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

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

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

P= atm P> atm

UNCONFINED AQUIFER

Confining beds

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.

ARTESIAN WELL

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

Potentiometric surface, water table maps

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

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

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

Transmissivity T = Kb T: Transmissivity, units: [L2/T] e.g., m2/d K: Hydraulic conductivity b: aquifer thickness

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

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.

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)

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.

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.

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.

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]

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

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

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

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.

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?

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

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

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

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

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

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

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

4.5 Hydraulic Properties of fractured Media

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

4.5 Hydraulic Properties of fractured Media Snow, 1968 Example 4.4