Pesticide Residues in Drinking Water

Pesticides enter surface and ground water primarily as runoff from crops and are most prevalent in agricultural areas. Pesticides are also used on golf courses, forested areas, along roadsides, and in suburban and urban landscape areas. Since World War II herbicide and insecticide application to crops has grown to an estimated 660 million pounds of active ingredient in 1993 (1). Without proper safeguards pesticides have the potential to seriously threaten many groundwater supplies in the United States. Approximately 50% of the U.S. population obtains its drinking water from groundwater sources and as much as 95% of the population in agricultural areas uses groundwater as its source of drinking water.

'Pesticide' is a general term for substances which are used to poison pests (weeds, insects, molds, rodents, etc.). The pesticides most acutely dangerous to man are insecticides and rodenticides, although pound for pound, herbicides are the most widely used type of pesticide(2). Not every pesticide is acutely toxic to humans or other non-target species.

On a national scale less than 2% of wells sampled in multi-state studies were found with pesticide concentrations above the established Maximum Contaminant Level (MCL) (3). Due to repeated detection of various pesticides in U.S. wells, the U.S. Environmental Protection Agency (EPA) recently proposed a State Management Program (SMP), which would control or ban pesticides with the greatest potential to contaminate groundwater(4). Five pesticides were initially selected due to the frequency of their occurrence: alachlor, atrazine, cyanazine, metolachlor, and simazine. According to the EPA they all have been detected in many states, and have the potential to reach levels which exceed health based standards. They are all associated with serious health effects including cancer.

The five selected pesticides are herbicides which are used to control broadleaf weeds and grasses. The EPA estimates between 200 and 250 million pounds of these herbicides are applied annually in the U.S. Atrazine, simazine, and cyanazine are applied to agricultural land before and after planting. Alachlor and metolachlor are applied to soil prior to plant growth (pre-emergent).

There are several factors which influence a pesticides' potential to contaminate water:

Groundwater contamination is higher when there is no crop or a young crop. A large actively growing crop has the ability to reduce pesticide concentration through a variety of mechanisms (5):

The Safe Drinking Water Act sets standards for drinking water and mandates the Environmental Protection Agency set Maximum Contamination Levels (MCLs) for a number of pesticides in public water supplies. Private water supplies are not monitored or regulated by this Act. Since pesticides are most prevalent in agricultural areas where most residents obtain their drinking water from private sources, it is up to the consumer or well owner to monitor contaminant levels. Testing and treatment options are available to the public. If you suspect contamination or if a neighbor has experienced contamination contact your health department.

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1. Aspelin, A.L. 1994. Pesticides industry sales and usage, 1992 and 1993 market estimates: U.S. EPA, Office of Pesticides Programs, Biological and Economic Analysis Div., Economic Analysis Branch Report 733-K-94-001, p.33.

2. Pimentel, D., L. McLaughlen, et al. 1991. Environmental and economic impacts of reducing U.S. agricultural pesticide use. In D.Pimentel and A.A. Hanson (Eds.) CRC Handbook of Pest Management in Agriculture. 2nd ed. vol I. CRC Press. Boca Raton. pp679-718.

3. Pesticides in Groundwater. 1996. Volume 2 of the series Pesticides in the Hydrologic Environment. Barbash, J.E. and E.A. Resek. Ann Arbor Press, Inc. Chelsea, Mich.pp 425.

4. Federal Register. Wednesday, June 26,1996. Part VII. Environmental Protection Agency. 40 CFR Parts 152 and 156.

5. Gustafson, D.I. Pesticides in Drinking Water. 1993. Pub by Van Nostrand Reinhold, New York, NY. pp 4.

This page was prepared by T.L. Pedersen , UCD EXTOXNET FAQ Team.
June 1997