Arsenic (As), chromium (Cr), and vanadium (V) are naturally occurring, redox-active elements that can become human health hazards when they are released from aquifer substrates into groundwater that may be used as domestic or irrigation source. As such, there is a need to develop incisive conceptual and quantitative models of the geochemistry and transport of potentially hazardous elements to assess risk and facilitate interventions. However, understanding the complexity and heterogeneous subsurface environment requires knowledge of solid-phase minerals, hydrologic movement, aerobic and anaerobic environments, microbial interactions, and complicated chemical kinetics. Here, we examine the relevant geochemical and hydrological information about the release and transport of potentially hazardous geogenic contaminants, specifically As, Cr, and V, as well as the potential challenges in developing a robust understanding of their behavior in the subsurface. We explore the development of geochemical models, illustrate how they can be utilized, and describe the gaps in knowledge that exist in translating subsurface conditions into numerical models, as well as provide an outlook on future research needs and developments.

Geogenic chromium (Cr) and vanadium (V) contamination of groundwater in the Piedmont region of North Carolina poses threats to public health. These contaminants are naturally derived from saprolite and aquifer materials, but geochemical variability in these materials makes it difficult to predict specific risks of Cr and V in well water. The objectives of this study were to (1) determine host phases of Cr and V in representative subsurface materials; (2) characterize contaminant binding parameters of chemically variable saprolites; and (3) examine the influence of saprolite chemistry on contaminant sorption, speciation, and phase associations. Isotherm experiments revealed that saprolite samples sorbed roughly an order of magnitude more V than Cr. Chemical extractions and synchrotron-based X-ray fluorescence showed that substantial Cr and V were bound with metal oxide/oxyhydroxides in native and Cr-and V-sorbed saprolites; however, electrostatically bound fractions were also present, representing potentially important sources of groundwater contamination. X-ray absorption-near-edge-structure spectroscopy indicated that sorbed Cr was found as reduced Cr(III), whereas sorbed V was dominated by the oxidized V(V) and intermediate V(IV) species. Results from this study could be used to help parameterize mechanistic models and improve prediction of the Cr and V contamination potential of shallow aquifers.

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 (R² = 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 vs. biotic roles driving Cr and V dissolution to groundwater.