Pesticides persist in ground water( Numerous studies over the past four ...)

Numerous studies over the past four decades have established that pesticides, which are typically applied at the land surface, can move downward through the unsaturated zone to reach the water table at detectable concentrations. The downward movement of pesticide degradation products, formed in situ, can also contribute to the contamination of ground water. Once in ground water, pesticides and their degradation products can persist for years, depending upon the chemical structure of the compounds and the environmental conditions.

Scientists at the U.S. Geological Survey (USGS) investigated the occurrence of selected pesticides and their degradation products in ground water during a study funded by the USGS National Water-Quality Assessment (NAWQA) Program. Specifically, the authors examined several of the factors that can influence the likelihood with which pesticides and their degradation products are detected in shallow ground waterincluding oxidation-reduction (redox) conditions and ground-water residence timesat four study 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 revealed that the pesticides and degradation products detected most frequently in shallow ground-water samples from all four areas were predominantly from two classes of herbicidestriazines and chloroacetanilides. None of the insecticides or fungicides examined were detected in ground-water 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 ground water that recharged between 1949 and 2004, and in monitoring wells spanning the full depth range (about 2 to 52 m) examinedfrom the shallowest to the deepest wellsin all four study areas. Comparisons of pesticide concentrations with a variety of environmental variables ind
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