Pesticides in Surface Waters

U.S Geological Survey Fact Sheet FS-039-97

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Pesticides Found in Surface Waters

Past studies have detected pesticides in surface waters in all regions of the nation. Of the hundreds of pesticide compounds that have been used in the United States, however, only 98 pesticides and 20 pesticide transformation products were included as analytes in the reviewed studies. Of these 118 compounds, 76 were detected in one or more surface water sites in at least one study. In Figure 3, detection frequencies for herbicides and insecticides are compared with agricultural use. The detection frequencies shown are for compounds targeted at numerous sites in the reviewed studies. Absence of a pesticide in Figure 3 does not necessarily mean that it does not occur in surface waters, but often that it was not studied.

In general, herbicides have been detected more frequently than insecticides, consistent with the greater use of herbicides. The most frequently detected herbicides include several triazines (atrazine, cyanazine, and simazine), acetanilides (metolachlor and alachlor), and 2,4-D. These compounds are among the highest in current agricultural use. Trifluralin and butylate were detected less frequently, despite relatively high use. These are volatile compounds that are usually incorporated into the soil when applied -- two factors that reduce the likelihood of transport to surface waters.

Many of the insecticides studied were rarely detected because of low use. Some with relatively high use were rarely detected for other reasons. For example, toxaphene has a high detection limit and aldrin degrades to dieldrin after application. The most frequently detected insecticides that are currently used are carbofuran and diazinon. Some organochlorine compounds that are no longer used were among the most frequently detected insecticides. While many of these detections occurred in studies conducted in the 1960's and 1970's, these compounds have also been detected in surface waters in more recent studies. This results from their storage in the bed sediments of rivers and lakes and their continued inputs from the atmosphere and soil contaminated from past agricultural applications.

While data are sparse in some areas of the nation, pesticide occurrence in surface waters is relatively well documented in the Mississippi River Basin and other parts of the Midwest. Several large regional studies have been conducted recently in this area. These include a 1989-90 survey of 147 streams in the Midwest, a study of the Mississippi River and six major tributaries during 1991-92, and a 1992 study of 76 Midwestern reservoirs. In addition, several Lake Erie tributaries have been monitored from 1983 to the present. All of these studies targeted triazine and acetanilide herbicides, several pesticide transformation products, and insecticides and other herbicides used in the Midwest. Herbicides were detected at nearly all sites in these studies, with atrazine, metolachlor, alachlor, cyanazine, and deethylatrazine (an atrazine transformation product) detected most frequently. Maximum concentrations of these compounds were in the micrograms per liter range, with concentrations generally highest in the smallest rivers. Herbicides of other chemical classes and insecticides were observed less frequently and at much lower concentrations in these studies.

Figure 3. Detection frequency for pesticides targeted at 50 or more sites in 127 studies conducted between 1958 and 1993. The relative agricultural use of each pesticide is shown in the left half of each graph. Numbers at right of bars represent the number of sites at which chemical was targeted.

For the most part, the available data are not sufficient to determine any long-term trends in pesticide occurrence in surface waters because of the lack of studies in which the same sites were sampled consistently over a number of years, and to the inherent variability of pesticide concentrations in surface waters. The increase in detections of triazine and acetanilide herbicides throughout the 1980's and 1990's may be more an indication of a trend toward more targeted monitoring than an actual trend in the occurrence of these compounds in surface waters. In the study of Lake Erie tributaries, for example, concentrations of several triazine and acetanilide herbicides were monitored for eight years in streams draining agricultural areas, with no discernible trend in concentrations over time (Richards and Baker, 1993).

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