Kinetic reactive transport explains distinct subsurface deposition patterns of pollutants in sediments. The case of the Sellafield-derived 236U, 137Cs and 239,240Pu in the Esk Estuary, UK.

The kinetics of the uptake of pollutants by solids in sediments interacts with transitional eddy diffusivity in the pore fluid, leading to different depth-distribution patterns. This work aims to gain insights into the still poorly understood behaviour in the marine environment of the anthropogenic ²³⁶U, a recently postulated tracer of water masses. It is hypothesized that the transition from mobile U(VI) to highly particle-reactive U(IV) in the anoxic zone of the sediment produces a subsurface deposition pattern. A novel numerical model for kinetic reactive transport in sediments, which merges diagenetic processes for transport and box models for the uptake, is used for concept demonstration. It is applied to synthetic environments with high eddy diffusivity to obtain the singular depth-distribution patterns of pulsed inputs of tracers that mimic the anthropogenic ^239,240Pu, ¹³⁷Cs, and ²³⁶U. While the first is retained in the upper cm, the second shows an exponential penetration pattern over few cm, and ²³⁶U is deposited with a Gaussian-like pattern centred below the transition to the anoxic zone. These patterns are then merged into a diagenetic model to compute the depth distribution at decadal or centennial scales of dissolved and particle-bound inputs of these radiotracers. It is successfully applied to a real case using literature data for a sediment core from the Esk Estuary, UK, affected by radioactive releases from the Sellafield nuclear reprocessing plant. This work provides insight into until now poorly understood field data and provides a novel view of broad implications in the study of the behaviour of pollutants in surficial aquatic sediments. [Display omitted] • Pore-fluid eddy diffusivity and pollutant uptake kinetics interplay in sediments. • Numerical model shows distinct subsurface deposition patterns for Pu, Cs and U. • TransItion U(VI) to insoluble U(IV) leads to maximum deposition in the anoxic zone. • ^239,240Pu, ¹³⁷Cs and ²³⁶U profiles in a sediment from Esk Estuary were explained. [ABSTRACT FROM AUTHOR]