SUPERIOR, Wis. (AP) - Federal regulators, citing public opinion, have withdrawn their support of a state plan that lets a Superior oil refinery emit sulfur dioxide above the legal limits.

The Environmental Protection Agency will make a final decision on Murphy Oil USA Corp.'s emissions by March, said environmental engineer Christos Panos of the agency's Chicago-based regional office.

State law prohibits sulfur dioxide emissions of more than four-fifths of a pound per million British thermal units, but the state Department of Natural Resources has let Murphy Oil emit at a rate of three pounds per million Btu since the mid-1980s, arguing that Superior's air quality would still remain within the limits set by the federal Clean Air Act.

The EPA moved last month to formalize the DNR agreement, contingent on public comment received through Sept. 15.

Eight letters of opposition were received, enough to make the EPA discuss the issue further, Panos said.

One group that opposed the Murphy Oil arrangement was the National Wildlife Federation, which said last week that the refinery's emissions led to, among other things, acid rain in northern Minnesota.

"It would be unconscionable to allow a company to continue emitting sulfur dioxide that will adversely affect this beautiful, pristine area if such emissions can be avoided," said federation attorney Nicolette Hahn.

Murphy Oil maintains that while the refinery has the highest sulfur dioxide discharge in the area, it does not pose a health hazard.

"We had hoped it wouldn't go this way," said refinery manager Greg Neve.  "We've fulfilled modeling requirements that prove our method of operating is protective of the environment."