The Millstream Aquifer features a highly transmissive calcrete unit that is principally recharged by flooding events in the Fortescue River supplemented by direct rainfall recharge on the outcropping aquifer and from ephemeral streams that drain water off the Hamersley Range to the south. Groundwater discharges from the aquifer through a number of river pools and springs that support diverse and culturally significant ecological communities.
The Millstream production borefield has been operated by the Water Corporation of Western Australia since the late 1960’s. Water pumped from production wells in the Millstream Aquifer is an important source of water for the Western Pilbara Water Supply Scheme.

Objectives
To understand the relationship between the rate at which water is extracted from the borefield and the associated impacts on the natural groundwater discharge sites.

Methods
A transient groundwater flow model was formulated and calibration was undertaken for the period 1968 to 1995. Calibration was followed by a verification (or validation) procedure which involved running the model over the period 1995 to 2007 with comparisons of key model outputs to observed aquifer responses.

A series of predictive scenarios were run over a one hundred year time frame using synthetic river flow and rainfall data sets. Climate dependent data sets were created for a series of climate change scenarios so that the impacts of future climate change can be illustrated.

Results
The predicted groundwater fluxes to the springs and river pools were found to be sensitive to the assumed future climate. A dry future climate change assumption results in a 25% reducion in safe extractive yield from the borefield.

Conclusions
The modelling results when combined with a series of ecological trigger levels provide a useful means of establishing sustainable extraction limits for the borefield under current and future climate change assumptions.