Fossil water or paleowater is groundwater that has remained sealed in an aquifer 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 geology seal the aquifer off from further replenishing from precipitation, the water becomes trapped within, and is known as fossil water.
The Nubian Sandstone Aquifer System is among the most notable of fossil water reserves. Fossil aquifers also exist in the Sahara, the Kalahari, and the Ogallala underlying the US Great Plains. A further potential store of ancient water is Lake Vostok, a subglacial lake in Antarctica.
Fossil water is a non-renewable resource.[1] Whereas most aquifers are naturally replenished by infiltration of water from precipitation, fossil aquifers get very little recharge.[2] The extraction of water from such non-replenishing groundwater reserves (known as low safe-yield reserves) is known in hydrology as "water mining".[3] 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 water table drops, forming a depression in the water levels around the well.[2] Water mining has been blamed for contributing to rising sea levels.[4]
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Aquifer drawdown or overdrafting and the pumping of fossil water increases the total amount of water in the hydrosphere, and may be responsible for up to one quarter of the Earth's total sea level rise since the beginning of the 20th century.[5]