Unit 8:
Groundwater

Overview

Groundwater refers to water that is found below the Earth’s surface in pore spaces between soil and sediment particles, and in fractures within rock formations. While groundwater constitutes only about 0.6% of water within the hydrologic cycle, it makes up about 20% of the world’s freshwater supply. Hence, groundwater is an indispensable economic resource, and its overconsumption can lead to major environmental problems. Environmental concerns associated with excessive groundwater extraction include ground subsidence, saltwater intrusion, and the mining effect. The mining effect refers to a situation in which groundwater extraction greatly exceeds the rate of recharge. Groundwater also maintains rivers, wetlands, and lakes, and it sustains subsurface ecosystems within karst environments as well as alluvial aquifers. Hence, groundwater resources must be conserved.

This unit begins by introducing the concepts of porosity and permeability as well as the water table. After reviewing the laws and principles that govern groundwater movement, the unit examines aquifers, which are geological formations saturated by water and through which water can be transmitted. Water can be extracted from aquifers through wells, springs, and streams. Since groundwater contamination is a serious environmental threat, the unit reviews a range of pollutants that commonly threaten groundwater resources and outlines how such contaminants behave below ground. After examining groundwater contaminants, the unit will go on to look at the features produced by groundwater below the Earth’s surface, concluding with an examination of the effects of hot water below ground.

Objectives

After completing this unit, you should be able to

define the terms porosity and permeability, and explain their importance in groundwater movement.
describe the significance of a water table within an aquifer.
explain Darcy’s Law and its relevance in explaining the movement of water below ground.
outline properties that make an ideal aquifer.
describe the behaviour of a water table during groundwater extraction from an open well.
explain factors that control the discharge of groundwater from a well.
outline the range of pollutants that pose the most common danger to groundwater resources, and describe how they behave below ground.
describe the effects of groundwater below the ground, including the development of karst features.
describe the features produced by hot water below ground.

Section 1: Introduction: Porosity, Permeability, and the Water Table

Section 1 introduces you to some fundamental concepts in groundwater hydrology. In addition to porosity and permeability, you will become acquainted with the modes of water storage in pore spaces below ground. The readings in Section 1 will highlight the different zones of water storage within a geological formation, including the saturated zone, water table, and unsaturated zone.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

“Introduction” (including the introduction to Chapter 11: Ground Water) (pp. 262-263).
“The Importance of Groundwater” (p. 263).
“The Storage of Groundwater” (pp. 263-267).

Study Questions

How does porosity differ from permeability? Which rock types have the highest permeability, and which have the lowest?
Explain how the water table is related to the saturated zone of a body of rock.
What is the capillary fringe?
In which zone do plants get their water within a rock or soil body?

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Section 2: The Movement of Groundwater in Aquifers: Discharge from Wells, Springs, and Streams

This section acquaints you with Darcy’s Law, which governs the flow of fluids (such as groundwater) though a porous medium. You will learn that three parameters control the velocity of groundwater: the permeability of the rock formation, its porosity, and the hydraulic gradient. The hydraulic gradient refers to the difference in hydraulic head (elevation plus pressure) between two points divided by the distance between them. As you will learn, Darcy’s Law can be used to explain the movement of water within aquifers as well as water flow through wells, springs, and streams.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

“The Movement of Groundwater” (pp. 267).
“Wells” (pp. 267-270).
“Springs and Streams” (pp. 270-271).

Study Questions

What does the “hydraulic head” of a drop of water refer to?
What are equipotential lines?
Briefly describe Darcy’s Law.
What is the relationship between the slope of the water table and its velocity?
What is the relationship between the water table and topography?
What is an aquifer? Which rock types make good aquifers?
What is the difference between an unconfined aquifer and a confined aquifer?
What is a cone of depression?
Outline the differences between a gaining stream and a losing stream.

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Section 3: Groundwater Pollution

Groundwater constitutes an important resource for water for many societies. Hence, keeping groundwater supplies free from contamination is an important facet of water resource management. In this section we look at the wide range of contaminants that can pollute groundwater. You will learn that in addition to the anthropogenic sources of pollution—household waste, pesticides from agricultural activities, animal waste, industrial effluent—natural sources can also cause pollution. After discussing contamination, we will look at prevention methods for avoiding groundwater contamination and at balancing groundwater withdrawal with recharge.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

“Contamination of Groundwater” (pp. 271-275).
“Balancing Withdrawal and Recharge” (p. 275-278).

Study Questions

Describe briefly how agricultural activities related to crop farming can contribute to groundwater pollution.
What is acid mine drainage?
Using examples, describe how some contaminants do not form plumes within the saturated zone.
Under what natural ideal conditions can human sewage in groundwater be purified quickly? What processes are involved in this purification?
What is saltwater intrusion?
Why is artificial recharge sometimes carried out? How is this done?

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Section 4: Effects of Groundwater Action

In Section 4 we examine the effects of groundwater below the Earth’s surface. Chief amongst these actions are the dissolution effects of water on soluble rock, by which subterranean cavities form. You will learn that the appropriate type of geology is required for the development of such features (e.g., limestone); we will examine some of the different features that develop under such conditions.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

“Geologic Effects of Groundwater” (pp. 278-282).

Study Questions

What is hard water?
How do speleotherms form?
How do sinkholes form?
What are geodes?

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Section 5: Effects of Hot Water below Ground

Heat from the Earth’s interior can heat up water in rock formations in the upper part of the Earth’s crust to produce hot groundwater. A number of effects are associated with hot groundwater, ranging from hot springs to geysers. The energy in hot groundwater can be harnessed to heat up buildings in temperate climates and to generate electricity. In addition to learning about some of the features and phenomena associated with hot groundwater, this section will also examine the generation of electricity using geothermal energy from groundwater at very high temperatures.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

“Hot Water Underground” (pp. 282-285).

Study Questions

What is the difference between travertine and sinter?
What advantages does geothermal energy have over fossil fuels as a source of electric power generation? What environmental problems are associated with the exploitation of geothermal energy?

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