Permeable Reactive Barrier (PRB) Technology: An Innovative Solution for the Remediation of Acidic Groundwater from Acid Sulphate Soil (ASS) Terrain

Abstract

The remediation of acidic groundwater contaminated with potentially toxic metals such as aluminium (Al) and iron (Fe) resulting from the oxidation of sulphidic materials in acid sulphate soils (ASSs) is a challenging geo-environmental problem that requires innovative engineering solutions. In low-lying coastal floodplains, the remediation strategies of groundwater manipulation (e.g. fixed-level weirs) and tidal buffering (e.g. two-way modified floodgates) are not feasible due to the risk of flooding during heavy rainfall events and their inability to prevent pyritic oxidation. In view of this in 2006, the first pilot subsurface permeable reactive barrier (PRB) using recycled concrete for the remediation of acidic groundwater (~ pH 3) was employed in ASS terrain in southeast New South Wales, Australia. While monitoring has confirmed the PRB has successfully neutralized the acidic groundwater to ~ pH 7.3 and removed ~ 95% of Al and Fe, this technology is not without its challenges. These have included the: (1) selection of the appropriate reactive material; (2) elucidation of the mechanisms involved in the neutralization of the acidic groundwater; (3) chemical armouring and possible clogging of the recycled concrete by Al and Fe oxy/hydroxide precipitates; and (4) thus, uncertainty regarding the longevity of the PRB. This paper will present details on the screening process of reactive materials, the installation of the PRB, the column experiments simulating the flow of acidic groundwater through the PRB for the determination of the predominant neutralization reactions occurring within the PRB, the long-term performance of the PRB and the current research strategy for determining its longevity.