DEEP WELLS 



III 



Soil 



Limestone 



Soil 



r 



Limestone 



for deep wells to become polluted by the entrance of surface 

 waters. In regions where the rock is within a few feet of the 

 surface, for instance, the casing may be carried only to the rock, 

 the fact that pollution can 

 enter the well through the 

 rock crevices being entirely 

 overlooked. (See Fig. 49.) 

 The chief precaution neces- 

 sary against this danger is 

 to carry the casing to a suffi- 

 cient depth to shut off all sur- 

 face waters entering through 

 fissures. It is hard to say 

 how deep it must be carried 

 to remove all danger of con- 

 tamination, but the crevices 



..... , FlG. 49. Diagram showing danger of pollu- 



are usually limited to the up- tion where casing is carried only to rock 

 per part of the rock (Fig. 49), 



and every additional foot of casing gives additional safety. Ten 

 feet of casing in the rock would materially reduce the danger, 

 while 25 feet would in most wells probably insure safety. The 

 safest plan, however, is to carry the casing from the surface down 

 to the water-bearing seam. The casing should always be set with 

 a tight joint at the bottom to prevent the entrance into the well 

 of surface waters that find their way downward along the outside 

 of the pipe. 



Again, it is not unusual to drill new wells in the bottom of old 

 dug wells and to allow the polluted surface waters to mingle with 

 the pure rock waters. 



Many towns situated on rock surfaces and using unprotected 

 wells of the type mentioned have been visited by epidemics of 

 typhoid fever, cholera and other diseases, leading to the loss of 

 many lives. 



Another source of pollution, less common and possibly less 

 dangerous than the preceding, arises from the fact that many 



