Groundwater
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Groundwater
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Aquiclude , Aquitard, Aquifuge k
Aquifer –k
Artesian Aquifer > ‹ –k
Cone of Depression » ^
Confined Aquifer ł –k
Darcy's Law r K SDischarge ‹ @
Dripstone @ ¡
Flowstone ‹ ¡
Hydraulic Gradient ^
Karst Topography ¡ * ˛
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Percolation V łPermeability V ł §
Porosity W
Recharge 6 Y
Replacement ⁄
Saturated Zone « k Sinkhole
Spring ¤
Unconfined Aquifer > –k
Water Table _
Zone of Aeration ; ¥
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0.000811,470aaaa
0.00011,120ØØØØ
0.00022,120^ ¿^ ¿^ ¿^ ¿
0.00112,900 ; ; ; ;
0.013176,400º rº rº rº r
0.022300,000 _ ˙ _ ˙ _ ˙ _ ˙
0.00116,500†F † F † F † F
1.723,400,000 1.74
24,064,000˙ ˙ ¿˙ ˙ ¿˙ ˙ ¿˙ ˙ ¿
96.51,338,000,000G'G'G'G'
-Ø - Ø - Ø - Ø OOOO HHHH km3
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Water In The Ground Groundwater is the water below the ground
surface occupying the pore spaces in rocks andsoils.
Groundwater is present everywhere beneath
land surface and ocean bottom.
Most ground water originates from precipitationand surface water.
Groundwater is always in motion.
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Water Table
Water table: the undulating surface
where pore water pressure =
atmospheric pressure
In fine-grained sediment, a narrowfringe immediately above the
water table is kept saturated by
capillary attraction.
In humid regions, the water table
is a subdued imitation of the
land surface above it.
Water table varies with
precipitation, infiltrtation rate,
evapotranspiration, and
groundwater flow rate.
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Porosity
Porosity is the ratio ofpore volume to totalvolume of a soil or rock .
open porosity or
effective porosity is theratio of accessiblepore volume to totalvolume .
Porosity determines the
amount of water that a
given volume of soil or
rock can contain.
Well-sorted
sediments
Poorly-sorted
sediments
Reduction of
porosity bycementation
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Rock Porosity
The porosity of a sedimentary rock is affected by
several factors:
Sizes and shapes of the mineral grains.
Compactness of their arrangement.
Weight of overlying rock or soil.
Extent to which the pores become filled with the
cement that holds the particles together.
The porosity of intrusive and metamorphic rocksgenerally is low.
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Groundwater Recharge and Discharge
Recharge is the process by which groundwater is replenished.
Discharge is the process by which groundwater reaches and flows to the
surface.
An area of the landscape where precipitation seeps downward and reaches an
aquifer is called a recharge area.
The water moves slowly toward discharge areas where groundwater flows tostreams, lakes, ponds, swamps or ocean.
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Darcy’s Law
K(h1-h2)V = ---------------
L
K is the hydraulic conductivity;
h1-h2 is the difference in altitude;
L is the horizontal distancebetween two points;
V is flow velocity;
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Spring A spring is a flow of groundwater emerging
naturally at the ground surface. Small springs are found in all kinds of rocks,
but almost all large springs flow from lava
flows, limestone, or gravel.
A change in permeability, a body of less
permeable rock adjacent to a permeable one,is a common explanation for the location of springs.
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Contacts between a porous
limestone and an impermeable shale.
Contact between a sand unitand an impermeable clay.
Contact between a jointed lava flow
and an impermeable mudstone.
Trace of a fault intersectingthe land surface.
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• ( L-( L-( L-( L- 1500 ß ß ß ß > K e — > K e — > K e — > K e —
• . » ‚ e vZL. » ‚ e vZL. » ‚ e vZL. » ‚ e vZL 1957 ¢ } § ¢ } § ¢ } § ¢ } §
` 1237 t K rt K rt K rt K r 4000 km
• 1966 ¢ } — ‹ ˘ /r¢ } — ‹ ˘ /r¢ } — ‹ ˘ /r¢ } — ‹ ˘ /r79999 m3 ???? 2001
@ — ¢ } ß -@ — ¢ } ß -@ — ¢ } ß -@ — ¢ } ß - 446 ‹ ˘ ? ‹ ˘ ? ‹ ˘ ? ‹ ˘ ?1.79999
m3
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Aquifer
An aquifer is a body of highly permeable rock or regolith
that can store water and yield sufficient quantities to supply
wells.
