: field monitoring and modelling

Abstract

International audienceCurrent ¹³⁷Cs levels in French forests are mainly due to global fallouts (GF) from nuclear weapon tests and deposition from Chernobyl accident. Since Fukushima accident, there has been an increasing interest in the behavior of this persistent radionuclide in the soil-tree system at short and mid-term after its deposit (95%) with a weak contribution of the organic layers (<1%).Stand-averaged ¹³⁷Cs activity concentrations in tree organs vary by more than an order of magnitude in the 3 stands, from typically ~0.1 to ~4 Bq/kg dw. The pattern is similar to that observed at short-term after Chernobyl and Fukushima accidents: roots ≥ leaves/needles = twigs ≥ bark ≥ branches = stemwood. In HET and CPS, ¹³⁷Cs and ¹³³Cs concentrations were linearly related (r≥0.9; p<0.001), suggesting a reached equilibrium. Whatever the isotope, the variability observed between individual trees is not correlated to tree characteristics (tree age, DBH or height) and remains under investigation at this stage.Normalized ¹³⁷Cs activity concentrations in stemwood (NC, m²/kg) of beech trees strongly vary between sites, from ~0.7 10^-4 m²/kg at HET site to ~5.1 10^-4 m²/kg at PS site. Such a variability could not be explained by differences in the ¹³⁷Cs soil exchangeable fraction. NC in stemwood of oaks and Scots pines are lower than those of companying beeches (1.1 10^-4 and 0.8 10^-4 m²/kg, respectively). Our results are 1 order of magnitude lower than those observed on the mid-term after Chernobyl, likely indicating a long-term decrease of ¹³⁷Cs bioavailability in soil.Investigations on potassium behavior, estimation of BGC fluxes for ¹³⁷Cs, ¹³³Cs and K, as well as on the selectivity coefficients for transfer processes, are under progress