Geology Reference
In-Depth Information
Chapter Summary
Groundwater is part of the hydrologic cycle and an
important natural resource. It consists of all subsurface
water trapped in the pores and other open spaces in
rocks, sediment, and soil.
Porosity is the percentage of a material's total volume that
is pore space. Permeability is the capacity to transmit fl u-
ids. Permeability is dependent on porosity, but also on the
size of the pores or fractures and their interconnections.
The water table is the surface separating the zone of
saturation (in which pores are fi lled with water) from the
overlying zone of aeration (in which pores are fi lled with
air and water). The water table is a subdued replica of the
overlying land surface in most places.
Groundwater moves slowly downward under the infl u-
ence of gravity through the pore spaces in the zone of
aeration to the zone saturation. Some of it moves along
the surface of the water table, and the rest moves from
areas of high pressure to areas of low pressure.
Springs are found wherever the water table intersects the
surface. Some springs are the result of a perched water
table—that is, a localized aquiclude within an aquifer and
above the regional water table.
Water wells are openings made by digging or drilling
down into the zone of saturation. When water is pumped
from a well, the water table in the area around the well is
lowered, forming a cone of depression.
In an artesian system, confi ned groundwater builds up
high hydrostatic pressure. For an artesian system to
develop, an aquifer must be confi ned above and below by
aquicludes with the rock units usually tilted so as to build
up hydrostatic pressure, and the aquifer must be exposed
at the surface so it can be recharged.
Karst topography largely develops by groundwater ero-
sion in many areas underlain by soluble rocks, and is
characterized by sinkholes, caves, solution valleys, and
disappearing streams.
Caves form when groundwater in the zone of saturation
weathers and erodes soluble rock such as limestone. Cave
deposits, called dripstone, result from the precipitation of
calcite.
Modifi cations of the groundwater system can cause seri-
ous problems such as lowering of the water table, salt-
water incursion, subsidence, and contamination.
Groundwater contamination by humans is becoming
a serious problem and can result from landfi lls, septic
systems, toxic waste sites, and industrial effl uents, all of
which affect the quality of the groundwater.
Hydrothermal refers to hot water, typically heated by
magma, but also resulting from Earth's geothermal
gradient as it circulates deeply beneath the surface.
Manifestations of hydrothermal activity include
fumaroles, hot springs, and geysers.
Geothermal energy is energy produced from Earth's internal
heat and comes from the steam and hot water trapped within
Earth's crust. It is a relatively nonpolluting from of energy
that is used as a source of heat and to generate electricity.
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Important Terms
artesian system (p. 334)
cave (p. 339)
cone of depression (p. 334)
geothermal energy (p. 352)
geyser (p. 351)
groundwater (p. 330)
hot spring (p. 348)
hydrothermal (p. 348)
karst topography (p. 339)
permeability (p. 331)
porosity (p. 331)
sinkhole (p. 335)
spring (p. 333)
water table (p. 331)
water well (p. 334)
zone of aeration (p. 331)
zone of saturation (p. 331)
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