wells; about one-third of the wells had 2 or more pesticides.
  
  Pesticide concentrations generally are below USEPA drinking-water standards. The risk to humans and the environment from these low-level exposures is unclear, and standards do not exist for many pesticides or for mixtures of pesticides.
  
  
• Herbicides-especially atrazine and its breakdown product desethylatrazine (DEA), and metolachlor, cyanazine, and alachlor-occur more frequently and usually at higher concentrations in streams and ground water in agricultural areas than in urban areas.
  
  
• Insecticides that were used in the past--especially DDT, dieldrin, and chlordane-still persist in streams and sediment. At least one guideline for sediment quality was exceeded at more than 20 percent of sites. This means that concentrations are high enough to be toxic to clams and other aquatic invertebrates and can affect the food supply of fish.