Aquifer heterogeneity within the large coastal sand island of Bribie Island, Queensland, Australia, has an affect on groundwater occurrence and migration. The stratigraphy of Bribie Island is complicated by the presence of low permeability humate-cemented indurated sand layers. Occurrences of indurated sand layers have previously been identified within many unconsolidated profiles along the east coast of Australia and around the world. Indurated sand layers are often discontinuous resulting in localised aquifer heterogeneity. However, their regional significance is commonly underestimated. The groundwater resource of Bribie Island is of commercial and environmental significance to the surrounding bay area. Recent development proposals for the groundwater resource necessitate an investigation into the nature of the water bearing properties of the island aquifer and in particular the presence of aquifer heterogeneity. Investigation of a "reference" transect across Bribie Island has involved the drilling and development of monitoring wells and the performance of hydraulic tests. This study demonstrates how detailed measurement of stratigraphy, groundwater levels, rainfall, barometric pressure and hydraulic testing can be used in conjunction to identify and assess aquifer heterogeneity within a sand island environment. Drill logs confirm the position of a palaeochannel within the sandstone bedrock that extends from the mainland continuing under Bribie Island. The overlying sediment profile is thickest within the palaeochannel. The Pleistocene and Holocene unconsolidated profile reflects a prograding barrier island/strandplain formation. The vertical sequence of sediments consists of units that range from offshore sandy silts to foreshore and beach medium-fine grained sands. An extensive indurated sand layer exists throughout the centre of the island. The greatest thickness of indurated sand is located centrally on the island beneath the main beach ridge system. The indurated layer at its thickest is approximately 5-8 m thick, but over much of the island the thickness is 1-3 m. The top of indurated sand layer is generally 1-3 m above mean sea level. Hydrographs from a network of groundwater monitoring wells illustrate that the groundwater resources across the reference transect can be divided into a shallow unconfined water table aquifer and basal confined aquifers. These upper and lower aquifers are characterised by different hydrological processes, physico-chemical properties, and water chemistry. The stratification of water levels across the reference transect and the relatively flat piezometric surface are in contrast with the classical "domed" water table aquifer expected of a barrier island. Stratified head gradients through the Bribie Island aquifers suggest groundwater migration to depth is impeded by the indurated sand layer. An elevated shallow water table results from the mounding of water above the indurated sand layer. The indurated sand layer is extensive across the reference transect. The elevated unconfined groundwater is usually stained with organic matter ("black water"), where as groundwater sourced from beneath the indurated sand layer is colourless ("white water"). The unconfined groundwater is also distinguished by low pH, low bicarbonate concentrations and high concentrations of organic carbon. Interaction between unconfined groundwater and surface water are also evident. Hydraulic tests indicate that each of the unconsolidated units across the reference transect has distinctive hydraulic characteristics. Estimates of vertical and horizontal hydraulic conductivity of the unconfined aquifer are two to three orders of magnitude greater than estimates for the indurated sand layer. Beneath the indurated sand layer hydraulic conductivities of the basal aquifers are also greater by two to three orders of magnitude than estimates for the indurated sand layer. The lower hydraulic conductivity within the indurated sand layer is responsible for the local semiconfinement of the basal aquifers.
| Identifer | oai:union.ndltd.org:ADTP/265270 |
| Date | January 2006 |
| Creators | Armstrong, Timothy James |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Timothy J. Armstrong |
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