Groundwater dynamics in an aquifer adjacent to a coastal lagoon

Abstract

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.ii Under particular conditions, the beach slope break combined with the tidal oscillation induced local circula...

Effects of alongshore morphology on groundwater flow and solute transport in a nearshore aquifer

“…The collected data were used for developing, calibrating, and validating the numerical models. Further details of the field methods and data collection can be found in Zhang [].…”

“…For example, fresh groundwater discharge to the sea has been shown to take place through a “tube” confined by a lower saltwater wedge and an upper saline plume on a two‐dimensional (2‐D) cross‐shore vertical section under the influence of tides and waves [ Boufadel , ; Michael et al ., ; Robinson et al ., ; Kuan et al ., ; Abarca et al ., ]. More recently, Zhang [] conducted a field investigation and numerical simulations based on a 2‐D model to demonstrate the combined effects of cross‐shore beach morphology and tides on the groundwater discharge location and local seawater recirculation under multiple beach slopes. All these studies have, however, assumed alongshore uniformity of the nearshore groundwater system, neglecting alongshore variations of beach morphology typically associated with adjacent surface water features such as a tidal creek.…”

Effects of beach slope breaks on nearshore groundwater dynamics

“…These two transects were approximately perpendicular to the shoreline, but not parallel to each other, because the shoreline curved with the alongshore morphological variation. Further details of the field methods and data collection can be found in Zhang () and Zhang, Li, Erler, Santos, and Lockington ().…”