Investigating Freshwater Lens Geometry and Groundwater Dynamics on Shackleford Banks, North Carolina: Implications for Freshwater Resources on Barrier Islands

Abstract

Freshwater resources on barrier islands are highly vulnerable to saltwater intrusion (SWI) driven by sea level rise, storm surge, and coastal flooding. This study examines freshwater lens geometry and salinity dynamics on Shackleford Banks, and undeveloped island located in the Cape Lookout National Seashore in North Carolina. Using analytical freshwater lens modeling, groundwater-level and electrical conductivity monitoring, frequency-domain electromagnetic induction (FD-EMI) surveys, and geospatial analysis, this research establishes baseline spatial and temporal patterns of subsurface salinity across geomorphic settings during a one-year observation period. Results indicate that island geomorphology (particularly island width) is the primary control on freshwater lens thickness and storage. Freshwater lenses systematically decrease in thickness from the wide, dune-dominated western portion of the island to the narrow, inlet-adjacent eastern segment. Seasonal precipitation drives short-term groundwater-level variability but does not override geomorphologically controlled spatial variability patterns. FD-EMI surveys reveal persistent organization of near-surface conductivity aligned with geomorphic units: interior dune systems consistently exhibit low conductivity indicative of fresh groundwater, whereas sound-side and ocean-adjacent environments display elevated salinity. Time-series and spectral analyses show that groundwater levels are predominantly recharge-driven, while salinity dynamics reflect localized marine influence near coastal boundaries. These findings support a model where geomorphology establishes a structural framework for freshwater availability, and climatic forcing governs temporal variability within that framework. The study provides a critical baseline for detecting future change under continued sea level rise and demonstrates the value of integrating hydrological and geophysical methods to assess freshwater vulnerability in barrier island systems.