Modelling the Performance of a Permeable Reactive Barrier utilised to Reduce the Risk of Acidic Groundwater in the Shoalhaven Floodplain

Acidic groundwater generated from acid sulfate soil (ASS) usually carries high concentrations of aluminium (Al) and iron (Fe), which create unfavourable conditions to living habitat. The ASS research team at the University of Wollongong, Australia implemented an innovative geotechnical engineering technique for the remediation of acidic groundwater through a permeable reactive barrier (PRB) using recycled concrete aggregates as the reactive material. This PRB was installed at the Shoalhaven Floodplain, southeast New South Wales (NSW), Australia in October 2006 and has proved effective in neutralisation of groundwater by increasing the pH from 3.6 to 7 and removing 99% of Al and Fe from groundwater to date. Dissolved Al and Fe were removed through continuous precipitation which would clog the pore spaces of the reactive medium by secondary mineral precipitation. This paper provides a complete evaluation of the performance of the PRB through field work and groundwater flow modelling coupled with geochemistry. The developed model (using finite difference codes: MODFLOW and RT3D) describes the chemical clogging due to mineral precipitates and the associated reductions in porosity and hydraulic conductivity of the reactive medium. The results obtained from numerical modelling, groundwater samples analysis and mineralogical analysis of barrier specimens confirm that the current PRB has performed well during the last seven years. Only a smaller amount of clogging was evident at the entrance of PRB with only a 3% reduction of hydraulic conductivity. This model would be beneficial for the environmental scientists and geotechnical engineers who have to deal with the ASS problems, especially in coastal Australia.