Mega‐Tidal and Surface Flooding Controls on Coastal Groundwater and Saltwater Intrusion Within Agricultural Dikelands

Climate change will increase sea levels, driving saltwater into coastal aquifers and impacting coastal communities and land use viability. Coastal aquifers are also impacted by tides that control groundwater‐ocean interactions and maintain an “upper saline plume” (USP) of brackish groundwater. Coastal dikes are designed to limit the surface impacts of high‐amplitude tides, but, due to ongoing sea‐level rise (SLR), low‐lying dikelands and underlying aquifers are becoming increasingly vulnerable to flooding from high tides and storm surges. This study combines field observations with numerical modeling to investigate ocean‐aquifer mixing and future saltwater intrusion dynamics in a mega‐tidal (tidal range >8 m) dikeland along the Bay of Fundy in Atlantic Canada. Field data revealed strong connectivity between the ocean and coastal aquifer, as evidenced by pronounced tidal oscillations in deeper groundwater heads and an order of magnitude intra‐tidal change in subsurface electrical resistivity. Numerical model results indicate that SLR and surges will force the migration of the USP landward, amplifying salinization of freshwater resources. Simulated storm surges can overtop the dike, contaminating agricultural soils. The presence of dikes decreased salinization under low surge scenarios, but increased salinization under larger overtopping scenarios due to landward ponding of seawater behind the dike. Mega‐tidal conditions maintain a large USP and impact aquifer freshening rates. Results highlight the vulnerability of terrestrial soil landscapes and freshwater resources to climate change and suggest that the subsurface impacts of dike management decisions should be considered in addition to protection measures associated with surface saltwater intrusion processes. Plain Language Summary Densely populated coastal communities are vulnerable to rising sea levels and ocean storms. Many of these coastlines are protected by dikes. These hard barriers prevent surface seawater flooding during high tides, but their subsurface impacts are less understood. Large tides play a key role in driving coastal surface and subsurface exchange. For example, subsurface tidal pumping creates a zone of unpotable groundwater in the “upper saline plume” that is maintained despite the presence of dikes. This study investigated how high tides, sea‐level rise (SLR), and storm surges influence groundwater‐ocean interactions. New data were collected along the Bay of Fundy, which has the wor