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concentrations/TMDLs. Two additional scenarios,using build-out assumptions, were <br /> completed assuming installation of denitrifying septic systems throughout the watershed with N <br /> removal rates of 25% and 45%. For these model runs,nitrogen removal rates were set by <br /> MassDEP with consideration of the monitoring results from the Cape Cod denitrifying septic <br /> system database maintained by the Barnstable County Department of Health and the <br /> Environment. Neither of these build-out scenarios met the threshold concentrations/TMDLs. <br /> The fifth scenario was an evaluation of potential natural nitrogen reductions if portions of the <br /> Santuit River were routed old cranberry bogs south of Santuit Pond. Monitoring associated with <br /> this scenario showed that under current conditions --20%of the nitrogen load entering the bogs is <br /> removed. After completing a more detailed characterization of the bogs,their volumes and the <br /> flows that exchange nitrogen among the bogs, SMAST scientists estimated that reworking of <br /> flowpaths could raise this removal rate by twofold or more. <br /> The current effort described in this Technical Memo began in mid-2007: the Sewer Commission <br /> selected five watershed wastewater scenarios to be assessed using both the Popponesset Bay and <br /> East Waquoit MEP models (i.e., a total of 10 model runs). In early 2008, the nitrogen loads for <br /> these scenarios were publicly presented and,in April 2008,were forwarded to the MEP <br /> Technical Team for review prior to modeling. <br /> The nitrogen loads from the five selected scenarios included updated land use and water use <br /> databases supplied by Stearns and Wheler(S&W) in consultation with the Sewer Commission. <br /> Land use and water use databases were updated for the Town of Mashpee, Town of Falmouth, <br /> and Town of Barnstable,but no changes were made to the MEP Town of Sandwich databases. <br /> All five scenarios are based on build-out conditions. Four of the scenarios are described in detail <br /> in the S&W Alternatives Analysisb and the fifth scenario(or third as listed below)was developed <br /> by Lombardo Associates, Inc. (LAI).' <br /> In brief, these scenarios are characterized by the following: <br /> • Scenario 1 -S&W: No expansion of existing wastewater treatment facilities(WWTFs); <br /> 4 new WWTFs with 3 ppm total nitrogen (TN)effluent and 5 effluent <br /> discharge sites; <br /> -380 new innovative/alternative(11A)septic systems with 10 ppm TN <br /> effluent <br /> • Scenario 2-S&W: Upgrade and expansion of existing facilities to a practical extent with <br /> effluent discharge of 3 ppm TN; <br /> 4 new WWTFs with 3 ppm TN effluent and 4 effluent discharge sites <br /> • Scenario 3-LAI: Nitrexrm-based decentralized approach with 16 WWTFs with 3.75 ppm <br /> TN effluent and 16 discharge sites; <br /> all new and expanded on-site septic system flow treated with on-site <br /> NitrexTM with 3.75 ppm TN effluent <br /> • Scenario 4-S&W: Fair Share based on facilitated discussions: <br /> 3 new WWTFs with 3 ppm TN effluent and 3 effluent discharge sites; <br /> --130 new innovativelalternative(UA)septic systems with 10 ppm TN <br /> effluent <br />