25 
After nine hours from the time of discharge, the dis- 
appearance rate measured by the Bureau of Sanitation changes 
sharply and is of the order of 15 hours for a 90% disappearance 
of coliforms. This value is based on samples collected close 
inshore near the surf zone and it "is believed to reflect the 
boundary effects obtained in and near the surf zone where the 
coliforms undergo a much slower rate of reduction than in off- 
shore waters". This same effect is reflected in the persistence 
of the beach counts observed on several occasions by the Bureau 
of Sanitation after the release of unchlorinated secondary treat- 
ment effluent for short periods of time from the Hyperion plant. 
The less rapid disappearance of coliforms in the surf zone was 
previously discussed (May 1 report) and it was pointed out that 
if sedimentation is an important factor in coliform disappearance, 
it would not be operative in the surf where turbulence would 
keep particulate matter in suspension. In fact, the breaking up 
of particles in the surf might in effect increase the apparent 
MPN's of the beach samples. The major significance of this 
effect is merely to shorten somewhat the time of travel available 
for effective coliform disappearance. In other words, the time 
of travel to the surf zone and not to the beach is the signifi- 
cant one. The difference between the two times, however, is 
small. 
Summary of Water Data 
The rate of disappearance of coliforms discharged from 
three ocean outfalls was determined by following patches of 
effluent tagged with fluorescein dye. The general similarity 
