Numerical investigation of solute migration and release from sediments driven by wave-induced accumulation of pore water pressure

Sediments with poor drainage conditions are prone to build-up of excess pore water pressure when exposed to dynamic wave loading. This results in accumulated seepage flow that may have a significant impact on migration of solutes in marine sediments. In this study, the complex interplay between solute transport and wave-induced seepage process is investigated through a pioneering approach that combines and synchronously solves the two-dimensional advection-dispersion equation governing nonreactive solute transport with the governing equations for wave-induced oscillatory and residual pore pressures. The simulated results have demonstrated that the accumulated seepage flow significantly contributes to the advective and dispersive fluxes of solute, where the advection leads to movement of the solute layer and the hydrodynamic dispersion primarily enlarges the thickness of solute layer. The distribution of residual pore water pressure along depth exhibits two patterns that are primarily influenced by the thickness of the seabed. When the seabed is thinner than 0.3 times the wavelength, the residual pore pressure increases monotonically with depth (Pattern I). In this situation, the solute always migrates upwards to the seabed surface. Otherwise, Pattern II depicts the distribution of residual pore pressure, where the peak value of residual pore pressure attains at a certain depth and may advance downwards with wave action time. In this case, the direction of solute migration in regions shallower than a certain depth (e.g., 0.6 times the thickness of sediment) may alternate over time but the solute can eventually be transported upwards to seabed surface. The parametric study shows that when the seabed thickness is 0.2–0.3 times the wavelength, accumulated seepage flow has less effect on solute migration, whereas decreasing relative density and shear modulus, as well as increasing soil permeability can enhance the effect of accumulated pore water pressure on solute migration