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Storage coefficient of an aquifer is the volume of water discharged from a unit prism, i.e., a vertical column of aquifer standing on a unit area (1 m2) as water level (piezometric level in confined aquifer—artesian conditions) falls by a unit depth (1 m). For unconfined aquifers (water table conditions) the storage coefficient is the same as specific yield, Fig. 4.4. The storage coefficient for confined aquifers ranges from 0.00005 to 0.005 and for water table aquifers 0.05 to 0.30.
Under artesian conditions, when the piezometric surface is lowered by pumping, water is released from storage by the compression of the water bearing material (aquifer) and by expansion of the water itself. Thus, the coefficient of storage is a function of the elasticity of water and the aquifer skeleton and is given by as-
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S = γwb(α + nβ) …(4.4)
Where, S = coefficient of storage, fraction; n = porosity of aquifer, fraction; b = saturated thickness of aquifer (m); γw = units weight of water (9810 N/m3); β = 1/Kw, reciprocal of the bulk modulus of elasticity of water Kw = 2.1 GN/m2 = 2.1 × 109 N/m2; and α = 1/Es, reciprocal of the bulk modulus of elasticity of aquifer skeleton.
The fraction of storage attributable to expansibility water-
Sw = 4.7 × 10-6 nb …(4.5)
The bulk modulus of compression of some formation material are given in Table 4.3.
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Table 4.3 Bulk modulus of compression of formation materials:
Example 1:
An artesian aquifer 20 m thick has a porosity of 20% and bulk modulus of compression 108 N/m2. Estimate the storage coefficient of the aquifer. What fraction of this is attributable to the expansibility of water?
The fraction of storage attributable to the expansibility of water (taking only the second term within the backets)-
Example 2:
In a certain place in Andhra Pradesh, the average thickness of the confined aquifer is 30 m and extends over an area of 800 km2. The piezometric surface fluctuates annually from 19 m to 9 m above the top of the aquifer. Assuming a storage coefficient of 0.0008, what ground water storage can be expected annually?
Assuming an average well yield of 30 m3/hr and about 200 days of pumping in a year, how many wells can be drilled in the area?
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ΔGWS = Aaq × Δ piezo. Surface × S = (800 × 106) (19 – 9) 0.0008 or
6.4 × 106 m3, or 6.4 M m3
Annual draft = (30 × 24) 200 = 0.144 × 106 m3 or 0.114 Mm3
Number of wells that can be drilled in the area = 6.4/0.144 = 44.5, say 44 wells 0.144
Of course, the well sites have to be investigated and there should be sufficient spacing for the wells.
Example 3:
An aquifer has an average thickness of 60 m and an aerial extent of 100 ha.
Estimate the available ground water storage if:
(а) The aquifer is unconfined and the fluctuation in GWT is observed as 15 m,
(b) The aquifer is confined, and the piezometric head is lowered by 50 m, which drains half the thickness of the aquifer.
Assume a storage coefficient of 2 × 10-4 and a specific yield of 16%.
Solution:
