Level and origin of 129I and 137Cs in lichen samples (Cladonia alpestris) in central Sweden

Lichen is a symbiosis between algae and fungi. They have for decades been used as bioindicators for atmospheric deposition of heavy metals, organic compounds and radioactive elements. Especially the species Cladonia alpestris and Cladonia rangiferina are important for the food chain lichen–reindeer–man.The concentration of 129I was determined in lichen samples (Cladonia alpestris) contaminated by fallout from atmospheric nuclear tests explosions and the Chernobyl accident. The samples were collected at Lake Rogen District (62.3°N, 12.4°E) in central Sweden in the periods 1961–1975 and 1987–1998, and analysed with accelerator mass spectrometry (AMS) at CNA (Seville) to study its distribution in different layers. Data on the 137Cs activity measured previously were also included in this study. The 129I concentration ranged from (0.95 ± 0.13) × 108 at g−1 in 1961 in the uppermost layer to (14.2 ± 0.5) × 108 at g−1 in 1987 in deepest layer. The 129I/137Cs atom ratio ranged between 0.12 and 0.27 for lichen samples collected in the period 1961–1975, indicating weapons tests fallout. For lichen samples collected between 1987 and 1998 the behaviour of 137Cs concentrations reflected Chernobyl fallout. The concentrations of the two radionuclides followed each other quite well in the profile, reflecting the same origin for both.From the point of view of the spatial distribution in the lichen, it appears that 129I was predominantly accumulated in the lowest layer, the opposite to 137Cs for which the highest amounts were detected systematically in the topmost layer of lichen. This vertical distribution is important for radioecology because lichen is the initial link in the food chain lichen–reindeer–man, and reindeer only graze the upper parts of lichen carpets.

Research highlights
► This work shows the results for 129I in lichens collected in 1961–1975 and 1987–1998 reflecting two different sources: fallout from nuclear tests and the Chernobyl accident.
► 129I has been found to be highest concentrated in the lower part of the lichens in contrast to 137Cs, for which the highest concentrations are in the top layer.
► The concentrations of 129I calculated for the different layers ranged from (0.95 ± 0.13) × 108 at g−1 in 1961 at the top layer to (14.2 ± 0.5) × 108 at g−1 in 1987 at the internal layer.
► The 129I/137Cs atomic ratio ranged between 0.11 and 0.25 for lichen samples collected in the period 1961–1975, indicating weapons tests fallout. For lichen samples collected between 1987 and 1998 the behaviour of 137Cs concentrations reflected Chernobyl fallout. The concentrations of the two radionuclides followed each other quite well in the profile, reflecting the same origin for both.