Environmental Science and Engineering Seminar

Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the PG&E Hinkley compressor station in the Mojave Desert near Barstow, CA. In-situ bioremediation using dilute ethanol commenced in 2010 and is expected to continue for up to 95 years. This treatment reduces Cr(VI) to trivalent chromium, Cr(III), which is rapidly sequestered by sorption to aquifer particle surfaces and by co-precipitation within Fe and Mn bearing minerals forming in place as reduction proceeds. Successful remediation requires that Cr(VI) removal is permanent and that no Cr(III) will re-oxidize back to Cr(VI) in the event of changing hydrologic conditions that may cause oxygen-rich groundwater to re-enter the treatment zone. We conducted multi-year laboratory microcosm experiments to explore the process of reductive sequestration and the potential for re-oxidation of Cr(III) to Cr(VI). Re-oxidation of Cr was observed in experiments with previously reduced material after just 14 days of exposure to oxygen. The nature and locus of Cr(III) sequestration determined its vulnerability to re-oxidation. We show that co-location of Cr with Mn at later stages of reduction influenced the susceptibility of Cr(III) to re-oxidation in microcosms. Site managers may need to plan for long-term monitoring and the possibility of active maintenance of anoxic conditions within the treatment zone to ensure permanent sequestration of Cr after biomineralization with ethanol ceases.