A field, laboratory, and modeling study of natural attenuation processes affecting the fate and transport of hexavalent chromium in a redox variable groundwater environment

Prior to this study, a framework for evaluating the natural attenuation (NA) of hexavalent chromium (Cr(VI)) in groundwater did not exist (USEPA, 2001). In this work, the NA processes for Cr(VI) were comprehensively evaluated within a redox variable (i.e. fraction or organic carbon (foc), redox) groundwater environment through field, experimental, and modeling studies. Chromium speciation and partitioning were correlated with the reductive capacity and redox conditions of the aquifer soils spatially distributed within the aquifer. The potential for competition for reducing power under specific cases within the aquifer was suggested by the data, highlighting the need to include contaminant interactions in natural attenuation assessments. ^ The sorption and desorption behavior of Cr(VI) in near-source soils were evaluated by performing a variety of analyses and experiments at the laboratory and field scales. The results of the field and laboratory experiments were consistent. The magnitude of the secondary source (diffusion limited) effect is not significant due to low concentrations of leachable chromium contamination. ^ Currently, NA processes, such as adsorption, reduction, and precipitation, for Cr(VI) cannot be independently quantified in a mechanistic manner for natural soils where multiple processes are present. Laboratory experiments were conducted to characterize and quantify the capacity and kinetics of the combined effects of NA processes for Cr(VI). Equilibrium and kinetic attenuation terms included linear sorption (Kd), estimated sorption capacity, non-linear Langmuir (KL, Q sorption parameters, and a pseudo first order rate of disappearance of Cr(VI) from aqueous:soil slurries. An operationally defined kinetic attenuation term, Specific Attenuation Capacity (AC) was shown to exemplify the difference in Cr(VI) equilibrium capacity and kinetic attenuation rates. The equilibrium and kinetic parameters were shown to relate to soil pH and soil foc. ^ A conceptual framework for conducting Cr(VI) NA evaluations was developed and applied, incorporating the findings of this Cr(VI) NA evaluation. The conceptual framework follows a multi-faceted approach that includes the development of a conceptual site model, an initial screening for NA, laboratory and field experiments, long-term attenuation capacity calculations and transport modeling, and development of a long-term monitoring plan. Further, the NA evaluation framework utilizes a statistical analysis framework to incorporate uncertainty of parametric values.