17 
dilution of fresh effluent with older effluent is occurring. 
The calculated MPN's are higher than the observed ones showing 
again an extra disappearance of coliforms in addition to that 
due to dilution. Assuming sedimentation to be an important 
factor in the extra disappearance, one might have expected the 
observed subsurface MPN's to be higher than the calculated MPN's, 
at least in some individual instances, since one might expect 
to trap settling particles in some of the subsurface samples. 
Actually, three out of the twenty subsurface samples had MPN's 
that were higher than the calculated, and in four instances, 
subsurface samples having higher counts than the water above 
them were observed. These limited observations provide some 
evidence for the occurrence of sedimentation which was evident 
aS a cause of coliform disappearance in the vicinity of the 
Orange County outfall (May 1 report). 
Dye Patch Experiments 
In addition to the two dye patch experiments done on the 
tracer cruise, six additional were run on three other trips 
during which primary effluent was being discharged from the 
Hyperion outfall. The MPN*s obtained are plotted against time 
for each individual run in Figure 5, and the geometric means of 
the MPN‘s of all the runs against time in Figure 6. The time for 
90% reduction in coliform count ranged from 13 hours for the most 
rapid to 43 hours for the slowest with 3 hours as the average. 
The last value is the best estimate available for the rate of 
disappearance to be expected around the proposed new outfall 
assuming the same type of primary effluent will be discharged. 
