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Hydrogeology, Conceptual Model and Groundwater Flow Within Alluvial Aquifers of the Tenthill and Ma Ma Catchments, Lockyer Valley, Queensland

Wilson, Andrew Scott (2005) Hydrogeology, Conceptual Model and Groundwater Flow Within Alluvial Aquifers of the Tenthill and Ma Ma Catchments, Lockyer Valley, Queensland. .

Abstract

The study focuses on the adjacent Tenthill and Ma Ma catchments which converge onto the heavily cultivated alluvial plain of Lockyer Creek.

Groundwater extracted from the alluvial aquifers is the primary source of water for intensive irrigation. Within the study the hydrogeology is

investigated, a conceptual groundwater model produced and a numerical groundwater flow model is developed from this. The hydrochemistry and

stable isotope character of groundwater are also investigated to determine processes such as recharge and evaporation.

Examination of bore logs confirms the Quaternary alluvium comprises a laterally continuous gravel aquifer with an average thickness of 4.5 m,

overlain by mixed sands and clays which form a semi-confining layer with an average thickness of 22 m. Variations in long term groundwater

hydrographs indicate the aquifer changes from confined to unconfined in some locations as water levels drop, while bores adjacent to creek banks display a rapid response to a flood event. Pump testing of bores screened in the gravel produces estimates of hydraulic conductivity ranging from 50-80 m/day and storativity of 0.00166 which are both within realistic bounds for this aquifer material.

Major ion chemistry of surface water collected during a flood is Mgdominated, similar to alluvial groundwater in the Tenthill catchment and the Lockyer plain, suggesting a strong connection between surface and groundwater in these locations. Alluvial groundwater salinity in Tenthill catchment is typically less than 3500 ìS/cm but may approach 6000ìS/cm on the Lockyer plain. By contrast Ma Ma catchment alluvial groundwater is Na-dominated with conductivity up to 12000 ìS/cm and more associated with groundwater from the underlying sandstone bedrock. Stable isotope analyses of alluvial groundwater from throughout both catchments and the Lockyer plain are compared with basalt and sandstone groundwater. A range of processes have been identified including recharge to alluvium from basalt groundwater and evaporated surface water; and alluvial-bedrock

groundwater mixing at some locations.

Integration of the components of the study enabled the production of a conceptual hydrogeological model of the Lockyer alluvial plain, proposing two hydrostratigraphic units; the gravel aquifer and the overlying mixed

sand and clay which acts as a semi confining unit. Hydrochemical and stable isotopic evidence suggests seepage from creek channels as the dominant recharge process. A single layer groundwater flow model using MODFLOW was developed, based on groundwater extraction data, to represent flow in the gravel aquifer. The model was calibrated to transient conditions with groundwater fluctuations, incorporating both drought and flood conditions. A sensitivity analysis for each of the aquifer properties

demonstrates the model is insensitive to variations within realistic bounds for the gravel aquifer material, however, the model is highly sensitive to changes in the chosen boundary conditions. Predictive simulations with

several annual extraction scenarios ranging from 1.75 to 0.5 ML/ha indicate the resulting minimum saturated aquifer thickness ranges from 0.03 to 1.4 m.

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ID Code: 16015
Item Type: QUT Thesis (Masters by Research)
Supervisor: Cox, Malcolm
Keywords: hydrogeology, conceptual model, groundwater, alluvial aquifers, Lockyer Valley, Queensland
Divisions: Past > Schools > Biogeoscience
Past > QUT Faculties & Divisions > Faculty of Science and Technology
Department: Faculty of Science
Institution: Queensland University of Technology
Copyright Owner: Copyright Andrew Scott Wilson
Deposited On: 03 Dec 2008 13:55
Last Modified: 29 Oct 2011 05:42

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