Stable isotopes of water in hydrogeology

Groundwater is a pivotal resource for many human populations and ecosystems. However, in groundwater sciences (hydrogeology), there remain many challenges in understanding key processes; including groundwater origins, water fluxes, and controls on water quality. The stable isotopes of water are naturally occurring, form part of the water molecule, can be measured at high spatial and temporal resolutions and have predictable fractionations that may be used to understand key water cycle processes. Such qualities mean that stable isotopes are a key tracer in defining the spatial and temporal heterogeneity of groundwater systems. This article provides an overview of the use of stable isotopes to analyze (i) groundwater recharge from both rainfall and irrigation water, and presents case studies that highlight the use of stable isotopes to determine the frequency, rapidity, and origins of recharge; (ii) groundwater flow pathways including paleo‐groundwater and mixing processes, and case studies of deeply circulating mineral waters, groundwater mixing, and origins of paleo‐groundwater; (iii) groundwater discharge via evaporation, surface waters, and case studies that highlight the modeling of dynamic tropical river inflows and submarine groundwater discharge; and (iv) groundwater salinity, including a comparison of processes driving salinization in two regional groundwater basins