22 Elements of Water Bacteriology. 



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 pene- 

 trates 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 

 bacteria 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 consid- 

 erable 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 impor- 

 tant because ordinary shallow weUs form a group by 

 themselves in respect to the possibility of aerial and surface 

 contamination, their water often being fairly rich in bac- 

 terial life. Egger (WolfEhugel, 1886) examined 60 weUs 

 in Mainz and found that 17 of them contained over 200 



