216 



Agricultural Bacteriology. 



Relation of water to soil. As was previously stated 

 the upper layers of the soil are rich in germ life. The 

 drainage water from such places will be teeming with 

 bacteria. The shallow dug well, loosely bricked up, is 

 filled during the wet seasons with water coming from the 

 upper layers of the soil. If material containing disease- 

 producing bacteria is placed on the ground near the well, 

 as in a privy vault, the bacteria may be carried by the 

 percolating water into the well. The distance between 

 the vault and the well that is required to prevent pollu- 



CESS POOL 



FIG. 21. POLLUTION OF A WELL. 



The well water may be polluted from a cess pool or 

 vault by the percolation of the ground water into 

 the well. (After Harrington.) 



tion of the well can not be stated in definite terms as it 

 will depend on the nature of the soil, whether it is close 

 and dense as in a clay soil, whether it is porous, as a sandy 

 soil, or whether the percolating water may form channels 

 in it, as in lime stone. The distance that may be tra- 

 versed by bacteria in the underground water also de- 

 pends on the slope of the underlying rocks. A well on 

 Tiigher ground than a privy is not necessarily protected 

 from pollution. 



