Groundwater is important for energy and food security, human health and ecosystems. The time since groundwater was recharged - or groundwater age – can be important for diverse geologic processes such as chemical weathering, ocean eutrophication and climate change. However, 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. This 50-year age threshold is also a useful metric for renewability of groundwater resources. In order to quantify the volume of young and modern groundwater, we combined geochemical, geological, hydrologic and geospatial datasets with numerical simulations of groundwater flow and age transport and analyzed tritium ages. Through a global but per-watershed analysis, we found that the total groundwater volume in the upper 2 km of continental crust is approximately 22.6 million km3, of which 0.1 to 5.0 million km3 is less than 50 years old.
Thus, while groundwater comprises a vast water resource that vastly exceeds all other water sources on our planet, the younger and more readily available portions of the subterranean water cycle are much smaller drops and are at most 6% of the total volume of groundwater in the upper crust. The volume of modern groundwater is equivalent to a body of water with a depth of about 3 m spread over the continents. This water resource dwarfs all other components of the active hydrologic cycle. A key application of our new estimate is quantifying groundwater storage and availability. In addition, the distribution of young groundwater storage can be used to delineate groundwater vulnerability to contamination, since recently recharged groundwater is more vulnerable to contamination by industrial or agricultural activities. Our estimates of spatiotemporally-constrained groundwater storage may also indicate hotspots for chemical weathering, landscape evolution and coastal eutrophication, helping us discern the critical role of groundwater in the Earth system.
M. Bayani Cardenas is Associate Professor and William T. Stokes Centennial Teaching Fellow, Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin - http://www.jsg.utexas.edu/researcher/bayani_cardenas/
This presentation is based on the upcoming paper: The global volume and distribution of modern groundwater, by Tom Gleeson, Kevin M. Befus, Elco Luijendijk, Scott Jasechko, and M. Bayani Cardenas, which has been reviewed and revised for publication.
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