Impact of labile organic carbon and manganese oxide on chromium and vanadium subsurface mobility: evidence from laboratory incubation experiments

Chromium (Cr) and vanadium(V) are redox-active, geogenic contaminants observed to co-occur in groundwater in the North Carolina (NC) Piedmont region. On a landscape-scale, factors controlling Cr and V solubilization to groundwater in the Piedmont are understood to be largely associated with the regional geology. However, the specific mechanisms mediating (bio)geochemical interactions among heterogeneous geologic materials and redox active chemical inputs in the subsurface are poorly understood. The specific goal of this research was to elucidate the chemical controls on the solubilization of Cr and V from saprolite - chemically weathered rock between soil and bedrock - to groundwater. We conducted 40-day batch incubation experiments using chemically variable saprolites from the NC Piedmont to evaluate dynamics of Cr and V solubilization as influenced by interactions between common chemical inputs. Organic carbon (citric acid) additions stimulated dissolution of Cr and V to the aqueous phase, with abiotic controls generating greater concentrations of Cr and V than biotic incubations. Addition of the oxidant manganese (Mn)-oxide suppressed solubilization of Cr and V from the saprolites. Across all experiments, dissolved Cr and V concentrations were positively correlated (<i>R</i>² = 0.81-0.99) with dissolved iron (Fe) concentrations. Overall, these results highlight how organic carbon inputs can modulate the cycling and solubilization of Cr and V in heterogeneous media, and our results may be impactful in making better predictive and vulnerability assessments plans, particularly in delineating abiotic <i>vs.</i> biotic roles driving Cr and V dissolution to groundwater.