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Pesticides in U.S. Water

September 15, 1999

Streams in areas with significant agricultural or urban development almost always contain complex mixtures of nutrients and pesticides, according to a report released by the U.S. Geological Survey (USGS). The report, “The Quality of Our Nation’s Waters,” looks at water quality in 20 of the largest and most important river basins in the United States. Researchers found 83 pesticides and breakdown products in water and 32 pesticides in fish or streambed sediment. Although the study targeted the broadest and most complete range of pesticides ever measured in a single assessment, researchers were not able to test for many important pesticide compounds because of analytical and budget constraints. Pesticides not measured include glyphosate, acephate, dimethoate, methomyl and thiodicarb.

The report documents that concentrations of individual pesticides in samples from wells and as annual averages in streams were almost always lower than current U.S. Environmental Protection Agency (EPA) drinking-water standards and guidelines. However, more than one half of the streams sampled had concentrations of at least one pesticide that exceeded a guideline for protection of aquatic life.

According to USGS, the potential risk to people and to aquatic life can only be partially addressed based on available standards and guidelines. The health picture is made more complex by the lack of standards or guidelines for many pesticides and their breakdown products or metabolites. Currently EPA has established standards and guidelines for only 46 of the 83 pesticides and breakdown products measured in this study. Of the thousands of possible pesticide breakdown products, few have been looked for in streams or ground water.

Adding to the complexity is the fact that existing standards were developed for individual chemicals and do not take into account exposure to mixtures of chemicals and seasonal pulses of high concentrations. The USGS analysis detected two or more pesticides in almost every stream sample and about one-half of the well samples. Some of the most frequently detected pesticides are suspected endocrine disrupters that may affect reproduction or development of aquatic organisms or wildlife by interfering with natural hormones.

Some of the highest concentrations of nitrogen and herbicides, including those most heavily used (such as atrazine, metolachlor, alachlor and cyanazine) were detected in USGS samples collected from streams and shallow ground water in agricultural areas. Some of the highest concentrations of phosphorus and insecticides (including diazinon, carbaryl and malathion) were found in urban streams. Of the urban streams studied by the USGS, nearly every one had concentrations of insecticides that exceed guidelines for protection of aquatic life.

Contaminants found in the USGS study include chemicals that are no longer in use, such as DDT, which was banned in the early 1970s. Persistent insecticides, such as the organochlorines DDT, dieldren and chlordane, are still found at elevated levels in fish and streambed sediment in many urban and agricultural streams across the United States. Although still present, there has been a reduction in concentrations of dieldrin and chlordane insecticides in whole fish since the 1980s, and concentrations of DDT in sediments have also decreased.

The USGS report shows that understanding patterns of contamination in relation to land use, pesticide use and the natural characteristics of hydrologic systems can potentially help reduce the amounts of pesticides that reach streams and ground water. It calls for local and regional management strategies to account for geographic patterns in land use, chemical use and natural factors.

“The Quality of Our Nation’s Waters–Nutrients and Pesticides,” USGS Circular 1225, is available as a PDF file on the USGS Web site: http://water.usgs.gov/pubs/ circ/circ1225/ or in printed form (single copies free) from: Branch of Information Services, P.O. Box 25286, Denver, CO 80225 or fax request to (303) 202-4693. Please specify USGS report C-1225.

For more information, visit the USGS pesticide home page at http://water.wr.usgs.ogv/pnsp/ — includes pesticide data, maps and other pesticide publications.

Sources: U.S. Geological Survey News Release, June 28, 1999, and “The Quality of Our Nation’s Waters,” USGS, May 1999.

Contact: NAWQA Program, USGS, 413 National Center, Reston, VA 20192; phone (703) 648-5716; fax (703) 648-6693; email nawqu_whq@usgs.gov.