A construction accident in a small town northeast of Frederick, Md., severed a sewer line and spilled 900,000 gallons of raw sewage into the ground, prompting state and local officials yesterday to warn residents that their water supply could become contaminated.

The problem is that the town is built on limestone and officials haven't been able to locate the spilled sewage or predict where or when it might show up, including possibly in the town's water supply.

Officials of Walkersville, about five miles from Frederick, said their testing shows that so far no contamination has reached the town's water supply, which is fed by three wells about a mile from the spill site.

But they are recommending that those with immature or compromised immune systems--infants, the elderly, people with AIDS and those receiving chemotherapy--boil tap water for at least a minute before drinking it.

State environmental officials who were in Walkersville yesterday to offer assistance said the long-term effect of the spill is of concern because of the amount of sewage and the unusual limestone geology in the area.

The town, like a number of areas in Western Maryland, is built over limestone, which is extremely porous and riddled with holes, fissures and channels through which fluids can travel quickly.

The configuration of the limestone at that spot, and the consequent flow pattern, is difficult to know, said Boyd Grove, a sanitarian in charge of the water supply program for the Maryland Department of the Environment. Eventually, there will come a time when the danger of contamination can be assumed to have passed, said Michael G. Marschner, chief of Frederick County Water and Sewer.

"The town's engineer and hydrogeologist will estimate when we can stop worrying," he said.

"If there was any question of a serious problem for the public at large, we would have put a boil-water notice in effect for everyone," Marschner said.

In addition to warning residents through the media, he said, county officials are also going house to house, distributing pamphlets warning of the possible health risks.

But because the implications of contamination are "potentially lethal" for the immuno-compromised, Marschner said, officials "decided to stay on the safe side and recommend that they boil their water and consult their physicians if they have questions."

Raw sewage contains bacteria and viruses, which can be neutralized at the treatment plant with chlorine, according to Elizabeth Pasierb, planning and zoning administrator for Walkersville. But two other contaminants in sewage, the microorganisms cryptosporidium and giardia, cannot be competely eliminated through chlorination, she said.

"They can cause mild flu-like systems in the general population but serious illness and even death in the at-risk population," Pasierb said.

At possible risk are the town's estimated 7,500 public water users, Pasierb said. An unknown number of people with private wells could also be affected, Pasierb said.

The spill didn't come to light until Monday, when an employee of the Frederick Town Bank in Walkersville phoned the county to report sewage in the basement. That was the only place the sewage has surfaced, as far as officials know, Marschner said.

"We found it in the main line; all the manholes were filled with it," Marschner said.

When his crews got to the construction site where the line was broken, "there was none of the sewage left at the site," Marschner said. "We figure it left through the cracks."

Where it went is the key question. Officials are monitoring the water treatment plant and have noticed no unusual chlorine demand, Pasierb said. Other tests will not be completed for several days.

An employee of Explosives Experts of Sparks, the Baltimore County company that was doing demolition work at the site Friday, refused to give his name yesterday but said the company is still puzzled over what happened.

"We don't understand why none of it surfaced at the time," the employee said. The company was the subcontractor for demolition work for utility lines for Sun Meadows, a 250-unit single-family home subdivision being developed by the East Coast Development Co. of Fairfax Station.

CAPTION: Town and county workers scattered lime and moved contaminated dirt yesterday on the construction site in Walkersville, where an 18-inch sewage pipe was damaged, spilling 900,000 gallons of raw sewage into the ground.

CAPTION: Michael G. Marschner said testing showed no contamination so far.