The drinking water supply for Vienna-area residents is "uncomfortably close" to exceeding accepted levels for a suspected cancer-causing substance, trihalomethanes or THMs, a consulting firm has told the town.

But the level of THMs -- a chemical compound similar to chloroform that is created when chlorine is added to most drinking water -- is not thought to pose an immediate health hazard, officials said yesterday.

Higher levels of THMs have been recorded in several area water systems in the past, including the Occoquan Reservoir, water supply for 600,000 Northern Virginia residents, and in Stafford and Spotsylvania counties and several Tidewater localities, according to Robert Hoehn, a professor at Virginia Polytechnic Institute and State University in Blacksburg who was an expert source for the Vienna study.

The Occoquan water is now treated with ammonia, which counteracts formation of THMs. Vienna spokeswoman Marie Kisner said yesterday that while the Town Council is not scheduled to take up the study until its December meeting, some form of ammonia treatment is expected to be approved.

The high THM levels in Vienna's water, 90 parts per billion -- just below the 100 parts per billion limit set by the federal Environmental Protection Agency -- would constitute a risk, said Hoehn, only if a resident drank two quarts of water containing that level of THMs every day for 70 years.

Vienna's water is "absolutely" safe, he said. "What I would worry about is that some residents might start using other water that is not as safe," because they are alarmed by the study.

Vienna residents drink Potomac River water treated at the U.S. Army Corps of Engineers' Dalecarlia Reservoir in the District of Columbia and then pumped through Falls Church. It sometimes takes several days for the treated water to arrive in Vienna, the study said, which gives the chlorine more time to create THMs by reacting with decayed organic substances -- such as minuscule bits of leaves.

The study said residents in the Fairfax Circle area west of Falls Church also drink water that has been in the pipelines "several days" and has high THMs concentrations.

State health officials could not be reached yesterday for comment on the study.

Chlorine has been used routinely in this country since 1912 to disinfect drinking water and prevent typhoid and cholera, said Hoehn, but studies have shown that the byproduct-THMs cause cancer in laboratory animals. Vienna only began testing its water recently under new EPA regulations, which went into effect last November.