01Properties of Aquifers

7/29/2019 01Properties of Aquifers

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Properties ofProperties ofAquifersAquifers

HsinHsin--yu Shanyu Shan

Department of CivilDepartment of CivilEngineeringEngineering

National Chiao TungNational Chiao TungUniversityUniversity


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Porosity of Earth MaterialsPorosity of Earth Materials

n porosity

Vv volume of void space in a unitvolume of earth material (L3)

V unit volume of earth material,including both voids and solids (L3)

V

V

nv

=


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Pores and PermeabilityPores and Permeability


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Engineering Grain SizeEngineering Grain Size

ClassificationClassification

305Boulder

Si z e Ra n g e ( m m ) N a m e


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Porosity Range ofPorosity Range of

SedimentsSediments

0.3-0.6Clay

0.35-0.5Silt

0.1-0.2Glacial till

0.2-0.35Sand and

gravel, mixed

0.25-0.5Well-sortedsand or gravel


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Effective PorosityEffective Porosity ne Porosity available for fluid flow


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Porosity of SedimentaryPorosity of Sedimentary

RocksRocks Primary porosity

Pores between grains

Secondary porosity

Fractures


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Primary porosity

Gravel


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Secondary porosity

Limestone


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Specific YieldSpecific Yield Sp e c i f i c Y i e l d Sy the ratio of the

volume of water that drains from asaturated geomaterial owing to theattraction of gravity to the totalvolume of the geomaterial


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Specific RetentionSpecific Retention Sp e c i f i c r e t e n t i o n Sr the ratio of

the volume of water a geomaterialcan retain against gravity drainageto the total volume of thegeomaterial.

n = Sy+ Sr


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Fig. 4.9 Pendular water clinging to spheres owing tosurface tension. Gravity attraction is pulling the

water downward


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Specific YieldSpecific Yield

221226Coarse gravel231326Medium gravel

252135Fine gravel

252035Gravelly sand

272035Coarse sand

261532Medium sand211028Fine sand

18319Silt

7312Sandy clay

205Clay

Average

Minimum

Maximum

Material


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Fig. 4.10 Specific yield of sediments from the Humboldt

River Valley of Nevada as a function of the median grainsize


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hA

hA-hB

hB

A B


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Bernoulli EquationBernoulli Equation

constant2

2

=++=

gPz

gvh

Bernoulli equation h hydraulic head (L, J/N)

First term velocity head (ignored inground water flow)

Second term elevation head

Third term

pressure head


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Hydraulic HeadHydraulic Head Total head

Elevation head Pressure head

Velocity head


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Hydraulic ConductivityHydraulic Conductivity Khydraulic conductivity (L/T)

A function of properties of bothporous media and the fluid passing

through it

k intrinsic permeability (L2)

A function of porous media only


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gkK=

density of fluid g acceleration of gravity

dynamic viscosity of fluid

1 cP = 0.01 dyn

s/cm2

1 darcy = 9.87 x 10-9 cm2


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Hydraulic ConductivityHydraulic Conductivity

iJHydraulicgradient

KkIntrinsicpermeability

kKHydraulic

conductivity

Geotechnica

l

Hydrogeolog

y

Ranges of HydraulicRanges of Hydraulic


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Ranges of HydraulicRanges of Hydraulic

ConductivityConductivity

10-2 110 103Well-sorted gravel

10-3 10-11 102Well-sorted sands,glacial outwash

10

-5

10

-3

10

-2

1Silty sands, finesands

10-6 10-410-3 10-1Silt, sandy silts,clayey sands, till

10-9 10-610-6 10-3Clay

Hydraulic

Conductivity (cm/s)

Intrinsic

Permeability(darcy)

Material

Measurement ofMeasurement of KK in thein the


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Measurement ofMeasurement ofKKin thein the

LabLab Constant Head Method

Falling Head Method Rising/Falling Head Method

Constant Rate of Flow


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Constant-Head Method


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Falling Head Method


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Types ofTypes ofKKTest ApparatusTest Apparatus Fixed-Wall Permeameter

No control of effective stress

Flexible-Wall Permeameter

Can control effective stress Consolidation Cell

Can control vertical effective stress

Other

Factors AffectingFactors Affecting


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Factors AffectingFactors Affecting

Measurement ofMeasurement ofKK Effective stress

Hydraulic gradient Volume of Flow


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Termination CriteriaTermination Criteria

