Hydrologic and Biochemical Processes Controlling Chromium Immobilization in a Low-Permeability Groundwater Zone

Abstract

Experiments and modeling were performed to investigate the coupled hydrologic and biochemical processes that control chromium (Cr) immobilization in a low-permeability groundwater zone. Bench-top flow cells were packed with a watersaturated high-permeability zone (HPZ) overlying a low-permeability zone (LPZ). Cr(VI) was initially flushed into the LPZ to establish a reservoir of this contaminant. Next, the electron donor acetate and the bacterium Geobacter sulfurreducens were flushed into the HPZ; they mixed with Cr(VI) in the LPZ and promoted its reduction. Experimental depth profiles show that approximately 80% of Cr(VI) introduced to the flow cell was immobilized as Cr(III) over 180 h within a small region on either side of the HPZ−LPZ interface. Groundwater flow and reactive transport in the flow cell were simulated using MODFLOW and RT3D, respectively, with dual-Monod kinetics defined in the custom reaction module of RT3D. Modeling results adequately matched experimental data and were extended to simulate Cr(VI) fate in a numerical flow cell with the same dimensions but with the LPZ replaced by a diffusion-controlled lower permeability clay matrix. For this scenario, approximately 99% of Cr(VI) in the LPZ could eventually be immobilized as Cr(III); this primarily occurred in the LPZ and mitigated Cr(VI) back diffusion to the HPZ. Overall, results from this work support acetate amendment to HPZs as an effective strategy to trap Cr(III) in LPZs and mitigate back diffusion of Cr(VI) into adjacent HPZs of a groundwater aquifer.