Practical approaches to seawater intrusion investigation and management

Author: Leanne Kaye Morgan

Morgan, Leanne Kaye, 2014 Practical approaches to seawater intrusion investigation and management, Flinders University, School of the Environment

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Abstract

Seawater intrusion (SWI) is the encroachment of saltwater into fresh coastal aquifers. It is a complex process that involves variable-density flow, solute transport and hydrochemical processes, which can make SWI assessment relatively difficult and expensive. The aim of this thesis is the development, application and critical assessment of practical (i.e., rapid, inexpensive) analytic modelling approaches for investigating and assisting in the management of SWI. The assessment of SWI vulnerability over large scales (i.e., national or continental) has generally been carried out using mainly qualitative methods, which consider only a subset of the factors thought to impact SWI. Recently, an alternative to the above-mentioned large-scale methods has been developed by Werner et al. (2012). The method is based on the steady-state, sharp-interface equations of Strack (1976), and therefore incorporates the physical mechanics of SWI, albeit under highly idealised conditions. In the first study of this thesis, the Werner et al. (2012) method is applied to the multilayered Willunga Basin, South Australia. Vulnerability is assessed both for current conditions and future stresses (i.e., increased extraction, sea-level rise and recharge change). Limitations of the method, associated with the sharp-interface and steady-state assumptions, are addressed using numerical modelling to explore transient, dispersive SWI caused by sea-level rise. The study provides guidance for an ongoing field-based investigation of SWI. Freshwater lenses on small islands are some of the most vulnerable aquifer systems in the world. However, there is currently little guidance on methods for rapidly assessing the vulnerability of freshwater lenses to the potential effects of climate change. In the second study of this thesis, the Werner et al. (2012) approach is extended to the case of freshwater lenses on islands, accounting also for land surface inundation associated with sea-level rise. The resulting equations provide general relationships between SWI vulnerability in freshwater lenses and hydrogeological conditions. Example applications to several case studies illustrate use of the method for rapidly ranking lenses according to vulnerability, thereby allowing for prioritisation of areas where further and more detailed SWI investigations may be required. The third study of this thesis considers the impact of SWI-induced changes in seawater volume on water-level trends (which are commonly used as a proxy for changes in aquifer storage), and coastal aquifer water balances. A steady-state, sharp-interface, analytic modelling approach was used to generate idealised relationships between seawater volume, freshwater volume and water levels. The approach assumes quasi-equilibrium conditions (i.e., steady-state conditions persist during temporal changes), which were evaluated using a selection of transient, dispersive simulations and found to be valid in the majority of cases. We conclude that changes in seawater volumes should be included routinely in coastal aquifer water balances. Also, temporal trends in coastal aquifer water levels may not provide an adequate measure of freshwater storage trends. In the fourth study of this thesis, physical processes associated with transient seawater movement into coastal aquifers are investigated. Specifically, sand tank modelling is used to assess whether SWI overshoot is a measurable physical process. SWI overshoot has been recently reported within numerical modelling studies of transient sea-level rise and SWI by Watson et al. (2010) and Chang et al. (2011) and involves the freshwater-saltwater interface temporarily extending further inland than the eventual steady-state position. This implies that steady-state SWI may not be the worst case, as is generally assumed. In this study, sand tank modelling of sea-level rise and SWI was carried out and photographs show, for the first time, that an overshoot occurs under controlled laboratory conditions. A sea-level drop experiment was also carried out, and overshoot was again observed, whereby the interface was temporarily closer to the coast than the eventual steady-state position. Numerical modelling corroborated the physical sea-level rise and sea-level drop experiments. This work demonstrates that commonly adopted steady-state approaches can under-estimate the maximum extent of SWI, due to the overshoot phenomenon.

Keywords: coastal aquifers,seawater intrusion,vulnerability,analytic solutions,sharp interface,overshoot
Subject: Environmental Science thesis, Environmental Studies thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of the Environment
Supervisor: Professor Adrian Werner