Ground-Water Occurrence and Flow
An aquifer is a ground-water reservoir composed of geologic units that are saturated with water and sufficiently permeable to yield water in a usable quantity to wells and springs. Sand and gravel deposits, sandstone, limestone, and fractured, crystalline rocks are examples of geologic units that form aquifers. Aquifers provide two important functions: (1) they transmit ground water from areas of recharge to areas of discharge, and (2) they provide a storage medium for useable quantities of ground water. The amount of water a material can hold depends upon its porosity. The size and degree of interconnection of those openings (permeability) determine the materials’ ability to transmit fluid.
Aquifers that are not completely saturated with water are termed unconfined aquifers (Figure 2.3). The upper portion of the aquifer, where the pore spaces are only partially filled, is referred to as the unsaturated zone. Confined or artesian aquifers are completely saturated, permeable geologic units overlain by low permeability confining layers that prevent the free movement of air and water between the layers. The water is thus confined under pressure and if tapped by a well rises to a level above the top of the aquifer, but not necessarily above the land surface. A perched aquifer represents a limited unconfined aquifer with an underlying confining layer that lies above and is separated from the regional water table by an unsaturated zone.
Figure 2.3 Schematic cross section of aquifer types.