Paleo-hydrological controls on continental-shelf groundwater salinity distribution offshore New Jersey, USA

• Characterizes hydrogeological heterogeneity using marine seismic and borehole data. • 3D paleo-hydrogeological modeling shows ORFG volume evolution with sea-level changes. • Interglacial periods cannot fully salinize ORFG, preserving relic pre-LGM freshwater. • Heterogeneity and density-driven flow create complex shelf fresh–saltwater patterns. Continental shelves host extensive offshore relatively fresh groundwater (ORFG) systems whose origin, distribution, and persistence are strongly influenced by paleo sea-level fluctuations; however, the paleo-hydrological controls governing their present-day salinity patterns remain poorly constrained. Here, we investigate the ORFG system offshore New Jersey, USA, by integrating high-resolution seismic profiles, borehole constraints, geochemical and isotopic data, and three-dimensional variable-density groundwater flow and solute transport modeling. Model results indicate that most ORFG was emplaced by topographically driven groundwater flow during sea-level lowstands over the past ∼ 100 kyr. During Pleistocene lowstands, enhanced hydraulic gradients across the exposed continental shelf promoted widespread seaward groundwater flow, leading to the development of low-salinity groundwater over large areas. Subsequent marine transgression led to partial salinization through diffusive and density-driven mixing with seawater. Our results further demonstrate that cyclical flushing and re-salinification of shelf sediments over glacial–interglacial timescales are inherently asymmetric, favoring the long-term retention of freshened groundwater. Compared with conventional layered modeling approaches, the incorporation of depth-migrated seismic stratigraphy and geostatistical property modeling substantially improves the characterization of ORFG spatial heterogeneity and volume distribution. However, these results are based on a single representative geostatistical realization and therefore do not explicitly quantify stochastic uncertainty. Overall, this study highlights the long-term legacy of paleo-hydrological conditions on continental-shelf groundwater systems and provides a process-based perspective for interpreting ORFG occurrence, evolution, and vulnerability along passive continental margins.