HGS Research Highlight - Holding and Allen 2015

This post features a study recently completed by Holding and Allen, 2015 that used HGS to investigate the impact of salt contamination of aquifers caused by wave overwash events. For more information on this study please contact the corresponding author: Shannon Holding (sholding@sfu.ca)

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

Authors: Shannon Holding and Diana M. Allen

 

Concentration distribution after inundation; the white lines represent geological heterogeneity.

Concentration distribution after inundation; the white lines represent geological heterogeneity.

Wave overwash events (such as storm surge and tsunamis) have the potential to result in severe consequences to the freshwater resources of small islands as a result of salt contamination of the aquifer. This study used numerical modelling to evaluate the freshwater lens response and recovery to overwash events for various island hydrogeological settings - or island types - that are observed worldwide. Due to the nature of wave overwash events, modelling the impact on a freshwater lens involves simulating density-dependent flow and solute transport across the land surface, the vadose zone, and the saturated domain. HydroGeoSphere represented the ideal code to simulate the coupled processes integrated across the surface and variably saturated subsurface domains. The model domains for each island type were developed as cross-sections representing the near-coast hydrogeological setting. A theoretical overwash event was simulated in three phases: 1) development of the freshwater lens to steady-state conditions; 2) a short temporal-scale saltwater inundation of the surface domain; and 3) recovery of the lens (saltwater migration through the system). Phase 1 models were initiated with high concentrations of salt and run with fresh recharge applied to the surface until a freshwater lens developed. Phase 2 simulated the inundation event corresponding to a saltwater source within the surface domain. During Phase 3, the surface boundary conditions reverted to the baseline (Phase 1) to allow the saltwater in the surface and subsurface to migrate through the domain, simulating freshwater lens recovery. Observation points were situated in the models to represent the typical depths of wells in order to evaluate the impact on (and recovery of) potable water supply for each island type.  The model results demonstrate the cumulative impact of different factors affecting the freshwater lens response and recovery for each island type, providing a generalised assessment of island susceptibility to overwash on a global scale.