Pesticides Found in Streams Across the United States

WASHINGTON, DC, March 6, 2006 (ENS) - Pesticides are present throughout the year in most streams in urban and agricultural areas of the United States, according to a new report released by the U.S. Geological Survey (USGS) on Friday. The report is based on data from 51 major river basins and aquifer systems from Florida to the Pacific Northwest, Hawaii and Alaska, plus a regional study in the High Plains aquifer system.

The pesticides are seldom found at concentrations likely to affect humans, but they may affect aquatic life or fish-eating wildlife in many streams, particularly those draining urban and agricultural areas, the USGS researchers learned. The toxic chemicals are less common in ground water, they found.

An Ohio Valley stream (Photo courtesy USDA NCRS)

The common occurrence of pesticide mixtures in streams, means that the total combined toxicity of pesticides in water, sediment, and fish may be greater than that of any single pesticide in a stream.

Senior author Robert Gilliom puts a high priority on understanding more about what happens when pesticides mix together in streams. "The potential effects of contaminant mixtures on people, aquatic life, and fish-eating wildlife are still poorly understood and most toxicity information, as well as the water-quality benchmarks used in this study, has been developed for individual chemicals."

"Studies of the effects of mixtures are still in the early stages," he said, "and it may take years for researchers to attain major advances in understanding the actual potential for effects."

More than 80 percent of urban streams and more than 50 percent of agricultural streams had concentrations in water of at least one pesticide that exceeded a water quality benchmark for aquatic life. Water quality benchmarks are estimates of concentrations above which pesticides may have negative effects on human health, aquatic life, or fish-eating wildlife.

"The USGS assessment provides the most comprehensive national-scale analysis to date of pesticide occurrence in streams," said Dr. Robert Hirsch, the USGS associate director for water. "Findings show where, when, and why specific pesticides occur, and yield science-based implications for assessing and managing pesticides in our water resources."

"While the use of pesticides has resulted in a wide range of benefits to control weeds, insects, and other pests, including increased food production and reduction of insect-borne disease," said Hirsch, "their use also raises questions about possible effects on the environment, including water quality."

Steams like this one in the Potomac River Basin contain frequently contain atrazine and metolachlor. (Photo courtesy USGS)

Gilliom said, "Pesticide use is constantly changing in response to such factors as regulations and market forces, and findings from this decade-long study need to be examined in relation to changes in use during and after the study."

For example, said Gilliom, levels of the herbicide alachlor declined in streams in the Corn Belt throughout the study period as its use on corn and soybeans declined, with no levels greater than its aquatic-life benchmark by the end of the study. The Corn Belt takes in Illinois, Indiana, Iowa, Nebraska, and Ohio and parts of adjoining states.

By contrast, both the use and the levels of atrazine, the most heavily used herbicide in the Corn Belt region, remained high throughout the study period, Gilliom said.

USGS researchers worked closely with the U.S. Environmental Protection Agency (EPA) during the 10 year study. The EPA uses the data extensively in their exposure and risk assessments for regulating the use of pesticides such as the reevaluation of diazinon, chlorpyrifos, cyanazine and alachlor.

Uses of three of these pesticides - diazinon, chlorpyrifos and cyanazine - have now been limited, and usage of alachlor was voluntarily reduced and this chemical has now been replaced by a registered alternative.

This stream flows near the city of Idana, Oregon (Photo courtesy City of Idana)

Insecticides, particularly diazinon, chlorpyrifos, and malathion frequently exceeded aquatic-life benchmarks in urban streams. Most urban uses of diazinon and chlorpyrifos, such as on lawns and gardens, have been phased out since 2001 because of use restrictions imposed by the EPA.

The USGS data indicate that concentrations of these pesticides may have been declining in some urban streams even before 2001. A case study of diazinon shows declining concentrations in several urban streams in the Northeast during 1998-2004.

In agricultural streams, the pesticides chlorpyrifos, azinphos-methyl, p,p'-DDE, and alachlor were among those most often found at concentrations that may affect aquatic life, with each being most important in areas where its use on crops is or was greatest.

The report, "Pesticides in the Nation's Streams and Ground Water, 1992-2001," Circular 1291 is available at http://water.usgs.gov/pubs/circ./circ1291, or by calling 1-888-ASK-USGS, or by fax 303-202-4693.

In-depth information about the pesticide assessment may be found at: http://water.usgs.gov/nawqa/ under "What's New."