Geology Reference
In-Depth Information
Area of
recharge
for aquifer
Artesian-pressure
surface
Flowing
artesian well
Nonflowing
artesian well
Shale
aquicludes
Sandstone
aquifer
◗
Figure 13.6
Artesian System An artesian system must have an aquifer confi ned above and below
by aquicludes, the aquifer must be exposed at the surface, and the rock units are typically tilted as
to build up hydrostatic pressure within the aquifer. The elevation of the water table in the recharge
area, which is indicated by a sloping dashed line (the artesian-pressure surface), defi nes the highest
level to which well water can rise. A wellhead below the elevation of the artesian-pressure surface will
be free-fl owing because the water will rise toward the artesian-pressure surface, which is at a higher
elevation than the wellhead. Conversely, a well will be nonfl owing if the elevation of the wellhead is at
or above that of the artesian-pressure surface.
system is used for irrigation. The aquifer of this artesian system,
the Dakota Sandstone, is recharged where it is exposed along
the margins of the Black Hills of South Dakota. The hydrostatic
pressure in this system was originally great enough to produce
free-flowing wells and to operate waterwheels. However, be-
cause of the extensive use of this groundwater for irrigation
over the years, the hydrostatic pressure in many of the wells is
so low that they are no longer free-fl owing and the water must
be pumped.
As a fi nal comment on artesian systems, we should men-
tion that it is not unusual for advertisers to tout the quality of
artesian water as somehow being superior to other groundwater.
Some artesian water might in fact be of excellent quality, but its
quality is not dependent on the fact that water rises above the
surface of an aquifer. Rather, its quality is a function of dissolved
minerals and any introduced substances, so artesian water re-
ally is no different from any other groundwater. The myth of its
superiority probably arises from the fact that people have always
been fascinated by water that fl ows freely from the ground.
Limestone, a common sedimentary rock composed pri-
marily of the mineral calcite (CaCO
3
), underlies large areas
of Earth's surface (
Figure 13.7). Although limestone is prac-
tically insoluble in pure water, it readily dissolves if a small
amount of acid is present. Carbonic acid (H
2
CO
3
) is a weak
acid that forms when carbon dioxide combines with water
(H
2
O + CO
◗
H
2
CO
3
). Because the atmosphere contains a
small amount of carbon dioxide (0.03%) and carbon dioxide
is also produced in soil by the decay of organic matter, most
groundwater is slightly acidic. When groundwater percolates
through the various openings in limestone, the slightly acidic
water readily reacts with the calcite to dissolve the rock by
forming soluble calcium bicarbonate, which is carried away
in solution (see Chapter 6).
2
→
In regions underlain by soluble rock, the ground surface may
be pitted with numerous depressions that vary in size and
shape. These depressions, called
sinkholes
or merely
sinks
,
mark areas with underlying soluble rock (
Figure 13.8).
Most sinkholes form in one of two ways. The fi rst is when
soluble rock below the soil is dissolved by seeping water, and
openings in the rock are enlarged and fi lled in by the overly-
ing soil. As the groundwater continues to dissolve the rock,
the soil is eventually removed, leaving shallow depressions
with gently sloping sides. When adjacent sinkholes merge,
they form a network of larger, irregular, closed depressions
called
solution valleys
.
◗
AND DEPOSITION
When rainwater begins to seep into the ground, it immediately
starts to react with the minerals it contacts, weathering them
chemically. In an area underlain by soluble rock, groundwater
is the principal agent of erosion and is responsible for the for-
mation of many major features of the landscape.
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