Modeling of Solute Transport in Pore Scale Sediment Beds: A Summary of Hydrodynamic Interactions Induced by Surface Wave, Bed Form, and Near Bed Turbulence

Solute transport in a pore scale sediment bed of river or lake has a significant effect on chemical mass balances and microbial activities in the water and sediment. The solute transfer between water and a pore scale sediment bed is often described by a "1-D vertical dispersion model" and estimated using molecular diffusion and porosity. However, surface waves, bed forms, and near bed turbulence create periodic pressure waves along the sediment/water interface, which in turn induces flows in the pores of the sediment bed. A coupled 2-D hydrodynamic and solute transport model has been developed to study the solute transport in the pore scale sediment bed, and the solute transport has been incorporated in a 1-D depth dependent "enhanced dispersion coefficient (D_E)". Typically, DE diminishes exponentially with depth in the sediment bed. It is a function of the near-bed coherent motion due to the turbulent current, relative dispersivity (longitudinal dispersivity/wave length), wave steepness, sediment hydraulic conductivity, and sediment porosity. Maximum values of D_E near the sediment surface can be much larger than molecular diffusion coefficients, e.g., D_E ∼ 10cm²/s in a gravel bed with pressure standing waves, D_E ∼1 cm²/s in a gravel bed under progressive surface waves, and D_E ∼0.1 cm²/s in a gravel bed under near bed turbulent current. The penetration depth due to turbulent is only about 1/5 ∼ 1/10 of that caused by the pressured surface wave. Therefore, the pressured surface wave is a dominant process, and the other processes can be ignored. However, the near-bed turbulent can enhance the transport at least 1 order magnitude than the underflow process along the sediment/water interface.