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A dilution of 1:400 (1 part effluent to 400 parts seawater) is used to summarize the 

 effects of the outfall. For nitrogen, concentrations of effluent diluted 1:400 are typically 

 smaller than the natural variability of the ambient nitrogen levels (see EPA, 1993, page 

 4-20). Thus for this nutrient, this dilution represents a reasonable level beyond which the 

 effect of the new outfall should be minimal. 



The effect of the effluent at a given location is illustrated by the frequency or 

 percentage of time that effluent more concentrated than 1:400 dilution is found at the 

 location. 



WINTER: During winter unstratified conditions, effluent is well-mixed vertically, and 

 surface values represent levels throughout the water column. Over a period from December 

 1990 to March 1991, effluent more concentrated than the 1:400 level from the existing 

 outfall occurs more than 80% of the time in a region which includes all of Boston Harbor, 

 and the South Shore as far as the North River (figure 4). In contrast, effluent more 

 concentrated than the 1:400 level from the proposed outfall is found more than 80% of the 

 time in a much smaller (10 km diameter) region offshore. For both outfall locations, effluent 

 more concentrated than 1:400 is found chiefly in western Massachusetts Bay and never found 

 around Stellwagen Bank or Race Point. 



SUMMER: During the summer stratified conditions, simulations show that effluent from the 

 existing outfall tends to remain in the lighter surface layer, while effluent from the proposed 

 outfall tends to remain in the heavier lower layer. For the surface waters, therefore, while 

 effluent more concentrated that 1:400 is found frequently in western Massachusetts Bay with 

 the existing outfall, it almost never occurs with the proposed outfall (figure 5). At 15 m 

 depth, the situation is reversed, with effluent more concentrated than 1:4(X) dilution found 

 typically in a 10 km diameter region around the proposed outfall, and almost never occurring 

 with the existing outfall (figure 6). The areal extent of the deeper waters typically affected 

 by the proposed outfall is significantly less than the areal extent of the surface waters 

 affected by the existing outfall. As in the winter months, effluent more concentrated than 

 1:400 is never found around Stellwagen Bank or Race Point. 



The entire 18-month simulation (January 1, 1990 to July 1, 1991) shows: 



* Stellwagen Bank was not frequented by effluent more concentrated than 1:4(X) (less 

 than 1 % of the time for the existing outfall and never for the proposed outfall) 



* Cape Cod Bay was infrequently visited by effluent more concentrated than 1:400. In 

 winter, these levels were exceeded less than 40% of the time from the existing outfall 

 and less than 20% of the time from the proposed outfall. In summer, these levels 

 were not exceeded by the existing outfall and less than 5 % of the time from the 

 proposed outfall. 



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