Gravel and sand generally are good aquifers. Many
sandstones and limestones are also good aquifers.
Aquifers are of two types:
– Confined (bounded by confining beds).
– Unconfined (an aquifer without overlain).
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The High Plains aquifer
An unconfined aquifer which lies at shallow depths beneath
the High Plains of the United States.
About 30 percent of the groundwater used for
irrigation in the United States is obtained from the
High Plains aquifer.
In parts of Kansas, New Mexico, and Texas, the water
table has dropped so much over the past half century
that the thickness of the saturated zone has declined bymore than 50 percent.
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The Dakota Aquifer
The Dakota aquifer system
in South Dakota provides agood example of a confinedaquifer.
Water that percolates into aconfined aquifer flowsdownward under the pull ofgravity.
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Artesian Aquifer An artesian aquifer is confined by rock layers that
restrict water flow, resulting in an aquifer that is
"pressurized." Water is virtually squeezed to thepressure level above the ground surface.
Water in an artesian aquifer could rise to the sameheight as the water table in the recharge area.
The well installed in an artesian aquifer is called an
artesian well.
A freely flowing spring supplied by an artesian
aquifer is an artesian spring.
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A Flowing (Artesian) Well
Maybe you've heard advertisements by water companies
wanting to sell you "artesian-well drinking water." Is this
water different from other bottled water taken from springs?
The water may not be different, but it comes to the earth's
surface a bit differently. A flowing well has water thatcomes up to the surface because of internal pressure in the
underground aquifer containing the water. An artesian
aquifer is confined by rock layers that restrict water flow,
resulting in an aquifer that is "pressurized." Water isvirtually squeezed to the surface by underground pressure.
This picture shows how strong artesian pressure can be!
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The Floridian Aquifer
A complex regional aquifer system in which both
confined and unconfined units are present, and in
which water locally reaches the surface by anartesian flow.
The aquifer system is restricted mainly to middleand late Tertiary limestones.
The age of groundwater in the well farthest from therecharge area has been determined by radiocarbon
dating : at least 19,000 years.
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Changes in the water table as a Result of Pumping
• When water is pumped from a new well, the rate of withdrawalinitially exceeds the rate of local groundwater flow.
• This imbalance in flow rates creates a conical depression in thewater table immediately surrounding the well called a cone ofdepression.
• The locally steepened slope of the water table increases the flow ofwater to the well.
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Mechanism of Land Subsidence
The weight of overlying sediments is supported by the
pore water pressure and the effective stress in an aquifer.
When groundwater is withdrawn, the pore waterpressure is reduced, and meanwhile, the effective stress
between mineral grains is increased, causing the
compression of sediments. As a result, the land surfacesubsides.
The amount of subsidence depends on:
How much the water pressure is reduced.
The thickness and compressibility of the aquifer.
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San Joaquin Valley kkkk
1. ” ˘ I m p† łˇ ” ˘ I m p† łˇ ” ˘ I m p† łˇ ” ˘ I m p† ł
2. T q k T q kT q kT q k H ˘ /r H ˘ /r H ˘ /r H ˘ /r 9
3. > d> d> d> d œœœœ 6 6 6 6
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10 m–
o C o C o C o C œ 1925 ?1977 9m–
œ ‚ 40 ' k –
˝ o ˝ o ˝ o ˝ o œ œ ›q ¢ Gq ek > A
‘|‘ |‘ |‘ | œ ‘ G KG s –
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Leaning Tower of Pisa
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Groundwater Contamination
Water circulating through sulfur-rich rocks may contain
dissolved hydrogen sulfide (H2S) that has the disagreeable odorof rotten eggs.
If the contaminated water percolates through sand or permeable
sandstone, it become purified within short distances. Sand
promotes purification by:
Mechanically filtering out bacteria.
