In the subsurface, the redox state of water strongly affects reactions of contaminants particularly nitrate, pesticides, arsenic, and chlorinated ethenes. In this session, we will examine the diverse physical and chemical factors that control subsurface redox conditions. These factors may include one or more of the following: (1) the transport and exchange of dissolved and gaseous chemicals in the unsaturated zone (2) the abundance and reactivity of reduced mineral species and organic carbon that vary with the geologic formation (3) chemistry of paleo and modern recharge waters (4) increased modern recharge and vertical mixing due to irrigation and ground-water pumping (5) anthropogenic fluxes of substantial quantities of redox sensitive solutes such as NO3− and dissolved organic carbon, and (6) the activity of microbes that facilitate redox reactions. We seek contributions describing the redox conditions of the subsurface and how these are related to geology, paleohydrology, and modern hydrology. Modeling studies that examine redox evolution and studies that show how the fate of contaminants is tied to the hydrology and geology are also welcome.