04/16/2026 CONSERVATION COMMISSION Minutes (2)

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Town of Mashpee Conservation Commission 16 Great Neck Road North Mashpee, MA 02649 • Septic Nitrogen Loading Calculation A nitrogen loading calculator developed by Mr. Colombo was presented as a screening tool to assist the Commission in evaluating septic system impacts on groundwater and nearby resource areas. The tool was developed in response to concerns regarding nitrogen calculation methods commonly submitted with applications,particularly those based on Cape Cod Commission worksheets, which he stated rely on average occupancy assumptions (e.g., 2.5 persons per household)rather than permitted system design flow. It was explained that septic systems are permitted based on the number of bedrooms and corresponding design flow (e.g., 220, 330, 440, or 550 gallons per day), and that this permitted flow governs the nitrogen load discharged to the subsurface regardless of actual occupancy. The calculator focuses specifically on nitrogen attenuation within the leach field and subsurface and does not incorporate additional factors such as impervious surfaces, which may reduce calculated averages but do not reflect the actual nitrogen concentration entering groundwater. The calculator was described as a transparent, data-driven tool, with all inputs visible and editable, allowing Commissioners to independently evaluate assumptions presented by applicants and their engineers. It was further noted that the tool is based on pre-programmed mathematical formulas and can generate results in real time. He emphasized that the calculator allows Commissioners to independently evaluate assumptions presented in applications. Key inputs and variables discussed included: • Effluent total nitrogen concentration (mg/L): Default values were noted to be approximately 35-36 mg/L for conventional Title 5 systems. It was stated that some innovative/alternative (I/A) systems do not reduce nitrogen and are still designed at similar concentrations, while others may achieve reductions (e.g., approximately 25 mg/L). • Design flow: Based on bedroom count(e.g., 440 gallons per day for a four-bedroom system), which governs nitrogen loading regardless of occupancy. • Soil characteristics: Included permeability (Ksat, expressed in feet per day),porosity(void space between soil grains), and first-order decay constants. These values were preloaded for various soil types (e.g., Carver, Mashpee soils) and automatically adjusted when soil types were changed. It was explained that: o Ksat represents the rate at which effluent moves through the soil (i.e., "speed" of flow) o Porosity represents the available void space for movement and treatment o Decay constants represent the rate of nitrogen reduction over time • Travel time: Calculated based on Ksat, porosity, and vertical separation,representing the time effluent remains in the leach field before reaching groundwater. • Vertical separation to groundwater: Typically five feet by regulation, with reduced separation increasing the likelihood of nitrogen reaching groundwater more quickly. • Recharge rate: Defaulted to approximately 26 inches per year, reflecting updated Cape Cod Commission guidance (noting older values of approximately 19 inches per year). • Lot size: Entered in acres and used to calculate nitrogen loading per acre. • System type: Included conventional, I/A, and Best Available Nitrogen Reducing Technology (BANRT) systems, with discussion noting that BANRT systems provide the highest level of nitrogen reduction but are significantly more costly. 8