75 



mixing processes due to their strength, but since they are periodic they essentially displace 

 material back and forth over the length of the tidal excursion with little net transport. The 

 tides have a large effect on the flushing of Boston Harbor, but they play little role in 

 transporting material over distances comparable to the size of Cape Cod and Massachusetts 

 Bays. 



Studies have shown that the mean flow of water in Massachusetts Bay is in a 

 counterclockwise direction (figure 3). The USGS modeling studies support this 

 interpretation. The modeling results indicate that this pattern is caused by the southward 

 direction of the coastal current flowing from the Gulf of Maine, in combination with mean 

 westerly winds. Western Massachusetts Bay, including the area around the proposed MWRA 

 outfall, is usually not included in this circulation but rather lies to the west of the coastal 

 current. During winter months (November through April), waters in the Massachusetts Bay 

 are well mixed vertically. During the late spring and summer months, however, freshwater 

 inflow from the Gulf of Maine and surface warming cause the water to become stratified, 

 and mixing between the upper layer and deeper colder layer is reduced. 



Simulations of effluent dilution were performed by tracking the concentration of a 

 passive tracer introduced with the fresh water at the outfall. Using observed treatment plant 

 flow data provided by the MWRA, the USGS modeled areas of effluent dilution from both 

 the present outfall near the mouth of Boston Harbor as well as the proposed new outfall in 

 Massachusetts Bay. Coastal circulation models are not appropriate for modeling the near 

 field turbulent entrainment of a rising plume, so the simulations performed with ECOM-si 

 are tuned to match the near field dilution (dilution occurring within 175 feet of the outfall) 

 predicted by EPA's ULINE model for near field dilution (EPA 1988). 



PREDICTIONS OF EFFLUENT DILUTION 



The results of the modeling of effluent dilution from the existing outfall in Boston 

 Harbor and from the proposed outfall in Massachusetts Bay shows that moving the outfall 

 into Massachusetts Bay: 



* greatly reduces effluent concentrations in Boston Harbor; 



* increases effluent concentrations in the immediate vicinity of the new outfall, but 

 overall reduces the size of the region in Massachusetts Bay frequently visited by 

 higher concentrations. 



These results reflect increased dilution of the effluent associated with the greater water depth 

 and the increased distance from the coast at the new location. 



