Alcoa has agreed to cap sediment in two sections of Badin Lake, east of Charlotte, that were contaminated years ago by its now-closed aluminum smelting plant.

The N.C. Division of Waste Management signed the agreement Thursday despite protests from former workers, residents and environmentalists that it does too little to protect the lake.

Alcoa will install a cap of sand, gravel and rock over sediment contaminated by low levels of polychlorinated biphenyls. PCBs are a family of chemicals, once widely used in industry, that were banned in 1979 because they cause cancer and other health problems.

For years, the state says, contaminated stormwater from the smelting plant flowed into the lake through two outfalls. The cap is intended to keep PCBs that sank to the bottom from recirculating in the water.

It’s unclear whether Alcoa is responsible for PCBs that have contaminated fish in Badin Lake.

North Carolina health officials issued an advisory about PCB contamination of catfish and largemouth bass in 2009. The advisory was based on tainted fish caught in the lake’s northwestern arm; the Alcoa works are on the lake’s southern end.

Alcoa points out that PCBs are widespread. A 2000-2004 federal study found PCB-contaminated fish in each of the 500 lakes sampled. But the Yadkin Riverkeeper, a watchdog group, says its own study linked Alcoa to PCBs found in Badin Lake and downstream.

Two North Carolina agencies are working with the Environmental Protection Agency to sample fish and sediment in Badin and the Yadkin River lakes upstream and downstream.

“We’re trying to answer the question, are the PCB issues broader than just the two spots at the end of the (Alcoa) outfalls?” said Elizabeth Cannon, chief of the N.C. Hazardous Waste Section. Results and analyses may be in early next year, she said.

The data will reveal whether PCBs pose a heath or environmental threat, Cannon said, but tracing contamination to a specific source “is very, very difficult.”

The sediment-capping project, she said, is a response to known contamination. State environmental regulators say they can force Alcoa to take more measures if needed.

Alcoa will lower the lake level by about 15 feet this fall and winter, scoop out contaminated sediment and install the cap over sediments that can’t be removed. The project is expected to take five weeks; Alcoa won’t say how much it will spend.

“The sediments have been closely monitored for decades and do not pose a risk to the environment or people who use Badin Lake,” Alcoa said in written responses to the Observer.

But a consultant for the Yadkin Riverkeeper wrote state officials last month that “the evidence indicates a high likelihood that the contaminated sediment hot spots are not all of the problem.”

The consultant, Environmental Stewardship Concepts of Henrico, Va., urged that a fuller investigation be conducted of pollutants from the smelter that were released over decades.

The health director for Stanly County, on Badin Lake’s west side, wrote that he also questions “the conclusion that there are no unacceptable current risks to wildlife and human health … without further environmental health and ecological study.”

Stanly has been locked in conflict with Alcoa for years over an expired hydroelectric license on the Yadkin.