The EPA today announced that Gowanus Canal in Brooklyn will become a Superfund site, against the wishes of the Bloomberg Administration, which fears the designation will stigmatize the site and prevent future real estate development.

The E.P.A. estimates that the federal cleanup will last 10 to 12 years and cost $300 million to $500 million.

The agency, which first proposed that the canal be designated a Superfund site last April at the urging of the New York State Department of Environmental Conservation, made its decision after reviewing comments from the public, city officials and others.

"After conducting our own evaluations and consulting extensively with the many people who have expressed interest in the future of the Gowanus Canal and the surrounding area, we have determined that a Superfund designation is the best path to a cleanup of this heavily contaminated and long-neglected urban waterway," Judith Enck, the E.P.A. regional administrator, said in a statement.