ACTIVITIES 55 



served fluctuations of the water table, which represent the 

 filling or emptying of the underground reservoir. In other 

 areas it is practicable, by observation of surface conditions, 

 to determine approximately the quantity of water that an- 

 nually enters or is discharged from the underground reservoir. 

 These observations comprise stream measurements to ascer- 

 tain the quantities of surface waters that percolate into the 

 ground, and measurements of the discharge of ground water 

 through springs and by evaporation from soil and plants. 

 These methods are laborious and difficult and none of them 

 are likely to afford precise results, but they are nevertheless 

 valuable, because they give a tangible basis for estimating the 

 supplies of ground water. 



In addition to determining the quantity and quality of the 

 waters a survey of ground waters discloses important factors 

 affecting the practicability of recovering them, such as hydro- 

 static pressure and the nature of the rock to be encountered 

 in sinking wells. The investigation of these elements is in- 

 volved in the study of the local geology, which is supplemented 

 by examinations and tests of the existing wells in the area 

 surveyed. 



The data gathered as to sources of ground water are natu- 

 rally of value chiefly in the arid and semiarid regions of the 

 West, where such supplies are needed both for domestic use 

 and for irrigation, but the importance of deep ground water 

 supplies over the whole country is being emphasized. It is 

 highly desirable to obtain supplies of domestic water from 

 sources other than the shallow wells, some of them open, that 

 are sunk near many houses. The water obtained from deep 

 wells has percolated through sands and other material for so 

 great a distance that its impurities have been removed by 

 filtration, and it possesses a sanitary value that cannot well be 

 overestimated, for it is free from the bacteria that cause 

 typhoid fever and the protozoa that cause amoebic dysentery,, 

 and its use obviates the necessity for shallow wells that may 



