MAY JUN JUL AUG 



NOV DEC 



FEB MAR APR MAY 



Fig. 3. The average monthly seawater temperatures at the Unit 1 & 2 intakes (ambient, bottom pair of lines 

 in each graph) and the Millstone Quarry cuts (effluent, the top pair of lines in each graph) during tfie four 

 six-month exposure periods. The monthly averages are from the 15th of one month to the 15th of the next 

 (— average temperatures during 3-unit operation, — average temperatures from November 1978 to May 

 1986, vertical bars represent the range of average monthly temperatures over this seven and one half year 

 period of 2-unit operation). 



12-15 °C. 



Effluent temperature regimes (i.e., annual tein- 

 perature ranges) have not changed substantially 

 since Unit 3 began operation; however, the volume 

 of cooling water has almost doubled. Prior to 

 1986, sampling sites at WP, FI, BP, and GN were 

 unaffected by the MNPS thermal plume; annual 

 average temperatures at these sites were within 

 0.6 °C of those at the intakes. From early spring 

 through early autumn, temperatures at the FI 

 sampling site were up to 2 °C above ambient, 

 but these elevated temperatures were attributed 

 to natural warming of shallow waters in Jordan 

 Cove by solar radiation. Similar insolation of 



shallow estuaries has been reported elsewhere 

 (Dale and Gillespie 1977; Dean and Officer 1977; 

 de Wilde and Berghuis 1979). 



Since Unit 3 began operation, the added volume 

 of cooling water has increased the areal extent of 

 the MNPS thermal plume, which on an ebbing 

 tide raises water temperatures in Jordan Cove (see 

 the I lydrothermal Studies section of this report). 

 During 3-unit operation in 1987, water tempera- 

 tures at FI and WP were close to ambient during 

 most of the tidal cycle (9-10 h), but maximum 

 AT's of 2-4 °C occurred for 2-3 hours per cycle 

 (Fig. 4). Insolation in summer might raise tem- 

 peratures an additional 2 °C. 



232 



