Dr. James Wittke
[email protected]
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Groundwater occurs in …
◦ Pores of soil and sediment
◦ Fractures in bedrock
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Largest reservoir of fresh water available to humans
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Present usage of
groundwater increased
significantly in past 80
years
Only 8% of water
withdrawn is
replenished by
recharge
Most used for irrigation
◦ 42% of total irrigation
water is groundwater
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Undersaturated zone
◦ Also called vadose zone
◦ Some water in open spaces (soil
moisture)
◦ Most water passes downward
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Water Table
◦ Top of saturated zone
◦ Capillary fringe extends upward
from saturated zone
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Saturated zone
◦ Also called phreatic zone
◦ All open spaces completely filled
with water
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Shape usually subdued replica of surface topography
Depth is variable
◦ Seasonal variations in rainfall
◦ Water moves slowly  piles up under high areas
◦ Variations in permeability
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May reach surface at streams, lakes, swamps
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Gaining streams
◦ Inflow of groundwater
through streambed
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Losing streams
◦ Outflow to groundwater
through stream-bed
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Stream’s nature can
vary along its length
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Porosity is percentage of
rock or sediment made of
pore spaces and fractures
◦ Loose sediments are most
porous
◦ Sedimentary rocks more porous
than igneous and metamorphic
rocks
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Determines how much
groundwater can be stored
May change over short
distances
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Permeability is ability of material to transmit fluid
◦ Depends on interconnectedness of porosity in rock or sediment
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Water under influence of gravity (amount available for
use)  specific yield
Water retained in cracks or as films on grains 
specific retention
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Aquitard
◦ May have high porosity, but with low permeability
◦ Impedes or prevents water movement
◦ Typically clay or shale
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Aquifer
◦ Permeable rock strata or sediment that transmits
groundwater freely
◦ Typically sands, gravels, sandstones
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Energy for movement
provided by force of gravity
Rate of movement is
exceedingly slow
◦ Typically few centimeters per
day
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Movement through ground
helps purify water…
◦ Filters out organisms
◦ Removes reactive chemicals
◦ Unreactive chemicals not
removed
Blue arrows are flow likes
(analogous to earthquake ray paths)
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Describes discharge (Q) through ground
Controlled by…
◦ Hydraulic gradient
◦ Conductivity (K)
◦ Cross-sectional area (A)
𝐾𝐴(ℎ1 − ℎ2 )
𝑄=
𝑑
Local vs. regional/subregional flow patterns
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Springs form where water table intersects surface
causing natural outflow of groundwater
Springs may be caused by aquitard creating localized
zone of saturation called perched water table
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Water 6-9°C warmer than
mean air temperature
Areas with high geothermal
gradient (igneous rocks
provide heat)
◦ Western USA, Iceland, New
Zealand, Japan
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Cold water sinks along
fractures; heated groundwater
returns to surface
Hot Springs in the United States
Silica dissolved in hot
water precipitates as
the water cools
Mammoth Hot Springs,
Yellowstone WY
Dissolved calcite precipitates as water cools
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Occur where
extensive
underground
chambers exist within
hot igneous rock
Groundwater heats,
expands, changes to
steam, and erupts
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Taps natural
underground reservoirs
of steam and hot water
Favorable geologic
factors include
◦ Potent source of heat
◦ Large and porous
reservoirs with channels
connected to the heat
source
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Geothermal energy is
not inexhaustible
Geysers
Geothermal
Field, CA
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Dug or drilled into
aquifers
Must penetrate
water table
Water in well rises
to level of local
water table and
must be pumped
to surface
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Artesian well – groundwater under pressure
rises above level of aquifer
Types of artesian wells
◦ Nonflowing – pressure surface is below ground
level
◦ Flowing – pressure surface is above the ground
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Not all artesian systems are wells, artesian
springs also exist
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Water table effected by pumping  drawdown
◦ Effect lessens with distance from well
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In many places water available to recharge
aquifer much less than amount being withdrawn
Effects:
◦ Cone of depression
◦ Ground subsidence
◦ Saltwater incursion
Water Withdrawals
(2000)
High Plains
Aquifer
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Extensive pumping
for irrigation has
lowered water table
Only gains observed
where surface water
is used for
agriculture (acts as
groundwater
recharge)
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Ground sinks when water is pumped from wells faster than
natural recharge processes can replace it
Withdrawing groundwater removes water support between
sediment grains and sediment undergoes compaction
Effect similar if hydrocarbons pumped from ground (Houston)
Subsidence caused by
pumping water for
agriculture
Sign on Snyder Hill
Road, Pima County,
1981
Ground cracks caused by pumping groundwater
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Problem in coastal
areas
Interface between
fresh groundwater
and salt groundwater
Excessive pumping
causes saltwaterfresh water interface
to shift
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Promoting recharge of
aquifer can stop/reverse
saltwater incursion
Pump water into ground
◦ Recharge ponds where runoff
accumulates and can infiltrate
(Above & below) artificial recharge basins
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Very permeable
aquifer – water may
travel long distances
without being
cleaned (A)
Sewage often
becomes purified as
it passes through few
dozen meters of
sand or permeable
sandstone (B)
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Heavy pumping can
modify water flow
Shifts direction of
flow
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Dissolution of rock by
groundwater produces
karst in soluble rocks (e.g.,
limestone)
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Limestone (dolomite) is common
rock type
Acidic waters dissolve calcite
◦ More dissolved CO2  more dissolved calcite
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Any process that reduces CO2 in
water or concentrates ions causes
calcite to precipitate
◦ Evaporation
◦ Organic activity (consumes CO2)
◦ Agitation of water (gas released when
water is “stirred”)
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Precipitated calcite called travertine
Huanglong NP, China
Travertine along
Havasupai Creek
AZ
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Acidic groundwater dissolves
soluble rock at or just below
water table  undecorated
caves
Water table drops
Speleothems form in zone of
aeration  decorated caves
◦ Composed of dripstone (travertine)
◦ Include stalactites (hang from
ceiling) and stalagmites (form on
floor)
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Landscapes largely
shaped by
dissolving power of
groundwater
Common features
include…
◦ Irregular terrain
◦ Sinkhole or sinks
◦ Lack of surface
drainage (streams) 
sinking and emerging
streams
◦ Towers
Karst of
western
United States
is mostly
inactive; it
formed 1000s
of years ago
when the
climate was
wetter.