Groundwater deeper than 500 m contributes less than 0.1% of global river discharge

Abstract

Groundwater is one of the largest reservoirs of water on Earth but has relatively small fluxes compared to its volume. This behavior 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 are 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 chloride cycle, providing ~7% of the flux of chloride to the ocean.

Acceleration of Deep Subsurface Fluid Fluxes in the Anthropocene

“…Over the past two decades, there has been an increase in the awareness of the microbial communities that inhabit the deep subsurface of depths of up to a few km (Bar-On et al, 2018;Magnabosco et al, 2018;McMahon & Parnell, 2014). This has been accompanied by questions about how the deep subsurface fits within the larger Earth system in terms of microbial life and associated water and geochemical fluxes (Ferguson et al, 2021(Ferguson et al, , 2023Lollar et al, 2019;Warr et al, 2018). As we stand at the precipice of the energy transition, we have the opportunity to develop the deep subsurface in a manner that allows us to study its natural functions and response to anthropogenic perturbations to minimize human impacts and build understanding, synergies and resilience.…”

“…CCS is arguably the most important of these projected uses in terms of reducing greenhouse gas emissions, with many of the studies examining the capacity to sequester carbon in the subsurface focusing on estimation of the volume of porosity in sedimentary basins suited for this purpose (Benson & Cole, 2008;Krevor et al, 2023;Zoback & Smit, 2023). Additional (Ferguson et al, 2023). Current geothermal projects are associated with smaller fluxes (C. E. Clark et al, 2010;IEA, 2021a).…”

“…Pores and fractures at these depths contain the largest volume of water aside from the ocean (Ferguson et al., 2021) and may contain ∼15% of the Earth's biomass (Bar‐On et al., 2018). Groundwater residence times exceeding one million years have been found in a variety of geological settings (Ferguson et al., 2023; Warr et al., 2018), indicating that these deep subsurface ecosystems have been isolated for prolonged periods of geologic time in this “hidden” part of the Earth system that has minimal interaction with the rest of the hydrologic cycle (Warr et al., 2018). Continental to global scale studies tend to treat the subsurface as a black box that is capable of storing or producing fluids without considering how fluxes and microbial communities might change within the subsurface.…”

“…Figure 2. Current oil and gas production involves similar fluid fluxes to natural deep (>500 m) groundwater flow with uncertainties based on 25th and 75 percentiles from a Cl flux-based estimate(Ferguson et al, 2023). Current geothermal projects are associated with smaller fluxes (C. E Clark et al, 2010;IEA, 2021a)…”

Streamflow Composition and Water “Imbalance” in the Northern Himalayas

“…Furthermore, the magnitude of such upflow of deep groundwater is deemed to be minor relative to total streamflow. Ferguson et al (2023) used a chloride mass balance approach to quantify the global contribution of deep groundwater (depth > 500 m) to global streamflow. They estimated that deep groundwater likely contributed <0.1% to global streamflow and concluded that deep groundwater was only weakly and sporadically connected to the rest of the water cycle on geological timescales, thereby supporting our compartmentalization of the terrestrial water cycle.…”