HGS RESEARCH HIGHLIGHT - Mega-Tidal and Surface Flooding Controls on Coastal Groundwater and Saltwater Intrusion Within Agricultural Dikelands
LeRoux, N. K., Frey, S. K., Lapen, D. R., Guimond, J. A., & Kurylyk, B. L. (2023). Mega‐Tidal and Surface Flooding Controls on Coastal Groundwater and Saltwater Intrusion Within Agricultural Dikelands. In Water Resources Research (Vol. 59, Issue 11). American Geophysical Union (AGU). https://doi.org/10.1029/2023wr035054
“[HydroGeoSphere] was selected for its ability to simulate surface flow dynamics during storm surges, along with variable-density, variable-saturation subsurface flow. HydroGeoSphere is increasingly used for coastal groundwater applications to consider perturbations from [sea-level rise] and tides. ”
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The study highlighted here makes full use of the density dependent flow modelling capabilities of HydroGeoSphere to investigate the impacts of climate change on groundwater-ocean interactions, and how sea-level rise, tides and storm-surges impact the long-term position of an upper saline plume in a coastal agricultural dikeland in Nova Scotia, Canada.
A presentation on this study was recently delivered by the primary author via webinar, so we’ll let the video speak for itself!
Plain Language Summary:
Fig. 5: HydroGeoSphere model calibration results (run 3, Table 1). (a) Pressure head distribution, (b) salt concentration (normalized to seawater) distribution and wedge location limit based on monitoring well electrical conductivity, and (c) the simulated and observed monitoring well groundwater levels (Figure 2c). Model domain is vertically exaggerated to visualize changes.
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 world's highest tides, and a numerical model was developed to represent surface flooding and saltwater intrusion dynamics. Results reveal high mixing rates between the groundwater and the coastal water body and demonstrate how future storm surges and SLR can drive flooding and salinize agricultural soils and underlying aquifers.
Abstract:
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.