|
(W/EH088) Evaluation of the box-approach in generic compartment models for sediment/water substance transfer. Sorensen, Peter1, Fauser, Patrik1, 1 ABSTRACT- The description of diffusive transfer seems not well established in the generic compartment model, where using a mean transfer length for diffusion damages the nature of diffusion. The full solution of the governing equation for sediment diffusion indicates an extremely high rate of substance uptake, as long the contamination in the sediment is limited. Contrary, initially the single sediment box approach yields a constant diffusion rate. After a specific period the single box approach i becomes more valid, however, that time period can easily be in the magnitude of many years! In case of steady-state where the emission of substance into the system is assumed equal to the sediment uptake by diffusion the one box approach will always overestimate the dissolved water concentration in relation to the more complete sediment description. Obviously the discrepancy in the diffusive transfer description will vanish as the transfer due to deposition of solids on the sediment surface starts to dominate in the total transfer of substance mass to the sediment. In many cases the deposition will dominate after a rather short time period and thus the concept in the compartment models will becomes acceptable simply because the diffusive transfer is negligible. Thus, in conclusion it appears that the one sediment box approach seems insufficient to include a realistic diffusion mechanism. But in many cases the deposition and thus the burial from the sediment box of fixed thickness will become the dominating factor in the sediment/water column exchange of substance and in that case the inadequate description of diffusion will not be important. Key words: Sediment transfer, diffusion, deposition, xeno-biotics |
