Two Classes of Herbicides Show Up in Groundwater Testing

A U.S. Geological Survey (USGS) investigation showed that pesticides and degradation products detected most frequently in shallow groundwater samples at four study sites were predominantly from triazines and chloracetanilides, according to a recent press release.

Specifically, the scientists examined several of the factors that can influence the likelihood with which pesticides and their degradation products are detected in shallow groundwater—including oxidation-reduction (redox) conditions and groundwater residence times—at sites across the United States. Results from the study were published in the May-June 2008 issue of the Journal of Environmental Quality.

The study was funded by the USGS National Water-Quality Assessment (NAWQA) Program.

None of the insecticides or fungicides examined were detected in groundwater samples. In most samples, the concentrations of the pesticide degradation products greatly exceeded those of their parent compounds. Pesticides or their degradation products were detected most commonly in groundwater that recharged between 1949 and 2004 and in monitoring wells spanning the full depth range (about 2 to 52 m) examined—from the shallowest to the deepest wells—in all four study areas. Comparisons of pesticide concentrations with a variety of environmental variables indicated that redox conditions, groundwater residence times, and the concentrations of dissolved oxygen and excess nitrogen gas from denitrification (the breaking down of nitrogen compounds such as nitrate) were all important factors affecting the concentrations of pesticides and their degradation products in all four groundwater systems.

The four sites were located in agricultural landscapes in Maryland, Nebraska, California, and Washington. They were selected for variability in overall land use, crops grown, climate, agricultural practices, irrigation, geohydrologic settings, and redox conditions. During the spring of 2004, water samples were collected from a network of 59 shallow single or clustered monitoring wells and analyzed for the occurrence of 45 pesticides and 40 pesticide degradation products, including herbicide, insecticides, and fungicides.

Greg Steele, senior author for the study, stated "Atrazine and its degradation product deethylatrazine both persisted in similar amounts at the Nebraska site, but in water samples from the other three study sites, there was little change with apparent age of water as the fraction as deethylatrazine generally exceeded 80 percent of the sum of atrazine and deethylatrazine. On the other hand, in three of the four areas studied (Washington excluded because it did not have any detections of metolachlor or its degradation products), the proportion of metolachlor in groundwater was far less than that for its degradation products."

The full article will be temporarily available for no charge. To see the abstract, visit

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