Areas with intensive fertilizer use and irrigation, such as in the Columbia Basin Irrigation Project (CBIP), showed the greatest impacts on ground-water quality. (The CBIP includes parts of Franklin, Grant, and Adams counties in eastern Washington.)

Shallow wells, generally those less than 150 feet deep, are the most susceptible to contamination, the USGS report says. A reassuring aspect of this finding, however, is that many public water supplies draw from wells at greater depths and are less susceptible to contamination from agricultural practices.

"From the standpoint of human health, we were most concerned about checking for nitrate and pesticides in drinking water," said Sandy Williamson, USGS hydrologist and chief of the study. "We found nitrate levels exceeding the maximum contaminant level in about 20 percent of all wells." (A maximum contaminant level-or MCL-is a drinking water regulatory standard that is set by the U.S. Environmental Protection Agency.)

"The story on pesticides is a mixed bag," Williamson said. "We found at least one pesticide in nearly half of the drinking water wells sampled, but pesticide levels were only a very small fraction of their MCLs." As a cautionary note, however, Williamson said that about half of the pesticides detected in Central Columbia Plateau wells do not have MCLs established.

A lack of information makes it difficult to assess the significance of finding pesticides in drinking water. "As scientists, we don't know enough yet about what happens when these pesticides are combined," he said. "In some of the very shallow wells that the USGS installed for monitoring purposes, we found up to seven different pesticides."

Encouraging news is that none of the newer pesticides, which break down more rapidly in the environment, were found at concentrations exceeding MCLs. Compounds that exceed drinking water standards were found in only 1 percent of the wells sampled-and those compounds have not been so
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Contact: Mr. Sandy Williamson
akwill@usgs.gov
253-593-6514
United States Geological Survey
22-Apr-1998