6.2 Perched Aquifers

An unconfined aquifer can also occur as a perched aquifer. Most commonly, perched aquifers form and are maintained by recharge that accumulates on aquitards in the vadose zone. Perched groundwater forms above a layer of lower permeability material within the vadose zone where the migration of percolating recharge is slowed to the extent that it saturates the porous material above an aquitard (Figure 45). If sufficient water is available for the development of water supplies, then this is referred to as a perched aquifer. Otherwise, it is referred to as perched groundwater. The lower permeability layer also becomes partly saturated below the perched zone (Figure 45), returning to unsaturated conditions below the saturated zone. The water continues to flow slowly downward under unsaturated conditions through the rest of the aquitard and back into the underlying sediments. A line along which water pressure equals atmospheric pressure forms within, or at the base of the lens resulting in an inverted water table so the entire perched system is surrounded by a water table. Where perched groundwater systems intersect the land surface, groundwater discharges as spring flow.

Figure showing a perched aquifer
Figure 45 – Perched aquifers form on lower permeability lenses (gray) that are aquitards in the vadose zone. Precipitation infiltrates and percolates downward to the regional water table, but its progress is slowed by the presence of low permeability layers and, if sufficient infiltration occurs (solid blue arrows), the vadose zone sediments overlying the lens become saturated and a water table forms. As water leaks into the low permeable layer a portion of the aquitard becomes saturated and then flow reverts back to unsaturated conditions. A water table forms the lower boundary of the perched system (inverted triangles). The left well shows the presence of unconfined saturated perched conditions (perched groundwater flows into the well). A well that is sealed through the perched zone, but open to the vadose zone below is dry because pore spaces outside of the well bottom are filled with air and water, and the vadose water is in tension (pressures are less than atmospheric) so does not flow into the well. The well on the right is sealed through the vadose zone, but open to the regional unconfined aquifer.

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Hydrogeologic Properties of Earth Materials and Principles of Groundwater Flow Copyright © 2020 by The Authors. All Rights Reserved.