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Recent advances in marine electromagnetic surveys have allowed geophysicists to interpret and map offshore freshwater resources with unprecedented resolution and to test inferences regarding onshore-offshore hydrologic connections. To date, however, little is known about the timing or isotopic composition of this unconventional water resource. Here, we reconstructed the Pleistocene hydrogeology of the U.S. Atlantic continental shelf using a cross-sectional paleo-hydrogeologic model to explore possible mechanisms and timing of freshwater emplacement offshore Martha’s Vineyard, Massachusetts. We considered two scenarios in which the Laurentide ice sheet extended different distances offshore, and a third scenario without any ice sheet. The hydrostratigraphic framework was constructed by integrating borehole lithology data, seismic data, and formation resistivity data. Model results were compared to formation resistivity data as well as borehole salinity, groundwater residence time, and stable isotope profiles. Neither of the ice-sheet scenarios provided a significantly better fit to the onshore isotopic and offshore salinity observations than the third scenario. All three model scenarios predicted freshwater emplacement within Tertiary and Cretaceous units. Pleistocene deposits were largely devoid of freshened groundwater. Simulated groundwater residence times for the midshelf region ranged between 104 and 106 yr at depths of <500 m. Simulated groundwater ages from wells completed within Pleistocene confined aquifers are consistent with measured groundwater ages within confined aquifers of Martha’s Vineyard and Nantucket Island (2750−5900 yr). Analysis of onshore 3H/3He dating data indicates that some wells contain a mixture of old and modern (<60 yr) groundwater. Calculated fossil groundwater in the midshelf region that included ice-sheet loading retained relatively low δ18O values, consistent with glacial meltwater recharge. Model results suggest that much of the freshwater emplacement occurred within the last glacial cycle and that the island and offshore hydrogeologic systems appear to be connected. 

Abstract First reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 10⁶ km³; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography‐driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three‐dimensional hydrological models. We show that such advances, combined with site‐specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub‐seafloor geomicrobiology.