Transcript Groundwater Systems

Groundwater
Systems
Chapter 13
Dynamic Earth
Eric H Christiansen
Major Concepts
• Groundwater is an integral part of the hydrologic system, and it is
intimately related to surface water drainage.
• The movement of groundwater is controlled largely by the porosity
and permeability of the rocks through which it flows.
• The water table is the upper surface of the zone of saturation.
• Groundwater moves slowly through the pore spaces in rock.
Major Concepts
• The natural discharge of groundwater is generally into springs,
streams, marshes, and lakes.
• Aquifers are saturated permeable rocks; they may be confined
between impermeable layers or unconfined and open to the surface.
• Erosion by groundwater produces karst, with caves, sinkholes,
solution valleys, and disappearing streams. Precipitation of minerals
from groundwater creates deposits in caves and along fractures and
cements many kinds of clastic sedimentary rocks.
• Alteration of the groundwater can produce many unforeseen
problems, such as pollution, subsidence, collapse, and disruption of
ecosystems.
Groundwater Systems
• Groundwater is simply water
below Earth’s surface.
• Two physical properties of a rock
largely control the amount and
movement of groundwater.
• Porosity, the percentage of the
total volume of the rock
consisting of voids.
• Permeability, the capacity of a
rock to transmit fluids.
Groundwater Systems
• Groundwater flows in an open,
dynamic system.
• Gravity is primary driving force.
• Water enters the system at the
ground surface through recharge,
and generally flows slowly through
connected pores in soil and rock.
• Locally, it dissolves soluble rocks
deposits minerals along fractures
and in caves.
• Leaves the system through
discharge.
Figure 13.01: The groundwater system is an open system of
water flowing below the surface but still under the influence
of gravity.
Porosity
 Percent of the total volume
that is open space.
 Affected by the size and shape
of particles.
 Increased by fracturing or
dissolution.
 Decreased by compaction and
cementation.
 Well sorted sediments have
higher porosity than poorly
sorted sediments .
Figure 13.02: Various types of pore spaces in rocks permit the flow of
groundwater.
Permeability
• A measure of how well
pores are connected
and how straight a path
a fluid follows
• Permeability is a
property of the rock
• Other liquids such as oil
flow through rock
• Density and viscosity
influence flow rate
Figure 13.02: Various types of pore spaces in rocks permit the flow of
groundwater.
The Water Table and Aquifers
• The water table is the upper
surface of the zone of
saturation.
• Aquifers are saturated
permeable rocks; they may be
open or confined.
Figure 13.04: The movement of groundwater in an unconfined
aquifer is directed toward areas of least pressure.
The Water Table
• Water is found at some depth
almost everywhere
• The water table is the boundary
below which all pore spaces are
filled with water
• The volume of pore spaces
decreases with depth
• Usable water is restricted to a
few hundred meters below the
surface
Figure 13.06: The base of an unconfined groundwater
reservoir is not an abrupt surface like the water table.
The Water Table
• Forms the boundary
between
• the Saturated Zone
• the Unsaturated Zone
• The unsaturated zone = zone
of aeration
Figure 13.03: The water table is the upper surface of the
zone of saturation. Water seeps into the ground through
pore spaces in rock and soil.
Unconfined Aquifers
 An aquifer that has highly
permeable material extending
from ground surface
downward to an aquitard at
its base.
 Recharge is from seepage,
lateral flow of groundwater,
or upward leakage through
the aquitard.
 Also called a water table
aquifer.
Figure 13.04: The movement of groundwater in an unconfined
aquifer is directed toward areas of least pressure.
Confined Aquifers
Figure 13.10: Flowing (or artesian) wells occur only when the top of the well is below the potentiometric surface and
require no pumping.
• Confining layers (aquitards) above and below a permeable zone
• Recharge at high elevation where permeable beds are exposed.
Major Aquifers in the United States
Figure 13.05: The major aquifers of the United States are shown on this map. Each aquifer consists of permeable rocks.
Data from: U.S. Geological Survey
Flow of Groundwater
 Groundwater flows due to the
force of gravity
 Recharge flows downward to
the water table
 Gravity creates water pressure
in the aquifer
 Groundwater then flows from
high pressure to low pressure
3D model and image provide courtesy of ctech.com
Flow of Groundwater: Unconfined Aquifers
 Recharge flows downward to
the water table which usually
mimics the ground surface
 Gravity creates water pressure
in the aquifer
 Groundwater then flows from
high pressure to low pressure
Figure 13.04: The movement of groundwater in an unconfined
aquifer is directed toward areas of least pressure.
