Global groundwater volumes in the upper 2 km of the Earth’s continental crust – critical for water security – are well estimated. Beyond these depths, a vast body of largely saline and non-potable groundwater exists down to at least 10 km —a volume that has not yet been quantified reliably at the global scale. Here, we estimate the amount of groundwater present in the upper 10 km of the Earth’s continental crust by examining the distribution of sedimentary and cratonic rocks with depth and applying porosity-depth relationships. We demonstrate that groundwater in the 2-10 km zone (what we call ‘deep groundwater’) has a volume comparable to that of groundwater in the upper 2 km of the Earth’s crust. These new estimates make groundwater the largest continental reservoir of water, ahead of ice sheets, provide a basis to quantify geochemical cycles, and constrain the potential for large-scale isolation of waste fluids.

Deep meteoric waters comprise a key component of the hydrologic cycle, transferring water, energy, and life between the earth's surface and deeper crustal environments, yet little is known about the nature and extent of meteoric water circulation. Using water stable isotopes, we show that maximum circulation depths of meteoric waters across North America vary considerably from 1 to 5 km, with the deepest circulation in western North America in areas of greater topographic relief. Shallower circulation occurs in sedimentary and shield-type environments with subdued topography. The amount of topographic relief available to drive regional groundwater flow and flush saline fluids is an important control on the extent of meteoric water circulation, in addition to permeability. The presence of an active flow system in the upper few kilometers of the Earth's crust and stagnant brines trapped by negative buoyancy offers a new framework for understanding deep groundwater systems.