Oct. 27, 2008 -- Pesticide concentrations in groundwater held constant or decreased over a recent 10-year period, according to a nationwide survey conducted by the U.S. Geological Survey.
The study sampled 362 wells across the country from 1993 to 1995 and again from 2001 to 2003.
"Looking at trends, it doesn't look like we're seeing any increases in concentration or detection frequency," said study author Laura Bexfield of the USGS in Albuquerque, N.M. "If anything, a couple of the compounds seemed to decrease. Overall, I think it's a good story."
Although more than half of shallow wells in urban or agricultural areas and one-third of deep aquifers contain measurable amounts of at least one pesticide, only six pesticide compounds were detected in 10 or more wells during each of the two sampling cycles.
Bexfield found a significant decline in atrazine -- one of the most-used pesticides -- and its breakdown product, and in prometon, which is used in non-agricultural settings to control weeds, near buildings and along transportation routes.
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For other pesticides, no trend could be measured, despite increases or decreases in the pesticides' use. The study, part of the National Water-Quality Assessment Program, was published in the current issue of the Journal of Environmental Quality.
The more widely used, water-soluble, or long-lived a pesticide is, the more likely it is to show up in groundwater, Bexfield noted. But, factors like when and how fast new water reaches a well can affect the amount of pesticide that is measured.
Surface waters such as streams and lakes tend to have higher pesticide concentrations than groundwater -- in other studies, the USGS detected one or more pesticides in 90 percent of urban, agricultural or mixed-use streams tested -- but the amounts fluctuate more; pesticides get flushed off of croplands in rainfall and then are gone.
"If it does get into groundwater, it's probably not going to go away for quite a while," Bexfield said.
"The big deal is once the groundwater is contaminated, it's a long-term contamination," agreed Paul Capel of the USGS and the University of Minnesota, Twin Cities. "The occurrence of any of them is an indication that there is something going on and we should pay attention before we screw up the resource. Generally, in terms of immediate health concerns, it's not a big concern because the concentrations are so low."
The vast majority of concentrations found by the scientists don't reach dangerous levels for drinking water, Bexfield added.
A previous USGS survey found that about one percent of wells studied exceeded human health benchmarks for pesticides derived from Environmental Protection Agency guidelines.
Related Links:
National Water Quality Assessment Program
"Pesticides in the Nation's Streams and Groundwater, 1992-2001
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