5. Geochemistry and Managed Aquifer Recharge Basics

Groundwater over time approaches chemical equilibrium with respect to the reactive minerals present in an aquifer. Aquifers may contain some minerals that are essentially unreactive in that their dissolution and precipitation rates are exceedingly slow under the temperature, pressure, and chemical conditions that occur in most near-surface groundwater environments. Anthropogenic aquifer recharge (AAR) very commonly results in the introduction of water into an aquifer that is compositionally different from the native groundwater and in disequilibrium with aquifer minerals. Geochemical processes potentially active in AAR systems can decrease hydraulic conductivity (cause clogging), increase hydraulic conductivity by dissolving grains and cements, naturally attenuate contaminants present in recharged water, and adversely impact water quality by the release of dissolved ions (e.g., arsenic and metals). At a minimum, it is important to have at least a qualitative understanding of the geochemical processes that might be active in a given system and their potential impacts on system performance. Major aquifer recharge systems may require detailed mineralogical and water chemistry evaluations and geochemical modeling to predict and develop strategies to manage active geochemical processes.