11/13/2025 CONSERVATION COMMISSION Minutes (2)

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Town of Mashpee Conservation Commission 16 Great Neck Road North Mashpee, MA 02649 • This settling reduces effective storage volume and shortens system longevity • The existing system (installed in 2015) is already performing poorly To address these issues, the new design proposes: • Plastic infiltrator chambers set on a stone base for stability • Stone surrounding and beneath the chambers for proper bearing • A pump chamber so the leaching field can be set above groundwater The proposed design requires a one-foot reduction from the standard five-foot groundwater separation. Mr. Ball has designed the system with 4.2 feet of separation to ensure that—despite minor variations during installation—it will reliably meet the required four-foot minimum. He confirmed the new leaching area will occupy essentially the same footprint as the old system and that the work area is lawn, with no significant vegetation impacts anticipated. Nitrogen Calculations and Variances Commissioners asked about nitrogen loading. Mr. Ball confirmed that: • Nitrogen calculations have been submitted • The model shows 15.749 ppm, unchanged from existing conditions • The calculation does not include anticipated nitrogen reduction from the Sludge Hammer unit because the standard spreadsheet does not incorporate that treatment type • The bedroom count and flows are unchanged • All variances requested are identical to those granted for the existing system • The distance between the leaching field and the house is approximately nine feet. Site Grades, Retaining Wall Consideration, and Low-Profile Alternatives Commissioners asked whether additional elevation could be gained by installing a small retaining wall. Mr. Ball provided the following information: • Grade at the house: approximately elevation 10 • Road elevations along Uncle Henry's: 7.3-8.0 (average approx. 7.5) • There is some slope toward the road, and a poly liner is included to prevent breakout downslope • A retaining wall was considered but was dismissed due to cost and complexity He further explained why reverting to another low-profile system was not advisable: • Their small pedestal supports tend to push into the sand • Even when geogrid is used beneath them, they remain less stable and reliable than stone-based systems • The current system's premature decline demonstrates these limitations Groundwater Levels, Buoyancy, and Flood Considerations Mr. Ball explained that: • The observed groundwater level is 1.4 feet lower than the estimated seasonal high used for design • The bottom of the new pump chamber will be: o —2 feet above observed groundwater o —4 inches above estimated seasonal high groundwater 15