Water is in constant flux and follows a complex hydrologic cycle. Evapotranspiration from land and evaporation from the ocean contribute to atmospheric moisture, which returns to the land in the form of rain and snow (precipitation). Snow accumulates in a winter snow pack, in glaciers, and on arctic ice-caps. Rainfall creates infiltration into the soil, as well as runoff into creeks, rivers, and lakes that drain back into the ocean. Soil moisture becomes available to plants that will transpire water. Excess infiltration water percolates to groundwater, which accumulates in underground basins and eventually flows back into rivers, lakes, man-made wells, or directly into the ocean. The elements of the hydrologic cycle are all interconnected.
When such water completely fills the void space of sediment pores or rock fractures, the pores or fractures are said to be saturated. In all but truly swampy areas, the ground just beneath the land surface is aerated: water occupies only part of the pore space. The water partially filling the voids is referred to as soil moisture, or vadose zone water. The boundary between groundwater and vadose zone water is referred to as the water table. The zone above the water table is referred to as the “unsaturated zone” or “vadose zone.”
A geologic formation from which significant amounts of groundwater can be pumped for domestic, municipal, or agricultural uses is known as an aquifer.
Aquifers sometimes are vertically separated by geologic formations that permit little or no water to flow. The
formation that acts as water barrier is called aquitard if it is much less permeable than a nearby aquifer but still permits flow (e.g., sandy clay). If the water barrier is almost impermeable (e.g., clay) and forms a more or less formidable flow barrier between multiple levels of aquifers, it is known as an aquiclude.
Aquifers can be of two major types: unconfined or confined. In an unconfined aquifer, there is no overlying aquitard or aquiclude. Where multiple levels of aquifers exist, the uppermost aquifer is typically unconfined. Vertical recharge by infiltration of rainwater or irrigation water downward to the unconfined aquifer is therefore not restricted. The water table at the top of the unconfined aquifer can migrate freely up and down depending on how much water is stored in the aquifer. The water level in a borehole drilled into an unconfined aquifer will be the same as the water level in the aquifer (if we ignore the effects of the capillary fringe).
A confined aquifer, on the other hand, is “sandwiched” between an aquiclude above and an aquiclude or aquitard (e.g., bedrock) beneath. As a result of “backpressure”, water in the confined aquifer is pressurized. Due to the pressure, the water level in a borehole drilled into a confined aquifer will rise significantly above the top of the aquifer. An artesian well occurs where the pressure is so large that the water level in a well drilled into the confined aquifer rises above the land surface—in other words, the well flows freely (if opened) without pumping. Note that a confined aquifer does not have a water table—it is completely filled with groundwater. The water levels in wells drilled into a confined aquifer correspond instead to the potentiometric surface of the aquifer, also known as pressure head or confined head. If the pressure head falls below the top of the aquifer, the aquifer is no longer confined; it becomes, by definition, an unconfined aquifer.
An aquifer confined by an aquitard rather than an aquiclude is referred to as a leaky aquifer, or a semiconfined
aquifer. In alluvial aquifers, the aquitard rarely is a contiguous layer of low-permeability clay, loamy clay, or sandy clay. Rather, it can be thought of as a local accumulation of multiple, horizontally discontinuous smaller clay “lenses” and other clay-rich or otherwise impermeable (or low permeability) layers. Though such lenses and layers are not contiguous, the overall effect on the regional aquifer below is identical to that of a solid, continuous aquitard.
Sometimes water collects above an impermeable layer or low-permeability layer within the unsaturated
(aerated) zone, forming a “perched” water table. By definition, a perched water table is a saturated groundwater zone separated from the aquifer below by a zone that is unsaturated or aerated. This should not be confused with an unconfined shallow aquifer that is separated from a deeper confined aquifer by thick, saturated layers of clay.