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GEOL: CHAPTER 13
Groundwater
Groundwater Introduction
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.6% of world’s water
22% of world’s freshwater
From precipitation that infiltrates ground
From streams, lakes, swamps, ponds
MOO!
Groundwater provides 80% of the water used for rural
livestock and domestic use, as well as providing 40% of
public water supplies.
Porosity
• Percentage of a material’s total volume
that is pore space
– Space between particles
– Cracks, fractures, faults, vesicles
• Water soaks into ground through pores
• Porosity varies by rock type and
fracturing and weathering history
Pore space
a. A well-sorted sedimentary
rock has high porosity,
whereas
b. a poorly sorted one has
lower porosity.
Openings resulting from solution
c. In soluble rocks such as
limestone, porosity can be
increased by solution, whereas
Fractures
d. crystalline metamorphic and
igneous rocks can be rendered
porous by fracturing.
Stepped Art
Fig. 13-1, p. 261
Permeability
• A material’s capacity to transmit fluids
• Depends on:
– Porosity
– Size of pores, with larger better
– Interconnections of pores
Permeability, cont.
• Aquifer: permeable layer that transports
groundwater
– Well sorted and well rounded sand and gravel
are best
– Limestone with fractures and bedding planes
enlarged by solution
• Aquiclude: prevents groundwater movement
– Shales, many igneous and metamorphic rocks
The Water Table
• The surface that separates the zone of
aeration from the underlying zone of
saturation
• Zone of aeration: The zone above the
water table that contains both air and
water in pore spaces
– Contains suspended water: adheres to
materials
The Water Table, cont.
• Zone of saturation: the area below the
water table in which all pore spaces are
filled with water
• Capillary fringe:
– Just above zone of aeration
– Water moves upward through capillary
action
The Water Table, cont.
• Water table configuration often roughly
replicates overlying land surface
– Rises beneath hills
– Lowest beneath valleys
• Also affected by regional differences in:
– Precipitation
– Permeability
– Groundwater movement
Groundwater Movement
• Gravity moves water downward
• Water moves downward along the slope
of the water table
• Water moves from areas of higher
pressure to areas of lower pressure
• Velocities average a few cm per day
Groundwater Withdrawals
• Discharge comes from:
– Flows into streams, lakes, swamps
– Springs
– Wells
– All lower the water table
• Recharge comes from:
– Rain and melting snow
– Recharge ponds/wastewater treatment
plants
Springs
• Water reaches the water table or other
impermeable layer and flows laterally
• Spring: when this lateral flow intersects
the surface
• Also occur with a perched water table: a
local aquiclude in larger aquifer
Springs form wherever laterally moving groundwater
intersects Earth’s surface.
Water Wells
• Dig/drill to zone of saturation; water
seeps in to fill hole and can be pumped
out
• Cone of depression in water table:
forms when water pumped out faster
than it is replaced
• Overuse of wells depletes groundwater
and lowers the water table
Artesian Systems
• Aquifer confined between two aquicludes
• Aquifer is exposed at surface
• Tilted rock units build up hydrostatic
pressure in the aquifer
• Elevation in recharge area defines
highest level to which water can rise
• Artesian-pressure surface: slopes away
from recharge area
Groundwater Erosion
• Limestone:
– Common sedimentary rock
– Underlies large part of Earth’s surface
– Composed primarily of calcite, CaCO3
– Easily dissolved by carbonic acid
– H2O + CO2 -> H2CO3
– CO2 present in air and soil
– Erosion creates karst topography
Major Limestone and Karst Areas of the World Distribution of
the major limestone and karst areas of the world. Karst
topography develops largely by groundwater erosion in areas
underlain by soluble rocks.
Sinkholes
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Depressions in regions underlain by
soluble rock
Form in 2 ways
1. Soluble rock below soil is dissolved by
seeping water; soil is eventually removed,
leaving shallow depressions that can
connect to create solution valleys
2. Cave roof collapses, creating a steepsided crater
This sinkhole formed on May 8 and 9, 1981, in Winter Park, Florida. It
formed in previously dissolved limestone following a drop in the water
table. The 100-m-wide, 35-m-deep sinkhole destroyed a house, numerous
cars, and a municipal swimming pool.
May 30th 2010 : Guatemala City
A huge sinkhole 60 feet wide and
300 feet deep formed in the middle
of the city. This one was man
made…
Karst Topography
• Formed by groundwater solution of
limestone and dolostone
• Caves
• Springs
• Sinkholes
• Solution valleys
• Disappearing streams
• High-relief landscapes: China
Features of Karst Topography: Erosion of soluble rock by groundwater
produces karst topography. Features commonly found include solution
valleys, springs, sinkholes, and disappearing streams.
Karst Landscape southeast of Kunming, China: The Stone Forest
Caves
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Groundwater erosion and weathering
Cavern: very large cave or system of caves
Mammoth Cave, Kentucky
Carlsbad Caverns, New Mexico
Lehman Cave, Nevada
Caves, cont.
