Tidal propagation and variable density processes in coastal aquifers

Author: Cristina Solorzano Rivas

Solorzano Rivas, Cristina, 2021 Tidal propagation and variable density processes in coastal aquifers, Flinders University, College of Science and Engineering

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Coastal aquifers are an integral source of freshwater supplying an increasing population, especially in coastal communities. Thus, understanding the groundwater dynamics in coastal zones is critical for the effective management of this precious freshwater resource. The study of coastal groundwater has the inherent complexity of the hydraulic connection of groundwater with the ocean. This thesis has selected three main complex processes involved in the fresh groundwater-ocean interaction that are not yet well understood: (1) the effects of dispersion in submarine aquifers caused by the mixing between subsea freshwater and seawater; (2) flow instabilities associated with mixed-convective processes induced by the occurrence of subsea fresh groundwater discharge through high-permeability seafloor sediments; and (3) the effects of tidal forces on groundwater levels in circular islands.

The first part of this study investigates the effects of dispersion in submarine freshwater distribution. It does so by comparing the Werner and Robinson (2018) analytical solution (i.e., the latest sharp-interface based analytical solution to offshore freshwater extent) with numerical model simulations, adding dispersion parameters. Results show that dispersion affects differently the interface tip position (i.e., where the interface intercepts the top of the aquifer) than it does to the interface toe (i.e., where the interface intercepts the bottom of the aquifer). The changes on the interface tip as well as on the interface width at the top of the aquifer caused by dispersion are dependent on the hydraulic characteristics of the submarine aquitard. This interdependence makes that the tip location and the interface width at the top of the aquifer have a non-monotonic relationship with increasing dispersion. Conversely, the submarine aquitard seems to have little influence at the bottom of the aquifer where both the toe location and the interface width have a monotonic relationship with dispersion, in a similar fashion to what it is observed in onshore aquifers. Additionally, our investigation shows that seawater circulation rates and submarine fresh groundwater rates increase with increasing dispersion, implying that heterogeneity causes larger groundwater fluxes to the ocean. We found that the effects of dispersion, especially in the submarine fresh groundwater discharge, are interrelated with the type of analytical case, showing larger changes in freshwater discharge with increasing dispersion in the analytical case associated with an onshore toe.

Our attempts to find a correction to the Werner and Robinson (2018) analytical solution to account for dispersion were not successful as a result of the complex interplay between dispersion, aquitard characteristics and the analytical case classification undertaken by the Werner and Robinson (2018) analytical solution. Nevertheless, this study provides insights into the controlling factors that can be considered when seeking dispersive correction coefficients for individual situations, such as individual correction factors for the tip and toe, as well as considering specific analytical cases.

The second part of this study investigates the subsea fresh groundwater discharge (SFGD) distribution through high-permeability seafloor sediments. Most studies related to predict offshore freshwater extent assume that low-permeability sediments protecting the submarine freshwater from overlying seawater intrusion are in direct contact with the ocean, implicitly assuming that SFGD is uniformly distributed. However, the distribution of SFGD through high-permeability sediments containing seawater is not well understood. This study demonstrates through variable-density and solute transport numerical modelling that mixed-convective processes govern the SFGD distribution through sandy seafloor sediments in the form of unstable buoyant freshwater fingers. Hence, mixed-convective theory, such as the non-dimensional Rayleigh number and mixed convection ratio, may be applied for the prediction of SFGD occurrence through high-permeability seafloor sediments. This investigation demonstrates that the temporality characteristic of unstable flow conditions in the form of permanent buoyant fingers is controlled by the lower boundary. This study is the initial step for further investigation to establish critical non-dimensional numbers to infer the occurrence of SFGD through high-permeability seafloor sediments.

The final part of this study investigates tidal propagation in circular islands. The aim is to determine the applicability of the widely used Ferris solution to tidal propagation, which is based on the assumption of a straight, infinite shoreline, in circular islands. The investigation is undertaken by comparing an existing analytical solution under radial flow conditions (‘circular solution’) with the Ferris solution. This comparison allowed us to obtain a correction factor to the Ferris solution to be applied in circular boundaries (‘corrected Ferris solution’). The advantage of the corrected Ferris solution over the circular solution is its direct application for the inverse problem of inferring aquifer diffusivity, while the circular solution requires iterative methods. This investigation demonstrates that the corrected Ferris solution and the circular solution predict within 1% and 2.5% of the known diffusivities for confined and unconfined conditions, respectively. This study contributes by extending the available tools to estimate aquifer parameters, especially in circular aquifers, facilitating the determination of properties that otherwise require more invasive methods such as pumping tests.

Keywords: offshore freshwater, density-dependent flow, dispersion, coastal aquifer, seawater circulation, submarine groundwater discharge, submarine fresh groundwater discharge, free convection, mixed convection, Rayleigh number, tidal propagation, phase lag, radial groundwater flow, Ferris solution, parameter estimation, circular islands 

Subject: Hydrology thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Professor Adrian Werner