Fossil water

"Fossil water" or paleowater is a somewhat broadly-used term to describe water that has been contained in some undisturbed space, usually groundwater in an aquifer, for millennia. Fossil water could also describe ancient water on other planets, or water sealed in subglacial lakes such as Antarctica's Lake Vostok. "Fossil groundwater" can be defined as "water that infiltrated usually millennia ago and often under climatic conditions different from the present, and that has been stored underground since that time."

Determining the time since water infiltrated usually involves analyzing isotopic signatures. Determining "fossil" status—whether or not that particular water has occupied that particular space since the distant past—involves modeling the flow, recharge, and losses of aquifers, which can involve significant uncertainty. Some aquifers are hundreds of meters deep and underlie vast areas of land. Research techniques in the field are developing quickly and the scientific knowledge base is growing. In the cases of many aquifers, research is lacking or disputed as to the age of the water and the behavior of the water inside the aquifer.

Large, prolific aquifers (notably the Nubian Sandstone Aquifer System and the Ogallala Aquifer) containing fossil water are of significant socio-economic value. Fossil water is extracted from these aquifers for many human purposes, notably, agriculture, industry, and consumption. In arid regions, some aquifers containing available and usable water receive little to no significant recharge, effectively making groundwater in those aquifers a non-renewable resource. Extraction rates greater than recharge rates result in lowering of the water table and can lead to groundwater depletion. Extraction of non-renewable groundwater resources is referred to as groundwater "mining" because of their finite nature.

In other regions that do receive significant precipitation and where recharge does occur, fossil water may be extracted and eventually replaced by younger water, in which case the water in those aquifers could be considered a renewable resource.

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