Washington Gas officials said yesterday that a change from domestic to imported natural gas was the "key contributing factor" in a rash of leaks in underground mains and service lines in Prince George's County over the past two winters.

A company-sponsored study, launched after a District Heights house exploded in late March, found that subtle molecular differences in the imported liquefied natural gas the utility began using in August 2003 were drying the rubber seals of aging metal couplings that link sections of pipe.

The utility, which serves almost a million customers in the Washington region, said it now expects to spend $144 million -- almost double its original estimate -- to repair an estimated 1,400 leaks in Prince George's and to replace thousands of old couplings.

Whether the replacement program -- which company officials said is two-thirds complete -- will translate into higher rates for customers is an open question. The company has not asked state regulators for an increase, but retains the right to do so.

The frequency of leaks began to soar in late 2003, soon after the company started supplying Prince George's with imported gas, mainly from Trinidad, brought in by tanker through Dominion's Cove Point liquefied natural gas terminal in Calvert County. The leaks tapered off as customer demand for gas fell in the summer, but they surged again this past winter.

The study's full results will be released at a news conference today, the company said. Washington Gas has scheduled a conference call this morning for industry analysts to discuss the findings and their financial implications for its publicly traded parent company, WGL Inc.

Washington Gas said it expects to be able to continue to use the imported gas through additional processing. It did not specify how.

"We believe that we can reverse seal deterioration by conditioning the Cove Point gas entering the system to prevent future drying of seals," Adrian Chapman, Washington Gas's vice president of energy acquisition and regulatory affairs, said in a statement.

Cove Point officials yesterday sharply disagreed with the Washington Gas interpretation of the study. An analysis performed by Dominion Resources Inc., which operates the terminal, found that the chemical makeup of gas from its facility closely matched gas generally used in the Washington Gas system.

"There was and is absolutely nothing wrong with the natural gas from Cove Point," Dominion spokesman Dan Donovan said last night. "The gas meets the stringent standards set by the Federal Energy Regulatory Commission and agreed to by Washington Gas and other companies that receive it."

Donovan added that the full Environ report, which Dominion engineers have reviewed, shows that the age of the equipment, particularly the 40- to- 50-year-old seals, was the real cause of the problem.

The Maryland Public Service Commission launched an inquiry into the leaks in April after the District Heights explosion, in which no one was injured. Residents complained of gas odor before the blast.

The company's findings are certain to intensify discussion in the gas industry about "interchangeability" -- the feeding of different natural gases, including imported liquid natural gas, into the U.S. interstate pipeline grid. Cove Point is one of four import terminals operating in the United States; the others are in Georgia, Louisiana and Massachusetts. Federal Energy Regulatory Commission officials have said they expect at least eight new terminals to begin receiving imported gas by 2010.

Until now, the focus has been mainly on how the imported gases, which tend to burn hotter, would affect household appliances, furnaces and gas turbines.

The Washington Gas study could force the industry to look more closely at how imports will affect the millions of miles of mains and service lines used to transport gas to customers.

"Some of the issues are going to be what kind of mechanical seals they're using, and how standard they are in the industry," said Ronald Gist, an analyst with the Houston energy consulting firm Purvin & Gertz.

The company analysis, headed by Environ International Corp., helps explain why the leaks were concentrated in Prince George's and not throughout the region, where Washington Gas serves nearly a million customers in Maryland, Virginia and the District.

Washington Gas relies on three interstate pipelines that supply domestic gas and a fourth that carries imported gas directly from Cove Point. The imported gas blends with domestic fuel by the time it reaches most Washington Gas customers. But about 300,000 customers, mostly in Prince George's, receive gas directly from Cove Point, before it has mixed with other supplies, according to Washington Gas documents.

Environ examined three potential factors in the leakage: winter ground temperatures, aging seals and gas composition.

"The age of the seals and winter ground temperatures also affect other areas of the Washington Gas service territory that employ mechanical couplings," the company said in a statement. "These areas have not experienced increased leak patterns."

Prince George's has a high concentration of the failing couplings, which were used widely from the 1950s through the 1970s -- a period of surging growth and home building in the county. Advances in plastics and fusion welding made the couplings outdated, but there are still thousands in use throughout the region, as well as the country.

An estimated 111,000 couplings are in Prince George's alone, Washington Gas spokesman Tim Sargeant said. He declined to say how many are in use throughout the company's service region.

Natural gas is made up almost entirely of methane, with a small percentage of heavier hydrocarbons such as ethane, propane and butane. The level of heavier hydrocarbons in the gas typically varies depending on where the gas comes from and how it is processed. Those variations in turn can affect a number of factors, including the temperature at which the gas turns to liquid -- known as hydrocarbon dropout -- and how hot the gas burns.