Jun Koarashi

Factors affecting vertical distribution of Fukushima accident-derived radiocesium in soil under different land-use conditions

The Fukushima Dai-ichi nuclear power plant accident in Japan, triggered by a big earthquake and the resulting tsunami on 11 March 2011, caused a substantial release of radiocesium ((137)Cs and (134)Cs) and a subsequent contamination of soils in a range of terrestrial ecosystems. Identifying factors and processes affecting radiocesium retention in these soils is essential to predict how the deposited radiocesium will migrate through the soil profile and to other biological components. We investigated vertical distributions of radiocesium and physicochemical properties in soils (to 20 cm depth) at 15 locations under different land-use types (croplands, grasslands, and forests) within a 2 km × 2 km mesh area in Fukushima city. The total (137)Cs inventory deposited onto and into soil was similar (58.4±9.6 kBq m(-2)) between the three different land-use types. However, aboveground litter layer at the forest sites and herbaceous vegetation at the non-forested sites contributed differently to the total (137)Cs inventory. At the forest sites, 50-91% of the total inventory was observed in the litter layer. The aboveground vegetation contribution was in contrast smaller (<35%) at the other sites. Another remarkable difference was found in vertical distribution of (137)Cs in mineral soil layers; (137)Cs penetrated deeper in the forest soil profiles than in the non-forested soil profiles. We quantified (137)Cs retention at surface soil layers, and showed that higher (137)Cs retention can be explained in part by larger amounts of silt- and clay-sized particles in the layers. More importantly, the (137)Cs retention highly and negatively correlated with soil organic carbon content divided by clay content across all land-use types. The results suggest that organic matter inhibits strong adsorption of (137)Cs on clay minerals in surface soil layers, and as a result affects the vertical distribution and thus the mobility of (137)Cs in soil, particularly in the forest ecosystems.

Comparison of the vertical distributions of Fukushima nuclear accident radiocesium in soil before and after the first rainy season, with physicochemical and mineralogical interpretations

Effect of intense rainfall on the distribution of Fukushima-accident-derived (137)Cs in soil was examined. Inventories and vertical distributions of (137)Cs in soils were determined at 15 locations (including croplands, grasslands, and forests) in Fukushima city in the post-rainy season, approximately 4.5months after the accident, and were compared with those in the pre-rainy season determined in our former study. The (137)Cs inventory levels scarcely changed between points in time spanning the first rainy season after the accident. Moreover, the majority of (137)Cs remained stored in the aboveground vegetation and in the upper 5cm of soil layer at undisturbed locations in the post-rainy season. A more quantitative analysis with the characterization of the vertical profile of (137)Cs using the relaxation length confirmed that the vertical profile was almost unchanged at most locations. Accordingly, it is concluded that rainfall during the rainy season had a limited effect on (137)Cs distribution in the soil, indicating the very low mobility. Chemical extraction of (137)Cs from selected soil samples indicated that (137)Cs in the soil was barely water soluble, and even the fraction extracted with 1M ammonium acetate was only approximately 10%. This further supports the low mobility of (137)Cs in our soils. Soil mineralogical analyses, which included the identification of clay minerals, suggested that smectite and mica could lower the exchangeable fraction of (137)Cs. However, no direct relationship was obtained between mineral composition and (137)Cs retention in the upper soil layer. In contrast, positive correlations were observed between (137)Cs extractability and soil properties such as pH, organic matter content, finer-sized particle content, and cation-exchange capacity. These results suggest that the mineralogical effect on the firm fixation of (137)Cs on soil constituents may be masked by the non-specific adsorption offered by the physicochemical properties of the soils.

Dynamics of decadally cycling carbon in subsurface soils

Subsurface horizons contain more than half of the global soil carbon (C), yet the dynamics of this C remains poorly understood. We estimated the amount of decadally cycling subsurface C (∼20 to 60 cm depth) from the incorporation of ‘bomb’ radiocarbon (14C) using samples taken over 50 years from grassland and forest soils in the Sierra Nevada Mountains, California. The radiocarbon content of all organic matter fractions (roots, low-density (LF), high-density (HF), and non-oxidizable HF) increased from the pre- to post-bomb samples, indicating ∼1–6 kgC m−2, or about half of the subsoil C, consists of C fixed since 1963. Low-density (LF-C) represented 20 years) for the arrival of ‘bomb’14C to this fraction. A two-pool (fast-cycling and passive) model including >20 year time lag showed that 28–73% of the subsoil mineral-associated C had turnover times of 10–95 years. Microbially respired C was enriched in bomb14C compared to both LF and HF fractions in 2009. Overall, we estimate that C fluxes through decadally cycling pools in the subsurface are equivalent to 1–9% (grassland) to 10–54% (forest) of the surface litterfall at these sites. Our results demonstrate the importance of decadally cycling C for ecosystem C balance, and that a lagged response of the large subsurface C stores to changes in environmental conditions is possible.

