Transcript 4-Geology and Groundwater

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
13
14
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: [L2/T] e.g., m2/d
K: Hydraulic conductivity
b: aquifer thickness
Darcy’s Law with T instead of K
16
17
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 m2/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) =L3/(L2 * L) = m3/m3
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
22
Storage in Confined Aquifers
SS in a confined aquifer reflects storage coming from
compression of granular matrix and expansion of
water
S s   w g (  P  n w)
w:
g:
n:
p:
w:
density of water
gravitational constant (9.81 m/s2)
porosity of aquifer
vertical compressibility of rock matrix
compressibility of water (4.8x10-10 m2/N)
23
24
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 + bSs
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
Vd

VT
Sr
Vr

VT
Sy + Sr = n
27
28
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 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
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
33
34
• Blanket sand and gravel aquifers
– E.g., fluvial deposits (alluvial aquifer):
long, narrow, thin aquifers
– Braided rivers
– Meandering rivers
– Alluvial fans
• Basin-Fill aquifers
35
36
37
• 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
39
40
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
45
4.5 Hydraulic Properties of fractured Media
Snow, 1968
3
b
k 
12s
b

s
Example 4.4
46