This video presents how groundwater impacts may occur in aquifers surrounding the coal seam gas formations in the Surat Basin in Queensland, Australia.
Similar to the way water in rivers and streams moves from higher to lower elevations, groundwater in porous formations – or aquifers – moves from high pressure to lower pressure. The pressure difference drives the way water moves, both within and between aquifers.
Groundwater in deeper aquifers of the Surat Basin is under pressure. For this reason, when a water bore is drilled, water rises to a level somewhere above the aquifer. The level it rises to is called the bore water level, also referred to as the groundwater level in the aquifer.
In the Surat Basin, within the CSG development area, the top of the Walloon Coal Measures is 200 to 800 metres below ground, but the water level is generally about 50 metres deep. Water levels in the Springbok and the Hutton sandstones – the aquifers above and below this coal formation – are generally about 30 metres below ground.
Gas is extracted by drilling a well into the coal formation and pumping groundwater out of the well to lower pressure, which in turn releases the gas. As the CSG development proceeds, water pressure in the coal formation continues to fall. This creates a pressure difference between the coal formation and the surrounding aquifers. Depending on how permeable the intervening formations are, water can move from those aquifers into the coal formation, lowering the water level in the aquifers. This is how groundwater is impacted.
Impacts on aquifers are predicted using a tool called a groundwater flow model. Models are built from available information and studies about groundwater flows and connectivity. The Office of Groundwater Impact Assessment has developed a regional-scale groundwater flow model to predict impacts from current and planned CSG development in the Surat Cumulative Management Area. Impact predictions are revised every 3 years and reported in the Underground Water Impact Report.
In the short term, impacts are predicted as the Immediately Affected Area, or IAA, which is the area of an aquifer where more than 5 metres of decline is predicted within the next 3 years. Shown here as a red line for the Walloon Coal Measures. The Long-term Affected Area, or LAA, is the area where more than 5 metres of decline is predicted to occur at any time in the future. The Walloon Coal Measures has the greatest predicted impacts in the Surat Basin, with declines of more than 500 metres predicted.
The Springbok Sandstone, which directly overlies the Walloon Coal Measures, is predicted to have water level declines around active CSG fields by generally about 20 to 50 metres.
The Hutton Sandstone, which is the aquifer below the Walloon Coal Measures, is only predicted to be affected in the south-eastern part by generally about 10 metres.
In the Condamine Alluvium, which sits directly above the Walloon Coal Measures, the predicted impact is less than 0.2 metres across most of the area.
There are some springs and watercourses that are predicted to be impacted in the northern parts of the CSG production areas, but the impacts are generally less than a metre.
For more information about this video, or if you have questions about the Underground Water Impact Report, visit our website.
Last updated: 23 May 2023