Groundwater Resources in Barrier Islands Are Vulnerable to Storm-Surge Salinization Through Various Dominating Processes, As Revealed by Data-Based Modeling

Groundwater resources on barrier islands sustain both human lives and ecosystem functioning worldwide. A critical threat to these vital freshwater resources is the projected increase in frequency of storm-surges with climate change. These extreme events have the potential to salinize the aquifer, which increases the mortality of stabilizing vegetation and potentially causes substantial erosion. It is important to understand which types of systems are most vulnerable to better plan mitigation efforts. To that end, we collected data on water level and salinity at three different barrier islands along the east coast of the US. These study sites span a range of barrier island typologies, including differences in topography, vegetation, overwash frequency and connection to the ocean. The relationship between the various processes and the vulnerability to salinization was quantified using transfer function noise models (Pastas) and cross correlation. It was found that changes in groundwater head can be well represented with bay water levels, ocean water levels, recharge (both precipitation and evapotranspiration), and overwash events (periods of time when bay or ocean water levels exceed the land surface elevation of the wells). The data-based models indicate that ocean/bay levels are the primary driver of water level change close to the shore while in the center of the barrier islands precipitation is a more significant driver. Substantial differences were found between the different sites in terms of the dominating factors and the overall vulnerability. A sheltered maritime forest site showed minimal impact from storm surge overwash while a less sheltered marsh and maritime forest site showed a clear relationship between the height and duration of overwash events and the total amount of salinity observed in the wells. Finally, at a barren high energy beach site, storm surge appeared to both salinize the aquifer from the top and raise the freshwater-saltwater interface from below. These findings have important implications for management of barrier island groundwater resources, which is a vital resource that is compromised by future changes in storm surge frequency and intensity.