Grant Ferguson, Jennifer C McIntosh , Scott Jasechko, Ji-Hyun Kim, James S. Famiglietti, Jeffrey J. McDonnell

Groundwater is one of the largest reservoirs of water on Earth but has relatively small fluxes compared to its volume. This behaviour is exaggerated at depths below 500 m, where the majority of groundwater exists and where residence times of millions to even a billion years have been documented. However, the extent of interactions between deep groundwater (>500 m) and the rest of the terrestrial water cycle at a global scale is unclear because of challenges in detecting their contributions to streamflow. Here, we use a chloride mass balance approach to quantify the contribution of deep groundwater to global streamflow. Deep groundwater likely contributes <0.1% to global streamflow and is only weakly and sporadically connected to the rest of the water cycle on geological timescales. Despite this weak connection to streamflow, we found that deep groundwaters are important to the global Cl cycle, providing ~7% of the flux of Cl to the ocean.