Abstracts

1
Civil Engineering, University of Victoria, Victoria, British Columbia, Canada
2
Department of Civil Engineering, McGill University, Montreal, Québec, Canada
3
Department of Geological Sciences, The University of Texas at Austin, Austin, Texas, USA
4
Department of Geography, University of Calgary, Calgary, Alberta, Canada
5
Geoscience Centre, Georg-August-Universität Göttingen, Göttingen, Germany
Groundwater is important for energy and food security, human health, and ecosystems.
Unfortunately, groundwater is being depleted globally impacting agriculture and the environment,
and even causing regional crustal deformation. Groundwater is also part of diverse
earth processes from chemical weathering to ocean eutrophication but groundwater
dynamics are not part of most earth system models. The time since groundwater recharge,
or groundwater age, ranges hugely from days to millions of years, which is important for
groundwater management and the role of groundwater in various earth processes. Yet,
the volume, distribution and lifespan of young groundwater remain unknown. Here we
show that only ~6% of global volume of groundwater is less than fifty years old (0.26 - 4.2
million km 3 ) and that current pumping rates can deplete this young groundwater in less
than fifty years in half the areas heavily irrigated with groundwater. We combine extensive
geochemical, geological, hydrologic and geospatial datasets with numerical groundwater
simulations to derive two consistent and independent estimates of the young groundwater
volume as well as provide the first data-driven estimate of total groundwater volume
(~22.6 million km 3 in the upper 2 km of the crust). Most young groundwater is found in
the first 250 meters underground but is very unevenly distributed across the Earth. The
regions with heavy groundwater irrigation and a