Morphologic, Atmospheric, and Oceanic Drivers Cause Multi‐Temporal Saltwater Intrusion on a Remote, Sand Island

• Published in: Water resources research 2023-10, Vol.59 (10), p.n/a
• Main Authors: Cantelon, Julia A., Robinson, Clare E., Kurylyk, Barret L.
• Format: Article
• Language: English
• Subjects: Atmospheric conditions; Baseline studies; Beaches; Climate; Climatic conditions; Coastal erosion; Coastal flooding; Coastal management; Coastal storms; Coastal zone; Coasts; Dunes; Electrical conductivity; Electrical resistivity; Environmental changes; Flooding; Floods; Fresh water; Freshwater; freshwater lens; Freshwater resources; geophysics; Groundwater; Groundwater data; Groundwater levels; Groundwater resources; Groundwater salinity; groundwater salinization; Groundwater table; Hurricanes; Hydrologic data; Inland water environment; island hydrology; Oceans; Precipitation; Precipitation patterns; Saline water; Saline water intrusion; Salinity; Salinity effects; Salinization; Salt water intrusion; Saltwater intrusion; Sand; Sea level; Sea level changes; Seasonal variation; Seasonal variations; Seawater; seawater intrusion; Storm surges; Storms; Transition zone; Water resources; Water table; Wave erosion
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2023WR035820

Small‐island populations disproportionately rely on fresh groundwater resources, which are increasingly threatened by salinization from changing ocean and climate conditions. This study investigates island groundwater dynamics and salinization over multiple timescales in response to marine, atmospheric, and morphologic drivers. New geophysical and hydrological data sets were collected on a remote sand island in the Northwest Atlantic Ocean between 2020 and 2022 and compared to historical baseline data from the 1970s. Data reveal saltwater intrusion due to multi‐decadal erosion, seasonal climate patterns, tidal forcing, and episodic flooding from Atlantic hurricanes. Long‐term dune erosion has caused the freshwater lens to thin and become asymmetrical; however, a lagged groundwater response causes the freshwater lens to be in disequilibrium with present island morphology. The maximum vertical lens thickness is seasonally constant, but the lateral transition zone along the coast thickens and moves seaward in spring when the water table is high from precipitation and frequent beach flooding. Groundwater level and electrical conductivity along low‐lying beaches significantly increase following Atlantic hurricanes due to seawater flooding, and reach a maximum of 1.93 m above sea level and 38 mS/cm, respectively. While groundwater levels recover quickly, conductivity (salinity) remains elevated due to the short intervals between winter flood events that outpace freshening from meteoric recharge. Results emphasize the importance of multi‐temporal groundwater dynamics and feedbacks between coastal flooding, erosion, and salinization. Field‐based studies considering multiple drivers and timescales of saltwater intrusion are critical for understanding and managing coastal freshwater resources in an age of rapid environmental change. Plain Language Summary Small island communities rely on groundwater for freshwater supply that is increasingly threatened by salinization. Along the coast, erosion, seasonal changes in atmospheric conditions, and sea levels control groundwater salinity changes over multiple timescales. We collected groundwater data and compared the present distribution and salinity of groundwater to results from monitoring conducted in the 1970s. In this work we show that decades of erosion, seasonal wave and precipitation patterns, tides, and intermittent seawater flooding drive freshwater decline on Sable Island, Nova Scotia, Canada. Long‐term erosion