Ch. 6] GROUND-WATER STUDIES 117 



GROUND-WATER STUDIES 



The study of the ground-water resources of any area is fundamen- 

 tally geologic. Extraction of ground water after it has been located 

 is dependent upon the character and attitude of the rock formations 

 that contain it, and upon the climate and opportunity for recharge. 

 As the sediments are the most important of all rocks as aquifers, the 

 sedimentary processes that produced the aquifers are of prime sig- 

 nificance. 



The first step in examination of ground-water resources of an area is 

 to map the geology and to become familiar with the kinds of materials 

 to be found. The stratigraphy of the whole section must be studied, 

 with special emphasis on the possible aquifers. Study of the outcrops 

 of aquifers is especially important in relation to recharge, because 

 outcrop areas are the places where the water enters to replenish that 

 which is withdrawn. Possible impermeable cover that might seal the 

 recharge areas is important. For example, relatively impermeable 

 glacial till impedes recharge in parts of the Cambrian sandstone aqui- 

 fers that supply the artesian water in eastern Wisconsin. Impermeable 

 strata also act as barriers which confine water under artesian pressure, 

 or they may cause the water to emerge as springs by preventing down- 

 ward movement of ground water, thus forcing it to move laterally. 



Indispensable tools in water-supply studies are well logs. Accurate 

 logs can be obtained only by examination of drill cuttings, where iden- 

 tification of sedimentary materials and their stratigraphic location for 

 correlation must be made, but electrical logs are useful as an aid. The 

 U. S. Geological Survey and most of the state geological surveys main- 

 tain files of carefully identified samples of cuttings from water wells. 

 They form an invaluable source of information for determining the 

 character of the formations, both aquifers and non-aquifers; for choos- 

 ing the best locations for new wells and for estimating probable 

 yields of water from them ; for forecasting the type of well construction 

 that will be most satisfactory in the area ; and for pointing out probable 

 difficulties that may arise during well construction. 



Methods for the quantitative examination of water resources have 

 been developed and improved greatly during the past fifteen years. 

 Basic mathematical concepts of the nature of the movement of water 

 through permeable materials are analogous to the transfer of heat 

 through a homogeneous medium (Theis, 1935). No natural aquifer is 

 isotropic; therefore corrections, determined from the examination and 

 evaluation of the sedimentary characteristics of the material, must be 



