Dilution Calculated from Radioactivity and Observed MPN's 
of Coliforms 
A total of three samples were taken in the boil during peak 
radioactivity and these were analyzed for the MPN's of coliforms, 
radioactivity, and chlorinity. From these data one can calculate 
the original coliform density of the undiluted primary effluent. 
The data from these calculations are presented in Table I. The 
values obtained are reasonable for a primary treatment effluent, 
although the geometric mean of 720,000/m1 is probably higher 
than would occur if a larger number of determinations were 
involved (see page 29). 
From the calculated MPN of the undiluted effluent, one can 
calculate the expected MPN for any dilution of the effluent as 
measured by radioactivity. The results of such calculations 
are shown in Figure 2. The three lines are based on the maximum 
count, the geometric mean of the count, and the minimum count 
of Table I, respectively. On the same graph are included the 
observed MPN's of all the surface samples analyzed for coliforms 
plotted against dilution calculated from the observed radio- 
activity of the respective sample. It can be observed that 
beyond a dilution of 1/100, the majority of the samples gave 
a MPN well below the minimum calculated range. These data show 
that factors other than dilution are operating to reduce the 
coliform population within a relatively short time after the 
effluent leaves the outfall. A similar conclusion was previously 
reached from calculations based on chlorinity (May 1 report), 
where an even greater difference between observed and calculated 
MPN's existed. The significance of the greater difference is 
discussed later. 
