Distribution and behaviour of naturally occurring radionuclides within a Scots pine forest grown on a CaF2 waste deposit related to the Belgian phosphate industry

Highlights: • Increased U and Pb solid-liquid distribution through acidification of soil. • Seasonal variability ²²⁶Ra:²¹⁰Pb ratio in soil, possible link with radon exhalation. • Tree roots serve as translocation barrier. • ²¹⁰Pb and ²¹⁰Po present in bark, needles and twigs. • Role for mosses as indicators for long-term exposure to ²³⁸U and its progeny. The distribution and behaviour of naturally occurring radionuclides within a vegetated part of a CaF₂ sludge heap from the Belgian phosphate industry was studied. A Scots pine forest plot was selected as study area. Trees were approximately 20 years old and showed a disturbed health state. Seasonal sampling campaigns of soil, roots, wood, inner and outer bark, needles and twigs gave insight on ²³⁸U, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po transfer and distribution between pine tree compartments. Soil samples were analysed for their texture, total organic and inorganic carbon, field capacity, pH and radionuclide content. Solid-liquid distribution coefficients (K_d) were experimentally determined for ²³⁸U, ²²⁶Ra (using Ba as analogue) and ²¹⁰Pb based on adsorption-desorption batch tests. Results indicated higher ²³⁸U, ²³²Th, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po activity concentrations in the deeper soil layers while the first 20 cm contained less radionuclides but had a higher level of organic carbon. Additionally, results indicated no seasonal changes in the ²³⁸U:²²⁶Ra ratio in the soil while the ²²⁶Ra:²¹⁰Pb ratio was significantly higher in spring compared to winter in the 20–60 cm soil layer. Pine tree roots served as natural translocation barrier for all radionuclides with high retention in the roots and low translocation to the above ground tree compartments. When considering the above ground compartments, ²¹⁰Pb and ²¹⁰Po were mostly present in the bark, needles and twigs. Furthermore, ²³⁸U and its progeny were highly accumulated in mosses. These results allowed us to establish more realistic soil-to-plant transfer factors. In addition, experimentally mimicking pore water acidification in the root zone resulted in lower ²³⁸U and ²¹⁰Pb K_d values compared to using a standard CaCl₂ solution. This study provides an integrated radioecological picture of knowledge and site specific data needed to study the long-term influence of vegetation on radionuclide dispersion in forest ecosystems.