Many recent studies show that submarine groundwater discharge may provide an important pathway for nutrients and contaminants to enter coastal ecosystems. To determine nutrient and contaminant loads associated with submarine groundwater discharge, there is a need for greater understanding of the flow dynamics and solute transport mechanisms in nearshore aquifers. Research is needed to quantify the effects of important factors, including terrestrial force, tidal force, beach morphology and aquifer properties, in both cross-shore and alongshore directions. Field investigations and numerical simulations were conducted in my Ph.D. research to advance our understanding of these effects. The findings from my work further demonstrate the complexity of nearshore groundwater systems and will benefit future studies of nutrient and contaminant transport and transformations associated with SGD.
This research started with field investigations at a carbonate sandy beach of Muri Lagoon on the east coast of Rarotonga, Cook Island (21°15'S, 159°44'W). Two field campaigns were carried out with measurements to provide real data for studying the groundwater flow dynamics in such a system. The study was motivated by an ultimate aim to quantify the N and P transformations along the subterranean estuary and loads to the lagoon. Measurements identified significant influence of beach morphology on variations of pore water salinity in the aquifer. The interactions of tidal force and beach morphology modify the groundwater flow and discharge location, and associated solute transport. Furthermore, the field data revealed that the presence of a shallow creek modified notably the beach profiles in both the cross-shore and alongshore directions. An unexpected saline pore water zone was observed landward of the intertidal zone indicating remarkable solute transport along the shore. The field results suggested that the effects of beach morphology can be significant in determining nearshore groundwater flow and solute transport processes. The alongshore variations (bi-directional beach morphology and presence of the creek), largely neglected in the previous studies, were found to play important roles in controlling the three dimensional pore water flow in such a system.
To further interpret the field data and analyse the nearshore groundwater system, numerical models were developed using a variable density and variable saturation groundwater flow code, SUTRA. Firstly, two-dimensional (2D) models were built to examine the effects of beach morphology. Different from previous studies on mild sloping beach morphology (~1/10), we focused on beaches with small slopes (~1/50 to ~1/100) and multiple slope breaks. We found that the groundwater discharge location was largely controlled by beach morphology interacting with the tidal force. The effects of varying tidal amplitude and inland heads on the discharging location were relatively small. Under particular conditions, the beach slope break combined with the tidal oscillation induced local circulation cells. Furthermore, under a flat beach profile or a profile with slope breaks, certain conditions could even lead to unstable flow.
To further investigate the unstable flow, more 2D simulations were conducted. The numerical results revealed a long-term oscillation in the intertidal zone under realistic conditions, which had not been reported previously. The associated flow patterns are distinctly different from that under a stable upper saline plume. Various factors involved, including the beach slope, inland head, tidal amplitude, hydraulic conductivity and dispersivity, were examined individually and in combination. The tidally driven seawater infiltration leads to an unstable condition with denser water overlying less dense water and ultimately is responsible for the onset of the oscillation. Three non-dimensional numbers – the gravity number, Rayleigh number and perturbation parameter were used to estimate and predict the extent of the long-term oscillation. At last, a three-dimensional (3D) model was developed based on the field data and subjected to the combined influence of tidal forcing, embedded creek and alongshore beach profile variations. A dynamic 3D flow simulated was characterized by significant drainage of land-sourced fresh groundwater across the landward section of the creek and saltwater infiltration through the seaward section of the creek as the tide receded. The circulating flow varied to a large degree, spatially and temporally. Simulated salt distributions also provided explanations for field observations. The results indicated pore water circulation in three dimensions linked strongly to the combined effects of alongshore variations of beach morphology (including the creek boundary) and tides.
The thesis comprises six chapters. The introductory chapter, Chapter 1, presents a literature review to identify the research gaps, and describes the research questions and objectives. In Chapter 2, field measurements and measurement results are presented to illustrate the complex nearshore pore water flow. The numerical simulations of the effects of beach morphology are detailed in Chapter 3. Further insight into the stability of groundwater flow in the intertidal zone is provided in Chapter 4. Chapter 5 focuses on the influence of alongshore variations on nearshore pore water flow and solute transport through numerical simulations by 3D models. In Chapter 6, conclusions are summarised together with recommendations for future research.