DUG WELLS 83 



drill in the well shows the presence of such open joints, a well in 

 slate or shale may usually be considered safe if not less than 100 

 feet from a source of pollution. 



The movement of water through limestone is almost entirely 

 by means of open passages. Some of these are only a minute 

 fraction of an inch in width, being no wider than joint and bedding 

 planes. In such passages the movement of water is very slow and 

 pollution is rarely carried far, 150 feet from a possible source 

 usually being a safe distance. Other passages, however, are of 

 considerable size, perhaps many feet in diameter, and may extend 

 for miles. One chamber in Mammoth Cave is several miles 

 long, and there is evidence that similar though perhaps smaller 

 channels exist at numerous other points. These openings are not 

 uncommonly occupied by flowing streams which, if polluting mat- 

 ter is introduced, may carry it for many miles. Such streams 

 may have connection with surface sink holes. Cornstalks and 

 other refuse from the surface not infrequently appear in wells 

 drawing water from limestone, and the waters are often muddy 

 after storms. Such occurrences are indications of surface con- 

 tamination and the waters should be avoided if possible. 



Practically no water passes through the body of granite, the 

 movement being mainly along joint or fault planes or through 

 pore spaces in the disintegrated upper portions of weathered 

 granite masses. Polluting matter may reach to considerable dis- 

 tances through joint or fault planes, as is indicated by the fact 

 that the salt water of the ocean finds entrance to some wells 

 located 500 feet, and in places even a quarter of a mile or more 

 from the shore. It is said that in the deep public well sunk in 

 granite at Atlanta, Ga., sufficient polluting matter entered through 

 a joint struck at 1160 feet from the surface to render the water 

 unfit for drinking. 



Best Situations for Dug Wells. - - The best locations for dug 

 wells are those points at which there can be no possibility of the 

 access of polluting matter. The usual distance to which such 

 matter travels has been discussed in the preceding section. 



