Chicago ? Yielding to years of pressure from environmental groups, one of the nation’s largest mercury polluters will stop using the toxic metal and switch to cleaner technology.

But the 200 tons of mercury on hand at the ERCO Worldwide chlorine plant in northern Wisconsin could end up on the world market and drift back to contaminate lakes and rivers.

ERCO, a subsidiary of the Toronto-based Superior Plus Income Fund, announced last week that it will convert its chlor-alkali plant outside Port Edwards, Wis., to a mercury-free manufacturing process in 2009. Only four other plants in the United States still use the liquid metal to create chlorine.

When mercury falls through air pollution into lakes, streams and oceans, it is converted into a potent neurotoxin that moves up the food chain from fish to people. The federal government estimates that 410,000 babies nationwide are born each year at risk for mercury poisoning because of high levels in their mothers’ bodies.