Environmental Isotopes in Hydrogeology

Abstract

Abstract In the last three to four decades, environmental isotopes have contributed immensely to studies and investigations in hydrogeology, complementing physical and chemical hydrogeology. Many hydrogeologic studies use stable isotopes of the water molecule to determine groundwater quality, origin, recharge mechanism, and rock–water interaction. A goodly number of the applications of environmental isotopes in hydrogeology have been in the arid and semiarid areas of the world, where water scarcity is most acute and poses constraints on economic development. A substantial amount of basic data and results of applied field investigations have already been published on applications of different isotopes for hydrologic purposes. Environmental isotopes provide indications of groundwater age and serve as natural traces for groundwater provenance. Stable isotopes of carbon, boron, nitrogen, and sulphur (i.e., 13 C/ 12 C, 11 B/ 10 B, 15 N/ 14 N, and 34 S/ 32 S) can give valuable information about reactions involving these elements and can also serve as pollution tracers. On the other hand, radioactive isotopes of some of these elements decay, providing us with a measure of the circulation time and, invariably, groundwater renewability. The most common of the radioisotopes, carbon‐14, is used to estimate groundwater residence time. Isotopes of the uranium series ( 234 U, 238 U, 226 Ra, and 222 Rn) are also useful tracers in isotope hydrogeology but are often not precise enough to establish the age of groundwater due to mineral–water interactions.