Sustainability of Tapping Ancient Groundwater

Fossil groundwaters make up a substantial fraction of the Earth’s fresh water and are being targeted for water supply wells at increasing rates. These groundwaters were recharged more than 12,000 years ago, often in climate conditions that were much different from those of today. Because of the long renewal times involved, fossil groundwaters have often been classified as nonrenewable. However, groundwater ages provide little insight into how water levels and fluxes will change as the result of pumping. The relationship between groundwater ages and these outcome-based metrics of renewability is not straightforward. Therefore, whether a groundwater is fossil or not may have little to do with its renewability. The hydraulic response of an aquifer system to pumping is not strongly related to groundwater age. The use of both modern and fossil groundwater can be unsustainable.