Assessing the Cr(VI) reduction efficiency of a permeable reactive barrier using Cr isotope measurements and 2D reactive transport modeling

In this study, we examine the efficiency of a permeable reactive barrier (PRB) installed in Thun, Switzerland, for the reduction of chromium(VI) using Cr isotope measurements and 2D reactive transport modeling. The PRB, constructed with iron shavings and gravel, was analyzed two years post-installation, revealing consistently elevated Cr(VI) concentrations downstream, surpassing Swiss threshold values. We utilized δ53/52CrSRM979 isotopic measurements to indicate Cr(VI) reduction by the PRB, observing positive fractionation in certain samples, which suggests reduction activity, while another sample indicated a bypass of the barrier due to the highest Cr(VI) concentration. This finding was further supported by a Rayleigh fractionation model estimating a reduction efficiency of approximately 15%. Our simulations indicated that partial bypassing of the Cr(VI) plume, attributed to inadequate permeability within the PRB structure, significantly influenced observed reduction rates. Consequently, achieving complete Cr(VI) reduction in highly oxygen-saturated aquifers with high groundwater velocities is challenging, and we recommend additional remediation measures to mitigate environmental impact.