Flow of Groundwater: Unconfined Aquifers
 Groundwater pressure can be
measured as hydraulic head-the elevation of the water table
at a given point
 Flow is from high to low
pressure or hydraulic head
 Hydraulic gradient is the
difference in head between two
points—the driving force
Figure 13.04: The movement of groundwater in an unconfined
aquifer is directed toward areas of least pressure.
Flow of Groundwater: Confined Aquifers
Figure 13.10: Flowing (or artesian) wells occur only when the top of the well is below the potentiometric surface and
require no pumping.
• Because water pressure builds in the confined zone, the groundwater
level (potentiometric surface) may be above the top of the aquifer
• Artesian aquifers have a potentiometric surface above ground surface
Natural and Artificial Discharge
• Groundwater discharge occurs
to lakes, streams and wetlands
• Maintains flow in streams during
dry periods
• Springs include any natural flow
of water from the ground
surface
• Intersection of water tableground surface
Figure 13.08B: The springs issue from the north wall of the
canyon and are fed by water that flowed underground.
© Charles Knowles/ShutterStock, Inc.
Natural Discharge: Springs
Figure 13.07A: A line of
springs develops on
valley walls where
impermeable beds
cause groundwater in
permeable layers to
migrate.
Figure 13.07C: Many
faults displace rocks so
that impermeable
beds are placed next
to permeable beds.
Figure 13.07B:
Springs form along
valley slopes where
cavernous limestone
permits the free flow
of groundwater to
the surface.
Figure 13.07D:
Surface water readily
seeps into vesicular
and jointed basalt
flows.
Artificial Discharge: Wells
• Before Pumping
• Water table is nearly horizontal
• During Pumping
• Water table is depressed as water is
withdrawn from the aquifer
• Forms cone of depression
Figure 13.09: A cone of
depression in a water table
results if water is withdrawn
from a well faster than it can
be replenished.
Thermal Springs and Geysers
Figure 13.13: Hot springs are common where
groundwater is heated in regions with young
volcanism or deep faulting.
• In areas of recent igneous
activity, rocks near magma
chambers can remain hot for
hundreds of thousands of years.
• Groundwater migrating
through these areas of hot rock
becomes heated and, when
discharged to the surface,
produces thermal springs and
geysers.
Geysers
Figure 13.12A:
Groundwater circulating
through hot rocks in an
area of recent volcanic
activity collects in caverns
and fractures.
Figure 13.12C: The
preliminary discharge of
water reduces the
pressure on the water
lower down.
Figure 13.12B: The
expanding steam forces
water upward until it is
discharged at the surface
vent.
Figure 13.12D: Eruption
ceases when the pressure
from the steam is spent
and the geyser tubes are
empty.
Erosion by Groundwater
• Slow-moving groundwater can
dissolve huge quantities of
soluble rock and carry it away in
solution.
• Subsurface dissolution forms:
• Caves
• Sinkholes
• Karst topography.
Figure 13.14: Importance of fractures in the evolution
of a cave system is revealed in the Redwall Limestone
exposed on walls of Grand Canyon.
Groundwater
dissolution of
salt in
formation,
Utah
Caves
• Shallow groundwater
dissolves carbon dioxide to
form a weak acid.
• The slightly acidic water
percolates through the
fractures and bedding
planes, slowly dissolving the
limestone and enlarging the
openings to form caves
Figure 13.20C: Dissolution is expressed vividly by the abundant
cavities so that the limestone outcrops resemble Swiss cheese.
Karst Topography
• Carbonate, sulfate, or salt beds
at or near the ground surface
• Humid climate
• Solution valleys with remnant
walls and towers
• Disappearing streams
• Sinkholes
Figure 13.18A: Initial stage.
Scattered sinkholes dotting
the landscape grow in size
and number as caverns
enlarge and their roofs
collapse.
Figure 13.18B:
Intermediate stage.
Individual sinks enlarge and
merge with those in
adjacent areas to form
solution valleys.
Figure 13.18C: Late stage.
Solution activity removed
most of the limestone
formation. Only isolated
knolls remain as remnants
of the former surface.
Karst Topography
Figure 13.19: Major areas of karst topography of the world are restricted to regions where outcrops of
limestone occur in humid climatic conditions.
Courtesy of USGS
Courtesy of John S. Shelton
Figure 13.17A: Sinkhole karst, Kentucky.
Figure 13.17B: Sinkhole in a karst terrain in Florida.
Figure 13.17C: In some karst regions, streams
disappear into subsurface caverns like this one in
China.
Figure 13.17D: Groundwater solution enlarged
these fractures in limestones in New Zealand.
Karst from Space
Figure 13.20D: Karst terrain. Karst processes form distinctive regional landscapes as shown on this map of southern China.
