224 CONSERVATION OF GROUND WATER 



3. Dependability. Yield from wells and springs generally 

 fluctuates less than stream flow in alternating wet and 

 dry climatic cycles. 



4. Uniformity in quality. Ground water is more uniform in 

 temperature and soluble mineral load than surface 

 water, and is generally free of turbidity and bacterial 

 pollution. 



The absence of these advantages has necessitated the devel- 

 opment of water from surface sources in many places where 

 ground water can be obtained only at excessive depth below 

 the surface or where it cannot be obtained in sufficient quan- 

 tity at any depth; where the ground water is deemed to be 

 fully appropriated or where it is of a quality that makes it 

 unsuitable for the use intended. Ground water has another 

 disadvantage as an alternate to surface water in that the use 

 generally requires expenditure of energy for pumping, 

 whereas the surface water may produce energy in addition to 

 supplying water for other uses. This is a formidable disad- 

 vantage in many areas. 



A principal disadvantage of ground water, however, is its 

 generally slow movement through rock materials, which miti- 

 gates against its successful use to meet the large requirements 

 of concentrated population and industry. Many cities that 

 once depended on wells for supply have been forced to develop 

 other sources more capable of supplying their needs as popu- 

 lation and water requirements increased. New York City 

 itself depended on wells, with increasingly unsatisfactory re- 

 sults, until its population reached 200,000 in 1830. Of the 100 

 largest cities in the United States (1950 census) only 19 depend 

 upon wells for their public supply; 16 of these are among the 

 60 cities that range in population from 105,000 to 250,000. 

 Of the 22 cities with population 250,000 to 500,000 only San 

 Antonio and Memphis obtain their municipal supplies from 

 ground water. Houston, Tex., is the only one of the 18 cities 

 of more than 500,000 population that depends upon wells for 

 all its public supplies, and it is turning increasingly to surface 



