Redox pathways in a petroleum contaminated shallow sandy aquifer: Iron and sulfate reductions

A comprehensive hydro-geochemical characterization was carried out in a petroleum-contaminated shallow sandy aquifer in South Africa. The results indicate the presence of a BTEX (benzene, toluene, ethylbenzene, and xylene) plume that has moved, although only slightly, along the regional hydraulic gradient from the spill site. Associated with the contaminant plume, spatial distribution pattern of terminal electron acceptors and metabolites indicates simultaneous occurrence of nitrate, manganese, iron and sulfate reductions resulting in overlapping redox zones within the aquifer. From the measured concentration of metabolic by-products, sulfate and iron reductions seem to be the dominant metabolic pathways, though. Incubation experiments conducted with hydrocarbon contaminated aquifer sediments and inherent microbial assemblages provide a sulfate reduction rate of 4272 nmol cm(-3) day(-1) and 96 nmol cm(-3) day(-1) for winter and summer, respectively. As oppose to this, iron reduction dominates in summer with measured respiration rate of 1414 nmol cm(-3) day(-1). In winter iron reduction was measured to be only 24 nmol cm(-3) day(-1). Based on the dissimilatory iron and sulfate reduction rate measurements, we predict that at the aquifer site, intrinsic BTEX oxidation is primarily occurring in winter and is coupled to sulfate reduction. Although widespread in the aquifer, the contribution of iron reduction for the removal of aromatic monocyclic hydrocarbons is relatively minor.