Aquifers Properties: Storativity

Cross-posted on GeoSelim here.

The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. It is equal to the product of specific storage and aquifer thickness. In an unconfined aquifer, the storativity is equivalent to the specific yield. Also called storage coefficient. The groundwater fraction which is of interest for abstraction is quantified by the storativity. In a confined aquifer, water release is related to the water-expansion and aquifer-compaction effect attributed to aquifer pressure changes due to pumping. This elastic water release is quantified by the storage coefficient as De Marlisy 1986):

S ≈ [ρ . g . n . (β1 . α/n)]. e

where ρ is the water mass/volume ratio, g is gravity acceleration, β1 is the water compressibility, α is the aquifer compressibility, n is the total porosity and e is the saturated thickness. When an unconfined aquifer is desaturated by well pumping, the quantity of extracted water is determined both by elastic water-release phenomena and by gravity-water phenomena. The storage coefficient is then:

S ≈ {[ ρ . g . n . (β1 . α /n)]. e} + ne

where the term [ρ.g.n.(β1.α/n)] represents the elastic phenomena and ne the specific yield parameter, which is:

n_e = gravitational water volume/ total volume

Because the value of the storage coefficient is several orders of magnitude smaller than the value of specific yield, the storativity of an unconfined aquifer is usually taken to be equal to the specific yield. It means that the water release (or storage) is due to the gravitational drainage of the aquifer because of the falling water level while pumping. This is why specific storage is commonly called drainage porosity, gravity porosity or effective porosity. Note that a confined aquifer that is desaturated by pumping (if the dynamic level falls below the bottom of the confining layer) becomes partially unconfined. The specific drainage (Sd) is the parameter that quantifies the volume of water that could potentially be released by a confined aquifer due to gravity (Lubczynski & Roy 2003). In a unconfined aquifer, the phenomenon of seepage takes place when there is a drop in the water level (particularly due to pumping). In a confined aquifer, the release of the water is not caused by gravity, but by decompression of the water and the reservoir generated by a reduction in head. As the compressibility of both aquifer and water is very small, we no longer refer to effective porosity but to a storage coefficient (S) defined as the volume of water released by a vertical prism of aquifer material of unit section as a result of a unit change in head; this factor is non-dimensional.

Reference and further reading:

ACTION CONTRE LA FAIM, 2005, Water, sanitation and hygiene for populations at risk, HERMANN ÉDITEURS DES SCIENCES ET DES ARTS, P 78 – 80.

Zekâi Sen., 1995, Applied Hydrogeology for Scientist and Engineers, Lewis Publication, Chapter 4, P 73-77.