AQUIFER STORATIVITY

A technical description of aquifer storativity (storage coefficient), specific storage, compressibility and their values

by Darrel Dunn, Ph.D., Hydrogeologist  

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(This is a technical page on aquifer storativity.  Click the link to see a nontechnical page on aquifer storativity.)

Specific Storage Tables

Aquifer storativity (also called storage coefficient) of a confined aquifer is defined as S=Ssb, where S is storativity (dimensionless), Ss is specific storage (L-1) and b is thickness (L) of the aquifer (Freeze and Cherry, 1979).  A confined aquifer is an aquifer that is confined between two aquitards (Freeze and Cherry, 1979).  Specific storage of a saturated aquifer is defined as the volume of water that a unit volume of aquifer releases from storage under unit decline in hydraulic head (Freeze and Cherry,1979).  Ranges of values for Ss for various subsurface materials have been published by Domenico and Mifflin (1965).  These ranges were calculated from the bulk modulus of compression (E) (ML-1T-2) using Sswα, where γw is the unit weight of water (MLT-2), and α is the vertical compressibility of the aquifer material (α is the inverse of E)(M-1LT2).  This formula for Ss assumes that water is incompressible.  Table 1 also shows specific storage values corresponding to the values for E calculated using Sswα.  A value of 62.4 lb/ft3 was used for the unit weight of water.  Domenico and Mifflin indicate that these values of specific storage may be high.  Their discussion implies that the values were based on Ss=kv/cv, where kv is vertical permeability and cv is the coefficient of consolidation.  Coefficients of consolidation were apparently obtained from laboratory consolidation tests and kv was from no-load vertical permeability tests.  The permeability under no load is likely to be greater than the permeability existing under effective overburden pressure at depth, possibly producing high values for Ss.  [Coefficient of consolidation (L2T-1) is a parameter used to describe the rate at which saturated clay or other soil undergoes consolidation or compaction when subjected to an increase in pressure.  See, for example, Foundation Engineering (Peck, and others, 1953).]