groundwater

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groundwater
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hydrologic cycle
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global distribution of water
need 400 million gallons for basic biological
needs per day for US need 1.8 trillion gallons
per day for other purposes in US ---domestic,
agriculture, industry
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water use in the US from 1950-2000 (ground water
in red)
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ground water withdrawal in 2000 from selected
states (data from US Geological Survey)
Alabama 440 Arkansas 6,920 California
15,400 Colorado 2,320 Florida 5,020 Kansas 3,790 M
issouri 1,780 Nebraska 7,860 Texas 8,970 (in
millions of gallons per day)
Arkansas use is quite high--state dependent on
ground water
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groundwater
lies beneath the surface
fills pore (empty) spaces and fractures in rocks
is resupplied by infiltration/percolation
is generally cleaner than surface water
is accessed by wells
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ground water definitions

• porosity

percentage of rock by volume that consists of
voids or openings
--measure of the rocks ability to hold water--
permeability
capacity of a rock to transmit fluid through pore
space/fractures
--rock is permeable if fluid flows easily--
same porosity, but different permeability
importance of connectivity of pore space
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loose sand has 30-50 porosity
compacted sand has 10-20 porosity
crystalline rock (e.g. granite) has 0-5 porosity
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water table
top of saturated zone
zone in subsurface where all pore space is
filled with water
zone in subsurface above water table that is
unsaturated
water level of most lakes and rivers corresponds
to local water table
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perched water table water table above zone of
saturation isolated
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water table
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ground water movement
movement through pores and fractures is slow
(cm/m per day) (flows in limestone caverns may be
much higher)
velocity depends on slope of the water table and
permeability of rock
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groundwater movement
hydraulic head (h) elevation pressure
no pressure at points A and B on water table
--on water table so no water is above them
elevation A gt elevation B hydraulic head of A gt B
h.g. ?h/L
water flows from zones of high high hydraulic
gradient to low hydraulic gradient
h at F gt G (??elevation of F, G lt ??pressure of
F, G
h at C gt D (greater pressure at C)
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remember that water flows to lower hydraulic
gradient
A
B
black lines show regions of equal hydraulic head
water flows perpendicular to the black lines and
in the direction of downward slope of water table
blue dots at A and B have higher hydraulic
gradient than associated red dots (more pressure)
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groundwater recharge
replenishment of groundwater by precipitation
water infiltrates from surface
infiltration increases cracks and pore
space vegetation (roots enhance infiltration),
shallow slopes (more time for water to sink),
greater precipitation (if fine particles do not
wash into pores)
water flows down through soil and rock in
response to gravity
some water emerges as groundwater discharge
some water remains stored in ground for long
periods of time
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groundwater through what does it flow?
aquifer
body of saturated rock or sediment through which
water can move easily
examples sandstone, conglomerate, sand, gravel,
fractured rock
aquitard
body of rock with low or no permeability,
which retards, or slows, flow of water across it
examples shale, clay, unfractured crystalline
rocks
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aquifers confined and unconfined
is impermeable
is filled with water
is separated from suface by an aquitard
has a water table that rises and falls during
wet and dry seasons
recharges quickly through precipitation,
has rapid movement of groundwater through it
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well
hole drilled or dug into ground to access water
in aquifer
in an unconfined aquifer, water level before
pumping is the water table
pumping draws water from pore space and may cause
drawdown
drawdown results in cone of depression (lowering
of water table)
if cone of depression grows too large, water
table may fall below well bottom
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example Mississippi Alluvial Aquifer system
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central Arkansas
map shows growth of region where drawdown
has produced a cone of depression at a depth of
110 feet
Stuttgart
when pumping began in 1930s, depth to water
table was about 50 feet
DeWitt
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artesian well
well in confined aquifer yields water that rises
above surface
water rises to water table of same aquifer where
it is unconfined
rocks must be tilted to do this
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artesian well
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how does an artesian well work?
potentiometric surface
level to which water would rise in a pipe in
absence of friction
consider water tower and building (operates like
Artesian well)
live up here low water pressure
potentiometric surface
live down here no problem
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springs
where water flows naturally from rock or sediment
water table is on the surface
dependent on precipitation
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spring along the Buffalo River
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streams gaining vs. losing
gaining stream
gets water from saturated zone and top of stream
is water table
losing stream
loses water to saturated zone and creates mound
on water table
continued loss of water results in dry stream
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groundwater in limestone caves, karst, sinkholes
caves
naturally formed underground chambers
acidic groundwater dissolves limestone (rainwater
becomes acidic as it passes through air and soil)
sinkholes
caves near surface that collapse
karst
landscape characterized by dissolution features
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caves
dripstones (speleothems) stalactites
(ceiling) stalagmites (floor)
most caves in limestone --other soluble rocks
also form caves-- Alabaster Caves, Oklahoma
(gypsum)
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sinkholes
collapse of near surface, underground cavities
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sinkholes may fill with water -- provide
resources for humans
Mayan civilization dependent upon them
common in Florida
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Arecibo Observatory is in giant sinkhole in
Puerto Rico
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sinkhole lakes
sinkhole lakes can disappear when debris that
clogs lower opening becomes unclogged
2002
2003
Lake Jackson, Tallahassee, Florida
dried up and re-filled at least 4 times in 20th
century
lake drains when material plugging hole in
subsurface is removed
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karst
rainwater infiltration
limestone dissolution
groundwater acidification
voids in limestone
precipitation of calcite (dissolved limestone)
lowering of water table
cave development
dripstone development
very sensitive to water table
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karst features
disappearing streams
natural bridges
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karst topography
southern China
north coast of Puerto Rico
Ozarks are karst
Jamaica (radar satellite image)
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groundwater contamination
infiltration of contaminants into water table
pesticides/fertilizers
landfill pollutants
industrial chemicals
heavy metals
bacteria/parasites from sewage
oil and gas
pharmaceuticals
animal waste
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groundwater contamination
expensive and difficult to clean up
contamination plume moves and grows
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groundwater contamination
different materials lead to different problems
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groundwater contamination
septic tanks
plume cleaned as it flows slowly through
sandstone
well produces clean water
plume flows quickly through fractured crystalline
rock
well is contaminated
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groundwater contamination
saltwater intrusion
created by over-pumping and movement of salt
water into aquifer
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contamination in karst
difficult to know where water goes
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balancing withdrawal and recharge
pumping gt recharge lowers water table (cone of
depression)
eventually yields dry wells and/or subsidence
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subsidence
sinking of ground surface in response to removal
of water
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collapse of confined aquifer from removal of water
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hot water underground
hot springs
water temperature is warmer than that of human
body
water heated by nearby magma or by deep
circulation to deep crust, which is warmer
hot water less dense than cool and rises on its
own
geysers
hot springs that periodically erupt hot water
and minerals
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geothermal energy
produced using natural steam or superheated water
no CO2 or acid rain is produced (clean
energy)
directly heats buildings
some toxic gases are emitted (e.g. sulfur)
superheated water can be corrosive to pipes
Iceland is a leader