On the large sand mass of Bribie Island, the sedimentary evolution related to sea level changes has resulted in marked spatial variations of aquifer properties. The main influence on groundwater behaviour is the induration of layers within the sandy sequences that comprise the island. The degree of induration controls vertical flows between the perched watertable and the underlying semi-confined regional aquifer. To identify heterogeneity within the indurated zone, an analysis of bore hydrographs was undertaken. The analysis shows that separate sections of the hydrological profile across the island display distinct unconfined or confined behaviour depending on the degree of hydrological discontinuity in the sandrock horizon. A two-dimensional (2-D) hydrogeologic conceptual model of Bribie Island is developed for the numerical modelling process. The two-aquifer system separated by the indurated layer ("coffee rock") is incorporated into the groundwater model. The indurated layer is spatially variable in thickness, continuity and permeability. An evaluation of the recharge and drainage parameters is conducted for improvement of model accuracy, using MODFLOW software to solve the quasi three-dimensional (3-D) flow model. The evapotranspiration (ET) parameter is tested due to the important role it has in the water balance of Bribie Island. The groundwater extraction and wastewater infiltration rates are evaluated separately for model input. Average hydraulic conductivities initially used in the numerical model for each aquifer do not match measured heads adequately. During initial model development, pilot point parameterisation of hydraulic conductivities was conducted providing a more complex distribution of the parameter and a better fit to observed water levels. Using spatial interpolation techniques, a gradual and realistic distribution of hydraulic properties is achieved rather than sharp changes between facies-related sedimentary sequences. The resultant visualisation of hydraulic conductivities supports the outcomes of the analysis of water level profiles. The model interprets and adjusts for the effects of the aquifer heterogeneity in respect to the hydraulic conductivity parameter. Areas of unconfined aquifer behaviour correspond to zones of higher hydraulic conductivities and are interpreted as regions of higher permeability due to a lesser degree of induration. Such zones of greater groundwater flow were found in the south of the island, the central swale and the northwest coastline, which act as connections to the basal aquifer enabling preferred recharge to it. Calibration is conducted in respect of the hydraulic conductivity, drainage and ET parameters. The calibration of simulated to observed hydraulic heads (objective function of 2.96 and correlation of 0.993) for the two layers achieved a close fit. The groundwater study demonstrates that both a statistical approach and numerical modelling are important in testing and refining the conceptual model. The modified conceptual model can be used as a basis for the construction of an improved and more reliable numerical model. The integration of geological information and spatial variability of aquifer parameters to produce a quasi three-dimensional model based on a two-aquifer conceptual model is a significant innovation of the modelling approach to Bribie Island. This approach contributes to a clearer understanding of the hydrogeological processes within the island. The investigation has contributed to a better understanding of the effects of broad and finer scale heterogeneities on groundwater dynamics in large sand mass.

Bribie Island is a large, heterogeneous, sand barrier island that contains groundwater aquifers of commercial and environmental significance. Population growth has resulted in expanding residential developments and consequently increased demand for water. Caboolture Shire Council (CSC) has proposed to increase groundwater extraction by a new borefield. Two aquifers exist within the Quaternary sandmass which are separated by an indurated sand layer that is ubiquitous in the area. A shallow aquifer occurs in the surficial, clean sands and is perched on the indurated sands. Water levels in the shallow water table aquifer follow the topography and groundwater occurs under unconfined conditions in this system. A basal aquifer occurs beneath the indurated sands, which act as a semi-confining layer in the island system. The potentiometric surface of the basal aquifer occurs as a gentle groundwater mound. The shallow groundwater system supports water-dependent ecosystems including wetlands, native woodlands and commercial pine plantations. Excessive groundwater extraction could lower the water table in the shallow aquifer to below the root depth of vegetation on the island. Groundwater discharge along the coastline is essential to maintain the position of the saline water - fresh groundwater boundary in this island aquifer system. Any activity that changes the volume of fresh water discharge or lowers the water table or potentiometric surface below sea level will result in a consequent change in the saline water – freshwater interface and could lead to saline water intrusion. Groundwater level data was compared with the residual rainfall mass curve (RRMC) on hydrographs, which revealed that the major trends in groundwater levels are related to rainfall. Bribie Island has a sub-tropical climate, with a mean annual rainfall of around 1358mm/year (Bongaree station). Mean annual pan evaporation is around 1679mm/year and estimates of the potential evapotranspiration rates range from 1003 to 1293mm/year. Flows from creeks, the central swale and groundwater discharged from the area have the potential to affect water quality within the tidal estuary, Pumicestone Passage. Groundwater within the island aquifer system is fresh with electrical conductivity ranging from 61 to 1018ìS/cm while water near the coast, canals or tidal creeks is brackish to saline (1596 to 34800ìS/cm). Measurements of pH show that all groundwater is acidic to slightly acidic (3.3-6.6), the lower values are attributed to the breakdown of plant material into organic acids. Groundwater is dominated by Na-Cl type water, which is expected in a coastal island environment with Na-Cl rainfall. Some groundwater samples possess higher concentrations of calcium and bicarbonate ions, which could be due to chemical interactions with buried shell beds while water is infiltrating to depth and due to the longer residence times of groundwater in the basal aquifer. A steady-state, sub-regional groundwater flow model was developed using the Visual MODFLOW computer package. The 4 layer, flow model simulated the existing hydrogeological system and the dominant groundwater processes controlling groundwater flow. The numerical model was calibrated against existing data and returned reasonable estimates of groundwater levels and hydraulic parameters. The model illustrated that: .. The primary source of groundwater recharge is infiltration of rainfall for the upper, perched aquifer (Layer 1). Recharge for the lower sand layers is via vertical leakage from the upper, perched aquifer, through the indurated sands (Layers 2 and 3) to the semi-confined, basal aquifer (Layer 4). .. The dominant drainage processes on Bribie Island are evapotranspiration (15070m3/day) and groundwater seepage from the coast, canals and tidal creeks (9512m3/day). Analytical calculations using Darcy’s Law estimated that approximately 8000m3/day of groundwater discharges from central Bribie Island, approximately 16% less than the model. .. As groundwater flows preferentially toward the steepest hydraulic gradient, the main direction of horizontal groundwater flow is expected to be along an eastwest axis, towards either the central swale or the coastline. The central swale was found to act as a groundwater sink in the project area.

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