GROUNDWATER MANAGEMENT IN BRIBIE ISLAND FOR URBAN EXPANSION

Peter Evans, Jerome Arunakumaren, Martin Doré and Troy Kasper
Bribie Island, a vegetated sand barrier island, is located close to Brisbane, Australia at the northern end of Moreton Bay and is separated from the mainland by Pumicestone Passage.

Since 1962 Bribie Island’s town water supply has been successfully largely sourced from groundwater drawn from an open trench system excavated into Quaternary age dune sands at the southern end of the island.  The landward migration of the seawater interface into the aquifer is inhibited through a process of release of highly treated sewage effluent to sand dune areas south of the extraction trench to preserve a mound of groundwater with an elevation above mean sea level between the coastline and the groundwater extraction trench.  An increase in local water demand from urban expansion on Bribie Island has now led to establishment of a larger capacity well-field for town water supply purposes in the central area of the island. 

Although relatively advanced software is available which can simulate complex seawater intrusion processes, there is frequently insufficient relevant data available to define the hydrogeological conditions that exist in coastal aquifers.  This raises questions regarding the predictive uncertainty of complex seawater intrusion models in the face of relatively limited data. The application of sophisticated seawater intrusion models does not necessarily provide sufficient confidence to support environmental and water resource management decision making regarding groundwater extraction in coastal aquifers.  The environmental implications of the expansion of urban areas of Bribie Island has been studied using a simple suite of software comprising MODFLOW, MT3DMS, MODPATH and Glover’s formula to describe the movement of seawater interface with discharge of freshwater. 

This simple process of numerical groundwater flow modelling combined with analytical modelling of seawater intrusion was used to investigate the relative impacts on the island’s groundwater resources for a series of alternative water resource development scenarios.  Since the response of complex natural systems to hydrological stress is not completely predictable, a robust process of adaptive management is currently undertaken for groundwater extraction on Bribie Island.  This adaptive management system is supported by both systematic hydrological and ecological monitoring including rigorous comparison of observed data with pre-determined thresholds.