Concern about traffic disruption and damage to the environment led a group of about 25 downcounty residents to dismiss several proposed alignments for a water main that will be installed underground over the next five years.

The residents -- members of the Citizens Advisory Committee formed by the Washington Suburban Sanitary Commission -- instead chose a tunneling method that will preserve trees and minimize traffic disruptions but will cost between $20 million and $40 million more than other options. But the residents were clear in what they wanted: Tunnel, don't dig, no matter what the cost.

"Cost is very important in that [we've decided] that it shouldn't be important," said committee member Bill Zwack, a Kensington resident who attended the meeting.

The meeting of the advisory committee followed three meetings in February where WSSC officials explained the water main project to those who will be affected most by it. The seven-foot-wide pipe will be carved through much of Montgomery's downcounty -- beginning in Potomac and ending in Kensington -- and will provide water to Prince George's County, as well as help to maintain water pressure in portions of Wheaton and Silver Spring.

The advisory committee is comprised of volunteers from Potomac, Bethesda, North Bethesda, Kensington, Garrett Park and Chevy Chase, and it includes two representatives from Prince George's County. The committee was formed to give residents an opportunity to participate in decisions for the project. Roger Johnson, a resident of Kensington Estates and a representative on the committee, said he was fascinated at the prospect of laying more than five miles of pipe across the most densely populated part of Montgomery County. WSSC originally considered two construction methods: one that would dig up to 350 feet deep then lower a machine that would dig the tunnel for the pipe; and another in which crews would dig a trench up to 20 feet deep along the length of the route, place the pipe inside then cover it.

"It's really kind of an interesting problem," Johnson said. "I started thinking about the scope of the problem, the number of trees, the amount of disruption," he said. "We cannot have Rock Creek Park disrupted. I wanted to make sure that didn't happen."

At the committee's first meeting on April 5, members were divided into four groups, which ranked criteria including construction costs and potential damage to the environment.

Johnson's group, for instance, decided that preserving mature trees and historical and cultural sites, and minimizing the impact on private property were most important. The group ranked cost and ease of construction as least important.

"We assume they're going to do their job right, so we're not going to worry about [ease of construction], said Wayne Goldstein of Kensington.

The rankings the groups came up with were then tallied by computer, which determined the construction method and route that most closely matched them. What matched was to tunnel underground for the pipe and do it at any cost.

Based on that feedback, WSSC will now study several tunneling options.

The options being considered all begin near the intersection of Tuckerman Lane and Interstate 270 and end at the intersection of Beach and Stoneybrook drives in Kensington.

"As we narrow this process, we start to focus on the most feasible alternatives," said John Mitchell, WSSC project manager. "We need to spend effort where it will bring the most fruit."

WSSC will study the three alignments recommended at last week's meeting, but it will also keep one of the trench-digging alignments for comparison purposes only, Mitchell said.

"Something would have to go extremely wrong for us to say, 'Now we have to look back at the open-cut pipeline.'"

The advisory committee will meet next in late May or early June.

WSSC officials are performing tests to determine how far underground the bedrock is located, and where specifically any pipes should be placed.

Because WSSC will use the tunneling method, officials must also decide where to locate three shafts necessary for construction. The largest of the shafts, called the working shaft, will be located in the middle of the alignment that is ultimately chosen. It will be the launching point for the machine that will dig the tunnel, and it will also be the place where rock and dirt extracted by the tunnel boring machine comes out of the ground and is hauled away. The working shaft will affect two to three acres when it is brought in.

Officials are considering a number of possible locations for the working shaft, including the cloverleaf at Connecticut Avenue and I-495, one corner of Georgetown Preparatory School property and part of the Renewable Natural Resources Foundation's property.

The other two shafts will be used for the retrieval of the tunnel boring machine, and will be smaller and less obtrusive.

One of the retrieval shafts will be at the intersection of Beach and Stoneybrook drives, but there are several options for the other. These include near the school bus depot or Daybreak Court.

Johnson said that his first instinct is to encourage WSSC to put the shaft near the Connecticut Avenue cloverleaf.

"If they were to put it on Tuckerman Lane near Georgetown Prep, that would cause a heck of a mess as well as a disruption for everybody," he said. "The Beltway is disrupted already."