1/20/2011 SEWER COMMISSION Minutes

Home Browse Search

Town of Mashpee Review of Proposed Plan Memo December 16, 2010 Page 2 of 3 Table 5. Threshold Comparison Results for AInslipee Sever Commission Scenarios: Popponesset Bay Contparison of TN concentrations for present conditions,threshold loading,and live modeled build-out landing scenarios for the Popponesset Bay system. MEP Threshold concentrations ror Popponesset Ray are 0,380 mg/L TN for celgrass(primary),,ind between 0.400,md 0.500 tng/L IN for infauma(secondary). The TMDL for all components of the Popponesset Bay system is 0.:38 m j I tot,d nitro!ett.'t Habitat MEP Scenario I Scenario 2 Scenario 4 Scenario 5 Scenario 3 threshold Present Threshold S&1V S&W S&W S&W LAI In L to /L rn L nt /L nt /L Ing/l, mg/L Po p tonessel Bay-head eelgrass 0.464 0.38, 0.394 0.386 0378 0.389 0,372 N4midto Rover- infauna 0.712 0.4-0.5' 0.0)1 0.570 0,52c c River- mid to leaver ) O.S)6 0.472 Sltcx string Bay-upper to lower infaunu 0.631 0.4-0.5 0.472 0.462 0,449 0.461 0.461 nckway E3ay- infauna 0.567 0.4-0.5 0.457 0.449 0.438 0,453 0.421 utter Note: shaded cells indicate Scenarios that meet MEP thresholds forceieruss/TNIDL or infatuta. Table 3. Threshold Comparison Results for Nilaslipee Sewer Commission Scenarios: Cast Wayuoit Bay Comparison of TN concentrations for present conditions,threshold loading,.bur live modeled Build-out loading scenarios for the East Wagooit Bay systcnt. Threshold concenlrations are 0.446 mo,TN for Mitt Pond(eelgrass),0.:380 rug/1-TN for Hamblin Pond (eelgrass),and less than 0.520 mg/L'fN for average of the upper-and tnid-river monitoring stations of the Quashnet River(infaunu).'` tlabital MEP Scen:u•io I Scenario 2 Scenario 4 Scenario 5 Scenario 3 Scenario 3R Tliwshold Present Threshold' SStW S&W SI&IW S&W LAI LAI m L nag L mg/L ing L ntJL mn L in L Ing1l, .leho Pond eel grass 0.60:3 0.446 0.429 0.435 0,437 0.434 0.472 0.429 Hamblin Pond eel mss 0.528 0.380 0.252 0.25.1 0.200 0,252 0,400 0.251 Quashnet River infauna 0,781 0,520 0.536 0.547 0.523 0.559 0.584 0.460 Note: shaded cells indicate Scenarios that meet threshold concentrations to support celerass or infaunal habitat(TIVIDL). Quashnet River The Proposed Plan does not appear explicitly state how much of Falmouth's portion of the Quashnet River is assumed to be sewered. The portion that is highlighted for possible service by the Town of Mashpee is not likely to be sufficient or representative of Falmouth's Fair Share. For the purposes of this analysis, it is assumed that all or most of the Falmouth septic nitrogen will be removed. Assuming the above, the nitrogen removal in the Proposed Plan is comparable to Scenario 3R for the Quashnet River. There may be enough excess capacity to avoid sewers in the areas north of the ponds due to their being significant excess capacity in Scenario 3R (See Table 3 above). The effect of not sewering this area on TMDL compliance would require further analysis. Mashpee River In Scenario 3R, 100% of the properties in the Mashpee River subwatershed were sewered with limited dispersal within the watershed. This included the entire portion of Sandwich within this subwatershed. Although these areas are highly attenuated and by far the least cost effective on a dollars per lb/day nitrogen removal basis, LAI's analysis concluded that nitrogen removal in this area was necessary. As can be seen in Table 5 above, despite removing virtually all the nitrogen from this subwatershed, LAI's Scenario 3R had little excess capacity. The GHD scenarios did not meet the TMDL requirements. Environmental Engincers/Consultants