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

I
.

. Pumping ground water from a well always
causes (1) a decline in ground-water levels
(heads; see Figure 7) at and near the well,
and (2) a diversion to the pumping well of
ground water that was moving slowly to its
natural, possibly distant, area of discharge.
Pumping of a single well typically has a
local effect on the ground-water-flow
system. Pumping of many wells (sometimes
hundreds or thousands of wells) in large
areas can have regionally significant effects
on ground-water systems.

. Ground-water heads respond to pumping to
markedly different degrees in unconfined
and confined aquifers. Pumping the same
quantity of water from wells in confined and
in unconfined aquifers initially results in
much larger declines in heads over much
larger areas for the confined aquifers (see
Box A). This is because less water is avail-
able from storage in confined aquifers
compared to unconfined aquifers. At a
later time, as the amount of water derived
from storage decreases and the system
approaches equilibrium, the response of
the system no longer depends upon being
confined or unconfined. The amount of head
decline at equilibrium is a function of the
transmitting properties of the aquifers and
confining units, discharge rate of the well,
and distance to ground-water-system
boundaries. Many aquifers, such as the
upper part of the deep flow subsystem
shown in Figure 6, exhibit a response
to pumping that is intermediate between
a completely confined and a completely
unconfined aquifer system.

I

I

I

,

-

-

land surface
(

Screened
interval
otwell
/

Water table
I
/ "

Unsaturated
20ne

Elevation A
and head
al point A,
in feet ...........

Unconfined
aquifer

Head ~t_
pointC,
*" in feet "'-

- - - 1=- - - ---= -~ - - -~3
~~~~~~~~; ~= ~~~-=
?~~~~~~=j~~l~:

~j-----
-----
- - - - _:
=-----=---=---=--=
=- .=-r>r;nfiiilng::::-=,
--=..--:':::-urtit--=..--=:
-----
------

Head a! /
pointS.
in feet

.

Confined
aquifer
-... Elevation
of point C,
infest

Elevation
of point B,
infeet

c

Undefined
interval

Sea level

Figure 7. The concept of "hydraulic head" or "head"
at a point in an aquifer.

Consider the elevations above sea level at points A
and B in an unconfined aquifer and C in a confined
aquifer. Now consider the addition of wells with short
screened intervals at these three points. The vertical
distance from the water level in each well to sea level
is a measure ofllydraulic head or head, referenced to a
common datum at each point A, B, and C, respectively.
Thus, head at a point in an aquifer is the sum of (a) the
elevation of the point above a common datum, usually
sea level, and (b) the heiglzt above the point of a column
of static water in a well that is screened at the point.
When we discuss declines or rises in ground-water
levels in a particular aquifer in this report, we are refer-
ring to changes ill head or water levels in wells that are
screened or have an open interval in that aquifer.

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