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.
