11/20/2025 SEWER COMMISSION Minutes (2)

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Town of Mashpee Sewer Commission t 16 Great Neck Road North Mashpee, MA 02649 TIMELINE FOR REVISED PHASING PLAN & REMOVAL CALCULATIONS Commissioners asked when they would receive updated nitrogen-removal figures and the refined mini-phase layout. GHD aims to deliver a draft at the December Sewer Commission meeting,possibly earlier if calculations are completed. The forthcoming draft will assume a specific phasing structure and will be ready for initial review and discussion. PERMEABLE REACTIVE BARRIERS (PRBs) At the request of the Wastewater Department, Mr. Drainville introduced the topic of permeable reactive barriers (PRBs)—subsurface trenches filled with reactive materials (often wood chips) used to reduce nitrogen as groundwater flows through. PRBs have long been used for contamination control and are being piloted in some Cape Cod communities for denitrification. Jessica Thomas (Roger Williams University) previously presented on PRBs to the Conservation Commission and recently consulted with staff regarding possible applications in Mashpee. Mr. Drainville explained they evaluated whether PRBs could enhance treatment around the sand beds but concluded that this is not a good use case. PRBs are only effective where a highly concentrated nitrate plume exists. Several Commissioners recalled that Mashpee investigated PRBs about 15 years ago and determined the Town did not have a plume concentrated enough to justify installation. They emphasized that PRBs require excavation, ongoing maintenance, and careful selection of location, and that they are not appropriate unless a single, strong nitrogen source existssomething that does not appear to be present in Mashpee. Mr. Drainville noted that the only reason PRBs were raised is because staff were asked to update the Commission after the topic resurfaced during a Conservation Commission presentation. PRBs can show short- term nitrogen improvements before the full sewer system's effects reach the estuaries, but only if an appropriate plume exists. Since Mashpee does not appear to have such a plume, the Commission agreed the topic will not be pursued further unless new data emerges. SUMMARY OF PHASING DISCUSSION FOR SELECT BOARD MEMBER WYMAN- COLOMBO At the request of the Chair, Mr. Drainville summarized the earlier phasing presentation for Select Member Colombo, who attended the meeting. He explained that: • GHD is finalizing a new updated phasing plan, • Phases will be broken into smaller sub-phases, • GHD will return next month with nitrogen-removal details, and