Small back-barrier sand islands are poorly known in terms of hydrogeology and have been overlooked in more extensive studies of coastal groundwater systems that include larger barrier island complexes. This study employs a three-fold sequential approach to aquifer characterisation in a back-barrier sand island. A three-dimensional stratigraphic model forms the foundation framework, being derived from a multidisciplinary approach to sedimentary analysis and the construction of a depositional chronology. A conceptual hydrostratigraphic model is formulated based on the translation of sedimentary facies to hydrofacies, combined with density dependent flow calculations and tidal oscillation measurements. Groundwater hydrochemical data and mineral geochemistry are integrated with the resulting hydrogeological model to examine water-rock interaction and solute transport mechanisms. The study area is Toorbul Island, a small back-barrier sand mass of ~5 km2 with a maximum surface elevation of ~3.5 m AHD, located in the Pumicestone Passage of Southeast Queensland. The island hosts a dual aquifer system consisting of an unconfined island freshwater lens, underlain by a semi-confined palaeovalley-fill aquifer. Groundwater in the semi-confined aquifer is hyper-saline, carrying high concentrations of dissolved metals, with iron, in particular, ranging from 40 to < 200 mg l-1. This is of significant interest for both human health and environmental management, because iron is an important nutrient source for toxic algal bacteria such as Lyngbya majuscula. Conceptual modelling demonstrates that iron oxides and hydroxides are the main source of iron in the semi-confined aquifer, with a contribution from ferruginous chlorite dissolution. Aqueous manganese and a proportion of the aqueous iron are derived from the dissolution of manganoan ilmenite. Ferric iron minerals also contribute a significant proportion of dissolved iron in the deeper regions of the unconfined aquifer. Aqueous iron in the shallow unconfined groundwater is limited by iron sulphides, which also regulate acidity and indirectly limit dissolved aluminium concentrations. Groundwater redox state governed by seasonal climatic fluxes is the most significant control on iron-bearing mineral phase stability. Transport of dissolved metals to the surrounding estuary and the adjacent barrier island groundwater system is limited by the rate of ion diffusion across transition zone boundaries. The overall conclusions derived from this research show that back-barrier islands should be evaluated as discrete hydrogeological entities. The stratigraphic complexity that may be apparent within these island landforms should not be underestimated and the model domain should not necessarily be treated as a homogeneous system. This complexity is exemplified by the relationship between the upper and lower aquifers on Toorbul Island and the associated distribution of groundwater compositional heterogeneity. The complex stratigraphy within the sedimentary pile is derived from the presence of a sub-surface palaeovalley and the sedimentary response to changing sea-level over time. Considering the current widespread distribution of estuarine systems, complex hydrogeology as exhibited by Toobul Island, may be common in many small back-barrier island groundwater systems. The aquifer characteristics and their influence on solute transport and delivery can have significant ramifications for the exploitation of the adjacent coastal plain and barrier island aquifers. The potential influence on the latter is of particular concern due to the pressure imposed on potable groundwater supplies by increasing population densities in coastal areas.

Utilizing the analytical solution of the steady state sharp interface saltwater intrusion model in coastal aquifers, a multi-objective optimization formulation of pumping rates and well locations in a coastal aquifer is formulated to solve problems in water management practice. The proposed optimization problem uses progressive genetic algorithm technique and the method developed is applied to the previous work of Cheng et al. [2000]. Through this analysis, several other applications are provided to demonstrate the use of the model in practical applications. This work is the first to optimize pumping rates as well as well locations simultaneously in coastal aquifer management. Known the limitation of the analytical solution, the work is expanded to cover the physics of saltwater intrusion in a more realistic way. This is variable density flow in a variably saturated porous medium. In this method, mixing between two fluids such as saltwater and freshwater can be described and the porous medium is also expanded to cover saturated and unsaturated zones together. One of the objectives is to develop a three dimensional physical model, verify the model, and apply to various applications in coastal aquifers. The developed model, TechFlow, is used to investigate instability issues associated with the numerical solution of the Elder problem in the perspective that includes physical instability issues associated with density differences used in numerical solutions, sensitivity of the solution to idealization irregularity, and the importance of accurate estimation of the velocity field and its association to the grid density levels that is necessary to solve the problem accurately. Saltwater intrusion hydrodynamics in a beach under the influence of tidal effects is also investigated using TechFlow. Based on the results of TechFlow with the use of various boundary conditions for the transport equation, the saltwater intrusion hydrodynamics in a beach under the influence of tidal effects shows unique dynamics. These solutions are primarily affected by density differences, tidal effects on a mild slope, variably saturated porous medium and finite domain solution condition. TechFlow is also used to investigate saltwater upconing beneath pumping wells both two- and three-dimensional applications.

