The Environmental Protection Agency announced yesterday that it will require stepped-up monitoring of surface water supplies that contain high levels of the widely used herbicide atrazine, but the action fell far short of demands by environmental groups to ban the use of the substance.

Atrazine is most commonly used to control weeds in corn, sugar cane and sorghum crops and on residential lawns in the Southeast. EPA officials said monitoring of surface water before it is treated in drinking water plants will begin this year at 200 sites where tests of treated water have shown levels of atrazine close to or above legal limits. The 200 sites are in 11 states in the Midwest and South.

Syngenta AG, a Swiss-based company that is the largest manufacturer of atrazine in the United States, will conduct the monitoring and will be required to take corrective steps if levels of the herbicide in drinking water remain too high, said Stephen L. Johnson, EPA's assistant administrator for prevention, pesticides and toxic substances.

If the amount of atrazine in drinking water is not reduced to acceptable levels, Johnson said, use of the herbicide in the affected watershed area will be banned "forever."

Johnson said eight of the 200 water systems currently have atrazine levels significantly above the legal limit and could be the first to face a ban on the use of the herbicide in surrounding fields. Two water systems in Missouri, two in Kentucky, and sites in Illinois, Indiana, Louisiana and Iowa are involved.

The announcement was the latest development in a long, contentious battle between the EPA and environmental groups over the health risks posed by the application of 76 million pounds of atrazine in the United States each year. Johnson called the surface water monitoring requirement "unique," but Jennifer Sass, a senior scientist with the Natural Resources Defense Council, said she was "flabbergasted" by the EPA announcement.

"We've reviewed the science on atrazine, and it is clear that it is dangerous at levels the EPA says are harmless," she said. "And we're shocked that EPA would abdicate its responsibility to protect the public and allow the manufacturer to write the rules."

Sass said several European countries have banned atrazine and that it should be prohibited in the United States. "We know it causes irreparable harm to exposed wildlife, it's a potential threat to human life, and it's in our water at unacceptable levels," she said.

Johnson and other EPA officials said the agency plans to develop a "very rigorous and comprehensive compliance system" to assure that the monitoring by Syngenta is accurate. "We'll be checking on them and will develop a process where they realize we're going to be there."

Tim Pastoor, head of global risk assessment for Syngenta, which describes itself as the world's largest agribusiness company, said the firm has been voluntarily monitoring surface water atrazine levels at many of the 200 sites since 1993. He said the agreement the company reached with the EPA was a "significant milestone."

Atrazine is used to control weeds in cornfields such as this one in Curran, Ill.