THE BACTERIA IN NATURAL WATERS 25 



offered in the Chicago Drainage Canal case and in the 

 lawsuit over the condition of the water supply of Jersey 

 City, to confirm the general conclusion that any water 

 which has been stored for 4 weeks is practically safe. 



Bacteria in Ground-waters. In general we have 

 seen that surface-waters tend continually to decrease 

 in bacterial content after their first period of contact 

 with the humus layer of the soil. In that other portion 

 of the meteoric water which penetrates below the 

 surface of the earth to join the reservoir of ground- 

 water, later to reappear as the flow of springs and wells, 

 this diminution is still more marked, since the filtering 

 action of the earth removes not only most of the bac- 

 teria, but much of their food material as well. The 

 numbers of bacteria in the soil itself decrease rapidly 

 as one passes downward. Kabrhel (1906) found several 

 million per c.c. in surface samples of woodland soil, 

 a few thousands or tens of thousands half a meter 

 below, and usually only hundreds in centimeter 

 samples collected at depths greater than a meter. 

 Many observers formerly believed that all ground- 

 waters were nearly free from bacteria, because often 

 no colonies appeared on plates counted after the 

 ordinary short periods of time. If, however, a longer 

 period of incubation be adopted considerable numbers 

 may be obtained. 



For convenience we may divide ground-waters into 

 three groups, namely: shallow open wells, springs and 

 " tubular " (driven) or deep wells. This division is 

 important because ordinary shallow wells form a group 

 by themselves in respect to the possibility of aerial and 



