Introduction to groundwater geochemistry and fundamentals of hydrogeochemical modeling

A wide diversity of physical and chemical processes control the distribution of species in waters in the vadose zone above the water table, and in the saturated zone below. The mineralogical composition of rocks or sediments, chemical reactions between solid, aqueous and gas phases, and oxidation/reduction (redox) processes are principal factors that influence the chemical composition of vadose-zone, ground- and surface waters, and the concentrations and mobilities of individual species. This chapter discusses the sampling of water, solids and gases, key steps in the investigation of groundwater systems, and then provides a brief summary of the thermodynamic fundamentals relevant for understanding geochemical processes in the vadose zone and in the aquifers and aquitards below it. This is followed by a short overview of important hydrogeochemical processes such as dissolution, precipitation, exsolution and redox processes, as well as sorption and ion exchange processes. These last two sections will help the reader understand the subsequent chapters on geochemical modeling. For more information on the fundamentals of aqueous and groundwater geochemistry, the reader is referred to standard textbooks such as Stumm and Morgan (1996) and Appelo and Postma (2005)