Effect of Cut-off Walls on Freshwater Lens: Oceanic Islands with Impermeable Surface.

The expansion of island infrastructure contributes to an increase in impermeable surfaces, adversely affecting the replenishment of freshwater lenses and thereby posing a threat to the water security of island residents. To counteract this, cut-off walls--a traditional method for preventing seawater intrusion--have proven effective in enhancing the volumes of freshwater aquifers. Employing a site-scale numerical model, this study investigates the influence of impervious surfaces on the morphology and distribution of island freshwater lenses and assesses the effectiveness of cut-off walls in preserving freshwater reserves in such contexts. Findings suggest that an increase in the extent of impermeable surfaces correlates with a decrease in the thickness of freshwater lenses and a displacement of their central point away from the impervious surface. Furthermore, the placement and depth of cut-off walls significantly influence their efficacy in increasing freshwater lens volumes and mitigating the negative impacts of impermeable surfaces on underground freshwater resources. The greatest effectiveness is achieved when these walls are positioned closer to the shoreline and at an optimal depth--identified in this study as 15 m for scenarios with 150 m of impermeable surface length. Beyond this depth, the increase in freshwater volume becomes marginally reduced. [ABSTRACT FROM AUTHOR]