4. Groundwater in the Murray-Darling Basin

As Table 2 indicates, there are large resources of groundwater in the MDB (Ife & Skelt 2004). 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, which are the major groundwater resources of the MDB. However, the resources are unevenly distributed and vary in quality. All three aquifer types are important groundwater resources in the MDB, though there are variations in terms of abstractions.

State	Sustainable Yield (`000 ML/a)	Total Liscence Allocations (`000 ML/a)	Actual Usage (`000 ML/a)	%Committed	%Used
NSW & ACT	5305.6	2440.0	1019.6	46	19
Victoria	2276.2	515.9	299.8	23	13
South Australia	1044.8	458.9	384.3	44	37
Queensland	350.0	356.0	265.8	102	76
Total	8976.6	3770.8	1969.4	42	22

Table 2. Groundwater Resources in the MDB (Source: Ife & Skelt 2004)

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. 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.

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4.1 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 (Figure 3). The GAB 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 2 kilometres thick in the Surat sub-basin and 3 kilometres thick in the Eromanga sub-basin. The GAB contains an estimated 64.9 billion ML of water. While the two sub-basins are separated by a basement high, the Nebine Ridge, they are hydraulically connected.

The GAB underlies predominantly arid and semi-arid areas, where often surface water resources are few and extremely unreliable. As a result, it is often 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.

Management zones were adopted in 2000 as part of the Great Artesian Basin Strategic Management Plan (GABCC 2000), which deals with many of the basin-wide management issues of wastage, heritage values and biodiversity. However, actual water management is still the responsibility of the relevant states (Queensland & New South Wales).

The major issue for the GAB is the falling artesian pressures reducing flow to natural springs and bores. The capping program now underway has shown some evidence of reversing this trend and, if completed, the capping program should lead to ongoing water availability for future users and the environment.

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4.2 The Murray Groundwater Basin

The 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 in part 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 in some areas over the last 25 years. However, a number of areas in southern NSW and northern Victoria have recorded a decline in the areas of high shallow groundwater during the recent drought.

There are large recoverable reserves of groundwater available in the Murray Basin aquifers. Use of groundwater as a proportion of the sustainable yield varies significantly across the basin, with use exceeding the sustainable yield in some areas. A major reason for this is variability of water quality across the Murray Basin.

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.

A wide range of ecosystems depend on groundwater, partially or completely, as a source of water. There are many examples of ecosystems that are dependent on groundwater in the Murray-Darling Basin
(SKM 2001).

The groundwater dependent ecosystems of the Murray-Darling Basin are a distinct component of the ecological values of the region and significantly add to the Basin’s biodiversity. These ecosystems are unique because of their dependency of at least some ecological processes on groundwater. Many of these ecosystems are poorly understood, despite their high conservation value. Moreover, these systems are facing increasing pressure from consumptive uses of groundwater and land use changes. Their fate is linked to the improvements being made
in groundwater management across the Basin.

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