Laboratory and Numerical Investigation of Saltwater Intrusion Processes in a Circular Island Aquifer

Abstract

A novel radial flow experimental system to study saltwater intrusion processes in an island aquifer is presented. The study investigated steady state and transient scenarios involving advancing and receding saltwater wedges in a circular island. The experimental results were simulated using the density‐coupled version of the MODFLOW‐USG code. The experimental data along with the model simulation results are employed to develop a new radial benchmark problem for testing density‐coupled models used for simulating saltwater intrusion processes. The experimental data for transient changes in toe position and freshwater storage level indicated an asymmetric pattern where the intrusion time scale is greater than the recession time scale. Numerical experiments were completed to further investigate this asymmetric effect and to intercompare the associated transient transport processes in circular and linear strip islands. We also analyzed the sensitivity of island geometry in controlling the freshwater storage levels under different recharge conditions. Modeling results show that for similar‐sized systems, circular islands are more efficient in storing freshwater than linear strip islands.

Effects of aquifer geometry on seawater intrusion in annulus segment island aquifers

“…The analytical solutions were validated by comparing their predictions with experimental data compiled from Memari et al (2020). Their experiments were carried out using a 15° radial 13 https://doi.org/10.5194/hess-2021-29 Preprint.…”

“…Fresh groundwater stored on oceanic islands is mainly recharged from precipitation and its availability can be impacted by a variety of factors, including island topography, rainfall patterns, tides, episodic storms and human activities (White & Falkland, 2010;Storlazzi et al, 2018). Among these, seawater intrusion is another important issue that greatly affects oceanic island freshwater storage and is thus of considerable interest (e.g., Werner et al, 2017;Lu et al, 2019;Memari et al, 2020).…”

“…Different from coastal aquifers where seawater intrudes into freshwater from one direction only, seawater intrusion occurs from two directions for narrow strip islands and from all directions for circular islands. Over the past few decades, seawater intrusion in oceanic islands has been extensively investigated in multiple ways, either directly by field observations (e.g., Röper et al, 2013;Post et al, 2019), or indirectly by experimental measurements (e.g., Stoeckl et al, 2015;Bedekar et al, 2019;Memari et al, 2020), numerical simulations (e.g., Lam, 1974;Gingerich et al, 2017;Liu & Tokunaga, 2019), and analytical solutions (e.g., Fetter, 1972;Ketabchi et al, 2014;Lu et al, 2019). Among these, analytical solutions are effective tools to assess the extent of seawater intrusion, despite that they cannot incorporate complex factors (e.g., dispersive mixing and transient oceanic dynamics) (Werner et al, 2013).…”

“…with a focus on the freshwater-seawater interface location and its corresponding watertable elevation. After comparing their predictions with experimental data compiled from Memari et al (2020), the analytical solutions are employed to investigate the effects of aquifer geometry on the freshwater-seawater interface location and watertable elevation in ASAs. Figure 2 shows the conceptual model of an ASA (a slice of atoll islands).…”

“…In order to focus on the effects of aquifer geometry on seawater intrusion, we assumed the same L * and L0 for the three aquifers, with L * and L0 equal to 1000 and 200 m, respectively. The sand characteristics were the same as in the experiments of Memari et al (2020). Two recharge events were considered (Cases 1 and 2, Table 1).…”