Evolution of Redox Processes in Groundwater

Abstract

Reduction/oxidation (redox) processes affect the chemical quality of groundwater in all aquifer systems. The evolution of redox processes in groundwater is dependent on many factors such as the source and distribution of electron donors and acceptors in the aquifer, relative rates of redox reaction and groundwater flow, aquifer confinement, position in the flow system, and groundwater mixing. Redox gradients are largely vertical in the recharge areas of unconfined aquifers dominated by natural sources of electron donors, whereas substantial longitudinal redox gradients can develop in unconfined aquifers when anthropogenic sources of electron donors are dominant. Longitudinal redox gradients predominate in confined aquifers. Electron-donor limitations can result in the preservation of oxic groundwater over flow distances of many kilometers and groundwater residence times of several thousand years in some aquifers. Where electron donors are abundant, redox conditions can evolve from oxygen reducing to methanogenic over substantially shorter flow distances and residence times.