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Pesticides and Degradation Products Detected in Ground Water

(Beyond Pesticides, July 3, 2008) The results of a U.S. Geological Survey (USGS) study  investigating the occurrence of selected pesticides and their degradation products in groundwater shows that these chemicals can persist for years, depending upon the chemical structure of the compounds and the environmental conditions. The study, funded by the USGS National Water-Quality Assessment (NAWQA) Program and published in the May-June issue of Journal of Environmental Quality, specifically examines several of the factors that can influence the likelihood of pesticides and their degradation products being detected in shallow ground water, including oxidation-reduction (redox) conditions and ground water residence times, at four study sites across the United States.

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.The study reveals that the pesticides and degradation products detected most frequently in shallow ground water samples from all four areas are predominantly from two classes of herbicides—triazines and chloroacetanilides. None of the insecticides or fungicides examined are detected in ground water samples in this study. In most samples, the concentrations of the pesticide degradation products greatly exceed those of their parent compounds.

Pesticides or their degradation products are 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) examined—from the shallowest to the deepest wells—in all four study areas. Comparisons of pesticide concentrations with a variety of environmental variables indicate that redox conditions, ground water residence times, and the concentrations of dissolved oxygen and excess nitrogen gas from denitrification (the breaking down of nitrogen compounds such as nitrate) are all important factors affecting the concentrations of pesticides and their degradation products in all four ground water systems.

The four sites selected for this study are 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.

“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% of the sum of atrazine and deethylatrazine,” states Greg Steele, senior study author. “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 ground water was far less than that for its degradation products.”

Water is the most basic building block of life. Clean water is essential for human health, wildlife, and a balanced environment. According to a Beyond Pesticides report, Threatened Waters: Turning the Tide on Pesticide Contamination, over 50% of the U.S. population draws its drinking water supply from ground water, which includes sources below the earth’s surface, including springs, wells, and aquifers. Once groundwater has been contaminated, it takes many years or even decades to recover, while streams and shallow water sources can recover much more rapidly. Herbicides are found more often in ground water than insecticides, but insecticides in ground water exceed drinking water standards more often than herbicides.

A 1989 study found residues of 39 pesticides and their degradation products in the ground water of 34 states and Canadian provinces. The pesticides were mainly herbicides used for agriculture and insecticides and nematicides used in soil treatments.

For more information about pesticide contaminated water, see Beyond Pesticide Daily News Blogs on water.


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