ABSTRACT
In this chapter, we introduce the overall picture of groundwater, its relationship with the hydrological cycle, and other terminology used in the mechanics of groundwater flow through porous media. The term groundwater, broadly speaking, refers to water that resides beneath the surface of the earth. However, to a groundwater hydrologist, or a geotechnical engineer, the term has a more restricted connotation: It implies a body of water that completely saturates the porous medium and the pressure at any point inside the body of water is equal to or above the atmospheric pressure. More on this aspect of water residing beneath the earth surface follows later in this chapter. We first look at some of the terminologies that are relevant for a further discussion. A water-bearing geological formation is called an aquifer if it stores enough water that can permeate through the geological formation under ordinary hydraulic gradients (or field conditions). Todd (1959) relates the term to its Latin rootsaquifer, combining form of aqua (water), and ferre (to bear). Thus, aquifer literally means water bearing. The important aspect of this definition is the fact that enough water can be economically mined from an aquifer. There are three more terms used in the definition of geological formation with regard to groundwater and its transmission. These are aquiclude, aquitard, and aquifuge. All these terms have their roots in Latin, according to Todd. An aquiclude is a geological formation that may contain a significant amount of water but is incapable of transmitting it under ordinary field conditions. A typical example is a clay layer. For all practical purposes, an aquiclude represents an impermeable formation. It generally forms the confining layer of a confined aquifer (to be discussed subsequently). An aquitard refers to a geological formation that transmits water at a very low rate compared with an aquifer. It often acts as a leaky formation between two aquifers. The term aquifuge on the other hand refers to a geological formation that neither contains nor transmits water. Typical examples include unfractured igneous rocks such as granite.
