Aquifers come in many shapes, sizes, and “flavors”. For example, some aquifer systems span hundreds – or even thousands – of kilometers across several states or nations and may include multiple individual layers of rock or sediment that total thousands of meters in thickness. Other aquifers may be restricted to a few kilometers within a stream valley, and be only a few meters thick.
Nonetheless, there are some important common features of different kinds of aquifers. In this section of the module, we provide a basic overview of aquifers: What is an aquifer? What are the different types of aquifers, and what is their anatomy?
Aquifers are geologic formations in the subsurface that can store & transmit water (Figures 1 and 2). As we will see later in the section titles Basic Aquifer Properties, there are specific rock and soil properties that govern these two functions. Adequate storage requires that there be sufficient void space between particles, in fractures, or generated by compressing the aquifer under pressure, to provide usable quantities of water. Adequate transmission requires that the void spaces where water occurs be well enough connected that it can percolate or flow under either natural or pumping-driven conditions, at a rate that will support sustained use.
These definitions are intentionally vague because they depend on the scale of intended use. For example, an aquifer that provides water for a large city will need to sustain higher pumping rates at wells (on order of tens of thousands of gallons per minute) than one that provides for a single-family (a few to perhaps ten gallons per minute). For reference, Penn State University relies almost exclusively on groundwater pumping for its water supply at the University Park campus, with a total extraction of ~2.5 million gallons per day (about 1750 gallons per minute) distributed among several pumping well fields.
In contrast to an aquifer, an aquitard, often also termed a confining layer or aquiclude, is a geologic formation in the subsurface that does not transmit water effectively – and therefore acts as a barrier to groundwater flow. In general, aquifers are usually composed of sediments or sedimentary rocks with grain sizes larger than fine- to medium sand (>~125 µm diameter), or of fractured rock. Aquitards are typically composed of fine-grained sediments or sedimentary rocks or those in which the pore spaces have been filled by mineral cements (silts, siltstones, shales, clays, cemented sandstones, or unfractured limestones).
In the simplest sense, you might imagine an aquifer formation that may be covered by a veneer of soil, and which extends downward from a few feet below the surface for several tens or even hundreds of feet (Figure 3). At some depth below the land surface, the interstices between soil or sediment particles, or the fractures in the rock, will be water-filled or saturated. Shallower than that depth, these interstices, or pore spaces, will be filled with air, water vapor, and some liquid water bound to the surfaces of the rock (Figures 3-5). This zone is the unsaturated zone, also known as the “zone of soil moisture” or the vadose zone. The water table marks the top of the ground water system and is formally defined as the depth at which the pressure in the subsurface is equal to the atmospheric pressure. Immediately above the water table, there is a narrow zone of saturation termed the capillary fringe. In this zone, water is wicked upward in pore spaces due to capillary forces. This is analogous to capillary tube experiments you may have seen or performed in physics or chemistry classes in high school; it occurs due to interaction between the polar water molecule and the surfaces of the solids, and is directly related to the fact that water has a surface tension (as you may remember from Module 1!). In the capillary fringe, pores are saturated, but pressures are sub-atmospheric, meaning that the water is under suction as it is pulled or wicked upward. The nomenclature “fringe” reflects the fact that slight variations in grain size lead to variations in the height that water is drawn (again, think to the capillary tube experiment, and effects of different tube sizes) (Figure 5).
Any precipitation or surface water that infiltrates to the water table must percolate through the vadose zone in order to recharge the aquifer. As we will see later in this module infiltration and recharge typically constitute only a small fraction – rarely more than 10% - of precipitation, because most water that falls in events is returned to the atmosphere by transpiration or evaporation, becomes runoff (i.e., if the capacity for infiltration is exceeded by the rate of precipitation), or is bound by soils in the vadose zone.