Scientists have found small amounts of potentially cancer-causing herbicides in rainwater from 23 Midwestern and Northeastern states, including Maryland.
Based on nine months of results through October 1990, a U.S. Geological Survey study confirms earlier research that the toxic chemicals can evaporate into the atmosphere and return to Earth as precipitation hundreds of miles away, much like acid rain.
"The concentrations are very small. I don't think there's any cause for concern for human and animal health," said Donald Goolsby, a USGS water quality specialist. "But we don't know all the effects."
Maryland was actually the site of a pioneering rainfall study a decade ago by the late Dwight Glotfelty, a researcher at the Beltsville Agricultural Research Center. He estimated that 10 percent of one common herbicide entering the Chesapeake Bay came from rainfall.
But Mr. Goolsby and his Denver-based team of scientists cast their net over half the country, analyzing samples collected weekly at 81 sites in an area bounded by Kansas, Virginia, North Dakota and Maine.
The two sites in Maryland used by the geological survey were at White Rock in Carroll County and at Wye in Queen Anne's County, Mr. Goolsby said.
They detected peaks of herbicide concentrations that coincided with the spring planting season -- highest in the five Midwest states where the chemicals are used extensively for corn and soybean weed control, decreasing in all directions away from the agricultural heartland.
Although they measured for at least four herbicides, the most prevalent were atrazine and alachlor, the latter marketed as Lasso and classified by the EPA as a "probable carcinogen." Other components of the rainfall included metolachlor, a close relative of alachlor.
Atrazine -- rated less dangerous to humans as a "possible carcinogen" -- is the most popular in a chemical arsenal that provided 645 million pounds of weedkillers for U.S. soil application in 1987. A total of 1,087 million pounds of all pesticides were applied that year.
And, in Maryland, the peak levels of atrazine detected by the researchers reached about 0.5 parts per billion of water, with an average over the spring season of 0.1 parts per billion.
The EPA has set the safe level for atrazine in drinking water at 3 parts per billion.
"At these very low concentrations, this rainwater probably doesn't have human health implications," said Rebecca Goldburg, a staff scientist with the Environmental Defense Fund. "But if it's this high in rainwater, how high is it in puddles and streams and lakes?"
Dr. Goldburg said the study "confirms the fears of environmentalists that herbicide use has become so heavy, the stuff is everywhere.
The highest levels of atrazine detected in the study -- in Illinois, Iowa, Indiana, Kansas and Nebraska -- averaged about 1 part per billion, but some individual readings reached 5 to 10 parts per billion, Mr. Goolsby said.
And a previous USGS study of herbicide contamination of rivers and streams at 150 sites in 10 Midwestern states found concentrations of atrazine year-round, peaking during the spring 30 to 40 parts per billion, more than 10 times the EPA's safe level for drinking water.
The majority of herbicides detected in local rainfall originates from the area where they are applied. But the detection of the chemicals in such pristine areas as northern Maine and Isle Royale National Park in Lake Superior is thought to be due to their long-distance travel.
Dr. Alan Taylor, the retired head of Beltsville's environmental chemistry laboratory, said the Glotfelty report -- published last year -- showed a small, unexpected peak of atrazine in Maryland during January and February.
"We concluded that it was coming from farms in states to the south of here," he said. The sampling stations employed by Dr. Glotfelty were at Wye, Sandy Point State Park and the Patuxent Wildlife Refuge.
One of the reasons that the 1981-1982 study was launched in Maryland was to investigate an alarming decline in bay vegetation. But Dr. Taylor said researchers "were never able to show that these materials were having an effect, not at these concentrations."
Rod Coggins, a spokesman for the Chesapeake Bay Foundation, said the new data "certainly has our attention. These airborne pollutants appear to become more significant the more we study them.
"But if I had to rate health risks [from chemicals], this would not be No. 1. We've got so much on our plate."