The tiny and frugal Loudoun County town of Hillsboro knew it was coming: someday it would have to chlorinate public drinking water. For several years it has set aside money for the day when state health officials would insist Hillsboro residents stop drinking pure mountain spring water, which has supplied the town for more than 100 years.

Last month, that day came after a rainy April in which water samples showed the water was not that pure and contained twice the coliform bacteria levels permitted in Virginia public water supplies.

Last week, the Town Council voted to award an $8,000 contract to install a water chlorination system on the rusty and rustic water pipes that carry spring water to 30 of the town's 41 homes, all built in the 19th century. The other 11 homes have their own wells and can continue drinking nonchlorinated water.

"The state ordered us to have this installed by June 30," Mayor Alexander Muir said. "But that's impossible, and we told them we'd do it as expeditiously as possible. Anyway, the water tests in May showed not a trace of coliform and so we know we're drinking good water again."

The April readings were bad because the heavy rains made it impossible "to go up and clean out the spring" in the mountains above the town near the West Virginia border, he said. He said he believes the pollution came from deer and other animal droppings above the spring.

The cost of the chlorination system will put the town budget in the red for the first time in its history, Muir said, but only on paper. The town has $33,500 in money market funds and certificates of deposit, and this fiscal year, which ends June 30, the town will have a budget surplus of $2,400, Muir said.

"These are small sums, but it's big money for our small town."