Geology Reference
In-Depth Information
Hot springs and geysers result when groundwater is
heated, typically in regions of recent volcanic activity.
Geothermal energy is a desirable and relatively
nonpolluting alternative form of energy.
from landfi lls, toxic waste, and agriculture has rendered the
groundwater supply unsafe.
As the world's population and industrial development
expand, the demand for water, particularly groundwater, will
increase. Not only must new groundwater sources be located,
but once found, these sources must be protected from pol-
lution and managed properly to ensure that users do not
withdraw more water than can be replenished. It is therefore
important that people become aware of what a valuable re-
source groundwater is, so that they can ensure that future
generations have a clean and adequate supply of this water
source.
INTRODUCTION
Within the limestone region of western Kentucky lies the
largest cave system in the world. In 1941, approximately
51,000 acres were set aside and designated as Mammoth Cave
National Park. In 1981, it became a World Heritage Site.
From ground level, the topography of the area is unim-
posing, with gently rolling hills. Beneath the surface, however,
are more than 540 km of interconnected passageways whose
spectacular geologic features have been enjoyed by millions
of cave explorers and tourists.
During the War of 1812, approximately 180 metric tons
of saltpeter, used in the manufacture of gunpowder, was
mined from Mammoth Cave. At the end of the war, the
saltpeter market collapsed, and Mammoth Cave was devel-
oped as a tourist attraction, easily overshadowing the other
caves in the area. During the next 150 years, the discovery of
new passageways and links to other caverns helped establish
Mammoth Cave as the world's premier cave and the standard
against which all others were measured.
The formation of the caves themselves began approxi-
mately 3 million years ago when groundwater began dis-
solving the region's underlying St. Genevieve Limestone to
produce a complex network of openings, passageways, and
huge chambers that constitute present-day Mammoth Cave.
Flowing through the various caverns is the Echo River, a
system of streams that eventually joins the Green River at the
surface.
The colorful cave deposits are the primary reason that
millions of tourists have visited Mammoth Cave over the
years. Hanging down from the ceiling and growing up from
the fl oor are spectacular icicle-like structures, as well as col-
umns and curtains in a variety of colors. Moreover, intricate
passageways connect rooms of various sizes. The cave is also
home to more than 200 species of insects and other animals,
including about 45 blind species.
In addition to the beautiful caves, caverns, and cave
deposits produced by groundwater movement, groundwater
is also an important natural resource. Although groundwater
constitutes only 0.6% of the world's water, it is, nonetheless,
a signifi cant source of freshwater for agriculture, industry,
and domestic users. More than 65% of the groundwater used
in the United States each year goes for irrigation, with indus-
trial use second, followed by domestic needs. Furthermore,
groundwater provides 80% of the water used for rural live-
stock and domestic use, as well as providing 40% of public
water supplies. In fact, of the largest 100 cities in the United
States, 34 depend solely on local groundwater supplies. These
demands have severely depleted the groundwater supply in
many areas and have led to such problems as ground subsid-
ence and saltwater contamination. In other areas, pollution
GROUNDWATER AND THE
HYDROLOGIC CYCLE
Groundwater , water that fills open spaces in rocks,
sediment, and soil beneath Earth's surface, is one reservoir
in the hydrologic cycle, accounting for approximately 22%
(8.4 million km 3 ) of the world's supply of freshwater (see
Table 12.1). Like all other water in the hydrologic cycle, the
ultimate source of groundwater is the oceans; however, its
more immediate source is the precipitation that infi ltrates
the ground and seeps down through the voids in soil, sedi-
ment, and rocks. Groundwater may also come from water
infi ltrating from streams, lakes, swamps, artifi cial recharge
ponds, and water-treatment systems.
Regardless of its source, groundwater moving through
the tiny openings between soil and sediment particles and the
spaces in rocks fi lters out many impurities such as disease-
causing microorganisms and many pollutants. However, not
all soils and rocks are good fi lters, and sometimes so much
undesirable material may be present that it contaminates the
groundwater. Groundwater movement and its recovery at
wells depend on two critical aspects of the materials that it
moves through: porosity and permeability .
TABLE 13.1 Porosity Values for Different Materials
Percentage
Material
Porosity
Unconsolidated sediment
Soil
55
Gravel
20-40
Sand
25-50
Silt
35-50
Clay
50-70
Rocks
Sandstone
5-30
Shale
0-10
Solution activity in limestone, dolostone
10-30
Fractured basalt
5-40
Fractured granite
10
Source: U.S. Geological Survey, Water Supply Paper 2220 (1983) and
others.
 
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