The volume and distribution of water below the Earth’s surface is globally quantified in a paper published online in Nature Geoscience this week. The study shows that modern groundwater, that which has been replenished in the past 50 years and has an important role in the hydrologic cycle, accounts for less than one-tenth of all groundwater.
Despite the reliance of human populations and ecosystems on groundwater, the amount and distribution of this resource has not been well defined at the global scale. In particular, a global estimate of modern groundwater has been lacking. This is an issue as modern groundwater is more renewable and more vulnerable to global change than older groundwater stores that can be millions of years old.
Tom Gleeson and colleagues combine various datasets, including measurements of the radioactive tritium introduced to groundwater about a half-century ago due to thermonuclear testing, with groundwater modelling to estimate the volume and distribution of groundwater stored in the upper continental crust. They estimate that the total volume of groundwater of any age is about 23 million cubic kilometres, which is enough to cover the global land surface in a layer 180 metres deep. However, they find that modern groundwater represents no more than 6% of global groundwater. Yet, modern groundwater still represents the largest component of the active hydrological cycle: there is three times more modern groundwater hidden below the ground than surface water atop it, and enough to cover the global land surface three metres deep.
Additionally, the authors find that modern groundwater is spread unevenly, with its distribution depending on geographic and environmental conditions (for example, there is very little beneath arid regions). They conclude that identifying groundwater stores that have been recently replenished will help find areas that are particularly vulnerable to contamination and land use changes.
In an accompanying News & Views article, Ying Fan writes that “this global view of groundwater will, hopefully, raise awareness that our youngest groundwater resources-those that are the most sensitive to anthropogenic and natural environmental changes-are finite.”