30 CONSERVATION OF GROUND WATER 



graphically and in depth from which the water may be ob- 

 tained. In each state are wells producing large quantities of 

 water, others yielding only a few gallons a minute, and still 

 others that have been abandoned because they penetrated 

 nothing that would yield suitable water. Well drillers and well 

 owners can outline areas respectively favorable or unfavorable 

 for wells on the basis of knowledge gained from past drilling. 

 Hydrologists have been able to apply this knowledge to many 

 unexplored areas and predict the possibilities of development 

 there, but for large sections of the country data are lacking, 

 and only the broadest of generalizations can be made as to the 

 ground-water resources. 



The occurrence of usable ground water is controlled in 

 large measure by the geology, and some geologic formations 

 are consistently better producers of water than others. But the 

 geology alone is not enough. Some very permeable gravels, 

 sands, or other rocks carry no water or are saturated with un- 

 usable water; some formations range widely in permeability 

 even within short distances, and some contribute certain chem- 

 ical or physical characteristics to the water which make it un- 

 desirable for specific purposes. Nevertheless the available 

 geologic data constitute the only source of information con- 

 cerning ground-water reservoirs in some parts of the country. 



A map has been compiled to show the areas in which mod- 

 erate to large supplies of usable water may be obtained from 

 wells (see Plate I between pages 32 and 33). The map shows 

 the areas where wells can yield 50 gallons per minute or more, 

 sufficient to supply small municipalities or small industries, as 

 distinguished from a well yielding only enough for stock or 

 domestic uses. Water has generally been considered usable 

 when it contains less than about 2,000 parts per million of 

 dissolved matter, but some areas are shown where water of 

 higher mineral content is actually being used. 



The depth of the aquifer below the land surface was not 

 taken as an essential criterion in the compilation of the map. 

 However, inasmuch as most waters at great depth are highly 



