Tools and methods for distinguishing and characterising sources of groundwater contamination in a complex urban re‐development precinct: Fishermans Bend, Melbourne, Australia
Creators: name: Hepburn, Emily
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 370799
subject: Hydrology not elsewhere classified

description: This research has stemmed from a major collaboration project between researchers at RMIT University, CRC CARE and environmental regulators at the Environment Protection Authority (EPA) Victoria, in Melbourne, Australia. The project was initiated after the state planning authority re‐zoned 240 hectares of under-utilised brownfield (former industrial) land at the Fishermans Bend precinct, located 1 km from Melbourne’s Central Business District (CBD). Despite extensive impacts to groundwater from historical (‘legacy’) landfills, former industrial sites and contaminated fill in the upper soil profile, the precinct is currently being transformed into residential land as part of Australia’s largest ever inner‐city re‐development project. <br><br>Arguably one of the most significant challenges for such large or ‘precinct scale’ urban re‐development projects is the technical assessment and management of contaminated groundwater. Whilst detailed assessments of groundwater contaminant sources will typically be conducted on a ‘site‐by‐site’ basis, in precincts such as Fishermans Bend, where large parcels of land containing hundreds of individual sites are being re‐developed, access to precinct‐wide data and an understanding of processes governing the behaviour of contaminants from different sources are critical for contamination assessment. Such understanding is vital in addressing common challenges such as distinguishing point sources from larger plumes in order to maximise the efficacy with which health and ecological risks can be assessed and managed.<br><br>The complex physical and chemical characteristics often associated with such settings, however, can make understanding the hydrogeological system challenging. For example, areas selected for urban re‐development are often located on coastlines, where surface water bodies and former swamps/wetlands can influence hydrogeological processes and seasonal dynamics. In addition, remnant underground infrastructure may artificially recharge and/or drain urban groundwater and multiple contamination sources may impact groundwater quality. <br><br>Legacy landfills in such settings are particularly common and were typically built with little or no leachate treatment and/or controlsystems, thus acting as ongoing sources of contamination to groundwater. Contaminants typically associated with landfill leachate in groundwater include ammonia, methane, bicarbonate, sulfate, and heavy metals. Recent research has also detected various perfluoroalkyl acids(PFAA),such as perfluorooctanoic acid (PFOA) in leachate‐impacted groundwater. Depending on the fate and transport of such contaminants and the potential exposure pathways, the associated ecological and human health risks may continue for a significant period after waste acceptance has ceased.<br><br>In this study, groundwater samples were collected over multiple seasons from 36 shallow bores installed by EPA Victoria across Fishermans Bend in November 2015. Data were stored and managed in ESdat and exported for visual display in ArcGIS. The data were then used to create a simple Geographic Information System (GIS)‐based Decision Support Tool (DST), designed to help environmental regulators and practitioners more effectively characterise the hydrogeological system and separate diffuse contaminant plumes from point sources. The DST is intended to complement rather than substitute single‐site assessments and environmental audits.<br> <br>Stable and radio‐isotopic indicators were analysed in a sub‐set of the groundwater samples in order to understand solute origins, groundwater flow paths, recharge and residence times at Fishermans Bend. Groundwater in the shallow aquifer was found to be predominantly recharged by modern rainfall with short residence times, indicated by fresh to brackish salinity and relatively high tritium (3H) activities. In contrast, the adjacent/lower aquitard was found to contain saline groundwater with molar ratios reflecting typical marine water, indicating relict salts emplaced as porewater at the time of sediment deposition. The presence of tritium above background levels in the aquitard suggests a component of modern recharge, likely sourced by ingress from the adjacent Yarra River.<br><br>The concentrations and proportions of a range of PFAA in groundwater surrounding legacy landfills at Fishermans Bend were determined, and relationships between PFAA and conventional indicators of leachate contamination (e.g. ammonia) were analysed. Different ratios of PFAA were found to reflect different contamination sources, including legacy landfills and an industrial point source. A new geochemical index was developed for enhanced detection of leachate impacts on groundwater, by incorporating PFAA (as PFOA/∑PFAA) into an existing method based on ‘leachate to native’ cation ratios (L/N ratios). The new ‘modified L/N ratios’ were able to distinguish statistically significant differences between leachate‐impacted and non‐impacted groundwater, where the standard L/N ratios could not.<br><br>A method for separation of different potential sources of heavy metals in groundwater at Fishermans Bend was developed using statistical data categorisation, analysis of soil leaching values and fill/sediment X‐ray fluorescence (XRF) profiling of the 36 boreholes. The method identified two major sources of heavy metals in groundwater: 1. point sources from up‐gradient groundwater contaminated by industrial activities and/or legacy landfills; and 2. contaminated fill (a combination of inert and contaminated waste material in the upper soil profile), where leaching of Cu, Mn, Pb and Zn was observed. The method can determine the likely contribution of these different heavy metal sources in groundwater, helping inform more detailed contamination assessments.<br><br>Overall, this research provides a comprehensive understanding of the hydrogeological system at Fishermans Bend, and provides novel methodologies allowing for the identification of different sources of contaminants in shallow groundwater. The findings have been published in four peer reviewed papers in internationally recognised journals, with one additional paper currently in press. <br><br>Novel contributions of this research include: <br><br>‐ Enhanced understanding of the typical concentrations/ratios of different PFAA in groundwater impacted by landfill leachate contamination, which can allow such contamination to be distinguished from point sources (such as industrial sites);<br><br>‐ A new geochemical index, incorporating cation and PFAA ratios, which can be further used to separate landfill contamination in groundwater from other sources;<br><br>‐ A new method using XRF profiling and statistical techniques that can separate different heavy metal sources in groundwater below heterogeneous soil profiles;<br><br>The findings of this research can be used to increase the consistency and effectiveness of future site contamination investigations undertaken at Fishermans Bend as re‐development progresses. This will help minimise health and environmental risks and maximise effective use of time and resources. The methodologies and findings can also be translated more broadly to the global context, providing new techniques that can assist in the delineation of different sources of groundwater contamination – an ongoing challenge in many settings worldwide.<br><br>