Global Isotope Hydrogeology―Review

• Published in: Reviews of geophysics (1985) 2019-09, Vol.57 (3), p.835-965
• Main Author: Jasechko, Scott
• Format: Article
• Language: English
• Subjects: Agriculture; aquifer; Aquifer storage; Aquifer systems; Aquifers; Chemical reactions; Climatic conditions; Contaminants; Data analysis; Discharge; Fossils; Geology; Groundwater; groundwater age; Groundwater data; Groundwater discharge; Groundwater quality; Groundwater recharge; Groundwater resources; Groundwater storage; Hydrogeology; isotope; Isotopes; Organic chemistry; Paleoclimate; Pleistocene; Rainforests; Rainy season; recharge; Reviews; River flow; Rivers; Scientists; Seasonal variations; Seasonality; Sedimentary basins; Stream discharge; Stream flow; transit time; Water pollution; Water resources; Water supply; Wet season; Winter precipitation
• ISSN: 8755-1209; 1944-9208
• DOI: 10.1029/2018RG000627

Groundwater 18O/16O, 2H/1H, 13C/12C, 3H, and 14C data can help quantify molecular movements and chemical reactions governing groundwater recharge, quality, storage, flow, and discharge. Here, commonly applied approaches to isotopic data analysis are reviewed, involving groundwater recharge seasonality, recharge elevations, groundwater ages, paleoclimate conditions, and groundwater discharge. Reviewed works confirm and quantify long held tenets: (i) that recharge derives disproportionately from wet season and winter precipitation; (ii) that modern groundwaters comprise little global groundwater; (iii) that “fossil” (>12,000‐year‐old) groundwaters dominate global aquifer storage; (iv) that fossil groundwaters capture late‐Pleistocene climate conditions; (v) that surface‐borne contaminants are more common in younger groundwaters; and (vi) that groundwater discharges generate substantial streamflow. Groundwater isotope data are disproportionately common to midlatitudes and sedimentary basins equipped for irrigated agriculture, but less plentiful across high latitudes, hyperarid deserts, and equatorial rainforests. Some of these underexplored aquifer systems may be suitable targets for future field testing. Plain Language Summary Water from the underground—groundwater—is the primary water supply to billions of humans. Understanding how groundwater originates and where it flows is important so that groundwater can be protected from pollution and overuse. One of the ways that scientists learn about groundwater is by measuring the abundances of heavier and lighter forms of elements in the groundwater. These isotopes help scientists map where groundwater comes from, measure how long it spends under the ground, and realize how important it is for generating river flows. This manuscript reviews the ways that measuring isotopes have helped scientists understand water resources and suggests ways that isotopes can contribute to understanding global groundwater resources even better. Key Points Global isotopic data quantify molecular movements and chemical reactions governing groundwater recharge, storage, and discharge Isotope measurements have been made in >100,000 well water and spring samples from >1,000 globally distributed aquifer systems This review presents global analyses of recharge seasonality, recharge elevations, groundwater age, groundwater discharges to rivers, and young versus old streamflow