National Water-Quality Assessment (NAWQA) Program
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.