p - p 

 E = St m X 100 



P - P 

 e m 



Thus, if the salinity of the effluent was 50.00 o/oo, and the receiving 

 water had a salinity of 35.00 o/oo and Station 3C had a salinity of 

 36.00 o/oo, the percent effluent at 3C was: 



36.00 - 35.00 inn - A a.-i'/ 

 50.00 - 35.00 ^ ^°° - ^-"^ 



This technique eliminated seasonal fluctuations and made measurements of 

 the influence of the desalination plant comparable all year. It had 

 some disadvantages, however. Percent effluent could only be determined 

 when the plant was operating while the data were being collected. During 

 the months when the plant was operating sporadically, it was difficult 

 to obtain many estimates of the concentration of effluent at the stations, 



Studies showed it took between 24 and 48 hours for the effluent to 

 reach all stations in the harbor and a similar time for the effluent 

 to disperse. During these times, the percentage of effluent was 

 either increasing or decreasing at the stations and a sample at any 

 one station would not necessarily be representative of effluent 

 levels during these sampling periods. Therefore, data taken within 

 48 hours following a start-up period gave relatively unreliable 

 results, especially for more distant stations. 



Since discharging effluent was required for calculation of the percent 

 effluent, data taken immediately after the plant shut down could not 

 be incorporated in the monthly averages even though some effluent was 

 still in the harbor. If the plant shifted its mode of operation, the 

 shift was not reflected in the more distant stations for 24 to 48 hours 

 and the calculated percent effluent was correspondingly wrong. When 

 the plant operated continuously (as it did in the first portion of the 

 study) the method worked exceedingly well. During months when the 

 operations were sporadic and unstable the averages were less reliable. 



A final problem was related to short-term differences that occurred 

 between measurements made in the harbor and on the surrounding grass 

 flats. During periods of rapid temperature or salinity changes, the 

 characteristics of the water in the shallows changed more rapidly than 

 in the deep water of Safe Harbor (due to the ratio of surface area 

 relative to volume). Thus, water at the shallower stations occasionally 

 had different characteristics from that which mixed with the effluent 

 at depth. Because of this shallow-water effect, it was possible to 

 obtain negative values for percent effluent present, at the shallower 

 stations after heavy rains or when there was a rapid change in 

 temperature. 



Despite these difficulties, the method worked within acceptable limits. 

 Figure 8 shows the mean monthly percent effluent at a station near the 

 discharge with 90 percent confidence limits of the mean, and Figure 27 



24 



