Bethlehem Steel has agreed to cut the amount of pollutants its Baltimore-area plant dumps into a Chesapeake Bay tributary.

The steelmaker’s Sparrows Point plant has been operating for 15 years under a side agreement to a 1985 wastewater discharge permit that allows it to dump thousands of pounds more pollutants a month than the permit allows.

Environmentalists had opposed a five-year state permit proposed in October for the plant, which is one of the largest dischargers in the state. The new permit — drafted after negotiations between the company, environmental groups, the Maryland Department of the Environment and the EPA — calls for reductions that went beyond what was originally proposed.

The new permit would cut by more than 90 percent the amount of lead and zinc the plant can discharge, replacing the 1985 permit that was never enforced. It would also sharply reduce the amounts of copper, nickel, chromium and cyanide.

The new permit also eliminates so-called “mixing zones” — as called for the the Chesapeake 2000 Agreement — that allow room for pollutants to be diluted at two of the company’s discharge pipes.

The permit includes a side agreement giving the steelmaker three years to build a treatment plant to allow it to meet the discharge requirements.

When completed, the treatment plant will achieve an 87 percent reduction in the discharge of metals and suspended solids — about 17 million pounds — annually, according to the MDE. Oil and grease will be reduced by 85 percent.

Bethlehem Steel spokeswoman Bette Kovach said the result was “standards that are realistic and protect the environment.”

The plant is located in Baltimore Harbor, which is one of the Bay Program’s three “regions of concern,” where states are supposed to prioritize efforts to reduce chemical contamination.