Groundwater refers to all the water below the earth's surface held in the saturated pore spaces and fractures of soil and rock. It forms an important component of the hydrologic or water cycle, in which water is constantly moving between the oceans, atmosphere and land areas.
An estimated 97% of the world's unfrozen fresh water occurs as groundwater. In many areas of Australia groundwater supplies are the only reliable sources of water for agricultural and human consumption.
Location of Tasmania's Groundwater
Tasmania has an extensive groundwater resource trapped by igneous, sedimentary and metamorphic rocks, as well as unconsolidated sediments. Tasmania's groundwater resources are not evenly distributed and quality and yield can be highly variable, depending on the aquifer type, the topographic location and the rainfall.
|The location of fractured and porus rock aquifers in Tasmania (Click image to enlarge)|
Fractured rock aquifers
(such as dolerite) are the main source of groundwater in the State, underlying some 85-90% of the land surface. Many of these rocks have relatively low porosities (less than 10%) and permeabilities and often produce low water yields.
Certain sedimentary rock aquifers (such as sandstones) in Tasmania provide high yields, possibly because of dual porosity effects which is where water is stored in pore spaces between sediment grains and also in interconnecting fractures. Where they occur, they tend to form highly useful supplies to farms, communities and to a minor extent, industry.
Intergranular aquifers (such as sands and gravels) are present in only 10-15% of the State. In general, these mostly unconsolidated formations have moderate porosity (up to 40%) and are relatively permeable. They provide useful water supplies for farms and small communities. The thickness and lateral extent of the water bearing units within them may be highly variable, which may limit their use as a resource.
The nature of Tasmanian aquifers means that there is always some uncertainty as to whether a borehole drilled will actually yield useable quantities of groundwater.
With intergranular aquifers, prediction of success of a proposed borehole can be made with some degree of certainty. For the fractured rock and dual porosity aquifers, the borehole has to encounter sufficient water-bearing fractures within that rock to produce a useable quantity of water before it can be termed a successful bore
The distribution of these fractures depends upon a variety of influences, and cannot necessarily be predicted from the surface. Prospectivity of these aquifers may therefore vary greatly, depending upon location.
Regional Groundwater Prospectivity Maps
, published by Mineral Resources Tasmania (MRT) in 2006, represent an initial summary of the available historical groundwater information.
Tasmania's groundwater resources are tapped by more than 10 000 bores and wells, supplying water for irrigation, town water, domestic use, stock watering, mining and other commercial purposes. While the majority of these are in use, there are also some which have been abandoned.
The best available information suggests that the sustainable groundwater yield across the whole State is of the order of 500 to 2530 GL (0.5 to 2.53 million million litres). With less than 5% of the sustainable yield estimated to be currently in use, Tasmania's groundwater resources are generally under-utilised. There are a number of local hotspots around the State however, where the density of bores and the demand for groundwater are relatively high, and pressures on the resource are occurring.
For groundwater monitoring information, see the Water Information System of Tasmania (WIST): http://wist.tas.go.au/ui
The Groundwater Information Access Portal is a database that allows you to find information about groundwater features in Tasmania: http://wrt.tas.gov.au/groundwater-info