114 foley. SEDIMENTATION AND GROUND WATER [Ch. 6 



Islands, and some other parts of the world are sources of large quan- 

 tities of ground water. Weathered and fractured metaraorphic rocks 

 in some areas are the only source of ground water, as, for example, in 

 parts of New England and the Piedmont Plateau of the southeastern 

 United States, but the quantities of water obtained from them are 

 small and sufficient usually for only small individual domestic sup- 

 plies. Intrusive igneous rocks also generally yield only small supplies. 

 As the sediments are the most important of all rock types as reser- 

 voirs from which ground water may be extracted, knowledge of their 

 classification, origin, and transportation is essential for the practical 

 development of most ground-water supplies. 



WATER-YIELDING CAPACITY OF SEDIMENTS 



To be of practical importance as a water-bearing formation, or more 

 properly a water-yielding formation, or aquifer, the material must be 

 of such a character that water can move through it rapidly enough 

 to furnish water to a well or spring. The amount of water that a 

 sediment contains when saturated is the same as its porosity, usually 

 expressed as a percentage by volume of the total volume of the rock. 

 The amount of water, also expressed as a percentage of the total vol- 

 ume, that the sediment will yield by gravity drainage is the specific 

 yield of the material; that retained by the material is its specific 

 retention. Estimates of supplies available in any formation based only 

 on porosity, without consideration of specific yield, are likely to be 

 entirely wrong. A thorough discussion of principles of the water- 

 yielding capacity of rock materials is given by Meinzer (1923). 



PERMEABILITY 



The permeability of a rock or soil with respect to water is its ability 

 to transmit water under pressure. Darcy (1856) verified the earlier 

 work of Hagen and Poiseuille, in which they demonstrated that the 

 rate of flow of water through capillary tubes is proportional to the 

 hydraulic gradient and showed its applicability to permeable filter 

 sands. The principle is stated as Darcy 's law and is sometimes ex- 

 pressed as 



Q = PIA 



in which Q is the quantity of water discharged in a unit of time, P is a 

 constant which depends on the character of the material, I is the hy- 

 draulic gradient, and A is the cross-sectional area through which the 