Oxidizing bacteria so they are rendered harmless.
Placing bacteria in contact with other organisms that
consume them.
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Contamination of Groundwater by Toxic WastesMunicipal and industrial wastes, Pesticides & herbicides,
Landfills and underground storage tanks
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Petrified wood is produced by thereplacement process.
ReplacementThe process by whicha fluid dissolvesmatter already presentand at the same timedeposits from solutionan equal volume of a
different substance
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Dissolution
Of all the rocks, the carbonate rocks (limestone,
dolostone, and marble) are among the most
readily attacked by the dissolution and hydrolysis.
The weathering attack occurs mainly along joints
and other partings in the carbonate bedrock. In temperate regions with a high rainfall, a high
water table, and a nearly continuous cover of
vegetation, carbonate landscapes are beinglowered at average rates of up to 0.01 cm/yr.
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Cave Formation Caves are resulted from the dissolution of
carbonate rock by circulating groundwater.
Limestone caves are generally believed toresult from dissolution by carbonic acid.
Some caves, like Carlsbad, may haveresulted from dissolution by sulfuric acid.
The rate of cave formation is related to the rateof dissolution.
A fully developed cave system may take
10,000 to 1 million years to produce.
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:………… pH ‰‰‰‰
CO32 -
+ H+ HCO3-
CaCO3 Ca2+
+ CO32 –
, ^ e^ e^ e^ ePH vvvv, CO3
2 – oB `oB `oB `oB ` HCO3- , l l l lCaCO3 >>>>
,ˆ ’x 3˝ˆ ’x 3˝ˆ ’x 3˝ˆ ’x 3˝ , +@ < y +@ < y +@ < y +@ < y ,# # # # • • • • , PH vZ /vZ /vZ /vZ /, HCO3-oB `oB `oB `oB ` CO3
2 – , l l l lCaCO3 – –– –
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Cave Deposits
Clay and silt, originally present as
impurities in limestone, gradually
concentrated as the rock was
dissolved.Flowstone is precipitated from flowing
water.
Dripstone is precipitated from dripping
water.
Stalactites are icicle-like forms of
dripstone hanging from the the
ceilings of caves.Stalagmites are blunt mounds
projecting upward from cave floor.
Columns are stalactites joined with
stalagmites.
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, P P P P
•¤ B c ˙ m•¤ B c ˙ m•¤ B c ˙ m•¤ B c ˙ m,))))¸ 2 ˆ & W ¸ 2 ˆ & W ¸ 2 ˆ & W ¸ 2 ˆ & W
, PH œ ˇœ ˇœ ˇœ ˇ , … … … …HCO3- a a a a CO32 – ,
CaCO3 ¤‚¤‚¤‚¤‚ – –– –
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The cavern formed in the saturated zone when the water table layat the former higher level. Uplift of the region caused streams todeepen their valleys. The water table then lowered in response tovalley deepening, leaving the cavern above the lower
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Sinkholes: large dissolution cavities
Some sinkholes are formed when caveshave collapsed, others are formed from
dissolution.
Many sinkholes are located at the
intersection of joints. New sinkholes are forming because of the
lowering of the water table due toexcessive pumping.
The floors of sinkholes lie below the water
table.
K t T h
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Karst Topography
Karst topography is a landscape in which caves and
sinkholes are so numerous that they form a peculiar
topography characterized by:Many small, closed basins.
Disrupted drainage pattern.
Streams disappearing into the ground.
Streams reappearing as large springs.
Karst topography was first described in the Karst region of
the former Yugoslavia, extending from Slovenia to
Montenegro.
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Hard Water and Soft Water
Hard water: water containing calciumand magnesium bicarbonatesdissolved from carbonate rock thatprevent the formation of lather with
soap. . soft water: water contains little
dissolved matter and no appreciable
calcium.
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Homework: > ’> ’> ’> ’
Hydraulic conductivity
Karst topography
Water table Artesian aquifer
Zone of aeration
*N- … ¢ P b p‹ ?N- … ¢ P b p‹ ?N- … ¢ P b p‹ ?N- … ¢ P b p‹ ? 121;;;;