Confined and Unconfined Aquifers

Aquifers can be either unconfined or confined.  Unconfined aquifers occur where the groundwater system is contained in permeable materials that extend to the lands’ surface and therefore is at atmospheric pressure.  Water levels in a well installed and screened in an unconfined aquifer define the water table, the upper surface of the saturated zone.

Confined aquifers exist where a low-permeability geologic deposit, such as clay, overlies the groundwater system.  In these settings, groundwater may be under a greater-than-atmospheric pressure.  In some cases “artesian” conditions may occur, where free-flowing wells discharge groundwater at the lands’ surface.  The pressure in an aquifer, called the head, can be determined by measuring the water level within a well.  In a confined aquifer the water level can lie above the top of the aquifer.  This elevation is commonly known as the pressure surface, but is more appropriately called either the piezometric or potentiometric surface.

Precipitation or surface water that percolates downward into the aquifer is called recharge.  Subsurface wastewater discharge and infiltration of stormwater runoff may also recharge groundwater.  This is known as artificial recharge, because the water is infiltrated as a result of land-use activity.

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