Groundwater recharge to coal basins in eastern Australia

Full text: The Bioregional Assessment Programme is investigating the potential impacts of coal seam gas and large coal mining developments on water resources and water-related assets. Part of this work is to develop numerical groundwater models which require an input of the groundwater recharge. The groundwater recharge is important in determining the overall water balance of a region and is particularly sensitive to the time after development ceases for the region to return to hydrological equilibrium. Many of these areas have poorly understood recharge mechanisms and little detailed past work to build upon. The chloride mass balance is the most widely used method for estimating recharge in Australia but is under- utilised in this industry. Each state has a database of thousands of measurements of chloride in groundwater that can be used to build an understanding of the recharge in a region. This investigation has grouped recharge estimates by stratigraphic formation in outcrop areas to develop regression equations between average annual rainfall and average annual recharge to enable the recharge estimates to be upscaled to the entire region under investigation. This work suggests that recharge to coal bearing formations is typically less than the formations that surround them. In the Sydney Basin the recharge to the Newcastle or Illawarra Coal Measures can be up to an order of magnitude lower than the Hawkesbury Sandstone or the sand dune systems that are productive aquifers, the coal measures have a recharge rate that is comparable to the Wianamatta Shale that is considered an aquitard. Similarly in the Clarence-Morton Basin, the Walloon Coal Measures recharge can be up to an order magnitude less than the Tertiary Volcanics. This investigation has demonstrated that we can do better than assigning recharge as an arbitrary percentage of rainfall and that this can be quite different between stratigraphic units. (author)