VGP Technical Report 21 - A regional baseline of quality and hydrocarbon occurrence in groundwater 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 Otway Basin as part of the Victorian Gas Program (VGP). These baseline groundwater measurements were collected from State Observation Bore Network (SOBN) bores using low-flow sampling techniques. This baseline study is the most complete deep groundwater chemistry dataset collected in the Otway Basin 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) conducted baseline characterisation studies of the groundwater and atmosphere in the onshore 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. Groundwater is water that is stored in porous geological formations. Groundwater is more heavily relied upon than surface water in many parts of the world and is a key source of drinking water in many places in Australia. To protect groundwater from contamination and overuse, it is important to understand how water gets underground, how it flows through the subsurface geology, and how long it remains underground. Hydrogeologists use geochemical methods to characterise groundwater quality and understand local and regional groundwater processes (such as where the water has come from and how long ago). Results for this project may be used by groundwater users, industry, water suppliers and regulators. The sampling equipment and methods used on this project 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. Of these 81 samples, 73 were collected from groundwater bores that are part of the Victorian SOBN and eight were collected from town water supply bores. Each groundwater sample was analysed for 144 analytes. Generally, the quality of the groundwater samples collected in the Otway Basin is good, with varying amounts of salinity to the north-east of the study area. Major ion chemistry showed that the quality of most groundwater samples is suitable for irrigation and stock use, and variable for drinking water use. Sample results from the major ion chemistry also show that evapotranspiration and carbonate water-rock interactions are the main controls on the groundwater chemistry of the Otway Basin. Stable water isotopes revealed groundwater recharge via rainfall is the main mechanism of replenishing the aquifers in the basin. Groundwater residence times in the basin generally increase with depth, with radiocarbon groundwater ages ranging from modern to older 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 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 bore construction or sampling, rather than actual occurrence in the groundwater. Bibliographic reference: Iverach, C.P., Bold, T.A., O’Neill,C. & Hocking, M., 2020. A regional baseline of quality and hydrocarbon occurrence in groundwater Onshore Otway Basin, Victoria. Victorian Gas Program Technical Report 21. Geological Survey of Victoria. Download: The downloadable version of this report is supplied in PDF format (19.7 MB), Att 1 data XLSX (110 kB) & accessible version (DOCX 9 MB). Related products:
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