チェルノブイリの森林土壌におけるセシウム沈着量の時間変化。ただし、半減期で減らなかったと仮定した場合の補正後の値。実際には物理学的半減期、Cs134なら2年で半減に注意。Cs134の放射能の面密度の場合(Cs137も換算可能）。森林土壌では、最初は周辺の草地に比べて放射能が弱い。しかしその後、事故後600日までは徐々に増え続け周囲と同程度になる。森林土壌と、森林上冠（林冠とか樹冠と呼ばれる）の合計は一定とされる。土壌で徐々に増える原因は、この論文では、塵芥類？の落下（図中のlitter fall）の効果は小さいが、天候の影響（風雨など）で針葉や小枝から放射性物質がはげ落ちる（図中のdeposition on forest floor）効果の影響と評価される。ただし森林の種類にも依存し、日本の場合に同じとは限らず再確認が必要と思われる。しかし考え方は参考になるかもしれない。参考資料はK. Bunzl, W. Schimmack, K. Kreutzer, R. Schierl, "Interception and retention of chernobyl-derived 134Cs, 137Cs and 106Ru in a spruce stand", Science of The Total Environment, Volume 78, January 1989, Pages 77-87. この図自体は文科省土壌採取プロジェクトの配布資料（公開資料）から。＊＊＊上記論文の要旨部分： Abstract The time dependence of the specific activity of Chernobyl-derived 134Cs, 137Cs and 106Ru was determined in vegetation and soil samples from an old spruce stand within a period of 600 days after the beginning of the radioactive fallout. The results show that 70% of the total activity of radiocesium and 60% of radioruthenium deposited in the spruce stand were retained intinially in the canopy. They were removed from the needles and twigs as a result of weathering (rain, wind, litter fall) and transferred to the forest floor, but only rather slowly (half-lives in the canopy: radiocesium, 90 days for the period 0–130 days, 230 days for the period 130–600 days; radioruthenium, 95 days for the period 0–200 days). The transfer of radiocesium and ruthenium to the forest floor by litter-fall was small when compared with that of weathering by rain or wind (radiocesium 7%, radioruthenium 8%, with respect to the total activity deposited in the canopy). The total deposition of radiocesium and ruthenium in the spruce stand was higher by 20 and 24%, respectively, than that observed in nearby grassland. The deposition velocity of radiocesium in the spruce stand was estimated at 5.5 mm s−1, higher by a factor of 10 than the figure for grassland. Similar values were found for radioruthenium. http://bit.ly/oISKY8