Investigating the impact of the Pleistocene sea-level lowstand on offshore fresh groundwater on the New Jersey shelf 

Abstract

<p>Offshore fresh groundwater reservoirs have been identified on continental shelves in several regions of the world. In many cases, sea-level change over geologic time-scales has been identified as a key factor in the emplacement of these freshwater systems. This numerical study analyzes a range of paleo-hydrogeological conditions on the New Jersey transect during the late Pleistocene, during which vast sections of the shelf were sub-aerially exposed. Coupled variable-density flow and heat transport simulations were conducted on a geologically representative 2D shelf model using SHEMAT-Suite. The model combines sequence stratigraphic interpretation of 2D depth migrated seismic lines and a stochastic facies distribution, with petrophysical properties derived from IODP Expedition 313 well data. The study considers a 60<sub></sub>000 year period of surface meteoric recharge, and the subsequent marine transgression from 12 000 years ago to present-day. A sensitivity analysis is conducted for key factors that influence offshore freshened groundwater emplacement during recharge phase: (1) topography-driven flow, and (2) permeability anisotropy. Systematically introducing anisotropy resulted in a 11 % – 31 % decrease in emplaced volume relative to the base-case. The results were analysed to determine whether the late Pleistocene sea-level lowstand drove enough freshwater emplacement that can explain the complex present-day observations. All of the simulated scenarios indicate that surface recharge lead to freshening of sediments across the entire transect during this period, even in case of high permeability anisotropy. The observations also suggest that the cyclical flushing and re-salinification of shelf sediments that takes place over glacial – interglacial cycles is an asymmetrical process, which favours storage of freshened pore fluid in the long run.</p>

The coastal transition zone is an underexplored frontier in hydrology and geoscience

“…Active groundwater flow is restricted to the more permeable layers when these have inlets and outlets that allow recharge and discharge, but groundwater may also be virtually stagnant in confined regions or low-permeability strata 20 . Recent numerical modeling suggests that mid-shelf offshore freshened groundwater (OFG) reservoirs 21,22 offshore the New Jersey, USA coast may be laterally compartmentalized by downward fingering of seawater 23,24 . Imaging the full vertical extent of the seawater's pathways is one of the challenges for deep-probing hydrogeophysics.…”

“…Three-dimensional modeling is expected to lead to insights into this phenomenon and many other dynamical processes of freshwater emplacement and preservation. Hydrological modeling can and has been used to simulate hydrologic processes over geologic timescales [23][24][25]88 . In fact, that is one of the great strengths of adding a modeling component to observationsit is a means to test hypotheses about the evolution of a system over time in order to understand how the present conditions came to be, and then how the system may evolve in the future.…”