VGP Technical Report 34 - Groundwater impact assessment - conceptual report, onshore Otway Basin, Victoria

Category: Victorian Gas Program	Product Code: MP-R-161884
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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 conceptual groundwater model has been constructed for the Victorian Otway Basin (South-West region) as part of the Victorian Gas Program (VGP). The construction of this conceptual model included a description of the regional setting, delineation of hydrostratigraphic units and identification of groundwater processes in the basin.

As part of the VGP environmental studies program, the Geological Survey of Victoria (GSV) developed a South-West region groundwater model to quantify potential groundwater and surface water impacts if any onshore conventional gas developments in the Otway Basin were to occur. The model is required to quantify 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, which broadly include:

construction of a conceptual groundwater model.
development and calibration of a numerical groundwater model.
application of possible gas development scenarios and potential impacts to existing groundwater conditions in the Otway Basin.

This report presents the first stage; the construction of the conceptual groundwater model framework to simulate any impacts of onshore conventional gas development in the Otway Basin.

This assessment has shown there is a complex system of aquifers and aquitards throughout the Otway Basin. Groundwater resources in these formations are relied upon substantially within the basin for town, irrigation, agriculture and domestic use. Freshwater aquifers are found at depths of up to 1 km below ground surface in strata that is Upper Cretaceous (80 Ma) to Quaternary (recent) in age. Some aquifers, such as the Lower Tertiary Aquifer (LTA), extend beyond the coast to many kilometres offshore.

Groundwater level trends in the Otway Basin are highly variable and are often dependent on local factors, including natural climate and ground conditions, as well as the impact of human activities. Large parts of the region have received below average rain in recent years, which has caused a decline in groundwater levels. The aquifers that have been most affected by this are the Upper Tertiary Basalt Aquifer (UTBA), Upper Tertiary Aquifer – Marine (UTAM), Lower Mid-Tertiary Aquifer (LMTA), Lower Tertiary Aquifer (LTA), Upper Cretaceous Sherbrook Aquifer (UKSA), and the Palaeozoic Basement (PBSE). Periods of reduced rainfall are generally accompanied by increased use of groundwater, which tends to accelerate the decline of groundwater levels in many of the aquifers.

Groundwater monitoring data has provided no evidence to suggest that historic gas production in the Port Campbell area (onshore and offshore) has impacted the overlying groundwater aquifers.

The conceptualisation of the groundwater system proposes that the regional stratigraphy can be described by a sequence of 12 model layers, which incorporate both aquifer and aquitard units. The area contains a total of 1367 groundwater production bores, 631 State Government groundwater observation bores and 20 petroleum wells. The area considered extends offshore to include the portion of the Victorian Otway Basin that is submerged approximately 50 km away from the coast. The conceptualisation information presented in this report is used for the construction and calibration of the South-West region groundwater model.

Bibliographic reference:
S. Torkzaban., M. Hocking., A. Gaal., S. Manamperi & C.P. Iverach., 2020. Groundwater impact assessment - conceptual report, onshore Otway Basin, Victoria. Victorian Gas Program Technical Report 34. Geological Survey of Victoria.

Download:
The downloadable version of this report is supplied in PDF format (PDF 66 MB).