Laura Banasiak

Installation of a lime injection barrier for the remediation of acid sulphate soil problems

Oxidation of naturally occurring pyrite (FeS2) in certain low-lying clayey soils generates sulphuric acid, hence the term acid sulphate soils. A horizontal alkaline barrier was installed by radial grouting, for the purpose of remediating leachate from acid sulphate soils and preventing further oxidation. The current research relates to a large-scale field trial of this technique and the effect on the groundwater composition. In coastal Australia, a pyritic layer commonly exists in the soil at shallow depth that is at risk of oxidation, hence the main objective was to inject the barrier above the pyritic layer to (1) stop infiltration of oxygen to the pyritic layer and (2) neutralize any acidity stored in the soil. Two fine-grained alkaline materials, lime and fly ash, were assessed in this study. Lime was selected for its neutralizing capacity, and the fly ash was selected to accompany the lime to enhance the pozzolanic reactions. The optimum mix ratio of lime, fly ash and water to form an ideal slurry and the optimum depth and pressure of injection were experimentally determined. For the large-scale field trial, the slurry was injected into a systematic grid of 22 holes to form the reactive barrier. The groundwater composition was monitored in a network of observation holes across the study site to determine the effectiveness of the barrier. The average groundwater pH was 3.25 prior to installation of the barrier, and it rose to 4.6 after the barrier was installed. The influence of the barrier on the groundwater pH was greater in observation holes close to the barrier than in those further away. The concentrations of aluminium and iron decreased in the groundwater after the installation of the alkaline barrier. The ratio of Cl/SO4 in the groundwater increased after the barrier was installed, which confirmed that the barrier had successfully controlled the subsequent pyrite oxidation in the soil.

Permeable reactive barrier (PRB) technology: An innovative solution for the remediation of acidic groundwater from acid sulphate soil (ASS) terrain

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.

Characterisation and assessment of recycled concrete aggregates used in a permeable reactive barrier for the treatment of acidic groundwater

The acidification of coastal waterways because of acid sulphate soil is an environmental, economic and social problem within Australia. A pilot-scale permeable reactive barrier (PRB), using recycled concrete aggregates as reactive material, was installed in low-lying acid sulphate soil terrain for acidic groundwater remediation. Column experiments were previously undertaken with synthetic groundwater to ascertain the dominant reactions occurring within the PRB. Results showed that armouring of the reactive material surface by precipitated Al- and Fe-bearing minerals significantly reduced its acid neutralisation capacity (ANC). The purpose of this current study was to validate this decline in ANC through characterisation of the virgin and armoured concrete aggregates, and precipitates that formed on the concrete. Samples of concrete aggregates and precipitates were analysed using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy-Energy dispersive spectroscopy (SEM-EDS) and X-ray micro-computed tomography (μCT). The conclusions drawn from these analyses are that Al-bearing (gibbsite 14.3%, boehmite 10.9%) and Fe-bearing (goethite 38.2%) mineral precipitates of diverse morphology form as a thin layer coating the aggregate surfaces. A reduction of CaO in the armoured concrete aggregates by 47% correlates with the reduction in ANC of the virgin concrete by 50% due to armouring.