|
Issues ConditionThreatening Processes |
|
At a glance The issue Tasmania's groundwater resources are extensive and are widely distributed.They generally occur either as a layer of water in the ground in a permanently saturated state, or as an aquifer. Groundwater is used throughout the State for a variety of uses including, water supply to both humans and stock, and for irrigation purposes. Groundwater is also a significant provider of various environmental services. There are intricate links between groundwater and surface water resources, whereby alterations to the water quantity or quality of either may in turn affect the other. Such changes can influence ecosystem functioning and the health of aquatic flora and fauna species. Groundwater yield and quality vary considerably across the State, largely related to the geology of the site and it's recharge characteristics. The National Land and Water Resources Audit represents the first significant Statewide assessment of Tasmania's groundwater resources. However, this assessment was based on existing data only. Over-extraction and point source and diffuse source pollution are the main risks to groundwater resources in Tasmania. It is also important to recognise the many connections between land use change and the quality and quantity of groundwater resources (e.g. land clearance and native vegetation conversion). Further details on these linkages are contained in Land Use and Inland Waters Issue Report. This 'At a glance' section provides an overview of the groundwater quantity and quality issue. More detailed information and references are available in the Groundwater Issue Report. Three indicators are presented within this report (see Indicators). A recommendation is also provided on Groundwater Management. Favourable News - The development status (groundwater use versus sustainable yield) of the majority of Tasmania's groundwater resources is considered by the National Land and Water Resources Audit to be generally low (NLWRA 2001). In 1996 (most recent data available), groundwater use in Tasmania was estimated to be 20,000 ML/yr. This was only 4% of the total estimated sustainable yield of 500,000 ML/yr for 1995 (ARMCANZ 1996) and less than 1% of the 2,500,000 ML/yr estimate for 2000 (NLWRA 2001). It was also only 6.5% of the total major useable groundwater resources of 312,000 ML/yr (the amount within the Groundwater Management Units and not within fractured rock aquifers). However, there is uncertainty associated with the estimates of sustainable yield, because of the limited groundwater data available in Tasmania (see 'Uncertain News' below).
- The Audit estimated that 63% of the total sustainable yield of groundwater and 87% of the sustainable yield of the major useable groundwater in Tasmania was below the salinity level of <1,500 mg/L (suitable for human consumption and crop irrigation) in 2000.
- The Audit also considered that the development status for most of the 17 Tasmanian Groundwater Management Units and Unincorporated Areas (areas of fractured rock aquifers) would still be low by the years 2020 and 2050, with the exceptions of the Sorell, Wesley Vale and St Marys Units. There is also significant uncertainty in this assessment, which is explained in the 'Uncertain News' section below.
- The compilation of available data on groundwater salinities by the Audit in 2000 indicated that levels were below 1,500 mg/L in most of the Groundwater Management Units and Unincorporated Areas, except for the Llandherne Unit and the North East Unincorporated Area.
- Based on the assumption that rising groundwater levels can indicate a risk of rising groundwater salinity in Tasmania, Mineral Resources Tasmania (MRT) found that only 1.7% (49) of the watertables located in 2,903 production groundwater bores across the State were at high risk of rising salinity. The watertables in 8.8% (255) of the bores were at medium risk and 89.5% (2,599) were at low risk of rising salinity. These data were mainly collected at the time of drilling and reflect the average of samples collected over 80 years (1922-99), but do not represent a structured sampling of Tasmania's hydrogeology.
- The implementation process of the groundwater reform recommendations put forward by the Council of Australian Governments is now being undertaken, including methods for risk mitigation.
- The Water Management Act 1999 provides for the licensing of extraction boreholes and for the development of integrated water management planning.
- The National Environment Protection Measure (NEPM) for the Assessment of Site Contamination was implemented as a State Policy in Tasmania in 2002. It includes criteria for groundwater investigation levels, and where criteria are not specified in the NEPM, other relevant guidelines are used (e.g. ANZECC Water Quality Guidelines 2000, the New South Wales Environment Protection Authority 1994 Guidelines for the Assessment of Service Station Sites, or the Environment Quality Objectives in the Netherlands 1999).
- MRT is the custodian of a groundwater database and carries out regular monitoring of a number of boreholes throughout the State.
- Catchment maps of resources are presently being prepared by MRT for the DPIWE to conduct initial resource assessments. However, they will be based on historical data, collected over a period of 50 years, and may not always accurately reflect present conditions.
- The DPIWE's Environmental Guidelines for the Use of Recycled Water in Tasmania (2002) specify requirements in relation to protection of groundwater (e.g. establishment of groundwater bores to monitor level and quality of groundwater).
Unfavourable News - There is only limited information available on groundwater quality and quantity, the hydrogeological properties associated with groundwater, and groundwater use in Tasmania. The limited data available restricts the accuracy of all aspects of the groundwater data compiled for assessment in Tasmania (NLWRA 2001).
