VGP Technical Report 25 - Geochemical interpretations of groundwater processes and methane origin, Onshore Otway Basin, Victoria.
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Product description:About the Victorian Gas Program: The Victorian Gas Program (VGP) is a comprehensive science-led program, incorporating geoscientific and environmental research to assess the risks, benefits and impacts of potential onshore conventional gas exploration and production. The program is also investigating the potential for further discoveries of onshore conventional and offshore gas in the Otway and Gippsland geological basins and assessing the feasibility of additional onshore underground gas storage in depleted reservoirs around the Port Campbell area. The VGP includes an extensive, proactive and phased community and stakeholder engagement program, through which the results of the scientific studies are being communicated. Executive summary: A regional assessment of groundwater chemistry, environmental isotopes, dissolved methane (CH4), and hydrocarbon occurrence has been conducted across the onshore Otway Basin as part of the Victorian Gas Program (VGP). These baseline groundwater measurements were collected from Victorian Government State Observation Bore Network (SOBN) monitoring bores using low-flow groundwater sampling techniques. This baseline study is the most complete groundwater chemistry dataset collected in the Otway Basin undertaken by the Victorian Government to date, building on existing data and providing a baseline measure of groundwater conditions. As part of the VGP environmental studies program, the Geological Survey of Victoria (GSV) has conducted baseline characterisation studies of the groundwater and atmosphere in the Victorian Otway Basin to determine current environmental conditions. This new groundwater chemistry dataset may support regulation of the sector by equipping government with a baseline it can use to monitor and report on any industry impacts. The groundwater sampling methods used are common to many groundwater geochemical studies conducted in Australia and internationally. In total, 81 groundwater samples were collected across the Otway Basin covering a depth below surface range of 8 m to 1500 m with each sample analysed for 144 analytes. Of these 81 samples, 73 were collected from SOBN monitoring bores and eight were collected from town water supply bores. Results from the major ion chemistry show that the Otway Basin has generally good groundwater quality, with most samples being suitable for irrigation and stock use, and variable for drinking water use. Results also showed that evapotranspiration and carbonate water-rock interactions were the main controls on the groundwater chemistry. Stable water isotopes revealed a primarily meteoric origin of water in the Otway Basin, suggesting surface infiltration after rainfall is the main mechanism of recharge to the basin. Groundwater residence times in the basin generally increase with depth, with radiocarbon groundwater ages ranging from modern (< 50 years) to more than 30,000 years. Methane occurs in very low concentrations in the groundwater of the Otway Basin with concentrations generally increasing with depth. Additional data, such as the isotopic composition of the CH4 and its relationship with ethane (C2H6) suggest that the CH4 measured has been produced naturally by microbes in the groundwater. Longer chain hydrocarbons and other petroleum by-products were detected in very few samples in very low concentrations. These are most likely the result of contamination during groundwater bore construction or sampling, rather than actual occurrence in the groundwater. Comparison between results from this study and previous study-site investigations yield comparable results, where any variations could be accounted by sampling method differences and laboratory variation. This report presents the chemical analysis of results and interpretations from groundwater samples collected in the onshore Otway Basin. It is intended to supplement the Otway Basin regional baseline technical report (Iverach et al., 2020a) and lend further context to results and interpretations presented within that overview report. Bibliographic reference: Iverach, C.P., Bold, T.A., O’Neill, C. & Hocking, M., 2020. Geochemical interpretations of groundwater processes and methane origin Onshore Otway Basin, Victoria. Victorian Gas Program Technical Report 25. Geological Survey of Victoria. Download: The downloadable version of this report is supplied in PDF format (25.6 MB), accessible version (DOCX 10 MB) & Att A1 link. Related products:
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