Some herbicide residues in groundwater used for drinking declined between 1993 and 2003 in test wells—apparently due in part to changes in how the chemicals are used—while other residue levels remained unchanged, a U.S. Geological Survey study published in the September-October edition of the Journal of Environmental Quality found.
Improvements in methods for detecting such trends could help efforts to reduce groundwater contamination, Laura Bexfield, a hydrologist in the agency's Albuquerque, N.M., office, said in her article, “Decadal-Scale Changes of Pesticides in Ground Water of the United States, 1993-2003.”
Chemicals Widely Detected
Bexfield's research focused on the herbicides atrazine and its degradate (breakdown compound) deethylatrazine, prometon, metachlor, simazine, and tributhiuron. The study found tiny declines in median concentrations of atrazine, deethylatrazine, and prometon in agricultural areas. Metachlor and simazine concentrations remained nearly constant in agricultural areas, according the study.
“Those [pesticides] are the ones that we tend to see the most” and for which there are enough data, Bexfield told BNA Oct. 20. She said that she considered a pesticide in her trend calculations if it showed up in at least the same 10 wells in both 1993 and 2003 samplings. Tributhiuron was “kind of a funny one” because it showed up in at least 10 wells in both samplings—but not the same wells, Bexfield said.
The data were taken from the USGS National Water-Quality Assessment Program, which provided consistent sampling and laboratory methods, Bexfield said. She noted that her paper is one of several by various agency experts that looked at different aspects of the issue, for instance nutrient concentrations.
Her study used data on pesticide concentrations in groundwater samples taken from 362 test wells clustered in 11 large geographical study units across the country that differed widely in geography, climate, and aquifers.
Bexfield said “the vast majority” of pesticide concentrations included in the study are below Environmental Protection Agency maximum permissible levels of contaminants for public drinking water.
Data May Reflect Various Causes
Bexfield noted that the pesticide concentration trends she found could result from a number of potential causes, including changes in the amounts of pesticides used or their formulations, natural recharging of groundwater, and the ability of pesticides to leach into groundwater.
In her study, Bexfield also recommended improvements to make future research more useful.
Among her suggestions were increasing the number of wells sampled, collecting information on more pesticide degradates, and estimating the age of the groundwater the samples are drawn from.
How long a body of water has been at a particular site is important to guaranteeing that “you're comparing the same thing between wells,” Bexfield said.
Better Testing Could Help
The ability to describe large-scale trends in groundwater pesticide concentrations “could help determine the effectiveness of efforts aimed at minimizing groundwater contamination, identify compounds of increasing concern, and estimate time scales over which changes might affect water quality,” Bexfield said. “There are pieces that are still missing or can be improved.”
The study, ‘Decadal-Scale Changes of Pesticides in Ground Water of the United States,‘ 1993-2003 is available at http://jeq.scijournals.org/cgi/reprint/37/5_Supplement/S-226.