The findings show that for both urban and agricultural areas, nonpoint chemical contamination is an issue. Much work still needs to be done in urban areas with point source contamination as well, including infrastructure improvements. Appreciable improvements in overall water quality, however, will depend upon effective management of point and nonpoint sources. The findings show that nonpoint chemical contamination is an agricultural and urban issue.
Contaminant concentrations vary from season to season and from watershed to watershed.
Even among seemingly similar land uses and sources of contamination, different areas can have very different degrees of vulnerability and, therefore, have different rates at which improved treatment or management can lead to water-quality improvements. Water Quality in Agricultural Watersheds Nitrogen and phosphorus in surface water commonly exceed levels that contribute to excessive algae. Excessive plant growth can lead to low dissolved oxygen, which can be harmful to fish and other aquatic life. Nitrate is often elevated above background levels in shallow ground water underlying farmland. These natural features enable rapid infiltration and downward movement of water and chemicals.