Ch. 6] PERMEABILITY OF SEDIMENTS 115 



water passes. The constant P is usually called the coefficient of perme- 

 ability. It has been expressed in various units by many investigators. 

 The coefficient of permeability used by the U. S. Geological Survey 

 was defined by Meinzer, who selected gallon, day, and square foot as 

 the units most applicable to ground-water work. Meinzer's coefficient 

 is the rate of flow of water, in gallons a day, through a cross-sectional 

 area of 1 square foot under a hydraulic gradient of 100 percent at a 

 temperature of 60° F. It is adapted for field use by correcting for the 

 prevailing temperature of the ground water in the area under study. 

 Wenzel (1942, p. 11) lists ten different permeability units in use in the 

 United States and conversion factors for changing these units into 

 Meinzer's units, or Meinzer's units into these units. 



The term "coefficient of transmissibility" was introduced by Theis 

 (1935, pp. 519-524) and is now in common use in water-supply work. 

 It is Meinzer's field coefficient of permeability multiplied by the satu- 

 rated thickness of the aquifer, and it is particularly useful because it 

 describes the ability of an aquifer as a whole to transmit water, an 

 essential factor in determining the actual amount of water that can 

 be obtained from an aquifer. It is also readily determined from data 

 collected during pumping tests. 



Permeability of Sediments 



The permeability of the sediments ranges from extremely low for 

 clay, the most nearly impermeable sediment as a source of water sup- 

 ply, to very high for coarse, clean gravel and cavernous limestone, the 

 most productive of all aquifers. Clay is useless as an aquifer not be- 

 cause it contains no water, for many clays have a porosity of more than 

 50 percent and are saturated with water, but because the interstices 

 between the grains are so small that essentially all the water is held 

 tightly by molecular attraction. 



Permeability of a sediment depends not only on the absolute size of 

 the constituent grains, but also on the sorting of the grains. A clean, 

 fine-grained sand may have a higher permeability than a coarse sand 

 or gravel in which the spaces between the larger grains are filled 

 with finer material. 



Coefficients of permeability have been determined in the hydrologic 

 laboratory of the U. S. Geological Survey on many hundreds of samples 

 of material from many states. Ten samples are listed in Table 1 to 

 illustrate the relation between grain size, sorting, and permeability. 

 The table includes the highest and lowest coefficients determined so far 

 in the hydrologic laboratory, but undoubtedly other materials exist 

 with higher and lower coefficients. 



