11 
from chlorinity. The differences between the calculated dilutions 
become even greater as the effluent moves away from the boil. 
Using the MPN‘'s determined for the boil samples and effluent 
dilutions calculated from the two types of measurements, one can 
calculate the MPN‘'s for the undiluted effluent (0"* sample), and 
for the dye patch samples taken after zero time. These data, 
along with the observed MPN‘'s are plotted in Figure 4 using the 
geometric mean of the results from the two dye patches in all 
cases. 
Since essentially no radioactivity existed in the area of 
the outfall previous to the one hour introduction of the radio- 
isotope tag (actually the background count was determined and 
corrected for), the decrease in radioactivity is a measure of 
the total dilution of this one hour's volume of effluent. If 
this effluent were being introduced into undiluted sea water, 
then the dilutions measured by radioactivity and chlorinity 
would be the same. However, as is seen from Figure 4, dilutions 
measured from chlorinity are much less, which must mean that the 
tagged effluent was being diluted mainly with the untagged 
effluent in the area. At eight hours, for example, the tagged 
effluent at its maximum concentration constituted only 1/16 of 
the total effluent in the sample. 
If the effluent field moved away from the outfall at all 
times, then dilution of the tagged effluent on its outer boundary 
would have to be with effluent of older origin and therefore 
lower coliform density. As previously mentioned, on the day of 
the experiment the tagged field curved back towards the outfall 
and there could have been some dilution of this field with 
