2.1 Water table aquifer
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 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.
In Canberra, the recharge is due largely to rainfall while discharge generally is to small streams. On an annual basis, in a groundwater system where groundwater abstraction is limited and in this case, an increase or decrease in rainfall will 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 on average, the annual change in storage is negligible and any groundwater abstraction is balanced by a decrease in evapotranspiration and discharge to surface water features.
The Australian Capital Territory Government, Environment and Sustainable Development Directorate use an estimated of 74 GL of groundwater recharge each year as a guide when setting limits for groundwater 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 the small size and geographic dispersion. Within the Canberra region these aquifers are considered to have poor continuity of supply.