Aqueous and Solid Phase Chemistry of Sequestration and Re-oxidation of Chromium in Experimental Microcosms with Sand and Sediment from Hinkley, CA

Cr(VI) contaminated groundwater at Hinkley is undergoing bioremediation using added ethanol as a reductant in a volume of the aquifer defined as the In-situ Reduction Zone (IRZ). This treatment effectively reduces Cr(VI) to Cr(III) which is rapidly sequestered by sorption to aquifer particle surfaces and by co-precipitation within iron or manganese bearing minerals forming in place as reduction proceeds. Successful mitigation of the extant Cr(VI) plume is projected to require 90 to 220 years, at which time ethanol loading will likely cease. This projection assumes that Cr(VI) removal is permanent and that no Cr(III) will oxidize back to Cr(VI) in the event of changing hydrological conditions resulting in oxygen rich water re-entering the IRZ. We undertook laboratory microcosm experiments to explore the process of reductive sequestration and the potential for re-oxidation. In reduction experiments, batch microcosms were prepared with materials from sites within the aquifer, up-gradient of the Cr(VI) regulatory plume. Control microcosms were prepared using cleaned quartz sand and iron (Fe)-coated sand. Unfiltered Mojave River groundwater and an added tracer of 50Cr were reacted with microcosm materials for two years. During this time, bio-reduction was stimulated by repeated additions of diluted ethanol to maintain reduced conditions within appropriate ranges; avoiding sulfate reducing or methanogenic conditions as much as possible while mimicking field conditions. To evaluate the potential for re-oxidation of Cr(III), additional batch microcosms of site materials and mixtures of Fe- and Mn-coated sand were first reduced for more than 1 year and subsequently exposed to air and allowed to oxidize for up to 2 years. Cr(VI) was released during oxidation of all materials. The amount of Cr(VI) released and when it was released varied among substrates. Our results showed that the nature and locus of Cr(III) sequestration influenced it’s vulnerability to re-oxidation back to Cr(VI). This data release includes three tables in *.csv format with the preliminary experiment (preliminary.csv), reduction experiment (reduction.csv), and the oxidation experiment (oxidation.csv) along with a Google Earth file of the well sites (wells.kmz).