Background and geographic area
The Millstream aquifer is a large, isolated groundwater system in north western Australia. The aquifer is recharged episodically by rainfall from cyclones. Seepage from the aquifer sustains approximately 20 kilometres of permanent wetlands, which are of high ecological and cultural significance and form part of the Millstream National Park. Fresh water from the aquifer is used to supply growing industrial and domestic water demand in the West Pilbara.
The long-term sustainable yield of the aquifer is low, but following wet years, the aquifer yield can be quite high with negligible impacts. As a result of this variability, it can be a challenge to manage abstraction from the aquifer to ensure no significant adverse impacts on environmental and cultural values and on the renewability of the resource itself.
The need for a secure, reliable supply is at odds with the variability of the water source, but the economic cost of not using this source is considerable. The only way the aquifer can be optimised is to take a risk based approach for the long term, manage to thresholds in the short term, and have alternative water sources for extended dry periods.
Application of a risk–based approach
Impact management criteria have been in place for many years and will be adjusted to incorporate new, more precise information. The criteria are based on a conceptual understanding of how aquifer discharge meets the needs of the ecosystems that depend on them. This relationship is interpreted through conceptual and statistical models that are based on correlations between measured aquifer water levels, surface flows and observations from field investigations. The long-term water level and discharge data sets, lithology of the aquifer, and a quantitative understanding of tree physiology and evapotranspiration rates informed the models.
Impact management criteria are used to control:
- the allowable rate of aquifer decline over different time frames
- the minimum aquifer level
- the minimum rates of spring discharges to the downstream environment.
Risk management criteria are used to set:
- the long-term average yield
- the maximum time frame for above average abstraction.
Water resources are managed through a license, which is the regulatory instrument that entitles the water user to a secure supply within the constraints of the criteria. Management measures include review of data against criteria, demand and supply forecasting, demand management, alternate pumping strategies, restrictions, contingency and drought management strategies, supplementation and ultimately cessation of abstraction.
The main strategy to protect the aquifer is, whenever possible, to use alternative water resources before the aquifer. This management approach depends on having an alternative source to the Millstream aquifer—the Harding Dam. The dam was constructed in the mid 1980s after the Millstream system experienced adverse impacts due to sustained high abstraction concurrent with impacts on the Millstream system.
In the mid-1980s thresholds were exceeded that led to tree deaths and temporary loss of habitat in wetland areas. In the absence of an additional supply source, this could occur again if there were another sustained dry period. While this is tolerable in the very short term, within the constraints of the long-term risk approach and the high variability of the natural system, the impacts of the mid 1980s show there is a tolerance threshold that should not be exceeded. With growing demand for water in the West Pilbara an additional water source is required. In late 2011, the Western Australian government announced that an agreement had been reached with Rio Tinto Iron Ore to develop a new 10 GL/yr borefield in the Lower Bungaroo valley. The new supply is expected to be online in mid-2013.
See National Water Initiative
for more information