CHAPTER 10 Groundwater
258
from the ground. Each is riddled with interconnected caves and passageways. This type of
karst topography forms in wet tropical and subtropical regions having thick beds of highly
jointed limestone. Here groundwater has dissolved large volumes of limestone, leaving only
these residual towers. Karst development is more rapid in tropical climates due to the abundant rainfall and the greater availability of carbon dioxide from the decay of lush tropical
vegetation. The extra carbon dioxide in the soil means there is more carbonic acid for dissolving limestone. Other tropical areas of advanced karst development include portions of
Puerto Rico, western Cuba, and northern Vietnam.
C O N C E P T C H E C K 1 0 . 1 2
How does groundwater create caves?
What causes cave formation to stop at one
level (depth) but continue or begin at a
lower level?
How do stalactites and stalagmites form?
Describe two ways in which sinkholes form.
4
3
2
1
Key Terms
aquifers (p. 245)
aquitards (p. 245)
artesian (p. 248)
caverns (p. 255)
cone of depression (p. 247)
confined aquifer (p. 248)
drawdown (p. 247)
gaining streams (p. 244)
geothermal energy (p. 254)
geysers (p. 252)
groundwater (p. 243)
hot springs (p. 252)
hydraulic conductivity (p. 247)
hydraulic gradient (p. 246)
karst topography (p. 256)
losing stream (p. 244)
perched water table (p. 247)
permeability (p. 245)
porosity (p. 245)
sinkholes (sinks) (p. 256)
speleothems (p. 256)
spring (p. 247)
stalactites (p. 256)
stalagmites (p. 256)
unsaturated zone (p. 243)
water table (p. 243)
well (p. 247)
zone of saturation (p. 243)
C H A P T E R
T E N
Groundwater
in Review
As a resource, groundwater represents the largest reservoir of fresh
water that is readily available to humans. Geologically, the dissolving action of groundwater produces caves and sinkholes. Groundwater is also an equalizer of streamflow.
Each day in the United States we use about 349 billion gallons of
fresh water. Groundwater provides about 79 billion gallons or 23
percent of the total. More groundwater is used for irrigation than
for all other uses combined.
Groundwater is water that completely fills the pore spaces in sediment and rock in the subsurface zone of saturation. The upper limit
of this zone is the water table. The unsaturated zone is above the
water table where the soil, sediment, and rock are not saturated.
The interaction between streams and groundwater takes place in
one of three ways: Streams gain water from the inflow of groundwater (gaining stream); they lose water through the streambed to the
groundwater system (losing stream); or they do both, gaining in
some sections and losing in others.
The quantity of water that can be stored in a material depends
upon the material’ s porosity (the volume of open spaces). The permeability (the ability to transmit a fluid through interconnected pore
spaces) of a material is a very important factor controlling the
movement of groundwater.
Materials with very small pore spaces (such as clay) hinder or
prevent groundwater movement and are called aquitards. Aquifers
consist of materials with larger pore spaces (such as sand) that are
permeable and transmit groundwater freely.
Groundwater moves in looping curves that are a compromise
between the downward pull of gravity and the tendency of water to
move toward areas of reduced pressure.
The primary factors influencing the velocity of groundwater
flow are the slope of the water table (hydraulic gradient) and
the permeability of the aquifer (hydraulic conductivity).
Springs occur whenever the water table intersects the land
surface and a natural flow of groundwater results. Wells, openings bored into the zone of saturation, draw groundwater out
and create roughly conical depressions in the water table
known as cones of depression. Artesian wells occur when water
rises above the level at which it was initially encountered.
When groundwater circulates at great depths, it becomes
heated. If it rises, the water may emerge as a hot spring. Geysers occur when groundwater is heated in underground chambers and expands and some water quickly changes to steam,
causing the geyser to erupt. The source of heat for most hot
springs and geysers is hot igneous rock. Geothermal energy is
harnessed by tapping natural underground reservoirs of steam
and hot water.
Some of the current environmental problems involving
groundwater include (1) overuse by intense irrigation; (2) land
subsidence caused by groundwater withdrawal; and (3) contamination by pollutants.
Most caverns form in limestone at or below the water table
when acidic groundwater dissolves rock along lines of weakness, such as joints and bedding planes. The various dripstone
features found in caverns are collectively called speleothems.
Landscapes that to a large extent have been shaped by the dissolving power of groundwater exhibit karst topography, an
irregular terrain punctuated with many depressions, called
sinkholes or sinks.
