Permeability, Impermeability, and the Selection of Representative Values

A large sector of hydrology deals with through-flow systems, that is, groundwater systems that are recharged. A term is needed that expresses the ability of different rock types to let water infiltrate and flow through. This term is the permeability coefficient, k. 

Henry Darcy, a French engineer, conducted experiments in 1856 in which he measured the flow velocity of water through a sand-filled tube. He found that the flow velocity (V) is directly proportional to the difference in hydraulic head (Ah), and is inversely proportional to the flow distance (Al)  

A convenient way to define the coefficient of permeability, in order to compare its value for different types of rocks, is to determine its empirical value for hydraulic gradients of 45°, that is, for Ah/Al=l. In such cases k=V. 

The permeability coefficient (k) has the units of velocity, that is, distance/time. It is determined either in laboratory experiments or derived from pumping tests. Both methods are semiquantitative, but are still highly informative, as the values observed for common rocks span more than seven orders of magnitude. A variety of units are in use—m/day being a common one. The following are a few of the average permeability or hydraulic conductivity values floating around in the literature, expressed in m/day  

10-7 clay, 10-6 sandstone, 10 2 limestone, 1 sand, 10 and gravel, 102 

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