The Contraction of Freshwater Lenses in Barrier Island: A Combined Geophysical and Numerical Analysis

Abstract

Coastal aquifers, vital for supplying freshwater to over one billion people worldwide, often face saltwater intrusion, with barrier island aquifers playing a crucial role in this coastal system. Despite the proliferation of studies exploring the impacts of sea-level rise, storm surges, and over-pumping, the effect of droughts on barrier island aquifers remains largely under-investigated. This lack of attention is particularly concerning given the heightened vulnerability of barrier island aquifers; they ultimately rely solely on aerial recharge compared to their inland counterparts. An in-depth understanding of recharge and salinization processes is imperative for sustainably managing these island water resources, especially considering climate change impacts. This study presents a new evaluation approach for the response of a freshwater lens to drought conditions, which incorporates in-situ observations, geophysical measurements, and numerical modeling. The study examines the response of a shallow unconfined aquifer on an Atlantic coast barrier island to the 2020 Northeast drought. It encompasses a density-driven flow model informed by time-lapse electrical resistivity imaging and in-situ measurements, including groundwater head and salinity collected from monitoring wells. Results from our approach indicate that freshwater lens volume is reduced by 11% during the 2020 Northeast drought. It returns to its previous volume over the spring season, indicating that the net water volume of the freshwater lens remained unchanged on an annual scale. The mean groundwater residence time on the island is approximately a year, indicating the barrier island aquifer is a hydrodynamically active zone. These findings highlight the vulnerability and resilience of shallow unconfined barrier island aquifers to droughts and climate change.

Keywords: Coastal Groundwater, Freshwater Lens, Barrier Island Aquifers, Drought, Electrical Resistivity, Variable-Density Numerical Model