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 reported.

Water quality experts with the U.S. Geological Survey said Monday 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 one part per billion, about one-third of the proposed federal drinking-water limit on atrazine, one of the herbicides studied.

However, 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 of April to July 1990.

The most prevalent chemicals were atrazine, alachlor and metolachlor, all suspected cancer-causing agents in humans.

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 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 the water of 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, Goolsby said.