Where to locate that area and other access points to the pipe will be the next big question facing Washington Suburban Sanitary Commission officials and the residents they've asked to help them.

"Next we'll be discussing the tunnel shaft locations, types of tunnel shafts and proposed locations of tunnel shafts," said Chuck Brown, spokesman for WSSC.

Representatives from WSSC briefed the Montgomery County Planning Board on the project's progress at the Planning Board's meeting Thursday.

The new water main will stretch from Interstate 270 near the intersection of Tuckerman Lane to a site near the Mormon Temple in Kensington. It will affect many communities in the downcounty, including Bethesda, Kensington and Garrett Park.

The pipe will deliver water from the Potomac water filtration plant to Prince George's County and will help keep water pressure at a safe level in parts of Montgomery County, including Wheaton and Silver Spring.

At a meeting in April, the Citizens Advisory Committee, a group of about 30 residents from areas that could be affected by the pipe's construction, helped WSSC narrow the possible methods for constructing the pipe from nine to three.

The committee was scheduled to meet again Tuesday night, after The Gazette's deadline, to help WSSC decide where to locate the shafts. Construction on the project is scheduled to begin in summer 2007 and last about three years.

But what is now being considered is where workers should dig shafts into the ground that will allow them to lower the digging machine, ventilate the tunnel, connect the new pipe to existing pipes, and raise the digging machine.

The three remaining options involve digging a 150- to 350-foot shaft, into which workers will lower a digging machine that will bore a tunnel the length of the pipe's proposed route. The tunnel method of construction was preferred over the other method, which involved digging up land the length of the route and installing the pipe and then covering it with dirt. The tunneling method, WSSC officials said, will cost about 20 percent more but it will be less disruptive to nearby homes, trees and traffic.

The largest shaft site is the working shaft, or central construction site. That's where most of the work in the three-year project will take place. The site will be busy 24 hours a day with machines bringing up the excavated rock and removing it from the site. The working shaft site will take up two to three acres and construction work will be ongoing for most of the three years. Sites being considered for the working shaft include the Bethesda bus depot, a grassy area near Kensington Parkway and the Connecticut Avenue cloverleaf.

Once the tunnel has been bored in the bedrock, workers will need to retrieve the boring machine, and they'll do that through a retrieval shaft. Retrieval shaft sites need about one acre and will involve periodic activity during construction. Retrieval shaft sites being considered include a site in Cabin John Regional Park at the northeast corner of Tuckerman Lane and I-270 and at the site near the Mormon Temple where the pipe project ends.

"[The site near the Mormon Temple] is where the existing 96-inch main starts," Brown said. "We're connecting two points."

Finally, the project requires a riser shaft, a site one-quarter to one-half of an acre that will connect an existing water main to the new one. Sites being considered for riser shafts include land near the intersection of Beach Drive and Knowles Avenue, and near the intersection of Beach Drive and Rockville Pike.

At Tuesday night's meeting, WSSC also planned to ask members of the committee to help narrow the three remaining options for the pipe's alignment.

All three remaining alignments begin at the intersection of I-270 and Tuckerman Lane:

*The first proposed alignment runs parallel to I-270 and then goes through Rock Creek Park before ending at Stoneybrook Drive;

*the second alignment runs under Tuckerman Lane and then along Saul Road; and

*the third alignment also goes under Tuckerman Lane before splitting off under Rock Creek Park, where it roughly parallels Beach Drive. It goes up to the CSX railway tracks, which it follows to Stoneybrook Drive.

WSSC officials said they would choose a final alignment by the end of the summer.

At the Planning Board briefing on the project Thursday, Chairman Derick P. Berlage said he appreciated WSSC's efforts to keep communities and the Planning Board informed.

"They're probably all controversial, but it's a clear presentation," he said.