Types of aquifers

In more detail, there are three main classifications of aquifers, defined by their geometry and relationship to topography and the subsurface geology (Figures 6-9). The simple aquifer shown in Figure 6 is termed an unconfined aquifer, because the aquifer formation extends essentially to the land surface. As a result, the aquifer is in pressure communication with the atmosphere. Unconfined aquifers are also known as water table aquifers, because the water table marks the top of the groundwater system.

A second common type of aquifer is a confined aquifer, which is isolated from pressure communication with overlying or underlying geologic formations – and with the land surface and atmosphere – by one or more confining layers or confining units. Confined aquifers differ from unconfined aquifers in two fundamental and important ways. First, confined aquifers are typically under considerable pressure, which may be derived from recharge at higher elevation or from the weight of the overlying rock and soil (known as the overburden). In some cases, the pressure is high enough that wells drilled into the aquifer are free-flowing. This condition requires that the water pressure in the aquifer is sufficient to drive water up the wellbore and above the land surface, and such wells are called artesian wells (Figure 7). Second, confined aquifers typically remain saturated over their entire thickness, even as water is removed by pumping wells. Water extracted from the aquifer comes only from the depressurization of the aquifer – a combination of depressurization and expansion of the water itself, and relaxation of the aquifer formation upon reduction in pressure (Figure 8).

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Figure 6. Schematic cross sectional diagram showing layered system with an upper unconfined aquifer above a confining unit, and underlain by a confined aquifer. Note the water level in the two wells: In the unconfined aquifer the water level in the well is the same as the height of the water table. In the confined aquifer, the water level is higher than the top of the aquifer – indicating that the aquifer is fully saturated and that the water is under pressure.
Source: USGS Water Science Photo Gallery

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Figure 7. Artesian well in Southern Georgia
Source: [link https://water.usgs.gov/edu/gallery/artesian-well-georgia.html 'USGS Water Science Photo Gallery'

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Figure 8. Schematic illustrating the effects of pumping from a well in an unconfined aquifer (left) and a confined aquifer (right). When unconfined aquifers are pumped, the water withdrawal leads to a drop in the water table, and the pore spaces become unsaturated. Pumping in confined aquifers decreases the water pressure, but the pore space remains fully saturated.
Source: W.M. Alley et al., 1999, USGS Circular 1186: Sustainability of Groundwater Resources

The third main type of aquifer is a perched aquifer (Figure 6). Perched aquifers occur above discontinuous aquitards, which allow groundwater to "mound" above them. Thee aquifers are perched, in that they sit above the regional water table, and within the regional vadose zone (i.e. there is an unsaturated zone below the perched aquifer). The dimensions of perched aquifers are typically small (dictated by climate conditions and the size of aquitard layers), and the volume of water they contain is sensitive to climate conditions and therefore highly variable in time.

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Figure 9. Schematic cross section showing occurrence of perched aquifers above an unconfined aquifer.
Source: D.T. Snyder, U.S. Geological SurveyScientific Investigations Report 2008–5059, Estimated Depth to Ground Water and Configuration of the Water Table in the Portland, Oregon Area

• Formative Assessment 1: Aquifers