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An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in the same geologic unit may be confined in one area and unconfined in another.
Aquifer compaction is a significant concern along with pumping-induced land subsidence. A large portion of the groundwater storage potential of many aquifers can be significantly reduced when longterm groundwater extraction, and the resulting groundwater level decline, causes permanent compaction of fine sediment layers (silts and clays).
In confined aquifers , the cone of depression is a reduction in the pressure head surrounding the pumped well. When a well is pumped, the water level in the well is lowered. By lowering this water level, a gradient occurs between the water in the surrounding aquifer and the water in the well. Because water flows from high to low water levels or ...
Aquifers can be unconfined, where the top of the aquifer is defined by the water table, or confined, where the aquifer exists underneath a confining bed. [5] There are three aspects that control the nature of aquifers: stratigraphy, lithology, and geological formations and deposits. The stratigraphy relates the age and geometry of the many ...
The above figures simulate possible coastal aquifers. In reality, it is complex. Due to complex geology - non-uniform rock layers and weathering, both confined and unconfined aquifers can be found within a coast. It is possible to have multiple confined aquifers at the bottom and an unconfined aquifers at the top of a coast.
The groundwater flow may be through either confined or phreatic aquifers, with smaller flow systems overlying or within. The residence time generally ranges from several decades to many centuries, implying the establishment of a complete chemical equilibrium with the aquifer. Mapping scales are between 1:250,000 and 1:2,000,000.
For a confined aquifer or aquitard, storativity is the vertically integrated specific storage value. Specific storage is the volume of water released from one unit volume of the aquifer under one unit decline in head. This is related to both the compressibility of the aquifer and the compressibility of the water itself.
Aquifers receive water through two ways, one from precipitation that flows through the unsaturated zone of the soil profile, and two from lakes and rivers. [2] When a water table reaches capacity, or all soil is completely saturated, the water table meets the surface of the ground where water discharge in the forms of springs or seeps .