Water Supply and Sewage Disposal. 217 



As the water percolates through the soil, the bacteria 

 are removed until at a varying depth beneath the surface 

 no organisms are found. The water that enters a well 

 from the lower layers of the soil may be free from bac- 

 teria. Such a supply is the most desirable if it can be 

 pumped from the well in the same condition as when it 

 entered. In order to obtain the water in this condition, 

 the well must be protected from all surface drainage. It 

 is very difficult to obtain this condition except in the 

 case of a drilled well which is cased with a threaded iron 

 pipe, and which is protected at the top so that no water 

 can enter the casing. A dug well, even if provided with 

 a tight wall to a considerable depth, is likely to receive 

 more or less surface water and hence is liable to pollu- 

 tion. 



The well should be so arranged at the surface that no 

 surface water, or water that is pumped onto the curb can 

 enter the well, for material containing disease-producing 

 bacteria may be brought on to the curb and in this way 

 pollute the water. The dug well, so common in the older 

 sections of the country, is a menace and should be re- 

 placed by the safer drilled well. The old-fashioned well 

 with bucket and sweep, which is so often regarded with 

 sentiment, is the most dangerous type of well, even 

 though the water is clear and sparkling. 



The water of a well that receives surface drainage is 

 not harmful at all times, but it may become so at any 

 time. The conditions needed to produce disease are all 

 present except one, viz, the deposition of material that 

 contains disease-producing bacteria within the zone from 

 which the water percolates into the well. This essential 

 condition may not be present for years, but at last it is 



