Pesticides found in streams across the US

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 US Geological Survey (USGS) on Mar. 3. 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. The USGS study, which covers the years 1992-2001, found that pesticides seldom occurred alone but almost always as complex mixtures. Most stream samples and about half of the well samples contained two or more pesticides, and frequently more. 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." The USGS found strong relationships between the occurrence of pesticides and their use. The good news is that some of the frequently detected pesticides, including the insecticide diazinon as well as the herbicides alachlor and cyanazine, are declining. 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 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. USGS measurements were compared to EPA drinking water standards and guidelines. Concentrations of individual pesticides were almost always lower than the standards and guidelines, representing less than 10 percent of the sampled stream sites and about one percent of domestic and public supply wells. 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 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.