Migration of radioactivity in multi-fraction sediments

Abstract

A new 3D radioactivity transport model coupled with multiscale circulation and multi-fractional sediment transport modules is presented. The sediment transport module simulates the transport of a mixture of one cohesive sediment fraction and a number of fractions of non-cohesive sediments of different sizes and densities. The model of radionuclide transport describes the key transport and exchange processes in the system of water-suspended and bottom multi-fraction sediments. Two-step kinetics with two successive reversible fast and slow reactions is used in the model. A noticeable feature of the model is approximation of the sediment and contamination profiles in the bed by multiple well-mixed layers to describe the vertical migration of radioactivity within bottom sediments due to erosion/deposition, molecular diffusion and bioturbation. The model accurately reproduced a laboratory experiment on the uptake of radiocesium by lake sediments. An analytical solution describing mutual adjustment of the concentrations of radioactivity in the pore water and in the multi-fraction sediment showed that activity was redistributed between different fractions of sediments far slower than between water and the total concentration in the sediment. The extended one-layer model of bottom contamination of multi-fraction sediments was derived from a general model and compared with a multilayer model. It was found, however, that the one-layer approximation was not capable of correctly predicting the inventory due to the fact that one-layer averaged concentration can essentially differ from the near-surface value in the multi-layer model. Radionuclide 123Environ Fluid Mech (2017) 17:1207-1231 DOI 10.1007 transport in channel with bottom depression was simulated to estimate the effects of erosion/deposition and the multi-fractionality of sediments on the transport process. It was shown that these factors affect the distribution of sediments by forming local maxima and minima of activity at the beginning and end of the depression, respectively, due to the redistribution of contaminated bottom sediments by flow. The developed model can also be used to simulate the transport of a wide class of toxic substances sorbed on sediments.

Dispersion of Particle‐Reactive Elements Caused by the Phase Transitions in Scavenging

“…The settling velocities for these particle aggregates and corresponding distribution coefficients depend on size in a complex way, in contrast to lithogenic particles for which simple semiempirical relations were obtained (Periáñez et al., 2019). Further studies should include processes in the bottom sediments (Maderich et al., 2017) and the interaction of contamination in the bottom layer and water column that can be responsible for the increase in concentration in the near‐bottom layers of the ocean (Livingston et al., 2001).…”

Temporal trends of 137Cs concentration in seawaters and bottom sediments in coastal waters around Japan: implications for the Kd concept in the dynamic marine environment

“…Application of average values to these models was one reason to avoid complexity. In addition, diagenetic processes such as a two-step process including fast and slow reversible adsorption-desorption in sediments and vertical migration of 137 Cs as particles or a dissolved form in the pore space were not taken into consideration [e.g., 29,30]. However, the conclusions drawn from the simple pre-and post-accident models based on geometric means in a wide area should hold in general; they are: (1) even before the Fukushima accident the water-sediment system with respect to 137 Cs had been in a transient state with a temporally changing K d approaching K d∞ ; and (2) once significant amounts of 137 Cs were added to the system, the system shifted to a newly established transient state for the K d(a) values approaching a higher K d∞ value.…”

The POSEIDON-R compartment model for the prediction of transport and fate of radionuclides in the marine environment

“…The POSEIDON-R equations are obtained by averaging the three dimensional transport equations (see [8]) for the dissolved radionuclide concentration in the water column and the concentration in three layers of the bottom sediment assuming that partitioning between dissolved and particulate fractions is described by a distribution coefficient K d (m 3 t −1 ). The equations for the water column layers, upper and middle sediment layer read as follows:∂C0ik∂t=∑m∑jFjimkV0ikC0jm−FijkmV0jmC0ik+γ0ikC0(i+1)k−(γ1ik+λ)C0ik+Lt,kh1kγ2kC1k+Qsik,∂C1k∂t=−(γ2k+γ3k+λ)C<...>…”