A large white plume of emissions rises mysteriously near Raleigh-Durham International Airport, leading some passersby to wonder for a moment whether a plane has gone down.

It's mainly steam vapors, however, billowing from the Ward Transformer industrial site, where environmental clean-up crews have begun decontaminating thousands of tons of soil tainted with toxic PCBs.

Pollution at the former transformer reconditioning plant is more widespread and deeper than tests revealed, and the cleanup will take months longer, pushing into next year, U.S. Environmental Protection Agency officials say.

"We're finding a lot more contamination than we thought we had," said Luis Flores, EPA's on-site project manager. "There are more than 50,000 tons of soil stockpiled awaiting treatment."

EPA now estimates the volume of soil that will eventually need treatment at 150,000 tons, more than double the original estimate. That number could still go higher. Crews have dug up about half the 20-acre site. They have yet to get to some areas -- including a treatment pond -- where they know high levels of pollutants exist.

The contamination is primarily polychlorinated biphenyls, which came from spills of oil used in electrical transformers in the 1960s and '70s. Ward Transformer built and reconditioned transformers at the site from 1964 until the end of 2005.

The company's methods of handling PCB-laced oil drained from the transformers led to widespread contamination of the land and nearby creeks, causing the site to be designated a high-priority cleanup under the federal government's Superfund Program.

A settlement, approved by the U.S. Department of Justice, calls for Ward and its customers, including Raleigh-based utility Progress Energy, to pay for cleanup of the contamination. An earlier estimated cost for the cleanup was about $10 million, but that may increase.

"Costs are expected to increase, but we will not have any specific cost information until a final clean-up plan is approved," said Cari Boyce, a spokeswoman for Progress Energy.

Clean-up goes on around the clock. After months of digging soil and demolishing buildings, contractors in mid-July began treating the tainted dirt by cooking it.

About 30 tons of soil an hour is fed by conveyor belt into a giant industrial dryer. The process, called thermal desorption, heats the soil to about 900 degrees, causing pollutants to vaporize and separate from the soil.

In a second chamber, the PCB vapors are then heated further to 1,900 degrees and destroyed. The gases are scrubbed, injected with water to cool them and vented through a stack as steam -- which is what motorists see as they drive near the airport.

The treated soil is tested for PCBs, and if it's not clean, it goes through the process again. Clean soil will be placed back in the ground.

EPA has used this method to clean dozens of Superfund sites. Flores said EPA experts on thermal desorption technology were present during testing last month to ensure that the equipment at the Ward site destroyed the PCBs and doesn't release harmful pollutants.

JoAnn Eskelsen, an environmental engineer with EPA's environmental response team and expert on thermal desorption, said the equipment is set up to destroy more than 99.99 percent of the pollutants and the emissions meet EPA's air quality standards.

"As far as what is in the stack, it's almost exclusively water vapor," Eskelsen said. "You may have trace amounts of carbon monoxide. There could be trace levels of particulates."

Janice Mrkonjic, an events planner who lives in Brier Creek Country Club subdivision, said she and her neighbors starting noticing the vapor about a month ago.

"Going in and out of the community, you see this smoke," Mrkonjic said. "It looks to be coming from the cylinder. My neighbors and I have commented, 'Where did that smokestack come from?'"

They've wondered what was coming out of it.

Nile Testerman, an environmental engineer with the N.C. Department of Environment and Natural Resources, said there is no cause for alarm.

"This system was used at another Superfund site in North Carolina, and it worked fine there," Testerman said. "As long as they meet the parameters, we are OK with the emissions. If they do not meet the parameters, the system shuts down."

The cleanup will not touch the pollution that has moved downstream and contaminated fish in Lake Crabtree, Crabtree Creek, Brier Creek and Brier Creek Reservoir, prompting officials to post signs along the waterways warning people to avoid eating the fish.

A plan recommended by the U.S. Environmental Protection Agency to clean up the waterways involves excavating contaminated sediment from tributaries downstream from Ward Transformer and from Little Brier Creek and Brier Creek. The streams, which total about 3 miles in length, would be restored once the contamination is removed.

The plan also calls for letting Brier Creek Reservoir, a flood control pond, and Lake Crabtree, a recreational lake, to recover naturally. EPA officials theorize that once they remove contamination from the Ward Transformer site, the clean sediment washing downstream into the lake will bury contaminated sediment. Levels of pollution in fish tissue would also decline in years ahead.

A decision will be announced in the fall on whether the cleanup will extend to the creeks downstream.

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