Environmental monitoring to inform sustainable management of groundwater extraction for urban water supply on Bribie Island

Groundwater resources on Bribie Island were mandated for additional development in 2006 as part of the Queensland Government's Regional Drought Management Strategy. Subsequently, a production borefield and water treatment plant were constructed at Banksia Beach during 2007/2008 to supply potable water to Bribie Island, and to augment supply to the South East Queensland region via connection to the water grid. An initial process of numerical groundwater flow modelling combined with analytical modelling of seawater intrusion was used to investigate the potential impacts of additional extraction of the island's groundwater resources. A comprehensive monitoring program was then developed to ensure that groundwater extraction would not induce unacceptable landward movement of the seawater interface, and prevent adverse impacts to groundwater dependent ecosystems to address State and Federal environmental regulatory requirements. To date, monitoring data indicate the response of the aquifer to extraction is in agreement with modelled predictions. Static water level across the northern borefield appears to maintain a positive relationship with rainfall, and these trends are likely to represent natural fluctuations in groundwater as the planned extraction volume has been significantly reduced during this period. Electrical conductivity data have demonstrated no discernible movement of the seawater interface since extraction began with observed fluctuations in EC of the shallow surface bores likely to be a result of local transport of salts with rainfall. The fluctuations in water levels across the monitored intermittently closed and open lagoon systems reflect natural variation due to runoff and marine influences and are not responsive to drawdown in the deep aquifer. Short-term changes in the key groundwater dependent ecosystems (GDEs) due to drawdown of the aquifer cannot be differentiated readily due to the natural variability of the ecosystems. The environmental monitoring program provides crucial feedback into the adaptive management strategy of the production borefield and water treatment plant (WTP) which helps safeguard the sustainability of the Bribie Island aquifer and associated ecosystems.