- Published: Tuesday, 14 August 2012 15:42
What is an Aquifer?
Although groundwater exists everywhere under the ground, some parts of the saturated zone contain more water than others. An aquifer is an underground formation or permeable rock or loose materials which can produce useful quantities of water when accessed by a well. Aquifers come in all sizes and their origin and composition is varied. They may be small, only a few hectares in area, or very large, underlying thousands of square kilometres of the earth’s surface. They may be only a few metres think, or they may measure hundreds of metres from top to bottom.
Many important Canadian aquifers are composed of thick deposits of sand and gravel previously laid down by glacial rivers. These types of aquifers provide water supply for many residents that live outside of urban centres that obtain their water supply from large rivers. To concentrate only on major (i.e., large) aquifers, however, is misleading. Many individual farms and rural homes depend on relatively small aquifers such as thin sand and gravel deposits of glacial or fluvial origin, as well as fractured bedrock intervals. Although some of these aquifers may not be significant, in total they make up a very important groundwater resource.
|Figure 1: Schematic showing the difference between unconfined (water table) and confined aquifers|
There are two different types of aquifers that exist beneath the landscape. Many people are familiar with the term “water table”. This is the water that is first struck when we dig below the surface. The water table represents the surface of groundwater within an “unconfined aquifer” – unconfined because the upper surface is open to the atmosphere. This is exemplified by the upper dashed and dotted line shown in Figure 1. As such, the water table represents the surface at which the water pressure is at atmospheric pressure. The second type of aquifer is a “confined aquifer”. These aquifers are so-called because they are confined from above and below by sediments of lower permeability. These lower permeability layers isolate the aquifer from direct interaction with the atmosphere or other overlying and underlying aquifer intervals. The sealing nature of the confining layers results in an increased pressure of the water in the aquifer, such that is a well is drilled and installed into a confined aquifer, the water level in the well will stabilize above the top of the aquifer sediments. The resulting surface provided by numerous wells completed in the same interval over a given area is called a “potentiometric surface” (i.e., lower dashed line in Figure 1).
Section 1: Introduction
Module 2: What is an aquifer?