Fresh groundwater far off the coast: 3D numerical simulations of groundwater flow at the New Jersey shelf

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The aim of this study is to understand groundwater circulation at the New Jersey shelf (NJS) based on numerical simulations. In the late 1970, groundwater with salinities significantly lower than seawater was initially discovered in the course of the Atlantic Margin Coring Project along the eastern U.S. continental margin. Such fresh groundwater reserves were shown to be present within a distance of locally more than 100 km from the coastline. A particularly detailed record of pore water salinity was recovered in the course of IODP Leg 313 at the NJS, revealing abrupt vertical salinity variations at each of the three drilling locations. Previous studies either explained the existence of fresh groundwater at the NJS by present-day seaward directed meteoric groundwater flow or by fresh water emplacement during the last glacial period. However, which of these processes finally resulted in fresh groundwater at the NJS is currently debated. Also, the cause for the observed vertical salinity changes is little understood. In order to answer these questions, we will build a detailed 3D hydrogeological model for simulating groundwater flow at the NJS. On the basis of numerical simulations, the following working hypotheses to be tested are defined: 1. Fresh groundwater at the NJS was emplaced during the last glacial period. 2. Present-day submarine fresh groundwater discharge does not extend up to 100 km offshore. 3. The distribution of fresh submarine groundwater is controlled by low-permeability deposits. An excellent data base is available for building a 3D hydrogeological model. It consists of numerous 2D seismic profiles, 3D seismic data (by December 2015) as well as petrophysical and logging data, acquired during ODP Legs 150, 174A and IODP Leg 313. Depth-migration of seismic data will be performed for building a 3D hydrogeological model in depth. Definition of individual model sequences will be based on the published lithostratigraphic framework established after IODP Leg 313. Each sequence will be subdivided into geological facies, where each facies type is characterized by petrophysical properties obtained from ODP/IODP logs and laboratory measurements. By using a multiple-point geostatistical approach for modelling facies distribution, petrophysical properties derived from AVO analyses of 3D seismic data can also be incorporated into the model. Testing the working hypotheses defined above by numerical groundwater flow simulations requires careful definition of the initial and boundary conditions of the model, incorporating Pleistocene sea level and temperature variations, present-day fresh-water discharge at the coast, the hydraulic head (eventually accounting for ice sheet topography) and heat flux. Results of this study may be applicable to the entire eastern U.S. Atlantic margin where fresh water is widely encountered and will yield a better understanding of submarine groundwater discharge in general.