Accumulation and variation of the natural radioactivity (Be-7, Pb-210 and K-40) in leaf samples

Abstract

The activities of the natural radionuclides Be-7 (half-life 53.29 d), Pb-210 (half-life 22.3 y) and K-40 (half-life 1.27 x 10(exp 9) y) in leaf samples of eight plants were measured. The leaf samples, Akagashi, Fuki, Kumazasa, Hirohatsuyagoke, Inutsuge, Kuromatsu and Hikagenokazura, were monthly collected at Mt Sefuri in Fukuoka Prefecture. Radioactivities of were determined by a gamma-ray spectrometry. The activities of these three natural radionuclides in the leaf samples were compared with the activities of these radionuclides in total deposition and aerosol samples. The total deposition and aerosol were collected at two sampling sites, Kyushu University (Fukuoka) and Mt. Sefuri. The concentrations of some elements in the leaf samples were determined by a neutron activation analysis. The activities of Be-7 and Pb-210 were generally high in spring (April/May) and low in summer (June to October) and then increase in winter in some leaf samples. Especially the activity of K-40 in Fuki and Kumazasa showed clear seasonal variation; decreased from May through next March. Seasonal variations of the activities of Be-7 and Pb-210 in the total depositions showed a similar pattern in Fukuoka and Mt. Sefuri; generally high in spring (March to May) and in winter (December to February). No seasonal variation of the activities of Be-7 and Pb-210 in aerosol samples, especially which in Mt. Sefuri was showed, due to limited data and limited sampling duration. The ability of leaf samples to retain the natural radionuclides was observed. In vascular plants, the highest capability of the leaf to absorb/retain Be-7 and Pb-210 is observed in Kuromatsu. This observation suggests Kuromatsu is the best as a biomonitor in these eight plants. Correlation of radionuclides in the leaf samples showed good correlation between Pb-210 and Be-7, lower and/or negative correlation between Pb-210 and K-40. The difference of correlation is attributed to the difference of the sources, where Pb-210 and Be-7 are originated from aerosol and K-40 is originally contained in leaf samples as an essential element and absorbed from soil through root of plants. The lower and/or negative correlations between elements and Pb-210 contained in the leaf samples indicates that these leaf samples can hardly be considered as reliable biomonitors of the atmospheric depositions of the trace metal.