Retention of potentially mobile radiocesium in forest surface soils affected by the Fukushima nuclear accident

The fate of 137Cs derived from the Fukushima nuclear accident fallout and associated radiological hazards are largely dependent on its mobility in the surface soils of forest ecosystems. Thus, we quantified microbial and adsorptive retentions of 137Cs in forest surface (0–3 cm) soils. The K2SO4 extraction process liberated 2.1%–12.8% of the total 137Cs from the soils. Two soils with a higher content of clay- and silt-sized particles, organic carbon content, and cation exchange capacity showed higher 137Cs extractability. Microbial biomass was observed in all of the soils. However, the 137Cs extractability did not increase after destruction of the microbial biomass by chloroform fumigation, providing no evidence for microbial retention of the Fukushima-fallout 137Cs. The results indicate that uptake of 137Cs by soil microorganisms is less important for retention of potentially mobile 137Cs in the forest surface soils compared to ion-exchange adsorption on non-specific sites provided by abiotic components.

Topographic heterogeneity effect on the accumulation of Fukushima-derived radiocesium on forest floor driven by biologically mediated processes

The accident at the Fukushima Daiichi nuclear power plant caused serious radiocesium (137Cs) contamination of forest ecosystems located in mountainous and hilly regions with steep terrain. To understand topographic effects on the redistribution and accumulation of 137Cs on forest floor, we investigated the distribution of Fukushima-derived 137Cs in forest-floor litter layers on a steep hillslope in a Japanese deciduous forest in August 2013 (29 months after the accident). Both leaf-litter materials and litter-associated 137Cs were accumulated in large amounts at the bottom of the hillslope. At the bottom, a significant fraction (65%) of the 137Cs inventory was observed to be associated with newly shed and less degraded leaf-litter materials, with estimated mean ages of 0.5–1.5 years, added via litterfall after the accident. Newly emerged leaves were contaminated with Fukushima-derived 137Cs in May 2011 (two months after the accident) and 137Cs concentration in them decreased with time. However, the concentrations were still two orders of magnitude higher than the pre-accident level in 2013 and 2014. These observations are the first to show that 137Cs redistribution on a forested hillslope is strongly controlled by biologically mediated processes and continues to supply 137Cs to the bottom via litterfall at a reduced rate.

Estimation of 14CO2 flux at soil-atmosphere interface and distribution of 14C in forest ecosystem

To realize the dynamical behavior of 14C among exchangeable carbon reservoirs in terrestrial environment, a method for in situ determination of 14CO2 flux at soil-atmosphere interface and a high flow rate CO2 sampler were developed. This method allowed us to collect integrated quantity of CO2 for determining 14C activity over an extended time period under environmental conditions with minimal site disturbance. The 14CO2 flux from ground surface was estimated to be 1.59 x 10(-5) Bq m (-2) S (-1) in a forest floor with the method. The specific activities of 14C in environmental materials such as some biological and air samples were also determined in the vicinity of the place, where the flux measurement was made, to discuss the behavior of 14C in the forest ecosystem. The results indicated that fresh pine needles had a similar 14C specific activity to the atmospheric CO2 at the same height due to its fairly rapid equilibrium, 14C specific activity in the atmospheric CO2 has a concentration gradient near the ground surface and, at least in this site, CO2 with high 14C specific activity was generated by decomposition of soil organic matter which may be accumulated in soil as a result of former nuclear weapons tests.

Carbon-14 transfer into rice plants from a continuous atmospheric source: observations and model predictions

Carbon-14 (14C) is one of the most important radionuclides from the perspective of dose estimation due to the nuclear fuel cycle. Ten years of monitoring data on 14C in airborne emissions, in atmospheric CO2 and in rice grain collected around the Tokai reprocessing plant (TRP) showed an insignificant radiological effect of the TRP-derived 14C on the public, but suggested a minor contribution of the TRP-derived 14C to atmospheric 14C concentrations, and an influence on 14C concentrations in rice grain at harvest. This paper also summarizes a modelling exercise (the so-called rice scenario of the IAEAs EMRAS program) in which 14C concentrations in air and rice predicted with various models using information on 14C discharge rates, meteorological conditions and so on were compared with observed concentrations. The modelling results showed that simple Gaussian plume models with different assumptions predict monthly averaged 14C concentrations in air well, even for near-field receptors, and also that specific activity and dynamic models were equally good for the prediction of inter-annual changes in 14C concentrations in rice grain. The scenario, however, offered little opportunity for comparing the predictive capabilities of these two types of models because the scenario involved a near-chronic release to the atmosphere. A scenario based on an episodic release and short-term, time-dependent observations is needed to establish the overall confidence in the predictions of environmental 14C models.

