Indicator: Groundwater quality and quantity
Lack of comprehensive data for ACT groundwater resources remains a concern. Knowledge of groundwater quality is particularly limited. More definitive, catchment-based data on groundwater quantity and quality are required.
Despite limited data, there is evidence that extraction exceeded the acceptable limit of 70% of sustainable groundwater yield in four of Environment ACT's 32 water management units: Fyshwick, Jerrabomberra, Woden and Woolshed.
Twenty new groundwater bores were licensed during the reporting period, nine of these in the inner urban Lake Burley Griffin subcatchment. Environment ACT data show a nearly fourfold increase in groundwater use from 2000–01 to 2002–03.
Knowledge relating to the groundwater resource continues to be limited in the ACT. It is assumed that groundwater reserves occur within confined fractured rock aquifers, but this disregards the possible presence of unconfined aquifers within unconsolidated sediments in the alluvial valleys of the Murrumbidgee and Molonglo Rivers.
Although salinisation is not considered to be problematic at this stage, salinity levels also constrain groundwater use for irrigation purposes (EACT 2003). The volume of groundwater available for extraction from aquifers at a catchment scale is limited to ten percent of groundwater recharge as defined in the Think water, act water (EACT 2003). However, there is some concern regarding the reliability of methods used to estimate recharge because of inherent assumptions and limitations (Salient Solutions and iCAM 2001), and this may have significant implications for determining sustainable yields. More spatial and temporal data that better characterise aquifers and their flow dynamics are required to improve these estimations.
This is particularly important given the increase in groundwater use (Table 1). The increase in groundwater use appears to be significant, although it is estimated that 7,000 megalitres of water are available for use. However, given the lack of quantitative data on the resource, this increase in use needs to be monitored carefully. In the meantime, when it is evident that the sustainable yield is being exceeded, this triggers the need for a detailed catchment scale hydrogeological study to be undertaken. To date, this has only occurred in the Jerrabomberra Creek catchment (Salient Solutions and iCAM, 2001).
|Table 1: Groundwater use, 2000-03|
|Year||Water use (megalitres)|
|Average annual use 2000–03||430|
Source: Data provided by Environment ACT 2003
Think water, act water (EACT, 2003) quantifies total runoff, allocations and uses for each subcatchment within the ACT. This includes groundwater and environmental flow allocations. It is therefore possible to identify allocation provisions and whether subcatchments are over-allocated.
Data provided by Environment ACT indicate extraction of groundwater exceeded the acceptable limit of 70% of sustainable yield in four of Environment ACT’s 32 groundwater management units: Woden, Jerrabomberra, Woolshed and Fyshwick Creeks. However, since the estimation of groundwater recharge and sustainable yield is still limited by the paucity of actual data, the apparent incidence of over-allocated subcatchments with respect to groundwater may be greater.
Anecdotal evidence indicates that in some of the more affluent suburbs of Canberra, the utilisation of bore water to offset the requirements of mandatory water restrictions is imposing additional stresses to the groundwater resource during drought. Twenty new groundwater licences were granted during the reporting period, nine of these in the inner urban Lake Burley Griffin subcatchment. This suggests that some residents and commercial users responded to water restrictions by (legally) utilising an alternative source rather than by restricting their overall use. As Stage 3 water restrictions were not implemented until after reporting period, it is important that this trend be monitored in the next State of the Environment Report.
Few data are available which characterise groundwater quality for State of the Environment reporting. Some bore data are available for the West Belconnen and Mugga Lane landfill sites which provide information on the physical, chemical and bacteriological properties of groundwater, as well as watertable depth. In addition to the accessions of nutrients, pathogens and chemical contaminants associated with leachates from the landfills, it is also evident that groundwater at these sites can be saline. Contaminated plumes associated with underground fuel storages at service stations exist at a number of locations across the ACT (see contaminated sites). The potential or actual impacts of these contaminated sites on surface waters need to be researched.
Environment ACT 2003, Think water, act water , ACT Government, Canberra.
Salient Solutions and Integrated Catchment Assessment And Management Centre (iCAM) 2001, Groundwater assessment of Jerrabomberra Creek catchment , Salient Solutions and iCAM, Canberra.
Additional data provided by Environment ACT.