The cost of living in the small, rural Loudoun County towns of Round Hill and Purcellville may go up.

Both towns get their water from springs in the nearby Blue Ridge Mountains and have water treatment systems considered inadequate by state health officials. They also have rusting 60-year-old underground water pipeline systems.

A study under way on joint water system improvements estimates that water bills would triple for Purcellville residents and sextuple for Round Hill residents to pay for a $715,000 water treatment plant to serve the two towns and a $980,000 underground water pipeline between them.

But those figures do not include the cost of increasing the water supply, which would cost a minimum of $160,000 to activate some large wells at an old slaughterhouse and about $1.3 million to enlarge Purcellville's reservoir.

Nor do the figures include the cost of rebuilding the towns' pipeline systems. That would cost about $427,000 in Purcellville and an undetermined amount in Round Hill. The estimates also do not include annual operating costs.

The adjacent towns now have separate water systems. But to improve them separately would be more expensive than the joint effort, officials of the towns said.

The two town councils have asked the engineering firm doing the study to develop more complete financial estimates. But the picture is gloomy, said Purcellville Town Manger Barry Lawrence, "particularly since there don't appear to be any federal or state grants available to help us."

Purcellville last year applied to the state for a $213,500 grant to pay half the cost of replacing and repairing its water lines. It was turned down, as were grant requests from all Northern Virginia towns, apparently because Northern Virginia is an area is considered less needy than other parts of Virginia.

Many Northern Virginia towns face similar water and sewer projects that they cannot afford. Nearby Lovettsville is trying to qualify for a $220,000 federal grant to repair its sewer system.

Round Hill Mayor Jeffrey Wolford said last week that "it's got to be partly political. How else can you explain absolutely no money from the federal block grant funds to the state coming here? Round Hill does not have high incomes. We have many elderly and many living just on Social Security."

But he added, "even if we somehow cut the cost in half with grants , this still would be an extremely expensive project . . . . The figures scare us."

It's possible that the towns, which have been studying their water problems separately and jointly for almost a decade, may continue to do nothing for a while longer, simply patching their systems when they break and forgetting about expansion, officials said.

State health officials in 1975 told the towns not to add more connections to their water systems until they improve the water treatment and supply, although both towns still have a handful of additional connections they can legally make. A few new homes in Round Hill have connected to the public sewer system but have drilled private wells, Wolford said.