Dissolved manganese affects the aesthetic quality of groundwater used for domestic purposes. Reducing conditions created by natural biodegradation of petroleum hydrocarbons in contact with groundwater can cause manganese to dissolve from aquifer sediments. However, the reaction is reversible by chemical oxidation downgradient of the bioremediation shadow. Geochemical modeling (MINTEQA2) predicts the redox conditions for manganese to precipitate from groundwater back to aquifer sediments. The reaction rate for manganese precipitation is related to the advective and dispersive transport of dissolved oxygen in groundwater. At a petroleum release site in central California, the reaction rate was observed to follow first-order kinetics. By knowing the site-specific aqueous geochemistry and reaction kinetics, measures can be taken with reasonable certainty to construct water supply wells to avoid producing water containing unacceptable levels of manganese.