Isotope Hydrogeology

Abstract

Investigations of subsurface fluids employ a wide variety of both stable and radioactive isotopic systems, informing a wide range of scientific study. Use of isotopes can improve understanding of fluid sources, fluid flow and solute transport, subsurface processes, solute origins, evolution, and reaction rates and mechanisms, paleoclimate, and fluid “age.” Advances in analytical methods and technology have greatly increased the number of isotopic systems now used to investigate subsurface fluid and solute transport. Here, the fundamentals of isotope investigations in the subsurface are explained and illustrated. Although not every isotopic system is discussed in detail, examples are provided that can aid application to other systems. An overview of the use of isotopes in determining fluid origins and processes affecting fluids in near-surface and deeper sedimentary and crystalline rock environments is provided. Next, an introduction to the use of isotopes to trace solute and contaminant origins and fluxes through the subsurface and interactions between subsurface fluids, fluids and minerals, and the critical zone is provided. Finally, an overview of radioactive and stable isotope analysis to determine subsurface fluid residence time explores the challenges and possibilities for quantifying rates of fluid and solute movement through the subsurface and interactions with the critical zone.