INTEGRATING REMOTE SENSING AND IN-SITU MONITORING TO UNDERSTAND OCEAN-AQUIFER-POND INTERACTIONS ON A REMOTE SAND ISLAND

Coastal environments lie at the forefront of climate change given the impact of sea-level rise and more frequent and intense coastal storms. Small, low-lying islands, which are particularly vulnerable to climate change, are often densely populated and heavily dependent on fresh groundwater resources. Coastal storms can temporarily flood low-elevation islands, damaging infrastructure and salinizing fresh aquifers critical for drinking water supply and irrigation.

Sable Island is a small, low-lying, sand island in the northwest Atlantic Ocean, over 150 km from mainland Nova Scotia, Canada. In 2013, the island was designated as a Canadian National Park Reserve to preserve its unique ecosystem of wild horses, grey seals, and endangered sea birds that live in the dynamic coastal dune environment. Approximately 20 freshwater ponds exist on the island in locations where the water table rises above depressions in the land surface. The states of these essential fresh surface water bodies are intrinsically tied to the groundwater conditions; thus, short-term and long-term changes in groundwater head impact pond water levels and aerial extent. Historical aerial imagery reveals a pronounced loss of freshwater (pond shrinking) over recent decades. Furthermore, flooding during coastal storms can overtop or erode the foredune and salinize freshwater ponds. To monitor ocean-aquifer-pond interactions, we installed stilling wells and piezometers equipped with Solinst depth loggers in 5 ponds and a transect of beach piezometers equipped with Solinst conductivity, temperature, and depth loggers. Near-infrared bands of Planet images were used to quantify pond area and beach flooding in over 80 images during the three-year monitoring period that included events such as Hurricanes Teddy (2020), Larry (2021), and Fiona (2022), as well as several winter Nor’Easters. For each pond, a volume-area-depth relationship was created, and estimates of pond area were compared to areas extracted from remotely sensed images. We used meteorologic and hydrodynamic data to determine the drivers that control pond areas, flood extent, and aquifer salinization. Results show that pond area responds to both variable climate conditions and seawater flooding, and demonstrate the efficacy of Planet imagery to analyze a remote coastal zone.