VGP Technical Report 37 - Groundwater impact assessment - Numerical model development and calibration, 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 Otway Basin have been assessed as part of the Victorian Gas Program (VGP). As part of the VGP environmental studies program, the Geological Survey of Victoria (GSV) 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 allows cumulative impacts from potential developments to be quantified, 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 possible gas development scenarios and potential impacts to existing groundwater conditions in the Otway Basin. This report presents the second stage: the development and calibration of a numerical groundwater model to simulate the potential impacts that hypothetical onshore conventional gas development may have on groundwater in the South- West region. Model calibration was undertaken by automated and manual methods to ensure aquifer parameters and calibration targets remained within acceptable ranges. Available groundwater monitoring data was considered for both the steady-state and transient model calibrations. Primary calibration targets were the watertable elevation and the modelled versus measured (i.e. residual) groundwater levels. Calibration of the steady-state model showed that modelled flow paths were generally in good agreement with flow paths expected from theoretical considerations developed in the conceptual model. Transient conditions were then simulated for the period 1980–2018, which revealed good correlation between modelled and measured data during the initial 20 years of simulation. This period is a transition from pseudo-steady conditions to temporal conditions and is when stress periods are at annual intervals. This suggests that the simulated steady-state solution describes the 1980 groundwater conditions adequately. However, calibration statistics improve during the 1990–1999 transient calibration period, with a scaled root mean square (RMS) of 2.99 per cent compared to the pre- calibration period scaled RMS of 3.60 per cent. Assumptions have been made in the calibration of the model as a result of limitations that determine the spatial and temporal variation uncertainties in the model. These assumptions include: • extrapolated stratigraphy. • that annual and quarterly stress periods adequately represent aquifer interactions. • that estimated aquifer hydraulic properties adequately represent those in nature. These assumptions provide a foundation for the development of the model. Whilst there is always some level of uncertainty, the best available data has been used to limit associated errors. Review of the spatial and temporal model calibration data, and the results of sensitivity analysis, has confirmed that the numerical groundwater model developed here is appropriate for assessing predicted groundwater level changes under the hypothetical onshore conventional gas development scenarios proposed. Bibliographic reference: TORKZABAN, S., HOCKING, M., BEVERLY, C., IVERACH, C.P. & GAAL, A., 2020. Groundwater impact assessment - Numerical model development and calibration, Onshore Otway Basin, Victoria. VGP Technical Report 37. Geological Survey of Victoria. Download: The downloadable version of this report is supplied in PDF format (PDF 77 MB). | ||||||
