The subsurface occurrence of ground water can be divided into two zones (Fig. 4.1):
(i) Vadose zone or unsaturated zone or zone of aeration, and
(ii) Phreatic zone or saturated zone or zone of saturation. In the saturated zone, all pores or voids are filled with water, whereas in the unsaturated zone, pores contain gases (mainly air and water vapours) in addition to water. The water table is defined as the upper limit of saturation at atmospheric pressure in the saturated zone.
A saturated geological formation capable of yielding water economically in sufficient quantity is known as an aquifer (or water-bearing formation or ground water reservoir). Ground water is constantly moving through an aquifer under local hydraulic gradients. Thus, aquifers perform storage as well as conduit functions.
Ground water may exist in aquifers in two different manners:
(i) Unconfined, and
(ii) Confined.
The unconfined condition occurs when the water table is under atmospheric pressure and is free to rise or fall with changes in the volume of the stored water. An aquifer with unconfined conditions is referred to as an unconfined or water table aquifer.
An aquifer which is separated from the unsaturated zone by an impermeable or very less permeable formation is known as confined aquifer (or artesian aquifer or pressure aquifer). Ground water in confined aquifer is under pressure which is greater than the atmospheric pressure.
The water level in a well penetrating a confined aquifer indicates the piezometric pressure at that point and will be above the bottom of the upper confining formation. Such wells are known as artesian wells, and if the water level rises above the land surface, a flowing well results (Fig. 4.2).
Following characteristics of the medium affect the availability and movement of ground water:
Porosity can be defined as the ratio of the volume of pores lo the total volume of the porous medium. It ranges from 0 to 50% for most of the rock materials. For aquifer considerations, porosities less than 5% are considered small, those between 5% and 20% are considered medium and those greater than 20% are considered as large.
The specific yield of a soil formation is defined as the ratio of the volume of water which the soil formation, after being saturated, will yield by gravity to the volume of the soil formation. The specific retention of a soil formation is defined as the ratio of the volume of water which the soil formation, after being saturated, will retain against the pull of gravity to the volume of the soil formation.
Obviously, the sum of the specific yield and the specific retention would be equal to the porosity of the given soil formation. The product of the average specific yield of a saturated water-bearing formation and its total volume gives the volume of water which can be recovered from the formation by gravity drainage. Fine-grained materials may have lesser specific yield than coarse material even though their porosity may be greater (Fig. 4.3) and Table 4.1.
In case of confined aquifers, there is no dewatering or draining of the material unless the hydraulic head drops below the top of the aquifer. Therefore, the concept of specific yield does not apply to confined aquifers and an alternative term, storage coefficient or storativity is used for confied aquifers.
Storativity or storage coefficient is defined as the volume of water an aquifer would release from, or takes into storage per unit surface area of the aquifer for a unit change in head. Its value is of the order of 5 x 10 to 1 x 10-5. For the same drop in head, the yield from an unconfined aquifer is much greater than that from a confined aquifer.
Example 4.1:
A ground water basin consists of 20 km2 of plains. The maximum fluctuation of ground water table is 3 m. Assuming a specific yield of 15%, determine the available ground water storage.
Solution:
Available ground water storage = Area of basin x depth of fluctuation x specific yield
= 20 x 106 x 3 x 0.15 = 9 x 106 m3.
Example 4.2:
In an aquifer whose area is 100 ha, the water table dropped by 3.0 m. Assuming porosity and specific retension of the aquifer material as 30% and 10% respectively, determine the specific yield of the aquifer and the change in ground water storage.
Solution:
Porosity = Specific yield + specific retention
... Specific yield = Porosity – specific retention
= 30-10
= 20%
Reduction in ground water storage = 100 x 104 x 3.0 x 0.2
= 60 x 104 m3