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During the development of the updated unified database,SMAST staff identified parcels that are . <br /> not proposed to be connected to either a WWTF or an VA septic system in any of the five <br /> scenarios. Wastewater from these parcels or others with standard Title 5 septic systems is <br /> assigned the standard MEP septic system loading factors. <br /> Table 1 provides a summary of the wastewater flows and effluent TN concentration by treatment <br /> technology for each of the scenarios. Table 2 provides the attenuated watershed nitrogen loads <br /> by subembayment for each of the scenarios based on the updated unified database. <br /> MEP Scenario Results and Discussion <br /> Scenario Results: Comparison to Threshold Nitrogen Concentrations <br /> Using the nitrogen loads based on the update unified database, Scenarios 3 and 4 meet the <br /> threshold values at the sentinel station for restoration of eelgrass in Popponesset Bay. Of these, <br /> only Scenario 3 yields water column TN concentrations within each of the three tributary sub- <br /> embayments that would be restorative of infaunal habitat(Table 3), although Scenario 4 is very <br /> close. <br /> In should also be noted that all of the scenarios yield water column TN concentrations restorative- <br /> of infaunal habitat in Ockway and Shoestring Bays, but all but Scenario 3 (and possibly 4) leave <br /> excess TN levels in the Mashpee River. This finding suggests that the collective wastewater <br /> treatment in the Ockway and Shoestring Bay watersheds may exceed what is necessary, but that <br /> the collective wastewater treatment in the Mashpee River watershed is insufficient in Scenarios <br /> 1, 2, 4 and 5. It should be noted that no other alternative nitrogen reduction approaches other <br /> than improved wastewater treatment were considered in these scenarios. <br /> Related to these findings, it is also worth noting,that Scenario 5 (Centralized approach)has the <br /> lowest nitrogen load to the lower Mashpee River(see Table 2), but the highest load to the <br /> freshwater portion of the Mashpee River. This result suggests that selection of effluent discharge <br /> locations in more optimal positions in the watershed may allow this and other scenarios to meet <br /> the threshold concentrations. <br /> Use of Alternative Septic Systems <br /> Review of Table 1 shows that there are significant differences among the scenarios regarding <br /> how alternative on-site septic systems are used. Scenario 3 assumes that 306,825 gallons per day <br /> (gpd)of wastewater flow in the combined Popponesset Bay/East Waguoit Bay study area is <br /> treated with alternative on-site septic systems. By contrast, Scenario 1 uses alternative on-site <br /> septic systems for 41,311 gpd of wastewater flow, Scenario 4 uses these systems for 5,621 gpd, <br /> and Scenarios 2 and 5 do not use alternative on-site septic systems. <br /> Effluent Concentration Assumptions in Scenarios <br /> Another issue that the Sewer Commission should consider is the effluent discharge <br /> concentrations-assigned to the various wastewater treatment technologies in the scenarios. As <br /> indicated in Table 1, standard Title 5 septic systems are assigned an effective effluent total <br /> nitrogen (TN)concentration of 23.63 mglL in MEP analyses. This concentration is based on an <br /> effluent TN concentration of 26.25 mg/L and, since water use is used as a proxy for wastewater <br />