COASTAL AQUIFER RESPONSE TO INCREASED TIDAL FLOODING FROM SEA LEVEL RISE

Sea level rise is affecting the frequency and severity of tidal flooding along coastlines. Increases in sea level lower the high tide threshold required for tidal flooding before low-lying coastal areas become permanently inundated. Tidal flooding occurs partly due to astronomical tides when the phases of short and long-period tidal constituents align to result in extreme high tide water levels at perigean, annual, lunar nodal, and other cycles. Groundwater salinity distributions are likely to be affected by tidal flooding, however the response of coastal aquifers to infiltration of seawater during tidal flooding events are not well understood. We used a numerical variable-density groundwater flow and salt transport model to investigate aquifer response to changes to the frequency and intensity of tidal flooding due to sea level rise. Model results showed that tidal flooding led to salinization of the coastal aquifer and disrupted the salinity structure of the lower saltwater-freshwater interface and shallow intertidal mixing zone. The size of the intertidal mixing zone increased with the long-term rise in sea level as the intertidal zone moved landward onto the land surface. The landward migration of the intertidal zone resulted in a shift from a stable pore water density configuration to an unstable case with density fingers that migrated through the aquifer. The results demonstrate the importance of sea level rise and tides on aquifer salinity distributions in areas susceptible to tidal flooding. The insights into the effects interacting short and long period hydrologic forcings on subsurface salinity distributions have implications for saltwater contamination of domestic water supply wells, chemical processing in coastal aquifers, and the chemical composition of submarine groundwater discharge.