Ch15 Groundwater

8/8/2019 Ch15 Groundwater

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Groundwater



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 ¡ * ˛



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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0.00022,120^ ¿^ ¿^ ¿^ ¿

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



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.



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.



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



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.



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.





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;



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.



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.



• ( 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



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).



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.



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.



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.



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!



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.



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.



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.



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



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 –



Leaning Tower of Pisa



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.



Contamination of Groundwater by Toxic WastesMunicipal and industrial wastes, Pesticides & herbicides,

Landfills and underground storage tanks







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



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.



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.



:………… pH ‰‰‰‰

CO32 -

+ H+ HCO3-

CaCO3 Ca2+

+ CO32 –

, ^ e^ e^ e^ ePH vvvv, CO3

2 – oB òB òB òB ` 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 òB òB òB ` CO3

2 – , l l l lCaCO3 – –– –





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.



, 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 ¤‚¤‚¤‚¤‚ – –– –



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



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



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.



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.



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;;;;

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Ch15 Groundwater