Performance monitoring of a permeable reactive barrier (PRB) for the remediation of acidic groundwater in acidic sulfate soil (ASS) terrain
The treatment of acidic drainage contaminated with potentially toxic metals such as dissolved aluminium (Al) and iron (Fe) resulting from pyrite oxidation in acid sulfate soil (ASS) is a challenging problem requiring novel engineering solutions. Traditional remediation techniques of lime neutralization, groundwater manipulation (e.g. fixed-level weirs), tidal buffering (e.g. modified floodgates) are unsuitable due to their inherent inability to prevent pyrite oxidation and treat acid previously generated, as well as the risk of flooding during heavy rainfall events. As an alternative, the first permeable reactive barrier (PRB) with recycled concrete aggregates as the reactive material was installed in 2006 in ASS terrain, southeast New South Wales, Australia. Despite variable environmental conditions in the field, continuous monitoring of the performance of the PRB has confirmed the successful neutralization of the acidic groundwater (∼ pH 3) to neutral pH (∼ pH 7.27) and near-complete removal of Al and Fe (∼ 95%). However, there has been a slow decrease in the groundwater pH within the PRB from an initial pH of 10.2 to 7.27 due to exhaustion of the recycled concretes alkalinity generating minerals as well as chemical armoring and partial clogging of the PRB by Al- and Fe-oxy/hydroxide precipitates, as indicated by laboratory column experiments and geochemical modeling. This paper outlines the long-term performance of the PRB to date and discusses the current challenges concerning its efficiency and longevity.