The Conservation Commission met in person Tuesday night at the Hopkinton Senior Center, where it approved improvements to a soil absorption system, plans by the MBTA to do sedimentation sampling, and two notices of intent for single-family homes.

The commission voted to approve a request for determination of applicability for improvements to the soil absorption system at 5 Constitution Court. It was filed by IMP Hopkinton LLC.

Frank Occhipinti of environmental engineering firm Weston & Sampson spoke on behalf of the applicant. The existing wastewater treatment system is “currently failing,” he explained. To rectify the situation, he proposed erecting a vertical geomembrane barrier. The plan for its installation was reviewed and approved by the Massachusetts Department of Environmental Protection.

Work would be performed outside of the resource areas but inside the buffer areas, Occhipinti noted.

Chair Jeff Barnes asked why the system was beginning to fail after only a couple of years. Don Provencher from Provencher Engineering explained that there were six leach fields, with five more in the back of the property. While he had planned to excavate only the primary fields, MassDEP recommended excavating the reserve areas as well. He described the soil quality as “glacial kill soil” that was replaced with sand.

“That just put a lot of pressure on the groundwater here,” he explained, causing some weeping first observed nearly two years ago. It was hard to determine because of the swale there.

To prevent further seepage, the barrier, which is 7 feet tall, will be placed there. The swale will also be redirected to tie into another swale on the western side of the property. A berm will be placed on the swale’s edge.

Barnes asked if the development there was exacerbating the runoff problem. Provencher didn’t believe so because the wastewater being generated “is within the design flow.”

Member Matthew Moyen asked if putting the barrier in would shift the flow to the opposite direction. Provencher did not expect that to happen, but barriers could be added to prevent seepage.

MBTA request for sedimentation sampling, vegetation removal at Cordaville Dam approved

The commission approved 6-0 a request by the Massachusetts Bay Transportation Authority for sedimentation sampling and vegetation removal at the Cordaville Dam at the Southborough border.

The applicant said the best strategy for mitigating any damage would be to come up with a joint plan with Southborough. The work primarily involves the Southborough side. Invasive species management was proposed, although it is more extensive on the Southborough side.

The commission agreed that mature trees should be replaced at a 1:1 ratio. Replication will take place within no longer than two years.

NOIs approved for Myrtle Avenue single-family homes

Two notices of intent were approved 6-0 for a single-family home at 4 Myrtle Avenue — a road to be constructed off Blueberry Lane — as part of a subdivision. Toll Brothers, the applicant, held off on its request for an NOI for a single-family home at 11 Fitch Avenue — also to be constructed off Blueberry Lane, but moved forward on an NOI at 13 Fitch Avenue. Toll Brothers will be appearing before the Planning Board at its next meeting to request the removal of Adams Street, which affects the 11 Fitch Avenue site.

The applicant’s representative, Ted Merchant, explained that measures will be put in place to update stormwater management practices to modern standards, even though this is not required, as a good-faith effort.

Barnes noted that the stormwater management permit has not yet been issued by the Planning Board. But he acknowledged the applicant’s commitment to upgrade the stormwater system. He described it as “a legacy project” because it is grandfathered in and does not have to meet current requirements.

Member Ted Barker-Hook mentioned concerns he had that permanent immovable barriers at the Chamberlain-Whalen development, which Toll Brothers owns, “have been ignored.” He described lawns being mowed up to the silt fence.

He said there is no space between the 50-foot buffer and the limit of work at 13 Fitch Avenue, worrying about potential erosion. Merchant proposed a super silt fence there.

After some discussion, the two NOIs were approved with the conditions of Planning Board approval for the stormwater management permit and a revised plan showing an alteration of the limit of work.