Generation of acidic groundwater attributed to pyrite oxidation in low-lying acid sulfate soil causes substantial damage to the soil-water environment and civil infrastructure. The installation of Permeable Reactive Barrier (PRB) is a frontier technology in the field of acid neutralisation and for removing toxic heavy metal cations, e.g. soluble Fe and Al. This study aims to assess the potential of limestone (calcite) aggregates as the PRB's main reactive material in low-lying pyritic land. During long-term laboratory column experiments, a significant capacity of limestone for removing the contaminant chemical species was observed. Nevertheless, the formation of secondary mineral precipitates upon geochemical reactivity within the granular media in the PRB caused armouring and chemical clogging that diminished the rate of reactivity, i.e. the treatment capacity of calcite aggregates, mainly at the entrance zone of the porous media. Flow properties were altered due to blockage of pores; for instance, hydraulic conductivity was reduced by 25% at the inlet zone. Non-homogeneous clogging towards the outlet was analysed, and the time-dependent effect on longevity of a limestone column was studied and quantified.
This study offers an analytical solution for radial consolidation that captures the bio-geochemical clogging effect in acid sulfate soils. Field sites and personal communication with industry practitioners have provided evidence of piezometers exhibiting retarded pore pressure readings that do not follow conventional soil consolidation and seepage principles when installed in coastal acidic floodplains. This retarded response together with a variation in pH, ion concentrations, and piezometric heads provided evidence of clogging at and around the piezometers. This paper uses the proposed bio-geochemical clogging model, which is an analytically derived system of equations to estimate the excess pore water pressure (EPWP) dissipation of piezometers installed in clogging-prone acid sulfate soils. The inclusion of the total porosity reduction attributed to biological and geochemical clogging improves the predictions of the retarded dissipation of excess pore pressure, especially after about 1 year. This method is validated for two previously identified acidic field sites in coastal Australia, where piezometers measured a very slow rate of dissipation. It is concluded that this model has potential to accurately monitor the performance of critical infrastructure such as dams and embankment foundations built on acidic terrain.
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