26 
between data obtained by this method and by three other indepen- 
dent methods, (using a radioactive tracer, running traverses 
across the sewage field, and running detailed grid patterns in 
the field) shows the validity of the dye patch procedure for 
determining the fate of coliforms in an effluent field. 
Calculations based on dilutions of effluent measured by 
radioactivity showed that even in effluent of very recent origin 
from an outfall, there is a greater disappearance of coliforms 
than can be accounted for on the basis of dilution alone. When 
Similar calculations are made using chlorinities as a measure 
of dilution, the extra disappearance over dilution is much greater, 
showing that fresh effluent undergoes considerable dilution by 
older effluent in the vicinity of an outfall. 
Data obtained from subsurface samples taken below the main 
effluent field show that sedimentation is an important factor in 
the extra disappearance of coliforms. The data available were 
not sufficient for a precise measure of this effect and it is 
not certain whether die-off of coliforms and other factors are 
also of significance in the extra disappearance. 
The disappearance rate differed with each type of effluent 
studied, being most rapid for the Orange County effluent and 
least rapid for the Whites Point effluent (although data on the 
Whites Point area are fragmentary). Primary effluent from 
Hyperion shows a more rapid rate of disappearance than secondary. 
The rate of disappearance may be related to the solid content 
of the effluents and their settling characteristics. The average 
value obtained for the Hyperion primary effluent, 3 hours for a 
90% disappearance, is a reasonable one to employ for calculating 
