Lichen (sp. Cladonia) as a deposition indicator for transuranium elements investigated with the Chernobyl fallout

The feasibility of employing carpet-forming lichens (sp. Cladonia) as a measure for the deposition of transuranium elements was investigated with the Chernobyl fallout. In Finland, the deposition of these elements after the accident was very uneven. The highest deposition values for 238Pu, 239,240Pu, 241Am, 242Cm and 243,244Cm were 5.7, 3.0, 1.3, 98 and 0.025 Bq m−2, respectively. The amount of deposited 239,240Pu was, however, only some percent of the fallout of the nuclear test explosions of the 1950s and the 1960s. Instead, practically no 242Cm was released into the environment during the weaponsʹ testing. The correlation between the refractory nuclides 238Pu, 239,240Pu, 241Am, 242Cm, 95Zr and 144Ce in lichen was high (r = 0.709–0.979), but the correlation between the transuranium elements and volatile137Cs was much lower (r = 0.227–0.276). The calculated biological half-lives of Pu and Am in lichen were 730 and 320 days, respectively. The 238Pu/239,240Pu activity ratio in the top parts of lichen samples was 0.54 ± 0.02 corresponding to a burn-up value of 12 M Wd kg−1. The 238Pu/239,240Pu activity ratios of 0.43 and 0.69 measured from two isolated hot particles correspond to burn-up values of 10 and 14 MWdkg−1, respectively. The deposition values for plutonium obtained with lichen-carpet samples are comparable to the values obtained with surface peat samples collected in May 1986. The 239,240Pu deposition values are also in good agreement with values obtained from regional 95Zr deposition and 239,240Pu/95Zr activity ratio data (r = 0.92).