Western Australia is fortunate in its endowment of earth resources: indeed it is well known as one of the world’s leading mineral provinces. Development of the huge reserves of iron ore, natural gas, mineral sands, nickel, gold, and diamonds have enriched WA, making it the powerhouse of the Australian economy. Less well known is the fact Perth, unlike other capital cities in Australia, has ready access to groundwater, another earth resource that is of vital importance to the future of the State.
Most members of the public, and many politicians, seem unaware of the fact Perth is much better off in relation to future low-cost water supplies than other mainland capitals. Perth can access large reserves of potable groundwater held in the Perth Basin, a large sedimentary basin that extends north-south from near Geraldton to Augusta and east to the Darling Fault. Many Perth residents do not appreciate that groundwater now supplies about 60 per cent of their domestic needs as well as 80 per cent of the total water used in the city and adjoining areas. Perth’s reliance on groundwater contrasts with the situation in Sydney, Melbourne, Brisbane, Adelaide, and Hobart - those cities are wholly dependant on surface water.
There has been much publicity in Perth during recent years about the low levels of water in dams that now supply about 40 per cent of Perth’s needs. The southwest of WA has experienced a 10 per cent decline in rainfall over the last 30 years, and this has resulted in major reductions in water flows into dams in the Darling Range. Some scientists, using computer-generated models, have predicted that rainfall will decline even further because of continuing “climate change”, with dire consequences for lifestyles in Perth and agriculture in the southwest. However, others have disputed the validity of those models, maintaining that it is impossible to predict reliably whether or not the climate will remain the same, or become drier or wetter than it is today.
The potential for building new dams in the Darling Range is low, because most fresh-water streams have already been dammed. Water storage in existing dams rose from 25 per cent of capacity in May 2005 to more than 40 per cent in early November 2005, the highest level achieved in the past five years. This is because Perth’s rainfall during 2005 was above average for the first time since 1996. However, even if future rainfall allows the dams to fill to capacity, this will still not be enough to satisfy rising demand associated with population increase and industrial expansion: Perth’s water needs are expected to double in less than 50 years. As a result, the percentage of dam water used in Perth must continue to decline, while the use of groundwater can be expected to increase correspondingly.
From 1962 to 1995, the Geological Survey of Western Australia carried out a strategic drilling program, involving more than 200 deep bores in 16 lines across the Perth Basin between Geraldton and Augusta, at a total cost of some $60 million (in today’s dollars). This was a far-sighted strategic program, designed to identify groundwater resources for future use in Perth and elsewhere in the southwest. It showed that the basin contains enormous resources of potable groundwater in the Jurassic Yarragadee Formation and younger near-surface formations. Most of the ground surface of the Perth Basin is covered with porous sand, which results in much of the rain falling on the area sinking below the surface, replenishing the underlying aquifers.
In 1996, the hydrogeology section of the Geological Survey (in the Department of Minerals and Energy) was transferred to the Water and Rivers Commission. Groundwater exploration then came to a halt, the groundwater database was not maintained, hydrogeological mapping virtually ceased, and the report required to integrate the results of the earlier Perth Basin drilling program was not prepared.
In 2001, the incoming Gallop Government recognised that the commission had not been adequately fulfilling its groundwater responsibilities, and as a result those responsibilities were transferred to the Department of Environment. That was another mistake, as the conservation culture of the Department of Environment was not readily compatible with groundwater exploration. Consequently the groundwater staff have again been moved - in 2006 they were transferred to the new Department of Water. It will be vitally important for this department to reinvigorate hydrogeology and initiate the programs of strategic drilling to define the groundwater reserves required to supply Perth and the southwest during coming decades.
In 1994, before hydrogeology was transferred to the Water and Rivers Commission, the Geological Survey had planned a program of drilling to further define groundwater reserves in the South West Yarragadee aquifer (south of Busselton). But as a result of the transfer, the program did not begin until 2003.
The new drilling confirmed that sustainable production of fresh groundwater (200mg/L of total soluble solids) could be maintained from the South West Yarragadee and overlying aquifers, at rates of up to 300GL per year. This compares with 100 GL per year currently supplied from dams in the Darling Range. It was announced in 2005 that water production from the South West Yarragadee aquifer for Perth’s integrated water-supply system would begin as soon as possible at an initial rate of 45GL per year.
The Government had previously announced a seawater desalination plant was to be constructed, costing nearly $380 million and producing 45GL per year, with completion scheduled for 2006. It is acknowledged by many water experts that if the results of the South West Yarragadee drilling project had been available two or three years earlier, a decision would not have been made to go ahead with construction of the desalination plant. The costs of water from the South West Yarragadee project are expected to be $0.85/kL, compared with $1.16/kL for the desalination plant. It has been suggested that the Government should consider mothballing the desalination plant after the South West Yarragadee project has come into production, retaining the plant as insurance against any future water shortages.
Most of Perth’s water is now produced from bores in the Gnangara Mound, north of the city. These bores tap the Yarragadee and younger aquifers. The groundwater is fed into the integrated water-supply system, some of which reaches as far as the Eastern Goldfields and parts of the wheat-belt. However, present production rates at Gnangara are unsustainable. Water levels have fallen, and production will need to be reduced after production begins from the desalination plant and the South West Yarragadee project.
Most of the North Yarragadee aquifer, which extends from north of Perth to Dongara, has still to be adequately evaluated. There can be no doubt that very large groundwater reserves exist in this large area, and they will eventually be developed to help meet the expanding water needs of Perth and elsewhere. Apart from the development at Gnangara, the only other part of the aquifer that has been developed to date is near Dongara, to supply Geraldton’s water needs. The salinity of North Yarragadee water (700mg/L) is higher than that currently supplied to Perth (400mg/L), but the quality can be improved by mixing with dam water or low-salinity water from the South West Yarragadee. Furthermore, the reverse-osmosis desalination process can operate at much lower costs when treating brackish groundwater as opposed to seawater.
For further information see the report sponsored by the Australian Academy of Technological Sciences and Engineering (ATSE), the Australian Water Association, and Engineers Australia, entitled Western Australia: water policy issues in climate uncertainty.