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.

Figure 1. Photo of the Casper aquifer in Eastern Wyoming, composed of cemented medium sandstone. The aquifer serves as the sole water supply for the town of Laramie, WY, population ~27,000.

Source: Demian Saffer

Figure 2. Photo of a fractured limestone member of the Casper formation. Sedimentary layers, or beds, are near-horizontal, and primary fractures are vertical.

Source: Demian Saffer

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.

Figure 3. Schematic diagram showing a vertical section from the land surface through the vadose zone, capillary fringe, and into an unconfined aquifer.

Source: From W.M. Alley et al., 1999, USGS Circular 1186: Sustainability of Ground Water Resources

Figure 4. Illustration of capillary action; water is drawn to different heights in tubes of different internal diameters.

Source: From USGS Water Science School. Photo Credit: C. Robinson, West Texas A&M Univ.

Figure 5. Cross-sectional diagram of a water Illustration of capillary action; water is drawn to different heights in tubes of different internal diameters, showing the vadose zone (unsaturated zone), water table, and saturated aquifer. Insets below show water occurrence in crevices or fractures in rock (left) or between sediment or soil grains (right).

Click Here for Text alternative of Cross-sectional diagram of water

Water (not ground water) held by molecular attraction surrounds surfaces of rock particles. All openings below the water table are full of ground water. The Unsaturated Zone is on the land surface and above the water table which contains the saturated zone, surface water, and ground water.

Source: U.S. Geological Survey, Water Science for Schools: Aquifers.