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 (S_{s}) and specific yield (S_{y}).
They are often determined using some combination of field tests (e.g., aquifer tests) and laboratory tests on aquifer material samples.
Contents

Storativity 1

Confined 1.1

Unconfined 1.2

Specific yield 2

See also 3

References 4
Storativity
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
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
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 \,
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 ([L^{3}]);

dV_w is the volume of water released from storage ([L^{3}]);

dp is the decline in pressure(N•m^{−2} or [ML^{−1}T^{−2}]) ;

dh is the decline in hydraulic head ([L]) and

\gamma_w is the specific weight of water (N•m^{−3} or [ML^{−2}T^{−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^{−2}T^{−2}])

n is the porosity of the material (dimensionless ratio between 0 and 1)

\beta_p is the compressibility of the bulk aquifer material (m^{2}N^{−1} or [LM^{−1}T^{2}]), and

\beta_w is the compressibility of water (m^{2}N^{−1} or [LM^{−1}T^{2}])
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/m^{2} or [MLT^{−2}/L^{2}])
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 S_{s}) 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
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

Finegrained sandstone


21


Mediumgrained 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
References

Freeze, R.A. and J.A. Cherry. 1979. Groundwater. PrenticeHall, 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 1662D. 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 19481960. U.S. Geological Survey Water Supply Paper 1839D. 42 p.
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