4 Geology and Groundwater

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44Geology and Groundwater Geology and Groundwater • Introduction

– Geology complexities are reflected in hydrogeology– Geology is the basis for any groundwater

investigation

• Topics of the chapter:– Aquifers and confining beds– Transmissive and storage properties of aquifers– Geology and hydraulic properties– Hydraulic properties of granular and crystalline

media– Hydraulic properties of fractured media

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4.1 Aquifers and Confining Beds4.1 Aquifers and Confining Beds

• Aquifer:A lithologic unit or a combination of lithologic units capable of yielding water to pumped wells or springs.

• Aquifer can cut across formations (independent of geologic units)

• Confining Bedsunits of low permeability that bound an aquifer– Examples are unfractured igneous rock, metamorphic

rock, and shale, or unconsolidated sediments such as clays

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Types of aquifersTypes of aquifers

• Confined aquifer (artesian):bounded by low-permeability beds on both sides (above and below)

• Unconfined (water-table):water table forms upper boundary

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P= atm

P> atm

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UNCONFINED AQUIFER UNCONFINED AQUIFER

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Confining bedsConfining beds

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ARTESIAN WELL ARTESIAN WELL

A well whose source of water is a confined (artesian) aquifer. The water level in artesian wells stands at some height above the water table because of the pressure (artesian pressure) of the aquifer. The level at which water stands is the potentiometric (or pressure) surface of the aquifer. If the potentiometric surface is above the land surface, the well is a flowing artesian well.

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ARTESIAN WELLARTESIAN WELL

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SPRING SPRING

A place where ground water naturally comes to the surface at the intersection of the water table and land surface.

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Potentiometric surface,Potentiometric surface,water table mapswater table maps

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Perched aquifer Perched aquifer

Unconfined aquifer developed above regional water table (lens) caused by a low-permeability layer Water table

Unconfined aquifer

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Types of confining bedsTypes of confining beds

Aquifuge, Aquitard, AquicludeNot favored (used) anymore

• Aquifuge: ultimate low-k unit, essentially impermeable. e.g., granite

• Aquitard: low-perm unit, capable of storing water, transmitting water between adjacent aquifers

• Aquiclude: confining bed

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4.2 Transmissive and Storage Properties4.2 Transmissive and Storage Properties

• Two most important aquifer characteristics:1. Ability to store groundwater2. Ability to transmit groundwater

• Transmissivity:Ease with which water moves through an aquifer(rate at which water is transmitted through a unit

width of aquifer under a unit hydraulic gradient

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TransmissivityTransmissivity

T = Kb

T: Transmissivity, units: [L2/T] e.g., m2/d K: Hydraulic conductivityb: aquifer thicknessDarcy’s Law with T instead of K

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exampleexample

• What is the transmissivity of an aquifer that has a thickness of 20 m and a hydraulic conductivity of 15 m/d?

• T = Kb = 20*15 = 300 m2/d

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Storativity (Coefficient of Storage) and Specific Storage

1. If water is removed from a confined aquifer:– Hydraulic head decreases - water level in wells falls– Fluid pressure decreases in the aquifer.– Porosity decreases as the granular skeleton

contracts (aquifer collapses slightly)– The volume of water increases

2. In unconfined aquifer, main source of water is drainage of water from pores

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Storativity (coefficient of storage)Storativity (coefficient of storage)

• Storativity (S): the volume of water that an aquifer releases from or takes into storage per unit surface area per unit change in head.

• Storativity is a dimensionless property

S = volume of water/(unit area) (unit head change) =L3/(L2 * L) = m3/m3

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Storativity contd.contd.

In confined aq. S ranges from 10-3 to 10-5

Specific Storage is the volume of water that an aquifer releases from or takes into storage per unit surface area per unit aquifer thickness per unit change in head

Ss = volume of water_______________________

(unit area)(unit thickness)(unit head change)

= 1/m

S = Ss b

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Storage in Confined Aquifers

SS in a confined aquifer reflects storage coming from compression of granular matrix and expansion of water

w: density of water

g: gravitational constant (9.81 m/s2)n: porosity of aquiferp: vertical compressibility of rock matrix

w: compressibility of water (4.8x10-10 m2/N)

)( wPws ngS

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Example 4.2

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Storage in Unconfined Aquifers

Pumping water from unconfined aquifer:– early stage: water comes from expansion of

water and compression of matrix– Later stage: water comes from gravity

drainageS = Sy + bSs

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Specific Yield and Specific Retention

• Specific yield of the aquifer is the amount of water per unit volume that will drain from an aquifer under the influence of gravity

• Specific Retention of the aquifer is the amount of water retained as a film on the surface of grains or held in small openings by molecular attraction

Sy + Sr = n

T

dy VVS

T

rr VVS

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Example 4.3

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Geology and Hydraulic propertiesGeology and Hydraulic properties

• Hydraulic properties of geologic material are related to rock type

material types to be examined:1. Unconsolidated sediments2. Semi-unconsolidated sediments3. Carbonate rocks4. Sandstone rocks5. Volcanic and other crystalline rocks

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Aquifers in unconsolidated sedimentsAquifers in unconsolidated sediments

• Blanket sand and gravel aquifers (alluvial) – Medium to coarse sand and gravel

• Basin-fill aquifers (valley-fill, wadi-fill)– Sand and gravel filling depressions formed by

faulting or erosion

• Aquifers in these materials are mainly unconfined

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Unconsolidated Unconsolidated

K depends on:– grain size, – mineral composition, – SortingK (clay) < 3 x 10-4 m/dK (coarse gravel) = 100 m/dK (well sorted) > K (poorly sorted)

Most aquifer in western Saudi Arabia are of this type

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• Blanket sand and gravel aquifers – E.g., fluvial deposits (alluvial aquifer):

long, narrow, thin aquifers– Braided rivers– Meandering rivers– Alluvial fans

• Basin-Fill aquifers

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• Aquifers in semi-consolidated Sediments

– Sandstone aquifers– Carbonate-Rock aquifers

• Enhancement of permeability and porosity by dissolution

• Karst aquifers

• Basaltic and other Volcanic-Rock aquifers

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4.4 Hydraulic Properties of Granular and 4.4 Hydraulic Properties of Granular and Crystalline MediaCrystalline Media

• Do rocks keep original porosity and permeability?

• What geologic processes change hydraulic properties?

• Original porosity >30% in many deposits– Porosity changes with depth (compaction)

– More clay, more loss of porosity

– More ss, less loss of porosity (resistance of compaction)

– Mineralogical alterations due to high T

– Cementation

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4.5 Hydraulic Properties of fractured Media4.5 Hydraulic Properties of fractured Media

• Originally impermeable rocks can be good aquifers due to fractures

• Fracture: a planar discontinuity in a rock or cohesive sediment

• Joints: macro-fracturess, no movement along plain

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• Fracture described by– Orientation

– Size

– Aperture (b): measure of width of fracture opening

• Fracture set

• Fracture density: number of fractures per volume

• Fracture frequency: number of fractures intersecting a unit length of borehole

• Fracture spacing: distance between two adjacent fractures

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3

12bks

bs

Snow, 1968

Example 4.4

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4 Geology and Groundwater