158 



u^.L-iaiiVui . .y jc:;s tlimi ^Ik'- ^re^.iivCi., L.j AoUrrti, o :.o:;<'._c 

 buoufliicy plune of frcsli, poorhj-di liileri , cffUtcnt potential ly could 

 Torn <>t the outf<vll site. Such « buoy«nt plune uould be resistflnt 

 to ucrticftl nixing and could conceiuftbly be fldoected into criticftl 

 lidbitflts by wind forcing. 



It is stflted in the fissessnent that the nixed layer depth 

 during sunner is 15-25 n. Since the diffuser stacks extend 2-10 n 

 off the bottom (uhich is «t 25 n), it appears that the effluent 

 uill be discharged into the surface nixed layer and uill not stay 

 belou the thernocline as stated. Since the surface layer during 

 the stratified tine of year can be transported considerable 

 distances due to t)ie reduced friction, the effluent plune could 

 possibly enter critical habitats. The modeling and laboratory 

 studies by Roberts of the initial diffusion apparently did not did 

 not take into account surface grauity uaoes or tidal currents. 

 These studies also haue apparently not been critically examined in 

 the field situation. 



The effects of different uind patterns on transport of 

 effluent has not been examined using the USGS 3-D model. The model 

 thus far has included real uind data ouer the period 12/1/1990- 

 3/29/91. Franks (1990) shoued that uind patterns differ from year 

 to year and can haue a large influence on offshore transport of 

 buoyancy driuen flous. Uind records from different years haue not 

 yet been used to force the USGS model, and the potential for uind- 

 forced offshore transport of the effluent into critical habitats 

 uas not examined. 



In addition to uind, the effects of different inflou patterns 

 from the Gulf of Maine uas not modeled. Such external forcing can 

 haue a large effect on the circulation patterns in the Bays region. 

 The degree of freshuater input into the Gulf of flaine uaries 

 substantially from year to year and can influence the inflou to the 

 Bays region. The site of the proposed outfall, being further 

 offshore than the current site, could be nore subject to uariations 

 in current patterns induced by uind forcing and Gulf of Maine flou. 



The effect of the magnitude of the subgrid-scale nixing rate 

 used in the USGS model has not yet been fully examined. If the 

 true nixing rate is considerably smaller than that used in the 

 nodel, the dilution of the effluent could be substantially less, 

 and the plume could potentially be carried substantially farther 

 than present nodel results indicate. 



B. Long-tern effects - The proposed outfall uill be in existence 

 for perhaps 20-30 years, yet the cunulatiue impact ouer time uas 

 not discussed. Tlie Assessment indicates that the initial fiue 

 uears of discharging primary-treated seuage could lead to 

 intermittent oxygen reductions in Stelluagon Basin belou State 

 standards, but that these uould be uithin the natural range of 

 uariability. Continued discharge of primary-treated seuage beyond 

 tlie fiue year period could potentially haue a significant negatiue 

 impact on the benthic enuironnent in the critical habitat. 



filthough the fissessnent prouides infornation on the expected 

 loading of clienical contaninants in the region around tlie outfall, 

 tliese predictions are based on dilution uith receiuing uaters . 

 Continued discharge of secondary-treated effluent ouer 20-30 years 

 could potentially add significantly to the estinated leuels if the 

 contaminants are accumulated in the surrounding sediments and 

 resuspended by strong uind euents such as storms. 



C. Relation to other outfalls - Uhile the Assessment compares 



