Also see : Groundwater ResourcesIntroductionGroundwater is water that occurs beneath the surface of the earth. It is available over most of Australia and in many parts of the country, especially the arid and semi-arid inland, it is of critical importance (Jacobson et al. 1983). It can be classified according to the rock types or aquifers in which it occurs: Surficial aquifers Surficial aquifers occur in alluvial sediments in river valleys, deltas, and basins, in lake or lacustrine sediments, and in aeolian or wind-formed deposits. They are essentially unconsolidated clay, silt, sand, gravel, and limestone formations, mainly of Quaternary age. These deposits are easily exploited and are the major sources of freshwater groundwater. Sedimentary aquifers Sedimentary aquifers occur in consolidated sediments such as porous sandstones and conglomerates, in which water is stored in the intergranular pores, and limestone, in which water is stored in solution cavities and joints. These aquifers are generally located in sedimentary basins that are continuous over large areas and may be tens or hundreds of metres thick. In terms of quantity, they contain the largest groundwater resources, although much of the water is of marginal quality. Nonetheless, these aquifers are of critical importance over much of inland Australia. Fractured rock aquifers These occur in igneous and metamorphosed hard rocks which have been subjected to disturbance, deformation, or weathering, and which allow water to move through joints, bedding plains, faults and solution cavities, and zones of weathering. Although fractured rock aquifers are found over a wide area, they contain much less available groundwater than surficial and sedimentary aquifers and, due to the difficulty of obtaining high yields, the quantities of water taken from them are relatively low.
Groundwater resourcesA distinction is drawn between major and minor divertible sources of groundwater (for definitions, see "Some necessary definitions" below)(AWRC 1987). Major divertible groundwater resources are those capable of supplying sufficient water to sustain a small town or irrigation development. This is assumed to be a minimum level of about 500 ML per annum. Less than this, the deposits are regarded as minor divertible groundwater resources. Although each minor groundwater resource may only provide a small volume of water, their number and widespread availability mean that the total volume is comparable to the major divertible groundwater resources. However, for the same reasons, only a very small portion of the minor resources can actually be used. Groundwater is also categorised according to its salinity, as fresh, marginal, brackish, or saline (based on AWRC 1987, Vol.1, 12).
Tables 1 and 2 list Australia's groundwater resources by aquifer types and Drainage Divisions. Table 2 provides a setting for the resources of the MDB. Groundwater in the Murray-Darling Basin |
 | As Table 2 indicates, there are large resources of groundwater in the MDB (MDBMC 1987, 81-85). They are present in all three aquifer types (Lovering et al. 1998). Covering by far the largest area are the sedimentary basins, in particular the Great Artesian Basin and the Murray Basin, the major groundwater resources of the MDB (Table 2 and Figure 1). However, the resources are unevenly distributed and vary in quality. Table 3 lists groundwater resources by major river basins (providing comparability and compatibility with data on surface resources). Because of the importance of minor sources of groundwater in the Basin, the data in the table are the sums of both major and minor sources. All three aquifer types are important groundwater resources in the MDB, though there are variations in terms of abstractions (Table 3). Abstractions from minor surficial and sedimentary sources are very small, being only 15 GL and 18 GL respectively (in 1983-84). |
However, of the total abstractions from fractured rocks of 124 GL, 85 GL were from minor fractured rock sources (Table 1). In spite of their size, the groundwater resources are not unlimited. Many of the potentially high-yielding aquifers receive a relatively low rate of natural recharge compared with the volume of groundwater they store. For example, the volume of water stored in the alluvial sediments in the lower Namoi Valley is about 20,000 GL, but the average yearly natural recharge is only about 30 GL. Recharge areas for the various high-yielding alluvial aquifers are the river beds and flood plains. For the Great Artesian Basin, the most important recharge areas are the wetter areas along the Great Dividing Range. For the Murray Groundwater Basin, recharge of the deeper confined aquifers occurs around the Basin's margins: the shallower unconfined aquifers also receive recharge over most of their surface areas.
