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The investigation of the transition zone between subsurface sediments and the water column, in addition to an enhanced understanding of the underlying processes, becomes more and more important when dealing with ecological questions not only in coastal areas.

Therefore, DiaTrans (diagenetic transport), a one-phase/multi-component model to simulate multi-dimensional fully coupled density-driven flow, transport and biogeochemical reaction processes in the subsurface underlying a seawater column, including bioturbation and bioirrigation, is presented. The components include water and an arbitrary number of dissolved constituents such as chloride, methane, sulphate or oxygen. The governing equations are discretized using a Finite-Volume-Method (FVM) on two-dimensional rectangular structured grids in an object-oriented framework. A fully-upwinding technique is employed for the advective fluxes. The sparse and non-symmetric system of highly non-linear equations is solved, utilizing the Newton-Raphson method with an inner linear preconditioned BiCGSTAB solver.

This implicit fully-coupled formulation of physical and biogeochemical processes, which has certain advantages, is a new approach not only in near-shore sediments, as generally decoupled methods are employed. The model accounts for different domain sizes and grid resolutions, unsteady conditions to simulate tidal cycles and variable physical parameters such as permeabilities and porosities which are important to cope with lateral heterogeneities. 

Typical benchmark tests have been carried out to verify DiaTrans' capability to model flow, transport and reaction processes in the subsurface. These include the Henry problem and the salt dome problem for density-driven flow and transport as well as an example for standard diagenetic biogeochemical reaction processes. 

As natural test cases for the numerical modeling different application examples are presented. First, the physical processes at sand boils, which account for submarine groundwater discharges at a site in the Wadden Sea of Cuxhaven, North Sea, Germany are investigated and the simulation results are compared to field measurements which have been carried out by the Alfred Wegener Institute for Polar and Marine Research (AWI), Bremerhaven, Germany. Second, reaction influenced methane concentration profiles at so-called vent and partial vent sites in Eckernförde Bay, Baltic Sea, Germany are simulated with a special focus on the effects of bioturbation and bioirrigation. For both applications, rather good matches are observed between model results and field data. The last application example deals with qualitative statements about the interaction processes at the sediment-water interface where the influence of surface water velocities on the fate of methane in the water column is investigated, considering biogeochemical reaction processes such as methane re-oxidation by oxygen and sulfate.

Because of its object-oriented nature, the model can easily be extended to additional physical processes, more complex biogeochemical reaction processes or enhanced numerics in the future. DiaTrans is considered as a sophisticated model for multi-dimensional density-driven flow, transport and biogeochemical reaction processes in porous media for both small and larger scale problems.


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