Nowadays society strongly depends on its available resources and the long term stability of the surrounding ecosystem. Numerical modelling has become a general standard for evaluating past, current or future system states for a large number of applications supporting decision makers in proper management. In order to ensure the correct representation of the investigated processes and results of a simulation, verification examples (benchmarks), that are based on observation data or analytical solutions, are utilized to evaluate the numerical modelling tool. In many parts of the world, groundwater is an important resource for freshwater. While it is not only limited in quantity, subsurface water bodies are often in danger of contamination from various natural or anthropogenic sources. Especially in arid regions, marine saltwater intrusion poses a major threat to groundwater aquifers which mostly are the exclusive source of freshwater in these dry climates. In contrast to common numerical groundwater modelling, density-driven flow and mass transport have to be considered as vital processes in the system and in scenario simulations for fresh-saltwater interactions. In the beginning of this thesis, the capabilities of the modelling tool OpenGeoSys are verified with selected benchmarks to represent the relevant non-linear process coupling. Afterwards, variable-density application and process studies on different scales are presented. Application studies comprehend regional groundwater modelling of a coastal aquifer system extensively used for agricultural irrigation, as well as hydro-geological model development and parametrization. In two process studies, firstly, a novel method to model gelation of a solute in porous media is developed and verified on small scale laboratory observation data, and secondly, investigations of thermohaline double-diffusive Rayleigh regimes on medium scale are carried out. With the growing world population and, thus, increasing pressure on non-renewable resources, intelligent management strategies intensify demand for potent simulation tools and development of novel methods. In that way, this thesis highlights not only OpenGeoSys’ potential of density-dependent process modelling, but the comprehensive importance of variable-density flow and transport processes connecting, both, avant-garde scientific research, and real-world application challenges.:Abstract Zusammenfassung Nomenclature List of Figures List of Tables I Background and Fundamentals 1 Introduction 1.1 Motivation 1.2 Structure of the Thesis 1.3 Variable-Density Flow in Literature 2 Theory and Methods 2.1 Governing Equations 2.2 Fluid Properties 2.3 Modelling and Visualization Tools 3 Benchmarks 3.1 Steady-state Unconfined Groundwater Table 3.2 Theis Transient Pumping Test 3.3 Transient Saltwater Intrusion 3.4 Development of a Freshwater Lens II Applications 4 Extended Inverse Distance Weighting Interpolation 4.1 Motivation 4.2 Extension of IDW Method 4.3 Artificial Test and Regional Scale Application 4.4 Summary and Conclusions 5 Modelling Transient Saltwater Intrusion 5.1 Background and Motivation 5.2 Methods and Model Setup 5.3 Simulation Results and Discussion 5.4 Summary, Conclusion and Outlook 6 Gelation of a Dense Fluid 6.1 Motivation 6.2 Methods and Model Setup 6.3 Results and Conclusions 7 Delineating Double-Diffusive Rayleigh Regimes 7.1 Background and Motivation 7.2 Methods and Model Setup 7.3 Results 7.4 Conclusions and Outlook III Summary and Conclusions 8 Important Achievements 9 Conclusions and Outlook Bibliography Publications Acknowledgements Appendix