In the arid, low rainfall environment around Olympic Dam, there are no reliable sources of surface water. Water used at Olympic Dam and in the town of Roxby Downs is artesian water and is extracted from the Great Artesian Basin. Artesian water is found in layers of porous rocks that may be up to hundreds of metres below the surface. These layers of water-bearing rock extend from the western slopes of the Great Dividing Range across nearly all of outback Queensland to the north east of South Australia. They cover a total area of 1.7 million square kilometres (22% of Australia) and contain 8.7 billion megalitres of water (a megalitre [ML] is a million litres; an Olympic swimming pool contains about 2 ML). This is equivalent to 4.35 billion Olympic swimming pools!
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The blue area on this map shows the extent of the Great Artesian Basin. |
This map shows the position of Olympic Dam compared with the southern margin of the GAB. |
Because of the higher elevation of the water-bearing rocks in the Great Dividing Range, the underground water in the basin builds up considerable pressure. This allows artesian water to flow freely from much of the basin, where it is used for a number of different purposes. The annual amounts of water used are summarised in the table below.
| Uses of Great Artesian Basin (GAB) water | Estimated annual amounts used (Megalitres per year; ML/a) | Percentage of total GAB water use |
|---|---|---|
| Pastoral bores - pipes sunk into the basin to allow water to be pumped out or to flow to the surface. | 495,000 | 87.0 |
| Town supplies | 20,000 | 3.5 |
| Petroleum industry - operations extracting hydrocarbons from the ground | 20,000 | 3.5 |
| Irrigation | 11,000 | 1.9 |
| Mining - includes all mining operations using GAB water as well as Olympic Dam | 16,000* | 2.8 |
| Industrial | 6,000 | 1.1 |
| Tourism | 1,000 | 0.2 |
| Total | 569,000 | 100.0 |
| *In 2000, the Olympic Dam operation used 10,560 ML, and Roxby Downs used 1,035 ML, making a total of 11,595 ML. This represents only 2.0% of the total amount of Great Artesian Basin water extracted from the basin in a year. | ||
The Olympic Dam mine site and Roxby Downs town lie to the south of the Great Artesian Basin. For this reason, water is pumped to the mine and to the town along underground pipe-lines from two bore fields that lie within the basin, 110 and 200 km north of Olympic Dam. The water from the borefields has a salinity greater than that permitted for domestic use. It is also more saline than is desirable for some aspects of minerals processing. Therefore the town's supply and some of the metallurgical plant's supply must be purified in a desalination plant before use.
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Part of the underground pipeline carrying artesian water from the borefields to Olympic Dam, during construction. |
Map showing the locations of Bore Fields A and B and Olympic Dam. |
Despite the enormous amount of water in the Great Artesian Basin, it is a finite resource, and care must be taken not to extract too much water from it. If the discharge from the basin becomes too great, the water pressure will drop, and flow from bores and natural springs may decrease or stop. This could have undesirable ecological and economic consequences. For example, many pastoral properties could become short of water for stock to drink and may go out of business. Wetlands and mound springs associated with natural springs may dry out and become damaged.
Measuring artesian water pressure in a bore in the Great Artesian Basin.
In order to minimise the effect on the Great Artesian Basin, the mining operation at Olympic Dam takes great care not to waste water, by using as little as possible for their purposes, and by recycling as much as possible.
This retention pond contains rainwater that is recycled through the processing plant.
Evaporation ponds, from which contaminated water is allowed to evaporate back into the atmosphere as pure water vapour.
Another important aspect of water management involves control of contaminated water. Some water from the processing plant contains acid and other chemicals, some of which are radioactive. This water cannot be recycled through the plant nor released to the environment. It is pumped through pipes to the Tailings Retention System (TRS). This is a complex series of storage cells, evaporation ponds and tailings ponds where the water evaporates as pure water back into the atmosphere, leaving solid waste tailings behind.
During 1994 seepage of water from the Tailings Retention System into the ground was identified. This was of concern to the mining company because of the threat to ground water quality immediately below the TRS. However, any radioactive pollutants in the seepage were filtered out by the clay liner at the base of the TRS. The acidity of the water was removed when it reacted with limestone beneath the TRS. The remaining water was of similar quality to the local ground water and so no environmental harm resulted. To ensure minimal future seepage from the TRS, a series of evaporation ponds was built from 1996. These are double-lined with plastic liners and clay liners underneath. Water is transferred (decanted) from tailings storage areas for evaporation from the ponds.
Waterproof plastic liners are used on the bottom of evaporation ponds to prevent leakage of contaminated water into underlying soil and rocks.
Other methods are used at different localities around the mine site to ensure that contaminated water does not enter the environment. For example, the processing plant is constructed on concrete slabs with concrete walls (bunds) around them. Any water spillage or rainwater is collected on the concrete and flows into drains to retention ponds. From here it is recycled through the processing plant. Similar techniques are used around workshops, vehicle washing areas, and storage areas for chemicals, fuels and oils.
Any minor water leakage from this leaching tank is prevented from contaminating the environment by this concrete apron and gutter.
