Unraveling the partial failure of a permeable reactive barrier using a multi-tracer experiment and Cr isotope measurements

At a Cr(VI) contaminated site in Thun, Switzerland, a permeable reactive barrier (PRB) was installed in 2008. Downstream Cr(VI) concentrations did not indicate any sign of its successful operation more than 2 years after PRB installation. The cause for this potential PRB failure was investigated by performing Cr isotope measurements and a multi-tracer experiment. The combination of reactive (Cr isotopes) and non-reactive tracers allowed characterizing the groundwater flow regime in the vicinity of the PRB in detail. In particular, it could be confirmed that most of the Cr(VI) load is currently bypassing the barrier, whereas only a minor Cr(VI) load is flowing through the PRB. Fitting of observed breakthrough curves using a conventional advection dispersion model resulted in average linear flow velocities of 13-15 m/day for the bypassing Cr(VI) load and 4-5 m/day for the Cr(VI) flowing through the barrier. Using a Rayleigh fractionation model a Cr(VI) reduction efficiency of 77-98% was estimated for the Cr(VI) load that is flowing through the barrier. In contrast, a value of 0-23% was estimated for the current overall PRB reduction efficiency. It is concluded that the PRB bypass and the low overall Cr(VI) reduction efficiency are caused by a limited PRB permeability inherited from skin effects that occurred during PRB emplacement.