A Proof‐of‐Concept Study of Using a Less Permeable Slice Along the Shoreline to Increase Fresh Groundwater Storage of Oceanic Islands: Analytical and Experimental Validation

Abstract

We propose a new concept that has the potential to mitigate seawater intrusion and increase the fresh groundwater storage of oceanic islands by creating a less permeable slice along the shoreline. We present a proof‐of‐concept study to examine its effectiveness through analytical and experimental studies. Analytical expressions for calculating the freshwater‐seawater interface location, water table elevation, fresh groundwater volume, and groundwater travel time are presented for both barrier and circular islands, which are found dependent on three different scenarios of interface locations. The analytical solution of the interface location in a barrier island is verified through sand‐tank experiments. Sensitivity analyses based on a simplified conceptual model of St. George Island in Florida, USA, indicate that the fresh groundwater volume monotonically increases with the decrease in the hydraulic conductivity of the coastal less permeable hydrogeologic unit. On the other hand, the increase of the coastal less permeable unit extent leads to an increased fresh groundwater volume. However, when the interface tip is on the aquifer bed of the coastal less permeable unit, a further increase of the less permeable unit extent only slightly increases the fresh groundwater volume, since the interface does not change any more and only the water table is elevated. We demonstrate here that the concept proposed has the potential in increasing the fresh groundwater storage of oceanic islands. Analytical expressions presented can improve our understanding of seawater intrusion in a dual‐unit oceanic island.

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

“…In line with previous studies, a variety of assumptions were made to facilitate deriving analytical solutions the freshwater-seawater interface and watertable elevation (e.g., Fetter, 1972;Strack, 1976;Lu et al, 2019). In addition to the assumptions mentioned previously, the temporal and spatial variability in recharge is not considered into analytical solutions.…”

“…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).…”

“…The analytical solutions of Ketabchi et al (2014) and Vacher (1998) assumed an island with two horizontal layers. More recently, Lu et al (2019) derived analytical solutions for the freshwater-seawater interface location and watertable elevation for both strip and circular islands with two adjacent layers, i.e., a less permeable slice along the shoreline of an island, and a more permeable zone inland. For more studies associated with seawater intrusion in oceanic islands, readers are referred to the comprehensive review of Werner et al (2017).…”

“…The freshwater-seawater interface was calculated using the analytical solutions for the three different aquifers. Note that Appendix A presents analytical solutions for seawater intrusion in strip aquifers deduced from Lu et al (2019). Figure 6 shows the freshwater-seawater interface calculated for Cases 1 and 2.…”