Wave overwash impact on small islands: Generalised observations of freshwater lens response and recovery for multiple hydrogeological settings

- Modelling overwash impact on freshwater lenses with different hydrogeological settings.
- Key factors and cumulative impact on freshwater lens response/recovery are identified.
- Generalised observations for susceptibility assessment provided for island types.

SummaryWave overwash events have the potential to result in severe consequences to the freshwater resources of small islands as a result of salt contamination of the aquifer. Due to the significant impact of overwash, it is important to characterise the susceptibility of small islands to these events. This study uses numerical modelling to evaluate the freshwater lens response and recovery to overwash events for various island hydrogeological settings (island types) observed worldwide. Models were developed for an example of each island type using a fully coupled surface-subsurface, density-dependent flow and solute transport modelling code. A theoretical overwash event was simulated, and the response and recovery of the freshwater lens were observed for 20 years. The freshwater lens response (degree of aquifer contamination) was largely determined by the vadose zone thickness. Lens recovery ranged from 1 to 19 years for the different island types, and was strongly affected by recharge rate. However, the recovery of potable water in the lens (and restoration of a water supply) was dominantly influenced by geological heterogeneities. The model results demonstrate the cumulative impact of the different factors affecting the freshwater lens response and recovery to the overwash event for each island type, and provide a generalised assessment of island susceptibility to overwash on a global scale, despite limited data availability for many small islands.