Groundwater Geochemistry and Isotopes provides a practical and theoretical understanding of geochemistry and environmental isotopes in hydrogeology with applications in both natural groundwater and in contaminated settings. This text teaches the thermodynamic basis and principal reactions involving the major ions, gases and isotopes during groundwater recharge, weathering, and redox evolution. This second edition brings additional case studies and new developments in the practical use of isotopes and geochemistry in understanding groundwater origin, evolution and anthropogenic impacts. Additional solved problems and example calculations are available as a free e-manual by request (idclark@uottawa.ca). The new edition develops the fundamentals of environmental isotopes and geochemistry and provides approaches for consultants and practitioners as well as undergraduates to address problems in natural and contaminant hydrogeology. Covers the basics of solutes, gases and isotopes in water, and concentration-activity relationships and reactions Describes tracing the water cycle, weathering, and the geochemical evolution of water quality Explores dating groundwater as young as a few years to over hundreds of millions of years Includes clear figures and calculations to enhance comprehension Uses case studies to demonstrate the integration of geochemistry and isotopes for contaminated groundwater.