Fossil Water
<p ><b>Fossil water</b> or <b>paleowater</b> is <a href="http://en.wikipedia.org/wiki/Groundwater" title="Groundwater" >groundwater</a> that has remained sealed in an <a href="http://en.wikipedia.org/wiki/Aquifer" title="Aquifer" >aquifer</a> for a long period of time. Water can rest underground in "fossil aquifers" for thousands or even millions of years. When changes in the surrounding <a href="http://en.wikipedia.org/wiki/Geology" title="Geology" >geology</a> seal the aquifer off from further replenishing from precipitation, the water becomes trapped within, and is known as fossil water.
The <a href="http://en.wikipedia.org/wiki/Nubian_Sandstone_Aquifer_System" title="Nubian Sandstone Aquifer System" >Nubian Sandstone Aquifer System</a> is among the most notable of fossil water reserves. Fossil aquifers also exist in the <a href="http://en.wikipedia.org/wiki/Sahara" title="Sahara" >Sahara</a>, the <a href="http://en.wikipedia.org/wiki/Kalahari_Desert" title="Kalahari Desert" >Kalahari</a>, and the<a href="http://en.wikipedia.org/wiki/Ogallala_Aquifer" title="Ogallala Aquifer" >Ogallala</a> underlying the US Great Plains. A further potential store of ancient water is <a href="http://en.wikipedia.org/wiki/Lake_Vostok" title="Lake Vostok" >Lake Vostok</a>, a<a href="http://en.wikipedia.org/wiki/Subglacial_lake" title="Subglacial lake" >subglacial lake</a> in Antarctica.</p>
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Fossil water is a <a href="http://en.wikipedia.org/wiki/Non-renewable_resource" title="Non-renewable resource" >non-renewable resource</a>.<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Fossil_water#cite_note-0" >[1]</a></sup>
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Whereas most aquifers are naturally replenished by infiltration of water from <a href="http://en.wikipedia.org/wiki/Precipitation_(meteorology)" title="Precipitation (meteorology)" >precipitation</a>, fossil aquifers get very little recharge.<sup id="cite_ref-elmhurst_1-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Fossil_water#cite_note-elmhurst-1" >[2]</a></sup> </p>
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The extraction of water from such non-replenishing <a href="http://en.wikipedia.org/wiki/Groundwater" title="Groundwater" >groundwater</a> reserves (known as low safe-yield reserves) is known in <a href="http://en.wikipedia.org/wiki/Hydrology" title="Hydrology" >hydrology</a> as "water <a href="http://en.wikipedia.org/wiki/Mining" title="Mining" >mining</a>".<sup id="cite_ref-2" class="reference" ><a href="http://en.wikipedia.org/wiki/Fossil_water#cite_note-2" >[3]</a></sup> </p>
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If water is pumped from a well at a withdrawal rate that exceeds the natural recharge rate (which is very low or zero for a fossil aquifer), the <a href="http://en.wikipedia.org/wiki/Water_table" title="Water table" >water table</a> drops, forming a depression in the water levels around the well.<sup id="cite_ref-elmhurst_1-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Fossil_water#cite_note-elmhurst-1" >[2]</a></sup></p>