09/04/2025 CONSERVATION COMMISSION Minutes

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7 Town of Mashpee Conservation Commission 16 Great Neck Road North Mashpee, MA 02649 w$XJI:YYF.µ'1 NOI 42 Seconsett Island Road, Peter & Marianne Frongillo. Proposed construction of 43- detached garage with ADU, I/A septic system upgrade, and mitigation plantings. 3361 (Representative: Cape & Islands Engineering, Inc.) • Presented by Raul Isardi of Cape and Islands Engineering on behalf of the applicant. The project involved two major components: (1) an upgrade of the existing septic system and(2) construction of a garage with an accessory dwelling unit(ADU) above. • Mr. Isardi explained that the resource areas within jurisdiction included land subject to coastal storm flowage, a coastal bank, a salt marsh across the street, and land under the ocean containing shellfish within Waquoit Bay. The project area is primarily lawn and driveway within the buffer zone to a coastal bank, which he described as a minor two-foot slope but nonetheless meeting the definition under the regulations. • The septic system upgrade replaces a non-compliant Title 5 system located only 23 feet from the bank and 40 feet from the salt marsh. The new system will consist of a denitrifying MicroFAST tank and pump chamber, with the leaching field relocated to the driveway area. This change increases the setback to the salt marsh from 40 feet to 139 feet and the setback to the bank from 23 feet to 67 feet, while also elevating the leaching field 5.6 feet above groundwater. Mr. Isardi emphasized the nitrogen- reducing benefits of the upgrade, lowering projected sitewide concentrations from 11.7 ppm to approximately 8 ppm. • The ADU will be built in a developed portion of the lot currently containing a shed, gravel driveway, and lawn, located outside the 75-foot buffer to the coastal bank. Some modifications to the driveway will be required, though the driveway will remain pervious gravel. The project includes 1,135 square feet of native mitigation plantings,replacing about 1,400 square feet of lawn. • Ms. Thornbrugh thanked Mr. Isardi for a clear presentation and asked several questions. She requested confirmation of how close the proposed mitigation planting would come to the coastal bank, and Mr. Isardi explained that about one-third of the plantings would be within the coastal bank itself, with the remainder in the adjacent buffer zone. Ms. Thornbrugh also confirmed that no trees would be removed and that utility work would be routed through an existing walkway to avoid damage to a large, mature tree in front of the house. • Ms. Thornbrugh expressed appreciation for the relocation of the septic system farther from the marsh but raised a question about whether the proposed garage could be designed on piles or with a carport to improve permeability. Mr. Isardi responded that the location was outside the flood zone and that the applicant preferred a concrete-slab garage with full walls, which was consistent with the site's use and did not create additional permeability concerns. • Mr. Colombo asked whether the application had been reviewed against the Cape Cod Commission's MORIS Data Viewer projections for 2050 flood exceedance. He noted that the viewer shows a one-in- five annual chance of flooding that could exceed FEMA maps by more than three feet of water. Mr. Colombo emphasized that Regulation 25 revisions require consideration of sea level rise and coastal resiliency. • He then asked Mr. Isardi to explain the buoyancy calculations provided in the submittal for the septic tank and pump chamber. Mr. Isardi explained that the 8.9 and 11.1 factors reflected the weight of the tanks relative to the upward buoyant force of groundwater, ensuring they would remain in place. He clarified that these figures did not address storm surge, only groundwater. Mr. Colombo expressed Page 11