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

Abstract 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 238 Pu, 239,240 Pu, 241 Am, 242 Cm and 243,244 Cm were 5.7, 3.0, 1.3, 98 and 0.025 Bq m −2 , respectively. The amount of deposited 239,240 Pu was, however, only some percent of the fallout of the nuclear test explosions of the 1950s and the 1960s. Instead, practically no 242 Cm was released into the environment during the weapons' testing. The correlation between the refractory nuclides 238 Pu, 239,240 Pu, 241 Am, 242 Cm, 95 Zr and 144 Ce in lichen was high ( r = 0.709–0.979), but the correlation between the transuranium elements and volatile 137 Cs 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 238 Pu 239,240 Pu 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 238 Pu 239,240 Pu 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 239,240 Pu deposition values are also in good agreement with values obtained from regional 95 Zr deposition and 239,240 Pu 95 Zr activity ratio data ( r = 0.92).