Groundwater-Pond Interactions on a Remote Sand Island Driven by Oceanic and Meteorologic Processes

Islands are hotspots of cultural and biological diversity; however, small-island populations and ecosystems face freshwater insecurity. Fresh surface water bodies on small islands are particularly limited in size and number and are less well studied than underlying freshwater lenses. Small islands with permeable geology often have a hydraulic connection among the ocean, groundwater system, and fresh surface water bodies, but these subsurface hydraulic linkages are poorly understood. Utilising in situ hydrologic and thermal monitoring and remote sensing, this study investigated how meteorologic (precipitation and evapotranspiration) and oceanic (waves, storm surge, and flooding) forcing drive pond level dynamics and groundwater–surface water interactions on a remote sand island (Sable Island) in the Northwest Atlantic. Stilling wells, piezometers, and sediment temperature rods were installed in five ponds from July 2020 to September 2021 and were collectively used to create area-depth relationships, infer pond bed hydraulic conductivity, and calculate vertical groundwater fluxes. PlanetScope satellite images during this period were classified to quantify pond surface areas and provide a link to in situ monitoring to assess changes following meteorologic and oceanic events. Meteorological and coastal hydrodynamic data were used to determine drivers controlling pond levels and groundwater–surface water exchange rates. Findings reveal that pond levels and vertical exchange fluxes are influenced by tides, precipitation, waves, and beach flooding, but have the highest correlation to oceanic forcing (high significant wave heights) that drive beach flooding and groundwater level changes. Remote sensing paired with pond level–area relationships proved effective for monitoring surface water dynamics in this remote area. Results expand our understanding of small-island hydrology and hydraulics and emphasise that surface water, groundwater, and the ocean are not separate but exist along a coupled hydrologic continuum. In a period of rapid environmental change, understanding fresh surface water dynamics on small islands is crucial to protecting vulnerable ecosystems from freshwater scarcity.