Evaluation of the deposition of ¹³⁷Cs in Japanese persimmon trees and yuzu trees from rainfall by collecting raindrops with sphagnum pads

The radiocaesium emitted due to the Tokyo Electric Power Fukushima Daiichi Nuclear Power Plant accident, initially migrated into fruit trees via the above-ground part of the trees. The behavior of the intercepted ¹³⁷Cs was evaluated for 6 persimmon and 22 yuzu trees. Radiocaesium can be deposited as dry or wet sediment. To measure the amount of ¹³⁷Cs in stemflow and raindrops, a collection pad comprised of sphagnum was used. This was attached to trees in a persimmon orchard and several yuzu orchards over 3 years, starting in 2016. Additionally, the effect of bark-washing on persimmon was examined by measuring the amount of ¹³⁷Cs trapped by the sphagnum pads on the leaf, calyx and main trunk following rainfall. The amount of ¹³⁷Cs mm^‑1 of precipitation retained by leaves was highest in 2016 in both types of fruit tree and decreased with time. The highest ¹³⁷Cs interception was detected on the same individual trees in 2016 and again in 2017. In the yuzu orchard in 2018, a higher concentration of ¹³⁷Cs in fruit than that in previous year were detected in the tree with more ¹³⁷Cs collected on the leaves than that in the previous year. This indicates the increase in the concentration of ¹³⁷Cs in fruit also depended on ¹³⁷Cs contamination on the leaves. These findings demonstrated that it is possible to identify trees which intercepted higher levels of ¹³⁷Cs during rainfall using sphagnum pads. Therefore those trees are most likely to have more contaminated leaves or fruits.