66
Representative value of specific yield is given in the following table :
Table 6.3 : Representative value of specific yield (after John son as in
Todd, 1995)
Material
Specific yield percent
Gravel, coarse
23
Gravel, medium,
24
Gravel, fine
25
Sand, coarse
27
Sand, medium
28
Sand, fine
23
Silt
8
Clay
3
Sandstone, fine grained
21
Sandstone, medium grained
27
Limestone
14
Dune sand
38
Loess
18
Peat
44
Schist
26
Siltstone
12
Till, predominantly silt
6
Till, predominantly sand
16
Till predominantly gravel
16
Tuff
21
Storage Co-efficient :
In confined aquifers changes in the pressure (i.e. Pumping of well) produce only
small changes in the storage volume. The water yielding capacity of a confined
aquifer can be expressed in terms of its storage co-efficient.
Storage coefficient is defined as the volume of water that an aquifer releases
from or takes into storage per unit surface area of aquifer per unit change in the
component of head normal to that surface. In most confined aquifers, values fall
