Page 128 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch03_061-117.qxd 8/17/10 7:48 PM Page 90

90 Chapter 3 Water Sources: Groundwater

Then,
[1>(4 3.14 397.4)][21.89 Q 1 8.08 Q 2 6.69 Q 3 ] 18.3 m
[1>(4 3.14 397.4)][8.08 Q 1 21.89 Q 2 8.08 Q 3 ] 18.3 m
[1>(4 3.14 397.4)][6.69 Q 1 8.08 Q 2 21.93 Q 3 ] 18.3 m
Then,
21.89 Q 1 8.08 Q 2 6.69 Q 3 91,341.6
8.08 Q 1 21.89 Q 2 8.08 Q 3 91,341.6
6.69 Q 1 8.08 Q 2 21.93 Q 3 91,341.6
Solving the three equations for the three unknowns, Q 1 , Q 2 , and Q 3 :
3
Q 1 Q 3 2,550 m /d
3
Q 2 2,289 m /d

When the areas of influence of two or more pumped wells overlap, the draft of one
well affects the drawdown of all others. In closely spaced wells, interference may become
so severe that a well group behaves like a single well producing a single large cone of
depression. When this is the case, discharge-drawdown relationships can be studied by
replacing the group of wells by an equivalent single well having the same drawdown dis-
tribution when producing water at a rate equal to the combined discharge of the group.
The effective radius of a heavily pumped well field could be a mile or more and
have a circle of influence extending over many miles. By contrast, lightly pumped,
shallow wells in unconfined aquifers may show no interference when placed 100 ft
(30.5 m) apart or even less. The number of wells, the geometry of the well field, and its
location with respect to recharge and discharge areas and aquifer boundaries are impor-
tant in determining the distribution of drawdown and well discharges. An analysis of
the optimum location, spacing, and discharges should be carried out when designing a
well field.

3.13 AQUIFER BOUNDARIES
Most methods of analysis assume that an aquifer is infinite in extent. In practice, all
aquifers have boundaries. However, unless a well is located so close to a boundary that the
radial flow pattern is significantly modified, the flow equations can be applied without ap-
preciable error. Nevertheless, in many situations definite geologic and hydraulic bound-
aries limit aquifer dimensions and cause the response of an aquifer to deviate substantially
from that predicted from equations based on extensive aquifers. This is especially true if
the cone of depression reaches streams, outcrops, or groundwater divides; geologic bound-
aries, such as faults and folds; and valley fills of limited extent.
The effect of aquifer boundaries can be incorporated into analysis through the
method of images. The method of images is an artifice employed to transform a bounded
aquifer into one of an infinite extent having an equivalent hydraulic flow system. The
effect of a known physical boundary (in the flow system) is simulated by introducing
one or more hypothetical components, called images. The solution to a problem can then
be obtained by using the equations of flow developed for extensive aquifers for this hy-
pothetical system.