DRINKING WATER 



89 



the spring they had washed down into the stream from 

 which the people in the town some distance away got 

 their drinking water. 



Spring water. Spring water, if from the side of a 

 sandy hill through which the water has filtered, is usually 

 safe, provided, of course, that it is protected from all 

 the surface water which has not been so filtered. 



Well water. Well water is usually good if the well 

 is entirely protected from the entrance of any water 



FIG. 56. A well contaminated by its surroundings because of the constant flow 

 of water toward it from all sides, owing to the removal of water from the well 

 itself. 



which has not been slowly filtered through at least ten 

 or fifteen feet of porous air-filled earth. Wells should 

 preferably be at least fifteen feet deep. Those which 

 are driven or bored are ordinarily much safer than open 

 wells because they are more adequately protected from 

 surface drainage. Many small towns obtain an ade- 

 quate and safe water supply from such sources. 



Lake water. Lake water is frequently used as a 

 source of supply for large cities. Like other surface 



