GeologyRocks

An artesian aquifer or well (also known as a confined aquifer) is an aquifer that is trapped between two impermeable layers. An unconfined aquifer is recharged rapidly from surface water. A confined aquifer takes much longer to recharge, but this also means it is not responsive to daily, annual or even millennial changes. An example of an artesian aquifer is South Dakota, USA (see diagram below). Note how the Dakota sandstone (the aquifer) is bounded top and bottom by an impermeable rock.

References

C. Plummer and D. McGeary, 1997. Physical Geology.

http://water.usgs.gov/pubs/FS/OFR93-643/