- Based on what little data there is available, the National Land and Water Resources Audit estimated that the projected extraction rates for the two Groundwater Management Units-Sorell and Wesley Vale, would exceed their sustainable yield (calculated for 2000) by the year 2020. St Marys will also potentially increase its groundwater use up to 65% of its 2000 sustainable yield by the year 2050. Each of these units is located in the Central and South East Groundwater Province of Tasmania. This assessment ties in with Mineral Resources Tasmania borehole data that indicates, over time in certain areas (e.g. around Devonport), the static water level in boreholes has been gradually falling. This is presumed to be a result of increased number of boreholes constructed, combined with land use changes (DIER 2001).
- The groundwater salinity levels (1,500-5,000 ml/L) in the Llandherne Unit and North East Unincorporated Area (as measured by the Audit in 2000) have the potential to limit the usage of the groundwater resources in these areas. The Sorell Unit may also be subject to similar problems, as it has an average salinity level just below this range (1,458 mg/L). The higher salinity level in the Llandherne Unit is of particular concern, because its shallow watertable (only 2 metres to the top of the aquifer) may indicate a relationship with dryland salinity (see Salinity Issue Report).
- Despite the Statewide picture of relatively low risk of increasing groundwater salinity associated with rising watertables, particular areas of the State do indicate a significant risk. Watertable depth trend bore data from 54 piezometers (a bore specifically designed to measure the groundwater surface) located in salt affected areas of Longford-Cressy and the Coal River Valley, indicated that over half (54%) of the bores were at high risk of salinisation, just under half (44%) were at moderate risk and only 2% were at low risk of salinisation in 2000.
- The Water Management Act 1999 provides for the licensing of extraction boreholes and for the development of integrated water management planning, but does not include any requirement for investigation of groundwater regimes and aquifers on a local or regional scale. A key aspect currently being considered is licensing of all high yielding bores.
- Contamination of groundwater from landfill sites (both current operating sites and closed sites) is a significant environmental problem facing Tasmania. A study of the effects of waste disposal on groundwater quality identified groundwater contamination at six of the ten sites that were investigated. If this percentage is extrapolated to the 176 sites identified around the State, potentially up to 100 waste disposal sites may have contaminated groundwater (Ezzy 2002).
- Currently there is no formal assessment of requirements to support environmental needs in relation to groundwater, nor formal allocation for environmental groundwater use.
Uncertain News The limited availability of groundwater data in Tasmania means that there is considerable uncertainty in the Statewide assessments that have been conducted to date on the quantity and quality of groundwater resources. Examples of areas of uncertainty are provided below. - There is uncertainty associated with the Audit's estimates for the groundwater sustainable yield in Tasmania, which stem from the lack of data available on, for example, groundwater through-flow, recharge based on hydrograph fluctuations, and the impacts of land use activities on groundwater resources (e.g. pollutant contamination). Consequently, the Audit highlighted the need for further investigation of sustainable yield in Tasmania.
- Groundwater extraction data are not routinely collected in Tasmania, with the most recent assessment based on limited data for the rate of groundwater use from 1996 (NLWRA 2001). To determine the development status of Tasmania's groundwater resources, it appears that the Audit had to rely on a comparison of 1996 water use data with estimates of sustainable yield for 2000. Without more recent and comprehensive groundwater use data it is uncertain how representative the 1996 assessment was for the rates of groundwater use in 2000.
- The Audit's estimates of future groundwater development were determined with the assumption of no change in land use patterns and based on very approximate estimates of sustainable yield. In some parts of Tasmania, land use change is significant (see Land Tenure, Land Use and Land Cover Issue Report) and impacts from land use can have significant effects on extraction needs and/or contamination of groundwater resources. Declining groundwater quality may prevent groundwater extraction rates from ever reaching a region's calculated sustainable yield. The effects of land use activities on surface waters (e.g. through water development), which are intrinsically linked with groundwaters, were also not considered.
- There are limited historical data to support trend analysis of groundwater salinity in Tasmania and there is insufficient information available at this time to link changes in salinity levels (where known) to pollution from human impacts.
- Presently in Tasmania there are limited groundwater monitoring systems suitable for assessing groundwater levels and trends for an accurate Statewide picture. The actual number of groundwater bores located throughout the State is unknown. Some data is available through Mineral Resources Tasmania for 2,903 production bores and 34 monitoring bores which they administer, but they do not represent a structured sampling of Tasmania's hydrogeology.
- Although 176 landfill sites have been identified around Tasmania (Ezzy 2002), the actual number of past and present sites is unknown. Only approximately 5.6% of these identified sites have been assessed for groundwater contamination.
- Because of the uncertainty in the estimates for the current and future development status of groundwater and the level of groundwater quality in Tasmania, it is not clear to what extent the environment has been or may be adversely affected.
|
|