258
from the ground. Each is riddled with interconnected caves and passageways. This type of
karst topography forms in wet tropical and subtropical regions having thick beds of highly
jointed limestone. Here groundwater has dissolved large volumes of limestone, leaving only
these residual towers. Karst development is more rapid in tropical climates due to the abundant rainfall and the greater availability of carbon dioxide from the decay of lush tropical
vegetation. The extra carbon dioxide in the soil means there is more carbonic acid for dissolving limestone. Other tropical areas of advanced karst development include portions of
Puerto Rico, western Cuba, and northern Vietnam.
C O N C E P T C H E C K 1 0 . 1 2
How does groundwater create caves?
What causes cave formation to stop at one
level (depth) but continue or begin at a
lower level?
How do stalactites and stalagmites form?
Describe two ways in which sinkholes form.
4
3
2
1
Key Terms
aquifers (p. 245)
aquitards (p. 245)
artesian (p. 248)
caverns (p. 255)
cone of depression (p. 247)
confined aquifer (p. 248)
drawdown (p. 247)
gaining streams (p. 244)
geothermal energy (p. 254)
geysers (p. 252)
groundwater (p. 243)
hot springs (p. 252)
hydraulic conductivity (p. 247)
hydraulic gradient (p. 246)
karst topography (p. 256)
losing stream (p. 244)
perched water table (p. 247)
permeability (p. 245)
porosity (p. 245)
sinkholes (sinks) (p. 256)
speleothems (p. 256)
spring (p. 247)
stalactites (p. 256)
stalagmites (p. 256)
unsaturated zone (p. 243)
water table (p. 243)
well (p. 247)
zone of saturation (p. 243)
C H A P T E R
T E N
Groundwater
in Review
As a resource, groundwater represents the largest reservoir of fresh
water that is readily available to humans. Geologically, the dissolving action of groundwater produces caves and sinkholes. Groundwater is also an equalizer of streamflow.
Each day in the United States we use about 349 billion gallons of
fresh water. Groundwater provides about 79 billion gallons or 23
percent of the total. More groundwater is used for irrigation than
for all other uses combined.
Groundwater is water that completely fills the pore spaces in sediment and rock in the subsurface zone of saturation. The upper limit
of this zone is the water table. The unsaturated zone is above the
water table where the soil, sediment, and rock are not saturated.
The interaction between streams and groundwater takes place in
one of three ways: Streams gain water from the inflow of groundwater (gaining stream); they lose water through the streambed to the
groundwater system (losing stream); or they do both, gaining in
some sections and losing in others.
The quantity of water that can be stored in a material depends
upon the material’ s porosity (the volume of open spaces). The permeability (the ability to transmit a fluid through interconnected pore
spaces) of a material is a very important factor controlling the
movement of groundwater.
Materials with very small pore spaces (such as clay) hinder or
prevent groundwater movement and are called aquitards. Aquifers
consist of materials with larger pore spaces (such as sand) that are
permeable and transmit groundwater freely.
Groundwater moves in looping curves that are a compromise
between the downward pull of gravity and the tendency of water to
move toward areas of reduced pressure.
The primary factors influencing the velocity of groundwater
flow are the slope of the water table (hydraulic gradient) and
the permeability of the aquifer (hydraulic conductivity).
Springs occur whenever the water table intersects the land
surface and a natural flow of groundwater results. Wells, openings bored into the zone of saturation, draw groundwater out
and create roughly conical depressions in the water table
known as cones of depression. Artesian wells occur when water
rises above the level at which it was initially encountered.
When groundwater circulates at great depths, it becomes
heated. If it rises, the water may emerge as a hot spring. Geysers occur when groundwater is heated in underground chambers and expands and some water quickly changes to steam,
causing the geyser to erupt. The source of heat for most hot
springs and geysers is hot igneous rock. Geothermal energy is
harnessed by tapping natural underground reservoirs of steam
and hot water.
Some of the current environmental problems involving
groundwater include (1) overuse by intense irrigation; (2) land
subsidence caused by groundwater withdrawal; and (3) contamination by pollutants.
Most caverns form in limestone at or below the water table
when acidic groundwater dissolves rock along lines of weakness, such as joints and bedding planes. The various dripstone
features found in caverns are collectively called speleothems.
Landscapes that to a large extent have been shaped by the dissolving power of groundwater exhibit karst topography, an
irregular terrain punctuated with many depressions, called
sinkholes or sinks.