Harnessing integrated hydrologic modeling to analyze the coastal impacts of groundwater-surface water interactions on beach surface stability and freshwater availability

Coastal aquifers supply freshwater to nearly half of the world's population, and their importance for sustainable development in coastal areas is immense. Due to the proximity to the ocean, salinization is typically the biggest risk for coastal groundwater resources. Furthermore, the interactions between groundwater and surface water during coastal flooding often result in surface instabilities arising from elevated groundwater heads. Here, integrated hydrologic modeling is used to examine the effect of groundwater-surface water interactions on the salinity distribution in aquifers and on the stability of beach surfaces. The processes considered include multi-scale fluctuations in sea level (tides, storm surges, and glacial cycles). Results show that modern salt distributions may change even if the current conditions remain stable, when considering short- and long-term cyclical processes that aquifers are likely still responding to. It is also found that during coastal flooding, critical hydraulic gradients may develop, potentially destabilizing the beach surface. The distribution of these critical gradients depends on beach topography, with a non-trivial relationship between surface elevation and the location of critical gradients. These results mean that the interactions between groundwater and surface water likely play a pivotal role in the hydrologic state of coastal systems, with important implications for water resources management and for natural hazard mitigation.