High Weedkiller Levels Found in River Checks

Golden said the data documented "atrazine levels that are sustained at pretty high levels for several weeks. That's definitely a cause for concern."

Peter L. deFur, a biologist at Virginia Commonwealth University, said "chronic low-level exposure" to atrazine can harm aquatic life. "I don't think low levels of atrazine exposures are safe," deFur said.

Charles Scott, field supervisor for the Fish and Wildlife Service's Missouri Ecological Services Field Office, said high levels of atrazine in northeastern Missouri could potentially affect several endangered and threatened species, including the pallid sturgeon, the Higgins' eye mussel, the fat pocketbook mussel and the decurrent false aster, a wetland plant. "It has a lot of biological impacts," Scott said of the pesticide.

The EPA has asked Syngenta to do additional monitoring at the two sites in northeastern Missouri where atrazine concentrations significantly exceeded 10 parts per billion, the level at which the agency believes it can impact aquatic systems. In these two watersheds, concentrations reached more than 50 parts per billion for days at a time.

Wood, the EPA spokeswoman, said the Indiana watershed did not trigger the agency's level of concern in 2006 and the company will be monitoring it for another year.

Pastoor, who noted that atrazine's effect of stunting plant growth is reversed as soon as the pesticide is taken away, said the fact that two watersheds showed high levels of exposure "doesn't mean there's a problem there. It just means there's a yellow flag that says you should take a look."

The two sites in question, he added, were prone to excessive runoff because they have an impervious clay soil that channels runoff into waterways, the land is sloped, and one of the farmers working the land had cleared much of the vegetation. Syngenta sales agents and local corn growers are trying to reform the practices of the farmer in question.

"We anticipate that site will significantly improve," Pastoor said, adding that the computer models Syngenta ran suggest there has been no ecological damage to the watersheds the company has monitored.

Hayes, who stopped working as a contractor for a coalition of chemical companies years ago and is now one of atrazine's most vocal opponents, said he does not think the federal government is surveying the pesticide enough in light of its pervasive influence.

"What's most disturbing about the information you're talking about is all that EPA requires Syngenta to do is monitor atrazine in a few key sites," Hayes said. "Industry's been allowed to have such a huge hand in the regulation of atrazine."