PUB00012

• Year: 1999
• Title: Sustainability of Ground-Water Resources: U.S. Geological Survey Circular 1186
• CWCB Section: Interstate & Federal
• Author: William M. Alley, Thomas E. Reilly, O. Lehn Franke
• Description: Illustration of the hydrologic, geologic, and ecological concepts to assure wise and sustainable use of ground-water resources
• Publications - Doc Type: Historical

East

West

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Figure C-2. Simplified hydrogeologic section for basin shown
in Figure C-l showing generalized ground-water-flow paths
under natural conditions. Confining units and their effect on the
flow system are not shown. (Modified from Morgan and Jones.
1999.)

At steady state, all of the ground-water with-
drawal comes from capture of surface water for each of
the three simulations. In looking at the distribution among
stream reaches. virtually all of the captured water is
from stream segment A when the water is pumped from
a well in the unconfined aquifer near that stream segment
(simulation 1). For a well located farther from stream
segment A (simulation 2), most of the capture is still from
stream segment A, but almost 25 percent of the capture is
from the more distant stream segments Band E. Finally,
if the water is withdrawn from the confined aquifer near
stream segment A (simulation 3), about 50 percent of the
capture comes from stream segment A, and the remainder
comes from more distant stream segments.

The results illustrate that, because the effects of
pumping propagate in all directions, several surface-water
bodies can be affected by the water-level drawdowns caused
by a pumping well. A well pumping from an unconfined aquifer
will tend to capture most of its discharge from the nearest
stream reaches. The presence of a confining layer between
the well and the streams causes the cone of depression of
the well to extend greater distances to capture the natural
discharge required to offset pumping. Morgan and Jones
(1999) demonstrated through additional simulations that,
as the depth of the well and the number of confining layers
increase, capture of discharge to streams and springs is
distributed over increasingly larger areas.

Tabte C-I. Streamffow capture along five stream segments for three pumping simulations: (I) pumping from well in unconfined
aquifer near stream segment A, (2) pumping from well in unconfined aquifer about 6,000 feet from stream segment A, and
(3) pumping from weli in a confined aquifer near stream segment A

Streamflow capture along stream segment
as percentage of pumpage
Simulation 1 Simulation 2*

Stream segment

A
B
C
D
E

97
<1
<1
<1

Simulation 3

70

12

<1

51
13
5
5
26

3
12

*Sum of percentages for simulation 2 is less than 100 due to rounding.

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