Chapter 14: Groundwater
Fig 14.5b
OBJECTIVES
•
Recognize that groundwater is a vital source of accessible
freshwater.
•
Describe how groundwater forms below the water table.
•
Explain the origin of aquifers, the controls of groundwater
flow, and the origin of oases and hot springs.
•
Describe how the movement of groundwater through
soluble rock creates caverns and a distinctive surface
topography.
OBJECTIVES
•
Explain how groundwater is exploited as a resource and
how the unique qualities of groundwater render it
susceptible to contamination by human activities.
•
Identify how plate tectonics governs the location and
overall shape of some aquifers, as well as groundwater
flow, by creating the mountains that influence topography
and act as recharge areas for aquifers, as well as the faults
that bring groundwater to the surface.
Groundwater
• Groundwater is a small fraction of Earth's total water, but it
constitutes 22% of freshwater resources.
Fig. 14.1
The Water Table
• The water table is the upper surface of the zone of saturation.
Fig. 14.2
Porosity and Permeability
• Porosity is the amount
of open space between
sediment particles.
• Permeability is the
amount of interconnectedness
between sediment
pores.
Fig. 14.3
Groundwater Supply
• Groundwater supply depends on rates of discharge and
recharge.
Fig. 14.5a
Wells and Aquifers
• Wells can be used to
remove groundwater from
an aquifer.
• If the water table drops,
wells may run dry.
•
•
•
•
Wells drilled into an aquifer must
penetrate below the water table. Note
the lower level of the water table during
the dry
season. The water table feeds wells and
springs, and changes in the level of the
water table from the wet to the dry
season affect
the availability of water. The change
shown here, for example, would cause
the shallow well (on the right) to dry up
during the
dry season.
Fig. 14.6
Unconfined Aquifers
• The upper surface of an unconfined aquifer is at the same level
as the water table.
Fig. 14.7
Confined Aquifers
• Confined aquifers can generate hydraulic head in artesian
wells.
Fig. 14.9
Heated Groundwater
• Heated groundwater can form geysers, hot springs, and
fumaroles.
Fig. 14.11
Sinkholes
• The dissolution of
underground rock layers
by groundwater can lead
to the formation of
sinkholes.
• Sinkholes come in two
types: solution sinkholes
and collapse sinkholes.
Fig. 14.15
Karst Topography
• In areas with soluble bedrock,
groundwater dissolution can lead to the
formation of karst topography.
• Karst topography is characterized by
sinkholes, monoliths, caverns, and
disappearing streams.
• As seen here, groundwater migrating
though the zone of aeration dissolves
limestone rock, then moves along the
water table to form interconnected
subsurface channels as dissolved rock is
transported to streams on the surface.
The stream continues to erode and the
water table drops, causing caves to form
in the abandoned channels.
Fig. 14.17
Risks of Overdrawing Wells
• As water is removed from a well, it produces a cone of
depression.
• In coastal regions, this can lead to saltwater contamination of
wells.
Fig. 14.19
Risks of Overdrawing Wells
• Because salt water is denser than
freshwater, it would normally sink
below any freshwater in an
aquifer.
• As freshwater is withdrawn and
cones of depression form, the
water pressure is reduced, which
allows the saline water to migrate
upward, causing a cone of
ascension.
Fig 14.21
Contamination of Groundwater
• Human actions can contaminate groundwater in a variety of
ways.
Fig. 14.23
Groundwater and Plate Tectonics
• Plate tectonics influences or has
influenced
• Temperature of groundwater
and existence of geothermal
resources
• Topography that affects
location of groundwater
resources
• Topography that affects flow
of groundwater, recharge of
aquifers
• Ancient flow of warm, saline
groundwater
Fig. 14.12
Summary
• Groundwater
• Is the most widespread and accessible source of
freshwater on Earth
• Is stored in aquifers, which are layers of porous and
permeable sediment or rock
• Can be found in confined or unconfined aquifers
• Can form geysers, fumaroles, and hot springs when it is
heated
• Can produce sinkholes, monoliths, disappearing streams,
and caverns when it dissolves underground rock layers
• Is susceptible to contamination from saltwater, landfills,
agricultural runoff, septic tanks, and other sources
• Is influenced by plate tectonics regarding location and flow