First results of operating and monitoring an innovative design of a permeable reactive barrier for the remediation of chromate contaminated groundwater

An innovative setup of a permeable reactive barrier (PRB) was installed in Willisau, Switzerland to remediate chromate contaminated groundwater. Instead of a conventional continuous barrier, this PRB consists of cylinders installed in rows: a single row for lower expected CrVI-concentrations and an offset double row for higher expected CrVI-concentrations. The cylinders are filled with reactive grey cast-Fe shavings mixed with gravel to prevent extensive precipitation of secondary phases in the pore space. The treatment of the contaminants takes place both within the cylinders and in the dissolved FeII plume generated downstream of the barrier. Monitoring of the contamination situation over a period of 3 a provided evidence of the mobilization, transport and behavior of the contaminants in the aquifer. Groundwater and reactive material were sampled upstream, within and downstream of the barrier by a Multi-Port Sampling System (MPSS) that revealed the geochemical processes as a function of time and space. Comprehensive chemical analyses included sensitive parameters such as CrVI, FeII/FeIII, redox potential, dissolved O2 and pH. Several campaigns using multiple optical tracers revealed a rather complex hydrological regime at different scales, thereby complicating the barrier performance.Results from the large 3D hydrogeochemical dataset show that the double row of cylinders successfully treated the chromate contamination. Remediation by the single row was not effective enough due to insufficient lateral overlap of the cylinders and their FeII-plumes. The low amount of precipitated secondary phases observed in the pore space of the reactive material reduced the risk of clogging the system and suggested a favorable longevity of the barrier. Limiting factors for the long-term operation are inferred to be the availability and accessibility of FeII within the cylinders and the concentration within the generated FeII-plume.