Identifying Redox Transition Zones in the Subsurface from a Site with Historical Contamination

Abstract Reactive iron mineral coatings found throughout reduction-oxidation (redox) transition zones play an important role in contaminant transformation processes. This research focuses on demonstrating a process for effectively delineating redox transition zones at a site with historical contamination. An 18.3 meter core was collected, subsampled, and preserved under anoxic conditions to maintain its original redox status. To ensure a high vertical resolution, sampling increments of 5.08 cm in length were analyzed for elemental concentrations with X-ray fluorescence (XRF), sediment pH, sediment oxidation-reduction potential (ORP), total volatile organic carbon (TVOC) concentration in the sample headspace, and abundant bacteria (16S rRNA sequencing). Over the core’s length, gradients observed ranged from 3.74 to 8.03 for sediment pH, -141.4 mV to +651.0 mV for sediment ORP, and from below detection to a maximum of 9.6 ppm TVOC concentration (as chlorobenzene) in the headspace. The Fe and S gradients correlated with the presence of Fe and S reducing bacteria. S concentrations peaked in the Upper Zone and Zone 1 where Desulfosporosinus was abundant, suggesting precipitation of iron sulfide minerals. In Zone 2, Fe concentrations decreased where Geobacter was abundant, potentially resulting in Fe reduction, dissolution, and precipitation of minerals with increased solubility compared to the Fe(III) minerals. Using complementary geochemical and microbial data, five redox transition zones were delineated in the core collected. This research demonstrates a systematic approach to characterizing redox transition zones in a contaminated environment.