4 5 6


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Head/Tail WaterPressure

Confining Pressure

Air Pressure

Top Cap

Porous Disc

Filter PaperWire Screen

Geotextile

Soil

Sample

Pesdestal

Burrete

Reser-

voir

S

40 0 0

Filter Paper

Porous Disc

0

1

2

3 7

8

9

10

Kg/cm

2

Vacuum

Pressure control and the flexible-wall cell setup


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Homogeneity and IsotropyHomogeneity and Isotropy


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Fig. 4.26 Grain shape and orientation can affect theisotropy or anisotropy of a sediment


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Fig. 4.27 Anisotropy of fractured rock units due

to directional nature of fracturing


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Fig. 4.28 Hetrogeneous formation consisting ofthree layers of differing hydraulic conductivity


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TransmissivityTransmissivity Amount of water that can be transmitted

horizontally through a unit width by thefull saturated thickness of the aquiferunder a hydraulic gradient of 1.

T=K B

T: transmissivity (L2/T or m2/d)

K: hydraulic conductivity (L/T)

B: saturated thickness of the aquifer (L or m)


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StorativityStorativity Storativity (S) or Storage coefficient

The volume of water that apermeable unit will absorb or expelfrom storage per unit surface areaper unit change in head.


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Specific StorageSpecific Storage Specific storage (Ss) or Elastic

storage coefficient The amount of water per unit volume

of a saturated formation that isstored or expelled from storageowing to compressibility of the

mineral skeleton and the pore waterunit change in head.


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)( ngS ws +=Specific StorageSpecific Storage

wdensity of water

g the acceleration of gravity

compressibility of aquifer skeleton

(1/(M/LT2)) compressibility of water (1/(M/LT2))

n porosity (L3

/L3

)


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Confined aquifer

S = B Ss

Unconfined aquifer

S = Sy + h Ss


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Water TableWater Table The surface at which pore water pressure

is equal to atmospheric pressure

Capillary fringe

Saturated zone

Ground surface

Hydroscopic waterpressure less than

atmospheric

Unsaturated zone

Pore water

pressure equal to

atmosphericPore water pressure

greater than

atmospheric


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Definition of AquiferDefinition of Aquifer A a q u i f e r is a geologic unit that can

store and transmit water at ratesfast enough to supply reasonableamounts to wells.


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Confining LayerConfining Layer A co n f i n i n g l a y e r is a geologic unit

having little or not intrinsicpermeability less than about 10-2

darcy.


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Aquifuge is an absolutely impermeableunit that will not transmit any water

Aquiclude is a formation that has very lowhydraulic conductivity, which hardly

transmit water Aquitardis a layer of low permeability that

can store ground water and also transmit

it slowly from one aquifer to another; alsoknow as leaky confining layer.


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Types of AquifersTypes of Aquifers Un f o n f i n e d a q u i f e r (Water-table

aquifer) Co n f i n e d a q u i f e r (Artesian aquifer)


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Unconfined AquiferUnconfined Aquifer An aquifer that is close to the ground

surface, with continuous layers ofmaterials of high intrinsic permeabilityextending from the land surface to the

base of the aquifer. Recharge from downward seepage

through the unsaturated zone, lateral

ground water flow, or upward seepagefrom underlying strata.


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Unconfined Aquifer


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Confined Aquifer


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Confined AquiferConfined Aquifer

An aquifer that are overlain by a confininglayer.

Recharge occurs in recharge area, wherethe aquifer crops out, or by slow

downward leakage through a leakyconfining layer.

Potentiometric surface is the surface

representative of the level to which waterwill rise in a well cased to the aquifer.

Confined Aquifer


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Confined Aquifer

Confined Aquifer


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Confined Aquifer

Perched Aquifer


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Perched AquiferPerched Aquifer

Water intercepted by a layer of low-

permeability material in morepermeable materials andaccumulated on top of the low-

permeability layer. Occurs above the main water table.


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Definitions


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DefinitionsDefinitions


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Vadose ZoneVadose Zone


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Vadose ZoneVadose Zone

The upper layer of the earth that

contain a three-phase system of solid,liquid, and gaseous material.

Also called the zone of aeration or

unsaturated zone.

Zone of AerationZone of Aeration


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Zone of AerationZone of Aeration

The zone where the soil moisture is

under tension

Capillary FringeCapillary Fringe


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Capillary FringeCapillary Fringe

Capillary pores in the zone ofaeration draw up water from thezone of saturation beneath the watertable.

In very fine-grained soils, thiscapillary fringe can saturate thesoil above the water table

Tensiometer reading is negative

Capillary fringe is a part of vadose

zone

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01Properties of Aquifers

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