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.

Biodegradation of petroleum hydrocarbons in the environment is well-documented, widespread, and naturally occurring (Wiedemeier et al., 1999). Where petroleum hydrocarbons undergo natural biodegradation in contact with groundwater, dissolved manganese may be found at relatively high concentrations in the groundwater (Deutsch, 1997). The presence of manganese can affect the aesthetic quality of groundwater for domestic use due to precipitation of the manganese as black deposits on fixtures and clothing (Driscoll, 1986). The U.S. Environmental Protection Agency (USEPA) has established a Secondary Maximum Contaminant Level (MCL) of 0.05 mg/L for manganese in drinking water (tap water). Some states, including California, have established similar limits for manganese based on aesthetic considerations (California Department of Health Services, 2000).

The persistence of dissolved manganese associated with the biodegradation of petroleum hydrocarbons is an important consideration in deciding whether engineered remediation of a petroleum release in contact with groundwater is necessary. The dissolution of manganese is reversible under oxidizing conditions. In this article, geochemical methods are used to predict the precipitation of dissolved manganese outside the region affected by petroleum biodegradation. These predictions are compared with monitoring results from a petroleum release site in central California.