118 foley. SEDIMENTATION AND GROUND WATER [Ch. 6 



made to compensate for departures of the actual aquifer from the ideal 

 isotropic aquifer. Grain sizes and sorting vary, both parallel to and 

 normal to bedding planes. No standard method of applying correc- 

 tions can be set up because each area has its own characteristics, and 

 at the present time the effect of sedimentary changes in aquifers on 

 transmissibility is not easy to evaluate. Research needs to be con- 

 tinued in many areas where detailed quantitative studies are in prog- 

 ress to determine the effects on transmissibility of such changes in 

 lithology. 



WELL DRILLING AND DEVELOPMENT 



The consulting engineer and the well driller are continually en- 

 countering problems in sedimentation during well construction and de- 

 velopment. There are few places where wells can be drilled where 

 there is not at least one of the problems of caving walls, quicksand, or 

 water-bearing material that must be screened out or retained with a 

 gravel wall and then properly developed (Bennison, 1947) . A thorough 

 knowledge of the sediments and careful examination of them as drilling 

 progresses can prevent much grief and, by indicating proper drilling 

 and development methods, may make the difference between a poor 

 well and a good one. The selection of the proper screen for a well in 

 formations where influx of sand must be inhibited requires a careful 

 sieve analysis of the material of the aquifer. The best screen size 

 seems to be one that will allow the finer grains near the screen to 

 pass through during development of the well, leaving the coarser 

 grains outside. This process increases the permeability of the aquifer 

 near the screen and increases the well capacity. If an artificially 

 gravel-walled well is to be constructed, the same careful analysis is 

 necessary to select the ideal gravel size or sizes that will retain the 

 aquifer material and yet allow maximum flow of water to the well. 



SEDIMENTS OF VARIOUS ENVIRONMENTS AS SOURCES 

 OF GROUND WATER 



In the following description of sediments of various environments 

 and their importance as sources of ground water, Twenhofel's classifi- 

 cation (1932, pp. 785-871) has been followed in general. Sedimentary 

 deposits of almost all environments are sources of ground water, some 

 much more important than others. The younger sediments are usually 

 more prolific in yield, because cementation has not progressed so far; 

 thus they commonly have a relatively higher permeability. However, 



