Seawater intrusion and contaminant transport in coastal aquifers are two major problems related to environmental and water resources protection in coastal areas and nowadays they are causing concern in many countries. The contaminants considered in this study are assumed to have a larger density than fresh groundwater, hence both seawater intrusion and contaminant transport in this context belong to the class of variable density groundwater flow and solute transport problems. This thesis focuses on the migration of contaminant plumes in unconfined coastal aquifers in the presence of seawater density and tidal fluctuation and on the importance and impact of boundary conditions in numerical modelling of that process.
In order to perform numerical modelling, an appropriate computer code is needed. A literature review shows that there are several codes available for solving the variable density groundwater flow and transport problem. In this thesis, the two-dimensional finite element code, 2DFEMFAT is adopted as a numerical simulator.
Laboratory experiments are designed in order to measure the location of the freshwater-saltwater interface in a porous medium and to show the migration of contaminant plumes in an unconfined coastal aquifer in the presence of a sea boundary condition. An imaging processing approach is developed to obtain the saltwater transition zone and to convert the dyed plumes into solute concentration contours. The successful application of this imaging approach generates detailed contours showing the width of the saltwater interface and the real diffusive front of plumes.
Experimental results for seawater intrusion under both a stationary and a tidal sea level are presented. These results are very helpful in testing mathematical solutions of the freshwater-saltwater interface. To the author's best knowledge, there have been no published results showing the width of a saltwater transition zone in a porous medium with a finite dispersivity. Since the boundary conditions and the physical parameters for these results are known, these results can be used as benchmarks to test mathematical solutions.
The code, 2DFEMFAT is tested against the experimental results for a steady-state saltwater interface and against the Elder free convection problem. The satisfactory performance of the code generates confidence in employing it as a simulator.
The impact of boundary conditions in numerical modelling of density-dependent groundwater flow and solute transport is exemplified by the salt dome problem involving groundwater flow over buried brine wastes and seawater intrusion modelling in the Gooburrum aquifers. In numerical solutions, arbitrary flow velocity may occur along the dome surface due to the highly density-dependent nature. This arbitrary velocity introduces unexpected solute mass into the flow field and subsequently results in an incorrect mass distribution and flow pattern. A hybrid transport boundary condition describing a solute flux normal to the interface is implemented. This boundary condition excludes the solute mass contributed by the arbitrary velocity occurring at the concentration-specified nodes. The presence and correction of this arbitrary velocity along the dome surface when applying a finite element model have not been noted in previous work on the salt dome problem.
The boundary conditions in seawater intrusion modelling are discussed. Usually, the hydrostatic equivalent freshwater heads are imposed at the seaward boundary connecting to the ocean. A problem was detected, however, in constructing a two-dimensional profile model for the layered Gooburrum aquifers at Bundaberg, Queensland. This problem arises because the head-specified boundary condition generates an unrealistic flow circulation in the lower Gooburrum aquifer and subsequently a very wide saltwater transition zone, which, compared with field evidence, is not correct.
Experimental results of contaminant transport near the sea interface show that the less dense contaminant tends to travel in the upper portion of the aquifer and is less affected by the intruded saltwater interface. It exits around the coastline where most of the groundwater does. The more dense contaminant migrates in a complicated manner. A diffusive front develops in the seaward direction and the front becomes more diffusive when the plume approaches closer to the saltwater interface. It is important to recognise this phenomenon, because a wide diffusion zone may develop at the field scale and pollute a large area near the coastline. The presence of the saltwater interface has a greater effect on the migration pattern of the more dense plume than the less dense one. Experimental results also show that the tidal variation of the sea level affects the shape of the plume, especially for the more dense ones. The overall travelling rate in the horizontal direction is not much influenced by tide.
Results of numerical modelling of contaminant transport in coastal aquifers are also presented. It is found that for the less dense contaminant, the seawater density can be safely neglected. For the more dense contaminant, the neglect of seawater density will result in fictitious migration paths. If the exact exit point and the exit concentration need to be simulated, the seawater density has to be included. Modelling of contaminant transport is also carried out under natural and pumping conditions for the Gooburrum aquifers as a representative field scale example. Results show that under natural conditions, the contaminant migrates in the seaward groundwater flow and exits at the coastline, while under pumping conditions, the contaminant will eventually reach the pumping wells. It is found that for a regional shallow aquifer, the plume is sensitive to the vertical dispersivity.