stations are shown in Fig. 1. Water samples were 

 collected just beneath the surface with a Van Dorn 

 bottle, chemically fixed and analyzed for dissolved 

 oxygen by the Winkler method (APHA, 1965). Water 

 temperatures at the time of sample collection were 

 obtained using an ARA electronic thermometer. 

 Saturation levels were calculated using a solubility 

 curve from the data of Weiss (1970). 



In general, all dissolved oxygen measurements 

 showed that Pilgrim Station intake waters during 

 the menhaden mortality were near saturation 

 (Table 1). Dissolved oxygen saturation levels in the 

 discharge channel and thermal plume usually ranged 

 from 120% to 140%. 



Mortality of menhaden continued until 

 April 24, 1973. However, after April 19, 1973, an 

 apparent decline in mortality occurred. Estimates 

 of menhaden mortality rates were made on several 

 occasions during the observed fish kill. Severely 

 stressed and dead menhaden were collected on the 

 surface of the thermal plume during 15-min periods. 

 Time collections of menhaden averaged 10.7 fish 

 per period (range of 5 to 17 fish per period) or about 

 0.7 fish per min. For each fish collected at the 

 water surface at least two additional menhaden 

 were observed in distress beneath the water surface 

 that were not collected. Applying an approximate 

 minimum mortality rate of about 2.7 fish per min, 

 an estimated 43,000 menhaden died due to gas 

 bubble disease during the period April 9 through 

 April 19, 1973. 



An attempt was made to decrease gas satura- 

 tion levels in the discharge water to those believed 

 to be non-lethal to menhaden (i.e., less than about 

 120% and 130% saturation for nitrogen and oxygen, 

 respectively) based on the tolerance observations 

 of a related species, Atlantic herring (C/upea 

 harengus harengus), reported by Stickney (1968). 

 The Massachusetts Water Resources Commission 

 (Division of Water Pollution Control) requested that 

 beginning April 20, 1973, Boston Edison Company 

 reduce Pilgrim Station power level by approximately 

 50% so that the temperature differential between 

 intake and discharge water would not exceed 15°F. 

 During the load reduction, studies of the dissolved 

 gas concentration and saturation of the station 

 intake and discharge water would be conducted. In 

 compliance with the Water Resources Commission's 

 request. Pilgrim Station power level reduction began 

 at 2400 hr EST on April 20, 1973, and by 0600 hr 

 EST on April 21, 1973, the station output was 300 MW 

 gross (At condenser = 14.85°F) and remained 

 approximately the same until 2145 hr on April 22, 

 1973, when the Water Resources Commission per- 

 mitted Boston Edison Company to begin increasing 

 the station power level back to full capacity. During 

 the station power reduction, discharge dissolved 



TABLE 1 Dissolved Oxygen Concentration and Percent 



Saturation of Pilgrim Nuclear Power Station Intake and Discharge 



Water During the April 1973 Menhaden Mortality 



'Saturations calculated from solubility data reported by Weiss 

 (1970) for a salinity of 30 ppt. 



78 Marcello, Fairbanks 



