Inorganic Hydrogeochemistry in the 21st Century.

Chemical and isotopic processes occur in every segment of the hydrological cycle. Hydrogeochemistry-the subdiscipline that studies these processes-has seen a transformation from "witch's brew" to credible science since 2000. Going forward, hydrogeochemical research and applications are critical to meeting urgent societal needs of climate change mitigation and clean energy, such as 1) removing CO2 from the atmosphere and storing gigatons of CO2 in soils and aquifers to achieve net-zero emissions, 2) securing critical minerals in support of the transition from fossil fuels to renewable energies, and 3) protecting water resources by adapting to a warming climate. In the last two decades, we have seen extensive activity and progress in four research areas of hydrogeochemistry related to water-rock interactions: arsenic contamination of groundwater; the use of isotopic and chemical tracers to quantify groundwater recharge and submarine groundwater discharge; the kinetics of chemical reactions and the mineral-water interface's control of contaminant fate and transport; and the transformation of geochemical modeling from an expert-only exercise to a widely-accessible tool. In the future, embracing technological advances in machine learning, cyberinfrastructure, and isotope analytical tools will allow breakthrough research and expand the role of hydrogeochemistry in meeting society's needs for climate change mitigation and the transition from fossil fuels to renewable energies. This article is protected by copyright. All rights reserved.