SOURCES AND SAFETY OF UNDERGROUND SUPPLIES 45 



the substances with which they may have originally been polluted 

 are frequently removed, at least in part. In coarser sands and 

 in gravel the water passes downward more rapidly, the conditions 

 are less favorable for filtration, and the water may remain polluted. 

 In general, however, waters from sands and gravels, if taken from 

 a considerable distance below the surface, are safe to use. 



The waters of clays, because of the fineness of the material, 

 come into contact with relatively large amounts of mineral matter 

 and frequently become mineralized, lime and salt being the most 

 common substances dissolved; as a rule, however, owing to the 

 filtration of the waters through the exceedingly fine material and 

 the slowness with which polluting matter progresses and the slight 

 distance to which it reaches in such material, they are free from 

 contamination. 



The water of till is generally uncontaminated because of the 

 natural filtration due to its slow downward penetration through 



Cesspool 



Well 



FIG. 23. Diagram showing pollution in till. 



the clay and sand of which the till is largely composed. In some 

 places, however, springs have formed more or less definite tubular 

 channels through the material, and if such a channel leads from a 

 cesspool or similar source of pollution the water becomes highly 

 charged with matter dangerous to the health. Once contaminated 

 it is likely to continue so for long distances, as little natural filtra- 

 tion takes place, because of the nature of the channel. (See 

 Fig. 23.) The water from till should be thoroughly tested by a 

 bacteriologist if there seems any likelihood of contamination. 



The waters in sandstones and conglomerates are very rarely 

 polluted, owing in the more porous varieties to the natural fil- 

 tration and in the compact varieties to the difficulty with which 



