1978 - present 



The results of the 1977 survey indicated that 

 the two-unit thermal plume approached, and ex- 

 ceeded, the 4000-ft limit specified by the NPDES 

 Permit for Units 1 and 2, for short periods as the 

 tide turned during the slack after ebb tidal phase 

 (Stoltzenbach and Adams 1979). Because of these 

 findings and recognizing that the original plume 

 predictions were based on non-transient models, 

 it became apparent to NUSCO that improved 

 modeling was necessary. 



During 1978 and 1979, Liang and Tsai (1979) 

 analyzed the data and calibrated an updated math- 

 ematical model to predict the thermal plume re- 

 sulting from the combined discharge of condenser 

 cooling water from all three units. Also, 

 Stohzenbach and Adams (1979) developed a near 

 field - intermediate field model and coupled it to 

 the existing transient far field model. This model 

 replicated the essential surface and subsurface fea- 

 tures of the MNPS thermal plume as observed 

 during two-unit operation and provided reliable 

 estimates of the surface and subsurface induced 

 temperature rise distribution down to a AT of 

 1.5°F (0.83°C). Conservative estimates of the 

 maximum extent (in terms of distance from the 

 discharge point) of 6°F (3.3°C), 4''F (2.2''C) and 

 1 .5°F (0.83°C) induced temperature increases were 

 5,000 ft (1,524 m), 8,000 ft (2,438 m) and 12,000 

 (3,658 m) respectively (Stoltzenbach and Adams 

 1979). The NPDES Permit now limits the 4°F 

 (2.2°C) temperature rise to 8,000 ft (2,438 m). 



Several factors were considered when selecting 

 the methodology and dates for the actual plume 

 mapping in 1987. After discussions with advisors 

 (J. Tietjen, N. Marshall, S. Saila, W. Pearcy) and 

 regulators, NUSCO determined that the configu- 

 ration of the thermal plume during late summer, 

 when ambient water temperatures were near max- 

 imum, would represent worst case conditions. 

 Mapping would have to include a dye survey so 

 that natural daily warming could be separated 

 from the heat load added by the Station. Also 

 because NUSCO wanted to determine the maxi- 

 mum extent of the three-unit thermal plume, the 



survey ^yould have to be completed when all 

 MNPS units were operating at near-maximum 

 capacity. Because of scheduled refuel outages 

 (Unit 1, July and August 1987 and Unit 3 to 

 begin 31 October 1987), a window of appropriate 

 conditions would occur in 1987 only during late 

 August. After that, water temperature would have 

 cooled considerably from its normal summer max- 

 imum. 



Materials and Methods 



As mentioned previously, Ocean Surveys, Inc. 

 (OSI), Old Saybrook, Connecticut, provided in- 

 strumentation and technical support to complete 

 the actual dye survey. Preliminary activities began 

 21 August 1987 and data collection began 23 Au- 

 gust 1987. Actual mapping of the thermal plume 

 during four major tidal phases (low slack, max 

 flood, high slack and max ebb) took place on 26 

 August 1987. The survey data were supplemented 

 with continuous in-situ temperature records taken 

 subsequent to the actual plume mapping. Details 

 of instrumentation and data aquisition techniques 

 used during the mapping survey are described in 

 the OSI report, which is the Attachment to this 

 section. A brief summary is given below along 

 with a description of methodology used by 

 NUSCO to collect in situ supplemental temper- 

 ature during October 1987. 



Dye injection 

 Dye plume mapping was accomplished using 

 Rhodamine WT flourescent tracer dye. Dye was 

 injected onto the water surface immediately down- 

 stream of the Unit 3 discharge. Mixing occurred 

 here and where the waters from Units 1, 2 and 3 

 met in the middle of the quarry. Dye concentra- 

 tion and water temperature were monitored con- 

 tinuously at one quarry cut using a calibrated 

 Turner Model 1 1 1 flourometer and a Yellow 

 Springs Instrument Company Series 700 

 thermistor, respectively. To insure that the dye 

 injection rate remained constant, NUSCO staff 

 checked the flow of dye, weighed the dye supply 

 reservoir and checked the flourometer every two 

 hours. Background levels of flourescence were 



328 



