Sea-level rise impact on fresh groundwater lenses in two-layer small islands

The fresh groundwater lenses (FGLs) of small islands can be highly vulnerable to climate change impacts, including sea-level rise (SLR). Many real cases of atoll or sandy islands involve two-layer hydrogeological conceptualizations. In this paper, the influential factors that affect FGLs in two-layer small islands subject to SLR are investigated. An analytical solution describing FGLs in circular islands, composed of two geological layers, is developed for the simplified case of steady-state and sharp-interface conditions. An application of the developed model is demonstrated to estimate the FGL thickness of some real-world islands by comparison with existing FGL thickness data. Furthermore, numerical modelling is applied to extend the analysis to consider dispersion effects and to confirm comparable results for both cases. Sensitivity analyses are used to assess the importance of land-surface inundation caused by SLR, relative to other parameters (i.e. thickness of aquifer layers, hydraulic conductivity, recharge rate and land-surface slope) that influence the FGL. Dimensionless parameters are used to generalize the findings. The results demonstrate that land-surface inundation has a considerable impact on a FGL influenced by SLR, as expected, although the FGL volume is more sensitive to recharge, aquifer thickness and hydraulic conductivity than SLR impacts, considering typical parameter ranges. The methodology presented in this study provides water resource managers with a rapid-assessment tool for evaluating the likely impacts of SLR and accompanying LSI on FGLs.