Takeshi Matsunaga

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.

Characteristics of humic substances in the Kuji River waters as determined by high-performance size exclusion chromatography with fluorescence detection

Direct measurement by high-performance size exclusion chromatography with fluorescence detection was applied to the characterization of humic substances in river waters from the Kuji River system, which runs through forest hills and an agricultural plain in Japan. The monitoring wavelength of excitation 320 nm and emission 430 nm corresponds to the fluorescence maxima for aquatic fulvic acid. Chromatograms of the river waters showed four peaks; each peak position was in good agreement among these samples. Peak height ratios for the samples from the upstream Kuji River and its tributaries were different from those of the midstream and downstream sections of the Kuji River, which may reflect differences in the characteristics of humic substances and other organic materials supplied from soil to river.

Discharge of dissolved and particulate137Cs in the Kuji River, Japan

Abstract The discharge of fallout137Cs in the water of the Kuji River was studied with the aim of understanding the migration behavior of137Cs in the river watershed. Under normal flow conditions, the radioactivity of dissolved137Cs in the water ranged from 0.05 to 0.12 mBq/l, while that of the particulate137Cs ranged from 0.02 to 0.09 mBq/l. A large increase in the particulate137Cs, up to 0.77 mBq/l, was observed under high flow conditions caused by a 40 mm precipitation event. It was found that the dominant speciation of137Cs changed from the dissolved to the particulate mode according to the flow conditions related to the precipitation event. The annual budget of137Cs in the watershed was estimated based on empirical relation between the load of137Cs and the flow rate. The accumulated137Cs in the watershed was evaluated as 4.2 × 103 MBq/km2. On the other hand, the annual losses were 1.7 MBq/km2 by the discharge into the river and 1 × 102 MBq/km2 by radioactive decay. This budget has proved, qualitatively, that the surface soil acts as a significant adsorbant for atmospherically-derived137Cs.

Characteristics of Chernobyl-derived radionuclides in particulate form in surface waters in the exclusion zone around the Chernobyl Nuclear Power Plant

Abstract The distribution of Chernobyl-derived radionuclides in river and lake water bodies at 6–40 km from the Chernobyl Nuclear Power Plant was studied. Current levels of radionuclides (Cesium-137, Strontium-90, Plutonium, Americium and Curium isotopes) in water bodies and their relation to the ground contamination are presented. The investigation of the radionuclide composition of aqueous and ground contamination revealed that radionuclides on suspended solids (particulate form) originate mainly from the erosion of the contaminated surface soil layer in the zone. Apparent distribution ratios between particulate and dissolved forms are compared to known distribution coefficients.

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.

The transfer capability of long-lived Chernobyl radionuclides from surface soil to river water in dissolved forms

Hydrologic runoff is one of the main processes in which radionuclides deposited in the surface environment migrate widely in both particulate and dissolved forms. This paper focuses on the transfer capability of long lived Chernobyl radionuclides from surface soil to river water in dissolved forms. First, concentration and speciation of radioactive Cs, Sr and transuranic isotopes, such as Pu and Am, were examined in undisturbed surface soil along the river in the exclusion zone (30 km zone) near the Chernobyl Nuclear Power Plant (NPP) in order to validate the radioactive contamination characteristics. Almost all radioactivities exist in the very top surface in the undisturbed soil layer. Sr-90 in the soil was estimated to be highest in the water soluble and exchangeable fractions, which were easily accessible to river water as a dissolved fraction. Pu isotopes and Am-241 are major radionuclides in free humic and free fulvic acid fractions. Secondly, surface soil near the Sahan River was extracted with distilled water, as an analogue of rain water, to estimate the dissolved fraction in runoff components from surface soil to river water. After a filtration procedure, extracted water was treated with ultra filtration techniques separating the molecular weight fractions of beyond and below 10,000 Da. Each fraction was measured for the radioactivity and the characteristics of organic materials including humic substances. Most Pu and Am exist in the molecular weight fractions beyond 10,000 Da, in spite of the fact that most of the dissolved organic fractions exist below 10,000 Da. This means that transuranic elements such as Pu and Am are associated with mobile high molecular weight materials like fulvic acids in water leachates.

Accumulation and potential dissolution of Chernobyl-derived radionuclides in river bottom sediment.

Areas contaminated with radionuclides from the Chernobyl nuclear accident have been identified in Pripyat River near the Chernobyl Nuclear Power Plant. The river bottom sediment cores contained 137Cs (10(5)-10(6) Bq/m2) within 0-30 cm depth, whose concentration is comparable to that in the ground soil in the vicinity of the nuclear power plant (the Exclusion Zone). The sediment cores also accumulated 90Sr (10(5) Bq/m2), (239,240)Pu (10(4) Bq/m2) and 241Am (10(4)Bq/m2) derived from the accident. Several nuclear fuel particles have been preserved at 20-25 cm depth that is the peak area of the concentrations of the radionuclides. These inventories in the bottom sediments were compared with those of the released radionuclides during the accident. An analysis using a selective sequential extraction technique was applied for the radionuclides in the sediments. Results suggest that the possibility of release of 137Cs and (293,240)Pu from the bottom sediment was low compared with 90Sr. The potential dissolution and subsequent transport of 90Sr from the river bottom sediment should be taken into account with respect to the long-term radiological influence on the aquatic environment.

Cesium-137 and mercury contamination in lake sediments.

The contribution of fluvial discharge and global fallout of 137Cs and mercury to sedimentation fluxes in Lake Shinji and Lake Nakaumi, Japan, was studied. The fluvial flux through soil erosion accounted for 11 to 27% of accumulated 137Cs in the sediments in the 1950s and 1960s, which were the periods of the most extensive atmospheric input, and for 90 to 100% in the 1990s. The vertical profiles of mercury concentrations in the lake sediments studied showed a maximum between 1959 and 1963, which was originated mostly from the extensive use of mercury-agrochemicals in paddyfields of the watershed in the past. These findings are representative examples of long-term contamination of lake sediments caused by the contaminated ground soil erosion.

The role of suspended particles in the discharge of 210Pb and 7Be within the Kuji River watershed, Japan

Abstract Fluvial discharge of atmospherically-derived 210Pb and 7Be was studied in the Kuji River watershed in Japan. The daily load of 210Pb and 7Be, associated with suspended particles, greatly increased during high-flow conditions. However, the activity concentrations of 210Pb and 7Be in the suspended particles were found to be rather independent of the flow rate. Due to this feature, the daily load of the activities associated with the suspended particles was mainly controlled by that of the suspended particles. Compared with the other reported values, the estimated mean residence time of atmospherically-derived 210Pb in the present study was rather short (about 800 years), possibly reflecting the high availability of the suspended particles eroded from the top soil in the watershed. The annual load of 210Pb and 7Be was predominantly of particulate form (approx 90%). These results suggest that the fluvial discharge of atmospherically-derived, highly adsorptive (kd ≳ 104 mg l−1) radionuclides in a river watershed is primarily due to the discharge of suspended particles, which strongly depends on the flow rate of river water and also on the location of the watershed.