04/16/2026 CONSERVATION COMMISSION Minutes (2)

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Town of Mashpee Conservation Commission 16 Great Neck Road North Mashpee, MA 02649 It was explained that these parameters are often not explicitly presented in typical application materials and that the calculator allows Commissioners to directly evaluate the underlying assumptions affecting nitrogen loading. The calculator estimated travel time and nitrogen attenuation through the subsurface. Discussion noted that, under typical Mashpee sandy soil conditions, travel times may have been less than one year(e.g., approximately 250-280 days in one example). It was further explained that effective denitrification generally requires two to three years of subsurface residence time, and that shorter travel times may result in little to no additional nitrogen reduction once effluent leaves the leach field. Additional outputs included: • Estimated nitrogen concentration at groundwater (e.g., values exceeding local thresholds in some scenarios) • Percent attenuation (noted to be limited under short travel time conditions) • Nitrogen loading per acre • Compliance indicators relative to nitrogen-sensitive area (NSA) thresholds It was further explained that nitrogen-sensitive area thresholds include approximately 10 mg/L for drinking water standards and lower targets (approximately 5 mg/L) associated with estuarine and coastal water quality goals. The calculator also included optional inputs for projected future conditions, including flood depth and exceedance probabilities for future years (e.g., 2030 and 2050). It was noted that, under certain modeled conditions, septic systems may become subject to periodic or permanent inundation, potentially resulting in direct discharge of effluent to surrounding resource areas. It was emphasized that the calculator is intended as a screening tool only, not a substitute for engineering design or site-specific analysis. A disclaimer was included noting that results are based on publicly available data and user inputs, and that applicants remain responsible for providing complete engineering and hydrologic documentation. Concerns were raised that certain I/A systems may not significantly reduce nitrogen despite their classification,particularly systems that rely on aerobic processes rather than denitrification. It was noted that some systems may improve system function (e.g., preventing biomat buildup) without meaningfully reducing nitrogen loading to groundwater. Mr. Colombo also described ongoing development of an expanded model intended to integrate watershed- specific data, including Total Maximum Daily Load(TMDL) thresholds, sub-watershed conditions, and regulatory data, with the goal of evaluating the relative contribution of individual properties to overall nitrogen loading within a watershed. Commission discussion included: • Potential use of the calculator as a reference tool during project review • Whether the tool should remain informal or be incorporated into review procedures • Legal considerations associated with use of a non-regulatory screening tool • The role of Commissioners in independently evaluating nitrogen data presented by applicants • Broader concerns regarding septic system performance, maintenance, and long-term effectiveness • Financial implications of advanced nitrogen-reducing systems and wastewater infrastructure 9