Transport of Reactive Tracer in Compacting Multi-Fraction Bottom Sediments

The one-dimensional transport of reactive tracer (radionuclide) in the multi-fractional aquatic sediments is considered. In the model a transfer of tracer in sediment is governed by the molecular diffusion of dissolved phase in pore water, biodiffusion, phase exchange between dissolved and solid phases of tracers, exchange between fast and slow reversible phases of tracer, and compaction under gravity force resulting in a decrease of porosity with sediment layer depth. The transport equations for 2-step reactions were completed by an equation for fractions of sediments assuming that mixing in the bottom sediment is intraphase. The compaction dynamics was parameterized using an approximation of porosity by travelling wave profile. The results of an idealized numerical experiment on the constant deposition of contaminated sediments show that in the case of 2-step reactions the exchange processes in multifraction sediments were far from equilibrium.