The Great Artesian Groundwater Basin |
The Great Artesian Basin (GAB) is one of the largest such basins in the world, with a total area of 1.7 million km2, covering 22 per cent of Australia (Table 4 and Figure 1). The Basin extends under the northern part of the MDB in Queensland and New South Wales. It is a multi-layered aquifer system, consisting mainly of sandstones alternating with impermeable siltstones and mudstones, and is up to 3,000 metres thick. The GAB contains an estimated 8,700 million ML of water.. The GAB underlies predominantly arid and semi-arid areas, where surface water resources are few and extremely unreliable. As a result, it is the only significant source of water for towns, farms and stock, as well as for mining and tourism. Without it, it is unlikely that these activities would be possible. However, the groundwater is generally unsuitable for irrigation because of its high sodium content, which makes it chemically incompatible with the soil (Habermehl 1980). Overlying parts of the GAB are large alluvial fan aquifers of Tertiary age associated with the major rivers, the Macquarie, Gwydir, Namoi, Border and Condamine. Increased attention is being given to the integrated management of the Basin's land and water resources by all of the responsible governments (Eigeland & Joshua 1996). |  |
The Murray Groundwater BasinThe Murray Basin, covering some 297,000 km2 is located in the southern part of the MDB and almost entirely within its boundaries (Brown 1989; Evans et al. 1990) ( Figure 1). It is a relatively thin saucer-shaped basin, between 200 and 600 m thick, consisting of Cainozoic age unconsolidated sediments and sedimentary rocks, primarily silts, clays and limestones. The only outlets are by way of the Murray and to the surface. The basin has limited storage capacity and the sediments are largely saturated. The thin and flat nature of the basin means that it can fill quite rapidly, and there is evidence that it has refilled six or seven times over the past 500,000 years. While previous fillings took 2,000 to 3,000 years, the current one is taking less than one hundred years, due essentially to the clearing of natural vegetation and its replacement by shallow-rooted plants, both in dryland and irrigated farming areas. Studies have indicated significant rises in groundwater levels over the last 25 years.There are large recoverable reserves of groundwater available in the Murray Basin aquifers, greater than was earlier believed, but at present, use is a very small percentage of the sustainable yield ( Table 5) (GWG 1996). A major reason is that water quality is highly variable. Of the shallow groundwaters, the best quality water is found around the Basin's margins, especially in the east and south-east and the south-west, in the South Australian Mallee. In other areas, especially adjacent to the course of the River Murray downstream of the confluence with the Murrumbidgee, the groundwaters are highly saline. Increased use of the groundwaters would contribute to the lowering of the watertables or at least help to alleviate the continuing rise.
Groundwater along the Dumaresq River ValleyThe Dumaresq River Valley illustrates the varying importance of all three aquifer types within a given region of the MDB (DBBRC 1989). Along the Dumaresq and other rivers, and the more extensive floodplains to the west, are the alluvial or surficial deposits of sand, gravel, clay and silt ( Figure 2). These provide useable quantities of good quality water, especially the deeper deposits. Elsewhere in the western part of the Dumaresq Valley are sandstone aquifers of the south-east rim of the Great Artesian Basin, while to the east are the fractured rock aquifers.Development of the groundwater resources is growing slowly but steadily, particularly in the area of the Cunningham Weir (GWG 1996b, 58-60).
ConclusionGroundwater resources are important throughout most of the Murray-Darling Basin, but especially in areas of limited and/or unreliable surface resources. Nonetheless, they require much better management than they have received, certainly until recently. Over recent years, much research work has been undertaken in the Murray Basin and this has been extended to the areas underlying the Darling River Basin. This work is increasing the understanding of the resources and assisting in the solution of the major groundwater, surface water and land salinisation problems (see Water and Land Salinity) (Anon. 1995).
