Küstenfernes Süßwasser: 3D numerische Simulationen von Grundwasserströmung am New Jersey Shelf

 

Flow of water across the seafloor into the ocean along Earth’s continental margins is commonly addressed as submarine groundwater discharge. Along the U.S. continental margin, significant volumes of groundwater with salinities far lower than seawater were discovered initially in 1976, during the “Atlantic Margin Coring Project”, and shown to occur as far as 100 km away from the coast (Hathaway et al., 1979). By now, it is clear that the occurrence of low-salinity groundwater at continental margins represents a global phenomenon.

Amongst all sites of known offshore fresh groundwater occurrence, the New Jersey shelf is considered the best documented example (Post et al., 2013). Pore water samples collected in the course of IODP expedition 313 at the New Jersey Shelf showed a complex vertical salinity distribution with several sharp, irregular boundaries between fresh and saline groundwater (Mountain et al., 2010). The origin of these freshwater reservoirs is still a matter of debate, being either considered to contain young fresh water currently transported offshore (van Geldern et al., 2013) or paleo freshwater emplaced during previous glacial periods (Lofi et al., 2013).

The aim of this project is to understand the groundwater circulation at the New Jersey shelf based on numerical simulations. The first stage of the project focuses on seismic interpretation and building a realistic 3D hydrogeological model. This is done in close collaboration with the seismic group at TU Bergakademie Freiberg, where key seismic profiles are currently reprocessed by enhanced depth-imaging methods. The second stage will consist of numerical modeling using the in-house code SHEMAT-Suite. The transient, coupled, nonlinear equations for fluid flow, energy and tracer transport can be solved in three dimensions. We recently benchmarked the code SHEMAT-Suite with the Henry problem for testing its ability to handle correctly diffusive and buoyancy-driven salt transport. Such a test is relevant for salt transport simulations at a continental margin scale – where fresh and saline groundwater bodies are in contact. Results of this study shall lead to a better understanding of mechanisms responsible for fresh water emplacement far off the New Jersey coast and may also be applicable to the entire eastern U.S. Atlantic margin where fresh water is widely encountered.

Funding for this project is provided by the Deutsche Forschungsgemeinschaft (DFG) within the Priority Programme 527 - "International Ocean Discovery Program" (IODP) under grant RE- 3863/2-1.

References:

Hathaway, J.C., Poag, C.W., Valentine, P.C., Miller, R.E., Schultz, D.M., Manheim, F.T., Kohout, F.A., Bothner, M.H., Sangrey, D.A., 1979. U.S. Geological Survey Core Drilling on the Atlantic Shelf. Science 206, 515-527.

Lofi, J., Inwood, J., Proust, J.-N., Monteverde, D.H., Loggia, D., Basile, C., Otsuka, H., Hayashi, T., Stadler, S., Mottl, M.J., Fehr, A., Pezard, P.A., 2013. Fresh-water and salt-water distribution in passive margin sediments: Insights from Integrated Ocean Drilling Program Expedition 313 on the New Jersey Margin. Geosphere 9, 1009-1024.

Mountain, G.S., Proust, J.-N., McInroy, D., Cotterill, C., and the Expedition 313 Scientists, 2010. Proceedings of the Integrated Ocean Drilling Program. Volume 313: Tokyo (Integrated Ocean Drilling Program Management International, Inc.), doi:10.2204/iodp.proc.313.2010.

Post, V.E.A., Groen, J., Kooi, H., Person, M., Ge, S., Edmunds, W.M., 2013. Offshore fresh groundwater reserved as a global phenomenon. Nature 503, 71-78.

van Geldern, R., Hayashi, T., Böttcher, M.E., Mottl, M.J., Barth, J.A.C., Stadler, S., 2013. Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin. Geosphere 9, 96-112.