Disappearance of Coliforms with Time 
The MPN's of coliforms for all surface samples are plotted 
against time in Figure 3. The straight line is an approximate 
best fit line drawn through the geometric means of the data 
grouped somewhat arbitrarily as shown on the graph. The line 
joining the geometric means of the data collected on the same 
day by the Bureau of Sanitation, Los Angeles, is also shown on 
the graph. These data provide the best estimates of the rate 
of coliform disappearance on the day of the tracer experiment. 
It is obvious that there is no significant difference between 
our data and that obtained by the Bureau of Sanitation. The 
average rate of decrease of MPN is of the order of one magnitude 
(90%) every 4 to 43 hours and the average count reduces to below 
ten in about 15 hours. 
On the day of the trip, the sewage field instead of con- 
sistently moving away from the outfall doubled back on itself 
while showing a net movement towards the beach (see NSE report). 
It is possible under these circumstances that the coliform 
population of the tagged effluent was reinforced by contributions 
from younger effluent, and this could account in part for the 
terminal decrease in disappearance rate shown by the Bureau of 
Sanitation data, and the three relatively high MPN's obtained 
by us between 13 and 16 hours. 
and Radioactivity 
The data in Table I show that a significantly greater 
dilution of the effluent is found when one calculates this value 
from radioactivity measurements as compared to values calculated 
