Gnangara Groundwater
Background
The Gnangara Mound is the name given to the superficial aquifer on the Swan Coastal Plain bound by Ellen Brook in the east, Swan River to the south, north to Gingin Brook, and the Indian Ocean.
The Gnangara System comprises:
- a superficial aquifer (up to 60 metres deep)
- the confined Leederville aquifer (up to 500 metres deep)
- the confined Yarragadee aquifer (over 700 metres deep).
The Gnangara System is the largest groundwater source developed in the State. The superficial aquifer provides water for public supply, agriculture, forestry, market gardens, local government authorities and garden bore users. The majority of the water abstracted from the deeper confined aquifers is used for public water supply.
The Gnangara Mound is a complex and dynamic hydrogeological ecosystem, impacted by a number of factors:
- drying climate;
- fire;
- urbanisation;
- dieback;
- multiple land uses; and
- water abstraction.
Water Quantity
The Gnangara System underpins the IWSS water grid and has provided up to 60% of the annual supply to Perth, the Goldfields and surrounding communities.
The Department of Water licenses the Water Corporation to access groundwater from the Gnangara system. A variable groundwater abstraction rule that provides for reduced groundwater use as surface water storage levels recover allows for up to 165 gigalitres of groundwater to be used from the Gnangara and Jandakot groundwater systems. The Gnangara system contributes up to 95% of the groundwater used within the IWSS water grid.
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The Water Corporation and the Department of Water jointly monitor pumping to understand the impact of water abstraction on the mound. The impact varies by location of the pumps. On average, the Water Corporation has drawn about 150 gigalitres a year from the Gnangara system for the water grid, over the past few years. This is about 40% of the estimated total water abstracted from the Gnangara system by all users.
In 2006, due to very poor rainfalls into surface water dams, abstractions totalled 162
gigalitres. Water is taken from eight separate schemes on the Mound.
| Groundwater Scheme | Abstraction 2006/07 (gigalitres) |
| Mirrabooka | 22.5 |
| Gwelup | 16.9 |
| 50.1 | |
| 32.6 | |
| Lexia | 6.4 |
| Yanchep & Two Rocks | 1.0 |
| Independent Yarragadee Bores | 32.8 |
| Total | 162.3 |
In wet years, the Water Corporation reduces abstractions from the mound as water is available from hills dams. In 2007 for example, due to better rainfalls that year, the Water Corporation only abstracted 135 gigalitres from the Gnangara system.
Water Quality
Each aquifer, borefield and bore has its own characteristics. Water from the superficial aquifer on the mound requires treatment to be suitable for drinking water supplies.
The Gnangara Land Use and Water Management Strategy (GLUWMS), prepared by the Western Australian Planning Commission, protects the public drinking water supply drawn from the central part of the Gnangara Mound. The Perth Coastal Underground Water Pollution Control Area has been proclaimed by the Department of Water to protect the public drinking water supply drawn from the western part of the Gnangara Mound.
Due to the impermeable layer that creates the aquifer, water from the confined aquifers is protected from the short term impacts of land use on water quality. The level of salinity is variable, with some bores being low at 200 mg/L and others reaching 1200 mg/L. Water for drinking generally should be less than 1000 mg/L and preferably be below 500 mg/l.
Temperature is a significant issue in the Yarragadee aquifer, with water being brought to the surface at up to 40 degrees Celsius.
This varying quality of water drawn from the Gnangara system is factored into operational decisions. For example, water which is salty and hot can be mixed with water low in these attributes. This is a low cost, highly effected method of providing safe, reliable drinking water to customers of the water grid, without the need for expensive additional treatment.
Current Situation
Water from the Gnangara system remains vitally important for a variety of users and the environment. Due to the large number of factors that can affect performance of the system an adaptive management approach is being used to manage user and environmental needs.
The quantity of water supplied into the water grid is currently determined by the variable groundwater abstraction rule and so varies each year depending on prevailing conditions.
The use of superficial and confined aquifers is also reviewed annually to satisfy operational constraints of the water grid and to minimise the environmental impact of abstraction.
This flexibility to alter abstraction strategies from year to year exists because there has
been significant investment in groundwater system assets, particularly into the confined aquifers in recent years.
Sustainability considerations
Economic | Social and Cultural | Environmental |
| Gnangara is an existing low cost water source – most replacement schemes would be more costly | Desire for increasing information and transparency regarding water use and ecosystem health of the mound | Complex interrelationships, including water supply for public use, may adversely impact wetlands and vegetation values |
| Current variable abstraction rule supports supply security in times of drought without additional investment | Some pressure to urbanise land on the mound currently using water for horticulture and other private purposes | Reduced water abstraction from this resource will necessitate the creation of substitute sources which may have environmental impacts |
| Gnangara is an existing low cost water source – most replacement schemes would be more costly | Gnangara is an existing low cost water source – most replacement schemes would be more costlyDesire for increasing information and transparency regarding water use and ecosystem health of the mound | Impact of reduced rainfall on long term water availability from the mound, requires further study. |
Potential source yield
The Gnangara System currently supplies around 150 gigalitres a year for public water supply. Without investment in new infrastructure this is expected to reduce to 110 -115 gigalitres a year on average.
Future development of Northwest Corridor Coastal groundwater schemes or artificial recharge techniques to enable increased use of existing groundwater infrastructure could provide significant additional capacity into the future.
Potential cost
Groundwater from Gnangara costs less than $1 a kilolitre.
The Future
Gnangara Sustainability Strategy
The State Government is providing $7.5 million in funding to undertake the Gnangara Sustainability Strategy. This across Government strategy is being led by the Department of Water in collaboration with:
- Department of Environment and Conservation
- Department of Planning and Infrastructure
- Department of Agriculture and Food
- Water Corporation.
The Strategy is investigating the future land use and water management opportunities
for the mound and is scheduled to be complete in mid-2009. This information will be essential in determining how this groundwater source can be used in the future.
Until 2011
In 2011 the Southern Seawater Desalination Plant is proposed to be available. This plant will significantly add to the supply available for the water grid.
Until this time however, the Water Corporation will remain very dependent on continued access to water from the Gnangara system, to reduce the risk of total sprinkler bans.
After 2011
It is anticipated that under a dry climate scenario, a sustainable average annual abstraction for public water supply from the existing developed groundwater schemes within the Gnangara system will be more than 120 gigalitres a year. The Water Corporation has allowed for this in current planning but the outcome of the Gnangara Sustainability Strategy should better define the best use of water from the Gnangara system.
A better understanding of the potential for additional water source development should also be an outcome of the strategy. At this time the Water Corporation is planning for future source development largely focussed on the North West Corridor Coastal Groundwater schemes that aim to capture groundwater before it discharges into the ocean from Quinns Rock to north of Yanchep.
The legislative framework to allow water trading exists in Western Australia. Once a clear management strategy for the Gnangara system is in place the Water Corporation will investigate water trading opportunities on the Gnangara Mound.
The use of groundwater replenishment techniques such as aquifer storage and recovery using treated surface water or groundwater or the use of highly treated wastewater as a means to recharge aquifers are being investigated and could prove vital in future sustainable management of the Gnangara System.
More Information
Key
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Potential source yield (in 50 year planning horizon)
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Potential cost (2007 $)