Tracing radioactive cesium in stem wood of three Japanese conifer species 3 years after the Fukushima Dai-ichi Nuclear Power Plant accident

Abstract

To understand the dynamics of accident-derived radioactive cesium (137 Cs) in stem wood that had a substantial amount of heartwood at the time of the Fukushima Dai-ichi Nuclear Power Plant accident, the radial and vertical distributions of 137 Cs activity concentration in stem wood of Japanese cedar (Cryptomeria japonica), cypress (Chamaecyparis obtusa), and larch (Larix kaempferi) were investigated. In addition, the natural distribution of stable cesium (133 Cs), rubidium (85 Rb), and potassium (39 K) concentrations was analyzed to determine the characteristics of 137 Cs distribution. Wood disks were collected from the tree stems of six cedars, three cypresses, and two larches at multiple heights in 2014, and the concentrations were measured every 2 cm in the radial direction. 137 Cs distribution in stem wood differed among tree species, sampling site, and vertical position of the stem within a tree. Statistical analyses suggested that the radial distribution of 137 Cs within the heartwood can be explained by the heartwood moisture content and the distance from the treetop, regardless of species, while the distribution between sapwood and heartwood was dependent on the heartwood cross-sectional area and was additionally different between larch and other species. Similarly, the heartwood/sapwood concentration ratios of stable alkali metals differed between larch and the other species. In the larch, the ratio was ca. 0.5 for all elements, but the ratio was over 1.0 and differed among elements in the other species. Consequently, the species-specific difference in the distribution of 137 Cs between sapwood and heartwood was considered to be due to different activity levels of radial transport toward the heartwood. The radial variation pattern of the 137 Cs/ 133 Cs concentration ratio showed that less 137 Cs was transferred to the inner heartwood compared with the 133 Cs distribution pattern in many trees; however, there was also a tree in which 137 Cs was excessively transferred to the inner heartwood compared with the 133 Cs distribution pattern. Such patterns may result from a combination of significant foliar uptake of 137 Cs and poor root uptake after the accident, in addition to the high moisture content of the heartwood.

Origin and hydrodynamics of xylem sap in tree stems, and relationship to root uptake of soil water

“…Although we observed a positive δD signal in the heartwood sap only in sugi reference tree S2, the changes in the tracer D 2 O concentration in the heartwood sap were slower and more irregular than those in the sapwood sap. Based on these results, the heartwood sap appears to be less mobile than the sapwood sap, and the heterogeneous distribution of the D 2 O tracer concentration was consistent with the 137 Cs distribution observed in the radial stem xylem 9 . In contrast to the sudden disappearance of the positive δD signal in the sapwood sap after 2 months, the positive δD signal in the heartwood sap gradually faded over 5 months (Fig.…”

“…If sugi and konara trees do not take up only near-surface water (to 5-cm depth), this would sharply decrease the radiation risk created by root uptake of ionic 137 Cs + together with soil water. Because more than 95% of the 137 Cs in fallout from the Fukushima nuclear accident has accumulated in the top 5 cm of the forest surface soil 1 – 9 , this would reduce the contamination risk for tree xylem.…”

“…We first reported that 137 Cs was absorbed into the xylem of trees through the bark and leaves by direct atmospheric uptake during the early stage of forest contamination 1 by contact with airborne materials. The latest field observations of 137 Cs distribution 2 – 9 show that the contamination of the xylem stem has gradually changed from decreasing in sapwood to increasing in heartwood in the forest environment of Fukushima and neighboring Tochigi and Ibaraki prefectures, with small differences among tree species and sites. Only 5 years after the accident 2 , 3 , 9 , the total inventory of 137 Cs deposited in forests has been partitioned among the soil (accounting for 81–91% of the total), litter on the ground surface (accounting for 6–18%); bark, branches, and leaves (accounting for 1–4%); and xylem (accounting for 0.1–2%).…”

“…The latest field observations of 137 Cs distribution 2 – 9 show that the contamination of the xylem stem has gradually changed from decreasing in sapwood to increasing in heartwood in the forest environment of Fukushima and neighboring Tochigi and Ibaraki prefectures, with small differences among tree species and sites. Only 5 years after the accident 2 , 3 , 9 , the total inventory of 137 Cs deposited in forests has been partitioned among the soil (accounting for 81–91% of the total), litter on the ground surface (accounting for 6–18%); bark, branches, and leaves (accounting for 1–4%); and xylem (accounting for 0.1–2%). In the soil reservoir, 95% of the 137 Cs has been stored in the top 5 cm, and the remaining 5% decreased exponentially to a depth of 20 cm.…”

“…The 137 Cs concentration in the heartwood of most sugi ( Cryptomeria japonica ) in Fukushima has exceeded that in sapwood, at least until 2016 2 , 3 , 5 – 7 , 9 . However, xylem contamination has been approaching a steady state 2 , 5 – 7 .…”