The USGS Water Science School
Pesticides are chemical and biological substances intended to control pests, such as insects, weeds, bacteria, and algae. Pesticides are heavily used on farmland, but in urban areas, the main usage is on residential and commercial properties. When storms hit, the runoff from yards and roadsides carry pesticides into local streams, where they may harm aquatic life and enter drinking-water supply intakes.
Results of USGS National Water-Quality Assessment (NAWQA) studies show that pesticides are widespread in streams and ground water sampled within agricultural and urban areas of the Nation. As expected, the most heavily used compounds are found most often, occurring in geographic and seasonal patterns that mainly correspond to distributions of land use and associated pesticide use. The frequency of pesticide contamination, however, is greater than expected. At least one pesticide was found in almost every water and fish sample collected from streams and in about one-half of all wells sampled. Moreover, individual pesticides seldom were found alone — almost every water and fish sample from streams and about one-half of samples from wells with a detected pesticide contained two or more pesticides.
For individual pesticides in drinking water, NAWQA results are generally good news relative to current water-quality standards and guidelines. Average concentrations in streams and wells rarely exceeded standards and guidelines established to protect human health. For aquatic life and wildlife, however, NAWQA results indicate a high potential for problems in many streams, particularly in urban areas, where concentrations of more than one pesticide often approached or exceeded established water-quality guidelines.
In urban areas, the wise usage of pesticides is the key to reducing pesticide problems that are increasingly occurring in our drinking water. Pesticides should only be applied when necessary and as recommended by the product labels. Persons applying pesticides should avoid spreading the product onto pavements, gutters, curbs, and storm drains.
Effects of urbanization Ground-water quality