Doctor of Philosophy
School of Civil, Mining and Environmental Engineering
Regmi, Gyanendra, Performance validation of a permeable reactive barrier (PRB) for treating acidic groundwater, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3833
The effectiveness of a permeable reactive barrier (PRB) to remediate contaminated groundwater from acid sulphate soil on the Shoalhaven Floodplain, southeast New South Wales (NSW), Australia was investigated. High concentrations of dissolved aluminium (Al3+), total iron (Fe), and sulphate (SO4 2-) in the groundwater along with low pH were evidence of acidic conditions due to pyrite oxidation at the study site. Groundwater manipulation using engineering solutions such as weirs and modified floodgates drains are not effective in low-lying ASS terrain, as they cannot remediate the acidity already present in the soil nor significantly prevent pyrite oxidation in areas far from nearby drains. This study combined laboratory, field and numerical analyses in order to determine the feasibility and performance of a PRB utilising zero-cost recycled concrete for the remediation of acidic groundwater in ASS terrain.
Long-term laboratory column experiments were carried out using synthetic and real groundwater from the study site. The column experiments investigated the acid neutralisation reactions occurring within the PRB and the precipitation of Al and Fe from the acidic groundwater. Three distinct pH-buffering reactions were ascertained: (i) the dissolution of carbonate/bicarbonate alkalinity from concrete at nearly neutral pH, (ii) the re-dissolution of aluminium hydroxide precipitates at pH ~4, and (iii) the re-dissolution of ferric oxyhydroxides minerals at pH < 3. However, carbonate/bicarbonate buffering was the most significant because of the maintenance of near neutral pH and complete removal of Al3+ and total Fe from the influent.
Chemical armouring and physical clogging, which are considered the major factors in reducing the efficiency of any reactive material, were also studied by evaluating the duration of buffering periods for maintaining neutral pH and also the changes in physical parameters (e.g. hydraulic conductivity and flow rate) due to mineral precipitation. Chemical armouring by secondary Al- and Fe- precipitates decreased the ANC of the recycled concrete by ~50% compared to its theoretical ANC. Furthermore, high concentrations of Al3+ and total Fe caused a rapid decrease in ANC efficiency due to accelerated armouring. Application of larger size concrete aggregates reduced the threat of physical clogging in the pilot-scale PRB. Furthermore, mineralogical and morphological analysis was carried out to characterise the recycled concrete used in the column experiments and the precipitates formed. Correlation between CaO reduction in the armoured concrete and the reduction in ANC validated the decline in ANC by chemical armouring. 3D image analysis was demonstrated to be a useful tool for the examination of the porous architecture, and the performance of PRB reactive materials in a novel yet quantifiable manner.
A comprehensive field study involved the monitoring of groundwater via piezometers and observation wells, installed up-gradient, within and down-gradient of the PRB, to observe changes in the level of the phreatic surface along with water quality parameters (e.g. pH, electrical conductivity (EC), oxidation reduction potential (ORP), temperature and concentration of anion and cations). Groundwater pH inside the PRB was maintained near neutral throughout the monitoring period. The concentration of Al3+ and total Fe were maintained below the Australian and New Zealand Environment and Conservation Council (ANZECC) (2000) criteria, in a similar manner to what was observed in the column experiments. Steady piezometric head observed within the PRB throughout the monitoring period confirmed that chemical and physical clogging did not occur within the PRB to an extent that would affect the permeability of the reactive material.
One-dimensional, simple reactive transport modelling was carried out based on data from a laboratory column experiment, mineralogical analysis of the recycled concrete and the PRB. Numerical modelling using MIN3P provides insights into the neutralisation mechanisms and geochemical evolution of groundwater along a flow path inside the PRB. The ability to make comparisons between the geochemically complex transport scenarios within the column experiments and pilot-scale PRB confirm that it can be used as an analysis tool for investigating the performance of PRBs in ASS terrain.
Overall, this study contributes a better understanding of the acid neutralisation processes occurring inside the PRB for the remediation of contaminated groundwater from ASS terrain and offers novel field, laboratory and modelling techniques to investigate and quantify these processes. The findings from the first pilot-scale PRB using recycled concrete as the reactive material confirms that it is a suitable environmentally friendly and cost-effective alternative to other conventionally utilised techniques (e.g. watertable manipulation, lime neutralisation) for the spot treatment of acidic groundwater in ASS terrain.