4 Geology and Groundwater

1
4Geology 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

2
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

3
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

4
P atm
Pgt atm
5
UNCONFINED AQUIFER
6
Confining beds
7
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.

8
ARTESIAN WELL
9
SPRING

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

10
Potentiometric surface,water table maps
11
Perched aquifer

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

Water table
Unconfined aquifer
12
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

13
(No Transcript)
14
(No Transcript)
15
4.2 Transmissive and Storage Properties

• Two most important aquifer characteristics
• Ability to store groundwater
• 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

16
Transmissivity

• T Kb
• T Transmissivity, units L2/T e.g., m2/d
• K Hydraulic conductivity
• b aquifer thickness
• Darcys Law with T instead of K

17
(No Transcript)
18
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 2015 300 m2/d

19
Storativity (Coefficient of Storage) and Specific
Storage

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

20
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

21
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

22
(No Transcript)
23
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 aquifer
• ?p vertical compressibility of rock matrix
• ?w compressibility of water (4.8x10-10 m2/N)

24
(No Transcript)
25

• Example 4.2

26
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

27
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

28
(No Transcript)
29

• Example 4.3

30
Geology and Hydraulic properties

• Hydraulic properties of geologic material are
  related to rock type
• material types to be examined
• Unconsolidated sediments
• Semi-unconsolidated sediments
• Carbonate rocks
• Sandstone rocks
• Volcanic and other crystalline rocks

31
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

32
Unconsolidated

• K depends on
• grain size,
• mineral composition,
• Sorting
• K (clay) lt 3 x 10-4 m/d
• K (coarse gravel) 100 m/d
• K (well sorted) gt K (poorly sorted)
• Most aquifer in western Saudi Arabia are of this
  type

33
(No Transcript)
34
(No Transcript)
35

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

36
(No Transcript)
37
(No Transcript)
38

• 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

39
(No Transcript)
40
(No Transcript)
41
4.4 Hydraulic Properties of Granular and
Crystalline Media

• Do rocks keep original porosity and permeability?
• What geologic processes change hydraulic
  properties?
• Original porosity gt30 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

42
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

43
4.5 Hydraulic Properties of fractured Media
44
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

45
(No Transcript)
46
4.5 Hydraulic Properties of fractured Media
Snow, 1968
Example 4.4