• Groundwater percolates through zone
of aeration:
– Dissolves carbonate rock
– Enlarges fractures and bedding planes
• At water table:
– Migrates toward surface streams
– Creates horizontal passageways
– Lowered streams create a lowered water
table, with new layer of cave formation
Caves, cont.
• Dripstone: calcite deposits
• Stalactites: hang from ceiling;
composed of calcite precipitation from
dripping water
• Stalagmites: calcite formation from drips
that hit cave floor
• Column: stalactite and stalagmite meet
• Drip curtains
• Travertine terraces
Groundwater Modification
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Groundwater has been rapidly
exploited with little regard for longterm effects
20% of all water used in U.S.
Lowering the water table
Saltwater incursion
Subsidence
Contamination
Lowering the Water Table
• Withdrawing groundwater faster than it
is recharged
• High Plains aquifer
– 30% of irrigation groundwater in U.S.
– 2-100 times more water taken out than is
recharged
– Water table is dropping
High Plains Aquifer
The geographic extent of the
High Plains aquifer and
changes in water level from
predevelopment through
1993. Irrigation from the High
Plains aquifer is largely
responsible for the region’s
agricultural productivity.
Saltwater Incursion
• Coastal communities
• Lens of freshwater above salty groundwater
• Excessive pumping draws saltwater into
wells: cone of ascension
• Recharge wells can counter the effects
Saltwater Incursion
Saltwater Incursion
Saltwater Incursion
Subsidence
• Removal of groundwater can cause poorly
consolidated sediments and sedimentary
rocks to pack more closely together
• San Joaquin Valley in California
– Groundwater used for irrigation
– 9 meters subsidence in some parts
• Oil extraction can also cause subsidence
Subsidence in the San
Joaquin Valley, California
The dates on this power pole
dramatically illustrate the
amount of subsidence in the
San Joaquin Valley, California.
Because of groundwater
withdrawals and subsequent
sediment compaction, the
ground subsided nearly 9 m
between 1925 and 1977. For a
time, surface water use
reduced subsidence, but
during the drought of 1987 to
1992, it started again as more
groundwater was withdrawn.
Oil Field Subsidence, Long Beach, California
The withdrawal of petroleum from the Long Beach,
California, oil field resulted in up to 9 m of ground subsidence
in some areas because of sediment compaction. In the next
photograph, note that the ground has settled around the well
stems (the white “posts”), leaving the wellheads above the
ground. The levee on the left edge of the photograph was
built to keep seawater in the adjacent marina from flooding
the oil field. It was not until water was pumped back into the
reservoir to replace the extracted petroleum that ground
subsidence finally ceased.
Groundwater Contamination
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Sewage
Landfills
Toxic waste disposal sites
Agriculture
Once pollutants are in groundwater, they
spread, making containment difficult
Hydrothermal Activity
• Hydrothermal: referring to hot water; heated
groundwater and its surface activity
• Fumaroles: volcanic gas discharge, such as
steam
• Hot springs
• Geysers
Hot Springs
• Any spring warmer than 37ºC
• Some at boiling point
• Heat usually comes from magma or cooling
igneous rocks
– Western states have most U.S. hot springs
• Heat can also come from deep circulation
and the geothermal gradient
Bath, England
One of the many
bathhouses in Bath,
England, that were built
around hot springs shortly
after the Roman conquest
in A.D. 43.
Geysers
• Hot spring that periodically ejects hot water
and steam
• Surface expression of interconnected
fractures that reach hot igneous rocks
• Percolating groundwater is heated
Geysers, cont.
• Water at bottom is at higher pressure and
higher temperature
• Rise in temperature or drop in pressure
turns water to gas, which erupts out of the
ground
• Cycle repeats
Old Faithful Geyser in
Yellowstone National Park,
Wyoming, is one of the
world’s most famous
geysers, erupting faithfully
every 30 to 90 minutes and
spewing water 32 to 56 m
high.
Hydrothermal Deposits
• Hot spring and geyser water contains high
levels of minerals that precipitate on the
surface
– Mineral composition depends on material the
groundwater flows through
• Travertine: calcium carbonate deposits
Hot-Spring Deposits in Yellowstone National Park, Wyoming Minerva
Terrace, formed when calcium-carbonate-rich hot-spring water cooled,
precipitating travertine.
Geothermal Energy
• Energy from Earth’s internal heat
• Typically steam heated underground is
used to drive turbines and generate
electricity
• Often cheaper than other methods
• Used in U.S., New Zealand, Iceland,
Philippines, Indonesia, other countries
Virtual Field Trip
• Hydrothermal activity
• Hot springs, their formation, and the
denizens that live in them
• Geysers and how they form
• Deposits associated with hot springs and
geysers