Washington Gas said yesterday that it can fix the damaged seals in underground pipes that have caused thousands of leaks in Prince George's County by mixing additives to the imported liquefied natural gas it receives.

The announcement drew a sharp rebuke from an industry trade group and the company that supplies the imported gas, which challenged utility's conclusion that differences in the chemistry of the imported gas caused the rubber seals to dry up and shrink, allowing the gas to escape.

Dominion Cove Point, which provides 8 percent of Washington Gas's annual supply, said the leaks were caused by old, poorly maintained metal couplings that link underground pipes together.

Dominion demanded that Washington Gas stop claiming that the gas supply was responsible for the leaks.

"It's not the gas, it's the pipes," said Dominion spokesman Dan Donovan.

"Blaming the natural gas for the leaks instead of the 40- to 50-year-old couplings is like blaming the rain for a leaky roof on the 40- to 50-year-old shingles," said Bill Cooper, executive director of the Center for Liquefied Natural Gas, a coalition of liquid natural gas producers, shippers and suppliers.

Washington Gas, which serves nearly a million customers in Maryland, Northern Virginia and the District, formally released a 41-page study yesterday describing what it says caused approximately 1,400 leaks in a 100-square mile area of Prince George's County. The Maryland Public Service Commission launched an inquiry into the leaks after a District Heights house exploded in late March.

Richard Schafer, the PSC's chief engineer, said yesterday that he is reviewing the report and is aware of Dominion's opposition to Washington Gas's findings.

He said the commission hopes to bring the companies and other stakeholders together to resolve the issues and perhaps conduct further research into the cause of the faulty seals.

Washington Gas is spending $144 million -- twice its original estimate -- to repair the leaks and replace the approximately 111,000 couplings in use throughout the county. The company said it plans to have all of the couplings replaced by December 2007. Repairs of the leaks, now two-thirds complete, will be finished by fall, officials said.

The study identified three factors contributing to the leaks: the age of the couplings, winter ground temperatures and the chemical composition of the imported gas.

The couplings were installed widely from 1958 to 1974, until advancements in plastics and fusion welding made them outdated. Still, hundreds of thousands remain in use throughout Washington Gas service territory.

Prince George's is the only service area that receives imported gas directly from Cove Point, before it is blended with domestic supplies from other interstate pipelines. Other areas where the couplings are in use have not reported an abnormal jump in leaks, James DeGraffenreidt Jr., chief executive of Washington Gas's parent company, WGL Holdings Inc., said yesterday.

That has led the utility to conclude that the change in gas composition is the "key contributor," DeGraffenreidt said.

DeGraffenreidt said possible solutions to the problem include blending the gas with domestic supplies or adding hexane and pentane to the fuel. The two heavy hydrocarbons, typically found in natural gas, are lost during the cooling process that turns the gas to liquid, making it more compact and easier to ship by tanker.

The absence of these hydrocarbons, the report said, was causing the seals to shrink. Reintroducing the hydrocarbons, the utility said, would cause the seals to swell.

DeGraffenreidt said the utility will continue to replace the outdated couplings until it can prove conclusively that it can reverse shrinkage of the seals.

The next step for the company will be determining where the gas will be remixed -- at Cove Point or somewhere along the Washington Gas system, before it reaches customers -- and who will pay the cost.

Donovan didn't say whether Dominion would agree to remixing the fuel, but he said most of its customers on the Eastern Seaboard and mid-Atlantic region generally don't want extra hexane in their supply because it can liquefy in the pipes, inhibiting gas flow.

Neither Washington Gas nor Dominion would say yesterday whether the two had discussed sharing the costs, but it is likely to be a point of contention between the two companies.

DeGraffenreidt said the utility has no immediate plans to pass the costs on to customers but has not ruled it out for the future.

He said the two companies have always worked cooperatively. He said he expected that to continue, especially with Washington Gas plans to double its consumption of liquefied natural gas from Cove Point over the next three years.

"I think [liquefied natural gas] is an important tool and resource for our customers," he said. "I would be surprised if we didn't get the same level of cooperation."

The Prince George County Council on Tuesday appointed a 11-person advisory committee to monitor the repair process.

"We're very satisfied with how Washington Gas has responded to this problem," said council Chairman Samuel H. Dean (D-Mitchellville). "But we wanted to make sure that citizens felt they had someone to watch over the process."

Washington Gas has proposed remixing imported gas it gets from Dominion Cove Point to stem leaks in Prince George's County.An explosion at a District Heights house in late March prompted the study.