Modelling soil - Vegetation - Atmospheric interactions of radon products in a Belgian Scots pine forest site

A soil-vegetation-atmospheric transfer (SVAT) model for radon and its progeny is presented to improve process-level understanding of the role of forests in taking-up radionuclides from soil radon outgassing. A dynamic system of differential equations couples soil, tree (Scots pine) and atmospheric processes, treating the trees as sources, sinks and conduits between the atmosphere and the soil. The model's compartments include a dual-layer soil column undergoing hydrological and solute transport, the tree system (comprising roots, wood, litter, and foliage) and the atmosphere, with physical processes governing the transfers of water and radon products between these compartments. A dose post-processor calculates dose rates to the trees from internal, external, and surface radiation exposures. The model parameterisation is based on measurement data from the Grote Nete Valley in the Belgian Campine region. The model results suggest that the tree intake of radon progeny is principally from the atmosphere, whereas radium is mainly taken-up from soil by the root uptake process, leading to an additional fraction of ingrown radon progeny in the tree by this route. It is also suggested that atmospheric uptake of radon is an essential mechanism when evaluating the tree uptake of ²¹⁴Po, ²¹⁴Pb and ²¹⁸Po and subsequent decay products. The model also indicates a slow uptake of radionuclides by the tree roots, with timescales in the order of years, leading to different dose rates for young and mature trees. The importance of foliar surface deposition, leading to a dominance of surface doses to the tree needles, is also highlighted. These mechanisms, ignored in most assessment models, are necessary improvements for assessment tools dealing with the impact of radon and its progeny in forests, with regard to legacy sites with ²²⁶Ra contamination.