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.
| Identifer | oai:union.ndltd.org:ADTP/265222 |
| Date | January 2006 |
| Creators | Spring, Ken |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Ken Spring |
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