Modeling dynamics of (137)Cs in forest surface environments: Application to a contaminated forest site near Fukushima and assessment of potential impacts of soil organic matter interactions.

A process-based model for (137)Cs transfer in forest surface environments was developed to assess the dynamic behavior of Fukushima-derived (137)Cs in a Japanese forest. The model simulation successfully reproduced the observed data from 3year migration of (137)Cs in the organic and mineral soil layers at a contaminated forest near Fukushima. The migration of (137)Cs from the organic layer to the mineral soil was explained by the direct deposition pattern on the forest floor and the turnover of litter materials in the organic layer under certain ecological conditions. Long-term predictions indicated that more than 90% of the deposited (137)Cs would remain within the top 5cm of the soil for up to 30years after the accident, suggesting that the forest acts as an effective long-term reservoir of (137)Cs with limited transfer via the groundwater pathway. The model was also used to explore the potential impacts of soil organic matter (SOM) interactions on the mobility and bioavailability of (137)Cs in the soil-plant system. The simulation results for hypothetical organic soils with modified parameters of (137)Cs turnover revealed that the SOM-induced reduction of (137)Cs adsorption elevates the fraction of dissolved (137)Cs in the soil solution, thereby increasing the soil-to-plant transfer of (137)Cs without substantially altering the fractional distribution of (137)Cs in the soil. Slower fixation of (137)Cs on the flayed edge site of clay minerals and enhanced mobilization of the clay-fixed (137)Cs in organic-rich soils also appeared to elevate the soil-to-plant transfer of (137)Cs by increasing the fraction of the soil-adsorbed (exchangeable) (137)Cs. A substantial proportion (approximate 30%-60%) of (137)Cs in these organic-rich soils was transferred to layers deeper than 5cm decades later. These results suggested that SOM influences the behavior of (137)Cs in forests over a prolonged period through alterations of adsorption and fixation in the soil.

Importance of root HTO uptake in controlling land-surface tritium dynamics after an-acute HT deposition: a numerical experiment.

To investigate the role of belowground root uptake of tritiated water (HTO) in controlling land-surface tritium (T) dynamics, a sophisticated numerical model predicting tritium behavior in an atmosphere-vegetation-soil system was developed, and numerical experiments were conducted using the model. The developed model covered physical tritiated hydrogen (HT) transport in a multilayered atmosphere and soil, as well as microbial oxidation of HT to HTO in the soil, and it was incorporated into a well-established HTO-transfer organically bound tritium (OBT)-formation model. The model performance was tested through the simulation of an existing HT-release experiment. Numerical experiments involving a hypothetical acute HT exposure to a grassland field with a range of rooting depths showed that the HTO release from the leaves to the atmosphere, driven by the root uptake of the deposited HTO, can exceed the HTO evaporation from the ground surface to the atmosphere when root water absorption preferentially occurs beneath the ground surface. Such enhanced soil-leaf-atmosphere HTO transport, caused by the enhanced root HTO uptake, increased HTO concentrations in both the surface atmosphere and in the cellular water of the leaf. Consequently, leaf OBT assimilation calculated for shallow rooting depths increased by nearly an order of magnitude compared to that for large rooting depths.

Monitoring of Low-Level Radioactive Liquid Effluent in Tokai Reprocessing Plant

The Tokai reprocessing plant (TRP), the first reprocessing plant in Japan, has discharged low-level liquid wastes to the Pacific Ocean since the start of its operation in 1977. We have performed liquid effluent monitoring to realize an appropriate radioactive discharge control. Comparing simple and rapid analytical methods with labor-intensive radiochemical analyses demonstrated that the gross-alpha and gross-beta activities agreed well with the total activities of plutonium isotopes (238Pu and 239+240Pu) and major beta emitters (e.g., 90Sr and 137Cs), respectively. The records of the radioactive liquid discharge from the TRP showed that the normalized discharges of all nuclides, except for 3H, were three or four orders of magnitude lower than those from the Sellafield and La Hague reprocessing plants. This was probably due to the installation of multistage evaporators in the liquid waste treatment process in 1980. The annual public doses for a hypothetical person were estimated to be less than 0.2 μSv y−1 from the aquatic pathway. Plutonium radioactivity ratios (238Pu/239+240Pu) of liquid effluents were determined to be 1.3–3.7, while those of the seabed sediment samples collected around the discharge point were 0.003–0.059, indicating no remarkable accumulation of plutonium in the regional aquatic environment. Thus, we concluded that there were no significant radiological effects on the public and the aquatic environment during the past 30-year operation of the TRP.