Page 94 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 77

per unit area. It is a dimensionless quantity. The response of a confined
aquifer to the change of water head is different from that of an unconfined
aquifer. When the head declines, a confined aquifer remains saturated; the
water is released from storage due to the expansion of water and the com-
paction of aquifer. The amount of release is exceedingly small. On the other
hand, the water table rises or falls with change of head in an unconfined
aquifer. As the water level changes, water drains from or enters into the
pore spaces. This storage or release is mainly due to the specific yield. It is
also a dimensionless quantity. For unconfined aquifers, the storativity is
practically equal to the specific yield, and ranges typically between 0.1 and
0.3. The storativity of confined aquifers is substantially smaller and gener-
ally ranges between 0.0001 and 0.00001, and that for leaky confined aqui-
fers is in the range of 0.001. A small storativity implies that it will require
a larger pressure change (or gradient) to extract groundwater at a specific
flow rate [2].
The volume of groundwater (V) drained from an aquifer can be deter-
mined from Equation (3.9):

V = S × A × Δh (3.9)

where S is the storativity, A is the area of the aquifer, and Δh is the change
in head.

Example 3.5: Estimate Loss of Storage in Aquifers
due to Change in Head
An unconfined aquifer has an area of 5 square miles. The storativity of this
aquifer is 0.15. The water table fell 0.8 ft during a recent drought. Please esti-
mate the amount of water lost from storage.
If the aquifer is confined and its storativity is 0.0005, what would be the
amount lost for a decrease of 0.8 ft in head?

Solution:
(a) Inserting the values into Equation (3.9), we obtain the volume of
water drained from the unconfined aquifer:
V = (0.15)[(5)(5280) ft ](0.8 ft) = 1.67 × 10 ft = 1.25 × 10 gal
2
7
3
2
8
(b) For the confined aquifer:
V = (0.0005)[(5)(5280) ft ](0.8 ft) = 5.58 × 10 ft = 4.17 × 10 gal
3
4
2
2
5
Discussion:
For the same amount of change in head, the water lost in the uncon-
fined aquifer is 300 times more, which is the ratio of the two storativ-
ity values (0.15/0.0005 = 300).