Page 106 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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3.8 Aquifer Characterstics 69

Ground surface Ground surface

Piezometric Surface
1´ Unit decline of
piezometric surface
S
Aquiclude
Water table
1´
S Artesian Unit decline Unconfined
b aquifer aquifer
Unit cross- of water table
sectional area
Unit cross-sectional area
Aquiclude Aquiclude
(a) (b)
Figure 3.3 Graphical Representation of Storage Coefficient. The volume of water that a unit
decline in head releases from storage in a vertical prism of the aquifer of unit cross-sectional
area. (a) Confined aquifer; (b) Unconfined aquifer. Conversion factor: 1 1 ft 0.3048 m

The physical processes involved when the water is released from (or taken into)
storage in response to head changes are quite different in cases in which free surface is
present from those in which it is not. A confined aquifer remains saturated during the
withdrawal of water. In the case of a confined aquifer the water is released from storage
by virtue of two processes: (a) lowering of the water table in the recharge or intake area
of the aquifer and (b) elastic response to pressure changes in the aquifer and its confin-
ing beds induced by the withdrawal of water. For this the storage coefficient is ex-
pressed as:

S = ugb[b + (a>u)] (3.9)
in which u is the average porosity of the aquifer; g is the specific weight of water; b is the
compressibility of water; and a is the vertical compressibility of aquifer material. In most
confined aquifers, storage coefficient values lie in the range of 0.00005 to 0.0005. These
values are small and thus large pressure changes over extensive areas are required to de-
velop substantial quantities of water.
A confined aquifer for which S in Eq. 3.9 is 3 l0 4 will release from 1 square mile,
2
64,125 gal (93,711 L/km ) by lowering the piezometric surface by 1 ft (0.3048 m).
A water table aquifer also releases water from storage by two processes: (a) dewa-
tering or drainage of material at the free surface as it moves downward and (b) elastic
response of the material below the free surface. In general, the quantity released by
elastic response is very small compared to the dewatering of the saturated material at
the water table. Thus the storage coefficient is virtually equal to the specific yield of the
material. In unconfined aquifers, the full complement of storage is usually not released
instantaneously. The speed of drainage depends on the types of aquifer materials. Thus
in water table aquifers, the storage coefficient varies with time, increasing at a dimin-
ishing rate. Ultimately it is equal to specific yield. Furthermore, since the dewatered
portion of the aquifer cannot transmit water, transmissivity of the aquifer decreases
with the lowering of the water table. Transmissivity is thus a function of head in an
unconfined aquifer. The storage coefficient of unconfined aquifers may range from 0.01