Specific storage

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In the field of hydrogeology, "storage properties" are physical properties that characterize the capacity of an aquifer to release groundwater. These properties are Storativity (S), specific storage (Ss) and specific yield (Sy).

They are often determined using some combination of field tests (e.g., aquifer tests) and laboratory tests on aquifer material samples.

Storativity[edit]

Storativity or the storage coefficient is the volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer. Storativity is a dimensionless quantity, and ranges between 0 and the effective porosity of the aquifer.

S = \frac{dV_w}{dh}\frac{1}{A} = S_s b + S_y \,

Confined[edit]

For a confined aquifer or aquitard, storativity is the vertically integrated specific storage value. Therefore if the aquitard is homogeneous:

S=S_s b \,

Unconfined[edit]

For unconfined aquifer storativity is approximately equal to the specific yield (S_y) since the release from specific storage (S_s) is typically orders of magnitude less (S_s b \ll \!\ S_y).

S=S_y \,

Specific storage[edit]

The specific storage is the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of aquifer, per unit change in hydraulic head, while remaining fully saturated.

Mass specific storage is the mass of water that an aquifer releases from storage, per mass of aquifer, per unit decline in hydraulic head:

(S_s)_m = \frac{1}{m_a}\frac{dm_w}{dh}

where

(S_s)_m is the mass specific storage ([L-1]);
m_a is the mass of that portion of the aquifer from which the water is released ([M]);
dm_w is the mass of water released from storage ([M]); and
dh is the decline in hydraulic head ([L]).

Volumetric specific storage (or volume specific storage) is the volume of water that an aquifer releases from storage, per volume of aquifer, per unit decline in hydraulic head (Freeze and Cherry, 1979):

S_s = \frac{1}{V_a}\frac{dV_w}{dh} = \frac{1}{V_a}\frac{dV_w}{dp}\frac{dp}{dh}= \frac{1}{V_a}\frac{dV_w}{dp}\gamma_w

where

S_s is the volumetric specific storage ([L-1]);
V_a is the bulk volume of that portion of the aquifer from which the water is released ([L3]);
dV_w is the volume of water released from storage ([L3]);
dp is the decline in pressure(N•m-2 or [ML-1T-2]) ;
dh is the decline in hydraulic head ([L]) and
\gamma_w is the specific weight of water (N•m-3 or [ML-2T-2]).


In hydrogeology, volumetric specific storage is much more commonly encountered than mass specific storage. Consequently, the term specific storage generally refers to volumetric specific storage.

In terms of measurable physical properties, specific storage can be expressed as

S_s = \gamma_w (\beta_p + n \cdot \beta_w)

where

\gamma_w is the specific weight of water (N•m-3 or [ML-2T-2])
n is the porosity of the material (dimensionless ratio between 0 and 1)
\beta_p is the compressibility of the bulk aquifer material (m2N-1 or [LM-1T2]), and
\beta_w is the compressibility of water (m2N-1 or [LM-1T2])

The compressibility terms relate a given change in stress to a change in volume (a strain). These two terms can be defined as:

\beta_p = -\frac{dV_t}{d\sigma_e}\frac{1}{V_t}
\beta_w = -\frac{dV_w}{dp}\frac{1}{V_w}

where

\sigma_e is the effective stress (N/m2 or [MLT-2/L2])

These equations relate a change in total or water volume (V_t or V_w) per change in applied stress (effective stress — \sigma_e or pore pressure — p) per unit volume. The compressibilities (and therefore also Ss) can be estimated from laboratory consolidation tests (in an apparatus called a consolidometer), using the consolidation theory of soil mechanics (developed by Karl Terzaghi).

Specific yield[edit]

Values of specific yield, from Johnson (1967)
Material Specific Yield (%)
min avg max
Unconsolidated deposits
Clay 0 2 5
Sandy clay (mud) 3 7 12
Silt 3 18 19
Fine sand 10 21 28
Medium sand 15 26 32
Coarse sand 20 27 35
Gravelly sand 20 25 35
Fine gravel 21 25 35
Medium gravel 13 23 26
Coarse gravel 12 22 26
Consolidated deposits
Fine-grained sandstone   21  
Medium-grained sandstone   27  
Limestone   14  
Schist   26  
Siltstone   12  
Tuff   21  
Other deposits
Dune sand   38  
Loess   18  
Peat   44  
Till, predominantly silt   6  
Till, predominantly sand   16  
Till, predominantly gravel   16  

Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:

S_y = \frac{V_{wd}}{V_T}

where

V_{wd} is the volume of water drained, and
V_T is the total rock or material volume

It is primarily used for unconfined aquifers, since the elastic storage component, S_s, is relatively small and usually has an insignificant contribution. Specific yield can be close to effective porosity, but there are several subtle things which make this value more complicated than it seems. Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized for a very long time, due to complications caused by unsaturated flow.


See also[edit]

References[edit]

  • Freeze, R.A. and J.A. Cherry. 1979. Groundwater. Prentice-Hall, Inc. Englewood Cliffs, NJ. 604 p.
  • Johnson, A.I. 1967. Specific yield — compilation of specific yields for various materials. U.S. Geological Survey Water Supply Paper 1662-D. 74 p.
  • Morris, D.A. and A.I. Johnson. 1967. Summary of hydrologic and physical properties of rock and soil materials as analyzed by the Hydrologic Laboratory of the U.S. Geological Survey 1948-1960. U.S. Geological Survey Water Supply Paper 1839-D. 42 p.