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

  poster Urheberrecht: © GGE

Figure 1 Map of study region showing IODP Expedition 313 well locations, 2D seismic lines and approximate model domain

 

Project description

The existence of submarine fresh groundwater has been recorded at continental shelves worldwide. Along the U.S. continental margin, significant volumes of groundwater with salinities far lower than seawater were initially discovered 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).

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, it has been postulated that these reservoirs either 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. We aim to test the hypotheses of fresh groundwater emplacement, as well as investigate the dynamic preservation and lifetime of fresh groundwater in the offshore environment. The mechanisms and time scales of fresh groundwater preservation are examined using numerical simulations on a geologically representative shelf model. The first phase of the project focuses on seismic interpretation and building the 3D hydrogeological model. The approach combines stochastic and deterministic methods, utilizing newly reprocessed seismic reflection data (Riedel et al. 2018), together with IODP Expedition 313 well log data and detailed core analysis (Mountain et al. 2010) to establish facies distribution trends within the shelf sequences. The second stage consists 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 benchmarked the code SHEMAT-Suite with the Henry problem for testing its ability to handle 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.

Publications

Thomas A.T., Reiche S., Riedel M., Clauser C. 2019. The fate of submarine fresh groundwater reservoirs at the New Jersey shelf, USA, Hydrogeology Journal, doi: 10.1007/s10040-019-01997-y.

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.

Riedel M., Reiche S., Aßhoff K., Buske S. 2018. Seismic depth imaging of sequence boundaries beneath the New Jersey shelf. Marine Geophysical Resources 1–16. https://doi.org/10.1007/s11001-018-9360-9

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.