About 96 percent of all unfrozen fresh water is found below the Earth’s surface and is known as groundwater. Groundwater systems globally provide 25 to 40 percent of the world’s drinking water. Groundwater represents an enormous resource that can only be managed through an understanding of the different types of aquifers and their rates of renewal.
Aquifers that are located close to the surface are often intimately connected with surface water systems. They may be replenished directly by infiltration of precipitation and sometimes surface runoff, and may discharge to waterbodies such as streams and lakes. When such aquifers are drawn down by pumping, they can cause a decrease in river flow, resulting in shortages of drinking water, water for agriculture, or water for the aquatic ecosystem. Deeper aquifers may also be replenished indirectly by water slowly leaking downward from shallower aquifers.
But not all groundwater is renewable. In fact, humans are using groundwater faster than it can be replenished in many regions of the world.
In some of these areas, most of the groundwater was recharged during ancient eras of dramatically cooler or wetter climates. Like the fossil fuels that were also created under long-vanished conditions, this “fossil water” is considered non-renewable.
Northern Africa, the Arabian Peninsula, Australia, Siberia, and the central United States contain large reserves of fossil groundwater. Fossil water can be a tremendous boon to those in arid or semi-arid climes. But it also poses a real question for water managers: Use it now, or save it for later?
Once non-renewable aquifers have been drawn down to a level at which it is not economic to pump the water, they cease to be a resource because they will not recharge for the foreseeable future. Drawing down this water can also have a variety of other impacts, such as increasing salinity of the water due to salt water intrusion and land subsidence.