In Situ Biostimulation of Cr(VI) Reduction in a Fast-Flowing Oxic Aquifer

Hexavalent chromium is among the most common and hazardous inorganic contaminants in soils and groundwater. A promising in situ bioremediation approach is the delivery of electron donors to stimulate microbially mediated Cr(VI) reduction, producing relatively insoluble Cr(III) precipitates. Even though this strategy has been implemented successfully in the past, it was primarily under favorable hydrological conditions, such as relatively slow groundwater flow velocities. To evaluate whether microbially mediated Cr(VI) reduction can be sustained at high groundwater flow velocities and under nearly oxygen-saturated conditions, we have conducted laboratory-scale column experiments and a field-scale pilot study using molasses as the electron donor. Despite the unfavorable conditions, both experiments provided clear evidence for microbially mediated Cr(VI) reduction. In particular, the well used for injection in the field experiment became anoxic after two months of injection and the Cr(VI) concentration decreased by a factor of six. After stopping the injection, these conditions prevailed for at least four months. In the column experiments, Cr(VI) reduction was accompanied by distinct Cr isotopic fractionation characterized by an enrichment factor of -1.25 parts per thousand. In contrast, Cr isotope data collected from the field experiment were ambiguous, which was because of the complexation of Cr(III) with organic carbon and the heterogeneous distribution of molasses due to the presence of narrow preferential flow paths. A substantial injectivity decrease likely caused by the formation of biofilm and the precipitation of Fe sulfides formed an additional challenge during the field experiment. Our data, however, also suggest that most of the identified challenges could be addressed by limiting the targeted dissolved organic carbon concentrations to about 10 mg/L. In conclusion, the injection of molasses constitutes a promising bioremediation strategy for the long-term treatment of Cr(VI) contamination even at high flow rates and under oxic conditions.