Groundwater is important for energy, human health and ecosystems. There are six quintillion gallons under the Earth, the equivalent to cover the entire planet under 600 feet of water.
The global groundwater age, or the time since groundwater was recharged, is very important for diverse geologic processes, such as chemical weathering, ocean eutrophication and climate change.
Images credit: University of Victoria/T Gleeson/K Befus/S Jasechko/E Luijendijk/M Cardenas
Measured groundwater ages range from months to millions of years.
The global volume and distribution of groundwater less than 50 years old—modern groundwater that is the most recently recharged and also the most vulnerable to global change—are unknown. Here we combine geochemical, geologic, hydrologic and geospatial data sets with numerical simulations of groundwater and analyse tritium ages to show that less than 6% of the groundwater in the uppermost portion of Earth’s landmass is modern.