1. 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

2. 4.1 Aquifers and Confining Beds
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
P> 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

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

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. Storage If water is removed from a groundwater reservoir:
Hydraulic head decreases - water level in wells falls
Fluid pressure decreases in the aquifer.
Porosity decreases as the granular skeleton contracts.
The volume of water increases as the water compresses.

19. Storativity (coefficient of storage)
Storativity (S): the volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head.
Storativity is a dimensionless property
S = L3/(L2 * L)

21. Storativity (coefficient of storage)
The equations for estimating storativity are different for confined and unconfined aquifers
In an unconfined aquifer, the height of the hydraulic head is shown by the water table.
Thus, a change in hydraulic head results in either increasing or decreasing saturation of the aquifer. A large change in hydraulic head results in a large change in the volume of water in the aquifer.

22. Storativity, contd.
In a confined aquifer, the height of the hydraulic head is given by the potentiometric surface, which is usually above the upper surface of the aquifer.
Water can be added or removed from the aquifer without affecting the saturation of the aquifer.
The potentiometric surface can rise and fall, but as long as it stays above the upper surface of the aquifer, there is no change in saturation in the aquifer.
In this case, the change in hydraulic head is being accomplished by a change in pressure.

24. Storativity, contd.
The amount of water that is absorbed or expelled from the aquifer is determined by the changes in water volume and porosity that result from the change in pressure.
Because the changes in water volume and porosity are relatively small, in a confined aquifer a large change in hydraulic head does not result in a large change in storage.

25. Specific storage
The amount of water per unit volume of a saturated aquifer that is absorbed or expelled due to changes in the compression of the fluid and medium caused by a change in hydraulic head is called - specific storage (Ss).
Ss has units of [1/L]

26. In a confined aquifer, the equation for storativity becomes:
where b is the thickness of the aquifer.
In an unconfined aquifer, a change in hydraulic head results in both a change in pressure in the saturated portion of the aquifer, as well as a change in the thickness of the saturated zone. In this case, storativity equals:
Sy is the specific yield of the aquifer - the amount of water per unit volume that will drain from an aquifer under the influence of gravity

27. Specific yield is usually several orders of magnitude larger than h x Ss so that in all but very fine grained units the h xSs component is ignored and storativity is considered equivalent to specific yield.
The volume of water that will be drained from or added to an aquifer as the head is raised or lowered:
V = S A h
A: area overlying the aquifer

28. Specific yield and Specific Retention
Specific yield: water released from aquifer by gravity drainage
Sy =Vd/VT
Specific Retention: water retained in aquifer due to molecular attraction
Sr = Vr/VT
Sy + Sr = n

29. Example Calculate Sy, Sr, porosity.
A fully saturated soil sample weighs 105 g. After the sample is drained by gravity, the weight of the sample is 85 g. After the sample is oven-dried, the sample weighs 80 g. The bulk density of the wet soil is 1.65 g/cm3, and the density of water is 1.0 g/cm3.
Calculate Sy, Sr, porosity.

31. Example 4.2
A confined aquifer is composed of dense, sandy gravel with a thickness of 100 m and a porosity of 20%.
Estimate the likely range for specific storage and storativity.
For a total head drop of 100 m in an area of 1 x109 m2, how much water is released from the storage?

32. 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

33. 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

34. 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

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

40. Carbonate-Rock aquifers
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

43. 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

44. 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

45. 4.5 Hydraulic Properties of fractured Media

46. 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

48. 4.5 Hydraulic Properties of fractured Media
Snow, 1968
Example 4.4