Site groundwater management strategies : groundwater metal remediation using Permeable Reactive Barriers

Permeable reactive barriers (PRBs) can be a successful site management tool for groundwater impacted by Metal Leaching and Acid Rock Drainage (ML/ARD). Using a sustainable biological and chemical treatment approach, they intercept and passively treat dissolved metal contaminant plumes, thereby reducing risks to the receiving environment and downstream receptors. Drawing on previous experience with PRBs containing an innovative mixture of organic leaf compost, limestone and zero valent iron, we will discuss strategies for remediation of metal contaminant plumes. PRBs are flexible and can be adapted to site conditions. PRB design considers contaminant concentrations and plume morphology; then optimizes PRB physical configurations and media composition to best match target treatment performance requirements. Tandem treatment walls can be designed to intercept high dissolved metal concentrations. Principals for design and installation in tidally influenced foreshore environments are also considered including the placement of targeted media mixes. Installation techniques successfully applied to date have included a slurry-assisted continuous trenching method through sand and gravels and rejuvenation of existing walls using a caisson method to sustainably extend the life of PRB sections. A review of PRB geochemical performance has demonstrated successful reduction of metal concentrations through bacterially mediated sulphate reduction with formation of sulphide minerals, formation of mineral oxyhydroxides, and adsorption and co-precipitation with iron oxyhydroxides. Removal rates have been typically greater than 99% (for example, copper and zinc reductions from magnitudes of 10’s mg/L to