Modelling cotransport of colloids and tracers in surficial sediments: applications to fallout radionuclides

Interpreting the depth distributions of radionuclides and other tracers in recent sediments is a key point in many environmental studies. There are cases of large penetration depths that cannot be explained by the kinetic reactive transport of the tracers in the dissolved phase. The cotransport of colloids and tracers has been studied in the context of column experiments, with a focus on the effluent. This work aims to develop a Lagrangian model for the cotransport of colloids and tracers in natural aquatic sediments, with a focus on their retention and the resulting depth distribution patterns. It makes plausible estimates of the magnitudes involved and studies their roles using real data from a variety of sedimentary environments. The retention coefficient for colloids is orders of magnitude lower than direct kinetic coefficients for the liquid-solid uptake and it co-varies with the dispersive transport, resulting in large penetration depths in times ranging from a few hours to a few months. The above depth distributions are used in a model of composite fluxes that operates at a centennial scale, as illustrated with fallout radionuclides using real core data. This can explain flattening and subsurface maxima in ²¹⁰Pb_exc profiles, and large penetration tails of high-k_d radionuclides that, when ignored, can lead to misuses of the ²¹⁰Pb dating models. In these examples, the colloidal bound fraction of the tracers ranged from 20 % to 40 %. These percentages are reliable in many aquatic environments, as supported by data from the literature.