WASHINGTON — TRACE levels of herbicides have been detected in rainwater throughout the Midwest and Northeast, with the highest concentrations occurring in a five-state region including Illinois, Iowa, Indiana, Kansas and Nebraska, scientists report. Water-quality experts with the United States Geological Survey said April 22 that herbicide residues were found in rainwater samples collected at 81 sites in 23 states, encompassing an area from Kansas to Virginia to the Canadian border.
The scientists said average herbicide concentrations for all 23 states were well below levels considered hazardous in drinking water. They said average concentrations were no higher than 1 part per billion, about one-third of the proposed federal drinking-water limit on atrazine, one of the herbicides studied.
But the study found herbicide contamination of rainwater was widespread, with residues detected in all 23 states and at all but two of the 81 collection sites over the period April to July 1990.
Donald Goolsby, lead author of the study, said while the survey did not find health-threatening chemical levels in rainwater, it was important in that it showed how herbicides can be widely distributed in the environment. ``The findings confirm a potentially important pathway -- airborne transportation -- for the migration of agricultural chemicals,'' said Mr. Goolsby, a water-quality expert in the Geological Survey's Denver office.
The potential for long-range transport of agricultural chemicals was first discovered in the 1970s when scientists detected trace levels of toxaphene -- a pesticide used on cotton fields in the Southwest -- in a lake on remote Isle Royale, an island in Lake Superior near the Canadian border.
Goolsby said his study found herbicide contamination of rain was most pronounced in the main farm-belt states of the Midwest, where pesticides are used extensively in the production of corn and soybeans.
Chemical concentrations reached as high as several parts per billion in some rain samples from Illinois, Iowa, Indiana, Kansas and Nebraska, said Goolsby, who also found residue levels declined in all directions away from the Midwest.
Goolsby said his survey built on previous studies that found substantial amounts of herbicide vaporize when they are applied to farm fields. The chemical vapors rise into the atmosphere, where they can be carried on winds for long distances.
The vaporization appears to occur primarily in two months following application of herbicides to fields in late spring, he said.
Goolsby said the rainwater study was part of a larger effort by the geological survey, the Environmental Protection Agency, and the US Agriculture Department to examine the impact of agricultural chemicals on water resources in the Midwest.
An earlier study by Goolsby found small but significant levels of herbicides in streams and rivers in the Midwest, with concentrations in some stream water samples exceeding federal safe drinking-water limits for certain chemicals.
In addition to the five Midwest states with heaviest chemical concentrations, the rainwater study included samples taken from sites in Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, North Dakota, Ohio, Pennsylvania, South Dakota, Vermont, Virginia, West Virginia, and Wisconsin.