Deposition by Groundwater
• The mineral matter dissolved by
groundwater can be deposited in
a variety of ways.
• Groundwater commonly
deposits mineral matter as
cement between grains in
permeable deposits such as
sandstone and conglomerates.
• The most spectacular deposits
are stalactites and stalagmites,
which are found in caves.
Figure 13.24: Calcite deposited by groundwater cements the
rounded quartz sand grains together, as shown in this thin
section of sandstone.
Deposition by Groundwater
Figure 13.25: Petrified trees litter the area, piled like giant jackstraws
about a rolling landscape on the Petrified Forest Member.
• Groundwater also carries
dissolved silica
• Locally it can replace plant
material to make
spectacular fossils.
• Petrified trees formed this
way in of the Petrified
Forest Member of the
Chinle Formation, Arizona.
Weathering and erosion
later exposed them.
Deposition by Groundwater
Figure 13.26: Mammoth Hot Springs, Yellowstone National Park.
Caves
Figure 13.22: Many varieties of cave deposits are shown in this
idealized diagram.
• Solution caves form
by groundwater
erosion
• Some partially fill by
deposition from
groundwater
• Stalagmites
• Stalactites
• Travertine
• Drip stones
Groundwater Resources
• Groundwater is a valuable
resource that is being exploited
at an ever-increasing rate.
• Ancient groundwater systems
have also produced valuable
mineral resources.
• Geothermal waters could
become a significant energy
source. Forms near shallow
magma systems and along faults. Figure 24.15: Geothermal energy stored in heated groundwater
is extracted in facilities such as this one in New Zealand to
generate electricity.
© Graham Prentice/ShutterStock, Inc.
Alteration of Groundwater Systems
• A variety of problems resulting
from human activities alter the
groundwater system:
• Pollution
• Saltwater encroachment,
• Changes in the position of the
water table
• Subsidence following withdrawl.
Figure 13.31: Dam constructed on permeable
limestone in western Wyoming never functioned; the
surface water seeped into the subsurface.
Waste Disposal and Groundwater
Figure 13.27A: Permeable sand and gravel overlying
impermeable shale creates potential pollution
problems: contaminants move with groundwater.
Figure 13.27B: An impermeable shale confines
pollutants and prevents significant infiltration
into the groundwater system in the limestone
below.
Figure 13.27C: A fractured rock body
provides a zone where pollutants can move
readily in the general direction of
groundwater flow.
Figure 13.27D: An inclined, permeable aquifer below a
disposal site permits pollutants to enter a confined
aquifer and move down the dip of the beds.
Saltwater Encroachment
Figure 13.28A: A lens of
fresh groundwater
beneath the land is
buoyed up by denser
saltwater below.
Figure 13.28C: Fresh
water pumped down an
adjacent well can raise
the water table; this
lowers the interface
between altwater and
fresh water.
Figure 13.28B: Excessive
pumping causes a cone of
depression in the water
table on top of the
freshwater lens.
• The relationship between fresh
water and saltwater on an island or
a peninsula is affected by the
withdrawal of water from wells.
• Excessive pumping causes a cone
of saltwater encroachment.
Modified Groundwater Drainage Systems
Figure 13.29A: Natural drainage of southern Florida in
1871 spread southward from Lake Okeechobee in a
broad sheet only a few centimeters deep.
Figure 13.29B: Canals diverted the natural flow of
surface water across the Everglades.
Subsidence
• Subsidence of buildings in
Mexico City resulted from
compaction after groundwater
was pumped from
unconsolidated sediment
beneath the city.
• Subsidence has caused this
building to tilt and sink more
than 2 m.
Figure 13.30: Subsidence of buildings in Mexico
City resulted from compaction after groundwater
was pumped from unconsolidated sediment.
Summary of Major Concepts
• Groundwater is an integral part of the hydrologic system, and it is
intimately related to surface water drainage.
• The movement of groundwater is controlled largely by the porosity
and permeability of the rocks through which it flows.
• The water table is the upper surface of the zone of saturation.
• Groundwater moves slowly through the pore spaces in rock.
Summary of Major Concepts
• The natural discharge of groundwater is generally into springs,
streams, marshes, and lakes.
• Aquifers are saturated permeable rocks; they may be confined
between impermeable layers or unconfined and open to the surface.
• Erosion by groundwater produces karst, with caves, sinkholes,
solution valleys, and disappearing streams. Precipitation of minerals
from groundwater creates deposits in caves and along fractures and
cements many kinds of clastic sedimentary rocks.
• Alteration of the groundwater can produce many unforeseen
problems, such as pollution, subsidence, collapse, and disruption of
ecosystems.