References Anon. (1995): "An invaluable database for the Murray Basin completed". Aus-Geo News, 28, 1. AWRC (1987): 1985 Review of Australia's Water Resources and Water Use. Australian Water Resources Council. Department of Primary Industries and Energy / Australian Government Publishing Service, Canberra. Brown, C.M. (Editor)(1989): "The Murray 1872-1989". BMR Journal of Australian Geology & Geophysics, 11, 127-395. DBBRC (1989): Groundwater in the Border Rivers Area. Dumaresq-Barwon Border Rivers Commission, Sydney. Eigeland, N. & Joshua, E. (1996): "Managing the Great Artesian Basin". Australian Journal of Soil and Water Conservation, 9(1), 21-26. GWG (1996a): Groundwater Development Potential in the Murray Basin. Technical Report No.1. Groundwater Working Group, Murray-Darling Basin Commission, Canberra. GWG (1996b): Murray-Darling Basin Status of Groundwater 1992. Technical Report No.2. Groundwater Working Group, Murray-Darling Basin Commission, Canberra. Habermehl, M.A. (1980): "The Great Artesian Basin, Australia". BMR Journal of Geology & Geophysics, 5, 9-38. Jacobson, G. et al. (1983): Australia's Groundwater Resources. Water 2000: Consultants Report No. 2. Australian Government Publishing Service, Canberra. Lovering, J.F. et al. (1998): "Salinity in the Murray-Darling Basin: a critical challenge for the 21st Century". pp. 215-230 in Weaver, T.R. & Lawrence, C.R. (Editors) Proceedings of the IAH International Groundwater Conference. Groundwater: sustainable solutions. International Association of Hydrogeologists, Brisbane. MDBMC (1987): Murray-Darling Basin Environmental Resources Study. Murray-Darling Basin Ministerial Council, Canberra.
Some necessary definitionsWater is measured in megalitres (ML), equivalent to one million litres, and gigalitres (GL), equivalent to 1,000 megalitres. Runoff and outflow were generally estimated using information from stream gauging stations. Where there was a lack of such information, they were estimated using rainfall and other data. The estimates were made for the runoff under natural conditions. In regulated systems, such as those in the Murray-Darling Drainage Division, simulation models were used to estimate the natural runoffs.
Mean annual runoffFor river basins with a runoff regime in which the flow increases downstream, the flow is greatest at the mouth of the river basin. In such cases, the mean annual runoff was defined as the outflow from the basin. In many river basins, however, particularly in the Murray-Darling Drainage Division, the flow in the rivers decreases downstream, often with little or no outflow from the basin. In these situations, mean annual runoff was defined as the combined mean annual runoff of each of the major catchments in the river basin, calculated at the point where the flow is greatest and excluding runoff from upstream basins.
Mean annual outflowBecause of the tendency for the outflow from some river basins to be less than the total runoff generated within them because of large channel losses, the mean annual outflows are also evaluated These are defined as the estimated outflow from a river basin under natural conditions. The basin outflow can be either to the sea or an adjacent basin but not to a sink or closed lake within the basin. This concept is of particular relevance to the MDB and its northern rivers. For the Division as a whole, the mean annual outflow at the mouth of the Murray is 12,200 GL under natural conditions. However, the mean annual runoff within the Division is 24,300 GL. This indicates that 50 per cent of the water that originates in the Division is lost through natural process before reaching the sea.
Divertible surface water resourcesA divertible resource is defined as the volume of water that can be diverted on a sustained basis into conventional water supply systems or to substantial private users, utilising existing storages and potential dam sites identified by investigation or indicated by preliminary reconnaissance. In general, it is any source capable of yielding more than 500 ML per annum.
Developed resourceThis is defined as the portion of the divertible resource currently available for use, estimated for storages existing or under construction, and including licensed withdrawals from streams. Source: AWRC 1987, Volume 1, 16-17.
Figure 1: Groundwater sources in the MDB
Figure 2: Groundwater sources in the Border Rivers area of Queensland and New South Wales
Table 1 Murray-Darling Basin groundwater resources and 1983-84 abstractions by aquifer types (source: AWRC 1987, Volume 1, 61-67)
Table 2 Groundwater resources in Australia, by Drainage Divisions, 1985. (source: AWRC 1987, Volume 1, 61)
Table 3 Groundwater resources in the Murray-Darling Basin, by river basins (source: AWRC 1987, Volume 1, 86-88, 119-122)
Table 4 Groundwater Resources of the Great Artesian Basin (source: AWRC 1987, Volume 1,141-158)
Table 5 Groundwater resources of the Murray Basin (source: GWG 1996, 4)
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