Soil degradation is the most serious environmental issue facing Australia today, and a major problem is the land salinisation and degradation of surface water quality in the Murray Basin of southeastern Australia. Most of the salinity problems are ground-water related, and it is only by understanding regional groundwater processes that effective management plans can be formulated to combat the spread of salinity throughout the basin. Regional aquifers of the Murray Basin can be put into three distinct provinces. The Riverine province underlies the Riverine Plain and the eastern Mallee region in New South Wales and Victoria, the Mallee-Limestone province underlies the Mallee south of the Murray River in South Australia and the Wimmera and western Mallee in Victoria, and the Scotia province occupies the most arid part of the basin east and south of the Barrier Range in South Australia and New South Wales. Groundwater flow in the three provinces is towards the Murray River, which is the sole conduit for the transport of groundwater and salts out of the basin. The major aquifers of the Murray Basin are the Renmark Group, Murray Group, Pliocene Sands and Shepparton Formation. The major impediment to flow , the mid-Tertiary low-permeability barrier, extends in an arc about 100 km wide through the centre of the basin. The barrier (a composite of the Winnambool Formation/Geera Clay) disrupts lateral throughflow in the Renmark Group aquifers and overlying Pliocene Sands. This disruption is one of the factors that controls the distribution of groundwater discharge zones, the extent of which is ultimately controlled by groundwater pressure in recharge zones. The Geera Clay is also a source of salts which diffuse downward into the lower Renmark Group aquifer. Land clearing since European settlement in the recharge zones has increased the rainfall infiltration capacity of soils, thereby increasing water pressure. This has resulted in rising water tables down-basin and reactivation of many ancient discharge zones, particularly in Victoria. However, land salinisation has been much more widespread in the recent geologic past. The Murray Basin aquifers contain about 4600 million megalitres (ML) of groundwater in storage, two-thirds of which is useful to humans. The total salt load in the aquifers and aquitards exceeds 100000 million tonnes and, under the current groundwater- surface water interactive flow regime, < 0.004% of this huge salt store is transported out of the basin annually by the Murray River. Recoverable reserves of groundwater in the unconfined aquifers total around 4.5 million ML/ year. The utilisation rate is only a few per cent of the sustainable yield in the unconfined aquifers and even less in the confined aquifers. Greater use of groundwater in recharge zones would ultimately reduce the rate of rise of the regional down-basin water table, though salt disposal considerations are the limiting factor on any development.
