Offshore fresh groundwater reservoirs occur on continental shelves in several regions of the world. But their origins are often poorly understood and an area of active research. In a numerical case-study, we analyze the extensive multi-layered freshened groundwater system on the New Jersey shelf. Coupled variable-density flow and heat transport simulations on a geologically representative 2D shelf model using SHEMAT-Suite yield a plausible range of paleo-hydrogeological conditions during the late Pleistocene. The model combines sequence stratigraphic interpretation of 2D depth migrated seismic lines and a stochastic facies distribution with petrophysical properties from IODP Expedition-313 well data. The study considers a 60 000-year recharge period for the subaerially exposed shelf transect, followed by the marine transgression from 12 000 years ago until today. Our sensitivity analysis studies three influencing factors during the recharge phase: (1) topography-driven flow, (2) permeability anisotropy and (3) enhanced terrestrial discharge. Most significantly, depending on the permeability anisotropy ratio, the emplaced volume decreases by 11 % to 31 % relative to the base-case. The results show that the lowstand period drove sufficient freshwater emplacement that can explain present-day observations. Simulated scenarios indicate that surface recharge freshens the sediments across the entire transect during this period, even for large permeability anisotropy ratios. The observations also suggest that cyclical flushing and re-salinification of shelf sediments during glacial – interglacial cycles is an asymmetrical process, promoting freshwater storage over geological time scales. Finally, the study indicates that offshore reservoirs may contain significant amounts of fresh groundwater.

Offshore freshened groundwater reservoirs occur on continental shelves in several regions of the world. Their origins are an active area of research, however, models often rely on simplified geometrical representations of subsurface geology. The New Jersey shelf hosts an extensive multi-layered freshened groundwater system that previous paleo-reconstructions have not reproduced. In this numerical case-study, we aim to characterize the New Jersey shelf system in the context of a geologically representative heterogeneous model. Our model combines sequence stratigraphic interpretation of 2D depth migrated seismic lines and a stochastic facies distribution with petrophysical properties of four boreholes. We employ a stochastic approach to generate both high and low onshore-offshore connectivity scenarios. The study considers a 58 000-year recharge period for the subaerially exposed shelf transect, followed by the marine transgression from 12 000 years ago until today. The results show that the lowstand period drove sufficient freshwater emplacement that can explain most of the present-day observations. The highest rates of recharge occurred during the periods of most rapid sea-level fall. Simulated scenarios indicate that topographically driven flow of meteoric recharge via surface-connected pathways is the key emplacement mechanism. Surviving freshwater systems exhibit lateral variability in salinity due to downward fingering of saline pore fluid. Freshwater preserved from the Last Glacial Maximum may extend up to 100 km from the coastline. The results also suggest that cyclical flushing and re-salinization of shelf sediments during glacial-interglacial cycles is an asymmetrical process, promoting freshwater storage over geological time scales.