VGP Technical Report 38 - Groundwater impact assessment - Conventional gas development scenarios, 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: The potential impacts of onshore conventional gas development on groundwater and surface water in the Victorian Otway Basin have been assessed as part of the Victorian Gas Program (VGP). A calibrated regional-scale groundwater model has considered combinations of future onshore conventional gas developments, to simulate the potential impacts that hypothetical development may have on groundwater in the Otway Basin. As part of the VGP environmental studies program, the Geological Survey of Victoria (GSV) has developed a South-West region groundwater model to quantify groundwater and surface water impacts of potential onshore conventional gas developments in the Otway Basin. The model was required to quantify individual and cumulative impacts from potential developments, including changes to groundwater levels and pressures and changes to river baseflows. The South-West region groundwater model was developed in three stages: 1. construction of a conceptual groundwater model 2. development and calibration of a numerical groundwater model 3. application of hypothetical gas development scenarios and potential impacts to existing groundwater conditions in the South-West region. This report presents the third stage: the results of hypothetical scenarios considered by the model to simulate the potential impacts that onshore conventional gas development may have on groundwater in the South-West region. Gas development phases were simulated under both steady-state (long-term average) and transient (time varying) conditions. The steady-state impact assessment of all prospective sites indicated the maximum possible extent of impact on groundwater levels. Transient simulations were then conducted to determine groundwater level changes within the life of the proposed gas production. Additionally, simulations were extended decades beyond the life of proposed gas production until equilibrium conditions were attained. The impact of each predictive scenario was assessed based on a comparison with the baseline run (no gas production; detailed in Torkzaban et al., 2020b) in either steady-state or transient conditions. Four gas production volume scenarios were considered; minimum, low-, medium-, and high-volume production cases. No significant impact on the watertable, aquifer storage volume or surface water flow was identified for any gas production volume scenario. The results suggest that gas reserve depressurisation associated with onshore conventional gas extraction has a negligible impact on groundwater level and storage in the overlying groundwater resource aquifers. Steady-state results show that maximum drawdown impacts extend furthest in three target gas formations: Waarre Formation, Pretty Hill Formation and the Heathfield Sandstone. Predicted regional drawdown in the overlying Dilwyn Formation was approximately 0.2 metres, due to the intervening units. Drawdown impacts of up to 20 metres in the target gas formations result in a drawdown impact of 0.1 metres in the overlying groundwater resource aquifer (Dilwyn Formation) in the transient simulation. Groundwater volume impacts vary depending on the gas production scenario, with a maximum removal volume from the Dilwyn Formation of approximately 660 megalitres (ML) (high scenario) and a minimum of 81 ML (minimum case scenario). Impact times were predicted for the medium and high production volume scenarios with a total recovery time of 15 years and 29 years, respectively. The average 30-year groundwater volume impact on the Dilwyn Formation is 5 ML/year, 14 ML/year and 34 ML/year for the low-, medium-, and high-volume production scenarios, respectively. The impact results presented here are insignificant compared to current groundwater extraction volumes in the basin by other sectors. Bibliographic reference: TORKZABAN, S., HOCKING, M., GAAL, A., IVERACH, C.P., BOLD, T.A. & BEVERLY, C., 2020. Groundwater impact assessment - Conventional gas development scenarios, Onshore Otway Basin, Victoria. VGP Technical Report 38. Geological Survey of Victoria. Download: The downloadable version of this report is supplied in PDF format (PDF 20 MB). | ||||||
