Offshore freshened groundwater (OFG) systems – results and implications from two continental margins

Offshore freshened groundwater (OFG) represents a significant but underexplored global water resource. Although OFG systems are strongly influenced by paleo sea-level fluctuations, the specific hydrological controls governing present-day salinity distributions remain difficult to constrain due to limited data availability and uncertainties in model parameterization.

We present our latest results from two OFG systems: (1) offshore New Jersey, USA, and (2) Canterbury Bight, New Zealand. Both studies integrate available data such as high-resolution seismic profiles, borehole constraints, geochemical and isotopic data, and paleo-hydrogeological modeling. Offshore New Jersey, OFG was primarily emplaced during sea-level lowstands over the past ~100 kyr, when large portions of the continental shelf were exposed to the atmosphere. Subsequent marine transgression led to partial salinization through diffusive and density-driven mixing with seawater. However, the duration of interglacial submergence has been insufficient to fully salinize the OFG, allowing relic freshwater from pre–Last Glacial Maximum and earlier interglacials to persist. In Canterbury Bight, simulations indicate that modern onshore recharge contributes only a limited fraction of the OFG. The majority of seaward OFG has a mean groundwater age of less than ~40 kyr, suggesting a dominant origin from local meteoric recharge during late-Pleistocene sea-level lowstands. In addition to diffusion and compaction-driven flow, topographically driven lateral flow across the continental shelf played a key role in OFG emplacement. OFG volumes and paleo-submarine groundwater discharge along the continental shelf have varied periodically with glacial–interglacial sea-level changes. Similar to New Jersey, the current OFG system in Canterbury Bight is at non-steady state and becomes gradually salinized by overlying seawater.

Overall, this study sheds light on the effect of changing paleo-hydrological conditions in shaping continental-shelf groundwater systems and provides a framework for assessing the occurrence, evolution, and vulnerability of OFG along passive continental margins.