Redistribution and Transport of Radiocesium via Branchflow and Stemflow through the Coniferous and Deciduous Forests after the Fukushima Dai-ichi Nuclear Power Plant Accident

概要

Chapter 1: Introduction
　Interdependent hydrological and chemical processes interact with the forest environment in many ways resulting in complex pathways for atmospherically deposited pollutants. Hydrochemistry and chemical processes such as branchflow and stemflow chemistry and isotope exchange, all vary as a function of the forest stand characteristics, canopy structure, seasonality, and meteorological conditions. Vast amounts of radioactive substances were deposited across forest environments after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011, which created an opportunity to provide insight on the hydrological and chemical processes influence on the atmospherically deposited pollutant pathways (i.e., radiocesium) in forests.

Chapter 2: Methodology
　This study aimed to fill a need for data regarding radiocesium fluxes via hydrochemical processes and investigated the distribution, transport, and cycling dynamics of radiocesium in the Japanese cedar (Cryptomeria japonica (L. f.) D. Don) and Japanese oak (Quercus serrata Murray) forest stands contaminated by the atmospheric deposition. Furthermore, a stable isotopes approach (δ18O, δD and d-excess) and electrolytes (pH, electrical conductivity, cation and anion) were employed to help elucidate on how branchflow and stemflow hydrochemistry controls radiocesium processes: washoff (dissolution), leaching (elution) and absorption/sequestration (storage). By collecting branchflow precipitation from primary branches and stemflow from lower portion of canopy and trunk-based vertically, the cycling of radiocesium via branchflow and stemflow and how it was altered by both canopy height and by season were examined.

Chapter 3: Vertical Variation of Water and 137Cs Cycled via Branchflow and Stemflow
　The descending order for branchflow and stemflow volume in the cedar tree per collection throughout the study period was: upper stemflow (8.60 L) > lower stemflow (4.41 L) > younger foliage (0.18 L). The order for oak tree was: upper stemflow (23.78 L) > middle foliage (9.43 L) > lower stemflow (7.87 L). Meanwhile the descending order for vertical distribution of volume-weighted means of 137Cs concentrations in each tree compartment per collection throughout the study period in cedar tree was: dead foliage (1.97 Bq L-1) > upper stemflow (1.76 Bq L-1) > lower stemflow (1.43 Bq L-1). The order for oak tree was: lower stemflow (4.30 Bq L-1) > upper stemflow (3.39 Bq L-1) > middle foliage (1.90 Bq L-1). While water flowpaths through canopies are complex, these results might be partly attributed to the differences in flowpath routing between branchflow (shorter flowpath) and stemflow (longer flowpath) and their corresponding residence times. Significant variability of 137Cs concentration via branchflow and stemflow was detected among the seasons (the largest during winter) and differed between the two forests, with the oak tree exhibiting higher 137Cs concentrations than the cedar tree. The inherent differences were caused by tree morphology and biomass, and capture of radiocesium substances during initial deposition.

Chapter 4: 137Cs Cycling Process via Branchflow and Stemflow
　A larger stemflow amount was generated in the oak tree (13.51 mm) compared to cedar tree (5.44 mm) during the lowest rainfall days in winter 2018 (total precipitation depth: 16 mm), indicating that the oak tree generated more branchflow and stemflow than cedar tree. Expressed per unit trunk basal area, the annual depositional flux of 137Cs generated from the cedar and oak trees was 375 and 2,810 Bq m-2, respectively, with the oak stand was 7.5 times greater than cedar. Of this total, 71% and 48% originated from the cedar and oak canopy, respectively, while the remainder originated from the trunk. The substantial flux of 137Cs by tree canopy suggests enhancement by foliar translocation and leaching processes. The 137Cs depositional flux varied between the tree compartment for both trees, depending on tree phenology (leafed, leafless) and tree physiology (water storage capacity), and seasonality of precipitation (dry and rainy seasons). Meanwhile, huge disparities in the annual 137Cs flux between oak and cedar stands resulted from differences in tree morphology and biomass, and capture of radiocesium substances during initial fallout caused. The cedar tree was controlled by the sequestration process whereas oak tree by leaching process; both species were influenced by the interaction between intercepted rainwater and tree biomass (particularly foliage at the canopy and outer bark at the trunk) during the flowpath.

Chapter 5: Hydrochemistry Mechanism of Branchflow and Stemflow
　The oak tree was generally enriched in δ18O and δD compared to the cedar tree. Findings between 137Cs concentrations and d-excess revealed a different interaction process occurred during the flowpath, with the oak trees contributing to a higher 137Cs concentration with a wider range of d-excess magnitude than the cedar trees. This suggests mixing and/or differential residence time of branchflow and stemflow during the flowpath caused by different tree morphologies particularly when it reaches the tree trunk. Hence, stable isotopes can be a good indicator for the 137Cs leaching via branchflow or stemflow. Meanwhile, the enrichment of 137Cs concentration and the branchflow/stemflow volume generated was related to electrolytes (electrical conductivity, pH, temperature). Furthermore, in relation to the different dynamic of 137Cs, the dominant ions in branchflow and stemflow were K+, Mg2+ and Ca2+ with the uptake or absorption and leaching mechanisms and a proclivity to leach in the lower part of the tree trunk of cedar stands. As such, the same leaching (elution) process for radiocesium and electrolytes (electrical conductivity, cation and anion) from both tree species was observed suggesting that branchflow and stemflow hydrochemistry controlled the dissolution/leaching of radiocesium. However, the other ecological variables that may influence the 137Cs leaching via branchflow and stemflow remain unclear due to the intricacies of the physico-chemical dynamics operating within tree bark.

Chapter 6: Overall Conclusion and Future Work
　Given the above findings, this study provides more detailed insights into the vertical variation of radiocesium cycling and its hydrochemical mechanisms involved at the tree scale and intra-interspecies in severely contaminated forested ecosystems. Optimistically, it will provide needed data on the conceptualization of radiocesium distribution, cycling and inventory in forests, and consequently allow proactive strategies to be developed to rehabilitate forests, create more specific boundaries and timing for exclusion zones.