4 Geology and Groundwater Introduction Geology complexities are

4 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 1

4. 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 Beds units of low permeability that bound an aquifer – Examples are unfractured igneous rock, metamorphic rock, and shale, or unconsolidated sediments such as clays 2

Types of aquifers • Confined aquifer (artesian): bounded by low-permeability beds on both sides (above and below) • Unconfined (water-table): water table forms upper boundary 3

P= atm P> atm 4

UNCONFINED AQUIFER 5

Confining beds 6

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. 7

ARTESIAN WELL 8

SPRING A place where ground water naturally comes to the surface at the intersection of the water table and land surface. 9

Potentiometric surface, water table maps 10

Perched aquifer Unconfined aquifer developed above regional water table (lens) caused by a low -permeability layer Water table Unconfined aquifer 11

Types of confining beds Aquifuge, Aquitard, Aquiclude Not 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 12

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4. 2 Transmissive and Storage Properties • Two most important aquifer characteristics: 1. Ability to store groundwater 2. 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 15

Transmissivity T = Kb T: Transmissivity, units: [L 2/T] e. g. , m 2/d K: Hydraulic conductivity b: aquifer thickness Darcy’s Law with T instead of K 16

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example • 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 m 2/d 18

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 19

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) =L 3/(L 2 * L) = m 3/m 3 20

Storativity 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 21

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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: g: n: p: w: density of water gravitational constant (9. 81 m/s 2) porosity of aquifer vertical compressibility of rock matrix compressibility of water (4. 8 x 10 -10 m 2/N) 23

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Example 4. 2 25

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 drainage S = Sy + b. Ss 26

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 27

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Example 4. 3 29

Geology and Hydraulic properties • Hydraulic properties of geologic material are related to rock type material types to be examined: 1. 2. 3. 4. 5. Unconsolidated sediments Semi-unconsolidated sediments Carbonate rocks Sandstone rocks Volcanic and other crystalline rocks 30

Aquifers in unconsolidated sediments • Blanket sand gravel aquifers (alluvial) – Medium to coarse sand gravel • Basin-fill aquifers (valley-fill, wadi-fill) – Sand gravel filling depressions formed by faulting or erosion • Aquifers in these materials are mainly unconfined 31

Unconsolidated K depends on: – grain size, – mineral composition, – Sorting K (clay) < 3 x 10 -4 m/d K (coarse gravel) = 100 m/d K (well sorted) > K (poorly sorted) Most aquifer in western Saudi Arabia are of this type 32

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• Blanket sand gravel aquifers – E. g. , fluvial deposits (alluvial aquifer): long, narrow, thin aquifers – Braided rivers – Meandering rivers – Alluvial fans • Basin-Fill aquifers 35

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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 38

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4. 4 Hydraulic Properties of Granular and Crystalline 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 41

4. 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 42

4. 5 Hydraulic Properties of fractured Media 43

4. 5 Hydraulic Properties of fractured Media • 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 44

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4. 5 Hydraulic Properties of fractured Media Snow, 1968 Example 4. 4 46