12/20/2012 SEWER COMMISSION Minutes

Home Browse Search

The Sewer Commission scenarios discussed in this memorandum were developed with.the assistance of GHD, Inc. staff. CSP/SMAST staff received the initial scenario details, including GIS coverages, from GHD along with a 2/15/12 Memorandum describing the scenarios.' All scenarios are based on the Unified Database that CSP/SMAST staff developed from GHD wastewater estimates and Town of Mashpee land use informations This database is based on buildout assumptions, rather than existing, land use conditions and includes land use within the Popponesset Bay and Eastern Waquoit Bay MEP watersheds, including portions of the Towns of Sandwich, Barnstable, and Falmouth, as well as the remaining parcels in Mashpee that are outside of the two estuary watersheds (Figure 2). Based on the Unified Database,there are 2.7 million gallons per day(MGD) of wastewater flow within this study area. For the purposes of the scenario evaluations, GHD assigned flow to specified subwatersheds even though the discharge sites might exist within multiple subwatersheds (e.g., Keeter, Back Roads, Willowbend). The scenario evaluations are based on the GHD subwatershed assignments for the discharge sites. Only wastewater discharge and treatment is altered in these scenarios; nitrogen loads from other sources (e.g., fertilizers, impervious surfaces, atmospheric deposition) are remain the same in each scenario. In addition, estuary inlet configurations and bathymetry are not altered and are as detailed in the respective MEP Threshold Reports. Table 1 provides the attenuated watershed nitrogen loads for each of the scenarios based on the unified database for each Popponesset Bay subembayment and stream input,while Table 2 provides the same information for each Waquoit Bay subembayment. Details of the wastewater in each of the scenarios in this memorandum are as follows: Option IA for Popponesset Bay and East Waquoit Bay: Option lA is based on projected build-out watershed nitrogen loads and the Rock Landing Well site is the primary discharge area for wastewater from the study area. This option includes the treatment of 38% of the study area wastewater flow(1.0 MGD)to 10 mg/L TN and the discharge of the treated effluent at the Rock Landing Well site,which is outside the two estuary watersheds. This option assumes that the Rock Landing Wells would be relocated to allow treated effluent to be discharged at this site. A total of 57%of the study area wastewater flow (1.5 MGD), including the Rock Landing site and portions of Falmouth, Sandwich and Barnstable sections of the study area, is discharged outside of the watersheds. The remaining 43% of the total study area wastewater flow is discharged within the subwatersheds to the estuary: 18%remains on standard septic systems (0.48 MGD) and 14% (0.37 MGD)is treated to 3 mg/L total nitrogen (TN) and is discharged at the Back Roads site within the subwatershed to Johns Pond (Figure 3). Most of the rest of wastewater flow remaining within the watersheds is treated to 3 mg/L TN at the existing smaller wastewater treatment facilities(WWTFs). Option 1B for Popponesset Bay and East Waquoit Bay Option 1B is based on projected build-out watershed nitrogen loads and keeps most of the wastewater within the estuary subwatersheds, while treating the discharged wastewater to 3 mg/L TN. This option keeps 87% of the total study area wastewater flow (2.3 MGD) within the estuary subwatersheds: 12% (0.32 MGD) remains on standard septic systems, 1% utilize cluster denitrifying septic systems @ 19 mg/L TN, and the remainder of flows are treated to 3 mg/L TN and are discharged at eleven (11) sites within the estuary watersheds (Figure 4). The Falmouth portion of the study area and a section of the Barnstable portion are discharged outside of the watersheds, while the majority of the Sandwich portion (0.37 MGD)is treated to 3 mg/L and is discharged within the Peters Pond subwatershed(i.e.,the"Golden Triangle" site and part of the Popponesset Bay watershed). 'J.Jefferson Gregg. February 15,2012. GHD Memorandum to Mashpee Sewer Commission regarding MEP Model Run Descriptions(Options 1A,1B,and 1C). 6 Eichner,E.,B.Howes,S.Kelley,and J.Ramsey.December 15,2009.MEP Technical Memo to Tom Fudala,Chair,Mashpee Sewer Commission regarding Report on Unified Database and Requested MEP Scenarios for the:(a)Popponesset Bay Estuary and(b)Eastern Basins of the Waquoit Bay System.