Contents preface (VII) introduction 1—37

Fig. 4.2 Aquifers and wells

Water released from an unconfined aquifer is the result of dewatering or draining of the aquifer material. In the case of confined aquifer, the release of water is the result of a slight expansion of water and a very small compression of the porous medium (2).

GROUND WATER AND WELLS

125

The availability, movement, and quality of ground water depend mainly on the characteristics of the medium. The 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 to the total volume of the porous medium. It ranges from 0 to 50 per cent 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 large (2). Porosity is, obviously, an inherent characteristic of the material independent of the presence or absence of water. For ground water studies, the interconnected pore space which can be drained by gravity should be used for determining the porosity and such porosity is known as effective porosity.
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.
These definitions of specific yield and specific retention implicitly assume complete drainage. 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. It may be noted that the time factor is not included in the definition of specific yield. However, the gravity draining of a formation decreases with time and may continue for years. Fine-grained materials may have lesser specific yield than coarse materials even though their porosity may be greater (Fig. 4.3 and Table 4.1).

Percent

50
45
40																			Porosity
35
	30
25										Specific yield
20
	15										Specific retention
10
5
0
	Clayandsilt	Sandy	clay		Finesand		Finesand		Mediumsand		Coarsesand		Coarsesand		Gravelsand		Finegravel	Mediumgravel	Mediumgravel	Coarsegravel	Coarsegravel	Boulders
1/16	1/8	1/4		1/2	1	2	4	8	16	32	64	128	256
										Grain size, d90 millimetres