Permeable reactive barrier (PRB) technology for the remediation of acidic groundwater from acid sulphate soil floodplain in south-east New South Wales

The release of acidic water contaminated with toxic metals such as aluminium (Al) and iron (Fe) resulting from pyrite oxidation within acid sulphate soils (ASSs) is a major environmental issue in Australia. Groundwater manipulation using weirs and modified floodgates is not effective in low-lying floodplains due to the high risk of flooding and their inability to prevent pyrite oxidation. Permeable reactive barrier (PRB) technology has been looked at as an alternative remediation technique in ASS terrain. Batch and short-term column tests were used to choose a suitable reactive material for the remediation of acidic water and the removal of Al and Fe, with recycled concrete deemed the most effective. Following these tests, a pilot-scale PRB was installed in ASS terrain on the Shoalhaven Floodplain, southeast New South Wales, Australia in October 2006. Despite a slow decrease in the performance of the PRB due to armouring effects caused by the precipitation of Al and Fe on the surface of the recycled concrete, continuous monitoring of the performance of the PRB over the last five years has shown an increase in pH to neutral (~ pH 7.3) and ~ 95% removal of Al and Fe from the groundwater. A long-term (30 months) column test conducted under controlled laboratory conditions for simulating the flow of acidic groundwater through the PRB determined that the predominant neutralisation reactions occurring within the PRB were the dissolution of carbonate/bicarbonate alkalinity, the redissolution of Al hydroxide precipitates and the re-dissolution of Fe oxyhydroxide minerals.