6 Aquifers and Aquifer Properties
Of all the words in the hydrologic vocabulary, there are probably none with more shades of meaning than the term aquifer. It means different things to different people, and perhaps different things to the same person at different times. It is used to refer to individual geologic layers, to complete geologic formations, and even to groups of geologic formations. The term needs to be viewed in the scale and context of its usage.
An aquifer is a saturated, permeable, geologic unit that can transmit and store significant quantities of water under ordinary hydraulic gradients, and yield water to wells and springs in economically usable amounts (Freeze and Cherry, 1979). The overarching concept is that aquifers are saturated geologic materials that store and transmit water in sufficient quantities and rates such that they can be sources of water for humans. This concept is found in early definitions and the concept that an aquifer is permeable enough to yield economic quantities of water to wells has been embraced by the science and water well industry communities (Freeze and Cherry, 1979). The term is sometimes used loosely to represent all formations of earth materials that contain groundwater, but a saturated clay formation may contain groundwater yet fail to meet the requirements to yield sufficient water for use. In this case, it would not be classified as an aquifer. The definition of an aquifer does not recognize less productive groundwater systems that may support vital ecological systems, though such systems may be critical to human wellbeing. When economics are tied to the definition, a 1000 m deep groundwater system may be of value for supplying cooling water to an electrical generation plant and thus viewed as an aquifer by this industry, but may not be considered an aquifer by someone looking for agricultural water to irrigate a crop.
Geologic units that don’t serve as aquifers are often referred to as aquitards, aquicludes or aquifuges. An aquifuge suggests that the material has no capacity to store or transmit water, it is impermeable. Though such settings may occur (e.g., a deep, unweathered granite), the term is not widely used. An aquiclude is a saturated geologic unit that has some storage and transmission properties, however, for practical purposes, no transmission occurs. Aquitards are units that store water and are less permeable than aquifers, so they slow the transmission of water. They may transmit water in regional layered flow systems where leakage passes through them from one aquifer to an overlying or underlying aquifer, yet their hydraulic conductivity is low enough that they cannot produce useful quantities of water. Modern usage has generally replaced the term aquitard with the term confining unit, confining bed and/or leaky confining unit. Most saturated geologic materials are classified as either aquifers or aquitards/confining units, rarely are units labeled aquicludes or aquifuges.
The most common aquifers are formations with hydraulic conductivity values in the upper half of the ranges shown in Figure 32. These include unconsolidated sands and gravels, limestones, and heavily fractured volcanic and crystalline rocks, and permeable sedimentary rocks such as sandstones. The most common aquitards are silts, clays and shales.
It is recognized that the definitions of aquifer and aquitard are imprecise with respect to values of hydraulic conductivity. The terms are often used in a relative sense. For example, in layers of sand and silt, the sands are considered aquifers, whereas in layers of silt and clay, the silts may be considered aquifers.
Aquifers are often named for regions or associated formations. The High Plains or Ogallala Aquifer of the central United States is the largest aquifer in the world and is named after the town of Ogallala, Nebraska near the outcrop location of aquifer material. Other large aquifer systems include the Nubian Sandstone Aquifer System in Africa, the Arabian Aquifer system in the Middle-East, and the North China Aquifer. All of these aquifers provide critical water for humans, agriculture and industries.
Aquifer systems are often thought of as having uniform properties, thicknesses and extent. However, heterogeneous and anisotropic properties with variable extent and thickness are nearly always the case. In the following discussion, simplified conditions are used to illustrate similarities and differences in aquifer types. There are three basic aquifer types: Unconfined, perched and confined.