2.2 Underlying aquifers
This water asset exists in the Canberra region but the volume could not be quantified due to the lack of both data and a suitable quantification approach. The reporting region comprises mainly of fractured rocks with a local groundwater flow system. Fractured rock aquifers are the dominant aquifer storage within the Canberra region. In fractured rock aquifers, groundwater is stored in the fractures, joints, bedding planes and cavities of the rock mass. The volume stored within these fractures is difficult to quantify, as the extent of fractures is generally unknown and variable in nature. On an annual basis, an increase or decreased in rainfall recharge will respectively increase or decrease evapotranspiration and discharge to surface water features rather than the volume of groundwater stored in the aquifer. It is therefore assumed that the annual change in storage is negligible. In Canberra the recharge is due largely to rainfall while discharge generally is to small streams. The Australian Capital Territory Government, Environment and Sustainable Development Directorate use an estimate of 74 GL of groundwater recharge each year as a guide when setting limits for ground water extractions; approximately 10% of this recharge is available for abstraction annually. Minor alluvial aquifers also exist within the Canberra region and generally occur in river channels and flood plains. Water in these aquifers is stored in gravel, sand and silt below the surface. The volume of water stored is limited and difficult to quantify due to their small size and geographic dispersion. Within the Canberra region they are considered to have poor continuity of supply.