Pollution 



301 



Figure 235. Top, mean prob- 

 able number of coliform bac- 

 teria per square centimeter of 

 bottom sediment in vicinity of 

 Whites Point outfall of Los 

 Angeles County, from which 

 the average sewage discharge 

 is 177 million gallons per day. 

 Note the presence of high 

 counts at more than 10 km 

 from the outfall. Bottom, dis- 

 tribution of black sediment 

 (solid dots) versus ordinary 

 green or gray sediment (circles), 

 free hydrogen sulfide (S), and 

 Chaetopterus variopedatus (C). 

 Transparency of overlying 

 water, shown by Secchi disk 

 readings in meters, exhibits a 

 pronounced decrease near out- 

 fall. Redrawn from Rittenberg, 

 Mittwer, and Ivler (1958, Figs. 

 3 and 4). 



118*20' 



12 3 4 MILES 



^n 1 ' I ' i H 



2 4 6 KM. 



118 20 



Sewage discharge alters sea water in other 

 ways than addition of bacteria. One way is 

 the addition of suspended soUd^ which mark- 

 edly reduce the transparency of the water 

 (Fig. 235). Reduction has been noted to a 

 distance of at least 4 km from outfalls. A 

 close inverse correlation exists between 

 Secchi disk readings (depth at which a 30-cm 

 white disk disappears from view) and the 

 contents of chlorine and ammonia. A trans- 

 parency meter for measuring horizontal 

 transparency of hght at various depths 

 through 1 meter of water served to track the 

 movement of effluent, which sometimes 

 spread out at an intermediate depth near the 

 thermocline. 



The low sahnity of the effluent dilutes the 

 sea water about 5 per cent directly over the 

 outfalls and progressively less at greater dis- 

 tance, but, owing to limitation in accuracy 

 of measuring chlorinity, the effects of dilu- 

 tion by sewage effluent cannot be traced as 

 far as can reduction of transparency. The 

 contents of phosphate and fixed nitrogen in 

 the effluent are of the order of 1000 times 

 their concentrations in average surface sea 

 water. However, collections of plankton 

 show only a slight average increase of dino- 



flagellate populations, uncertain increases of 

 diatoms, copepods, tunicates, tintinnids, and 

 annelid larvae, and no increase of other 

 forms. No blooms have been related to sew- 

 age discharge. Perhaps the minor effect of 

 the great concentration of nutrients on 

 plankton populations results from the rapid 

 dispersal of nutrients by currents and diffu- 

 sion as compared with the time required for 

 much plankton growth. Abundant heavy 

 metals discharged in sewage have no identi- 

 fiable effect on the sea water or its life. 

 Oxygen demand of the sewage is great, about 

 70 times the oxygen normally present in the 

 same volume of surface sea water, when the 

 oxygen demand is taken as the total of the 

 ordinarily measured 5-day biological oxygen 

 demand (160 ppm) plus the oxygen required 

 to oxidize the ammonia and particulate or- 

 ganic matter that is discharged. Neverthe- 

 less, little effect has been noted in the 

 receiving waters and certainly no stagnation 

 occurs, probably because of the rapid mixing 

 and the liberation of oxygen during photo- 

 synthesis by photoplankton. 



With respect to benthos, greater effects of 

 sewage discharge are discernible. As noted 

 by Hartman (1956), within 3 km of the 



