Klas Rosén

Migration of radiocaesium in Swedish soil profiles after the Chernobyl accident, 1987–1995

The study site comprises temporary and permanent grassland in areas in central and northern Sweden which were strongly affected by the Chernobyl fallout in 1986. The aim of the study was to investigate the vertical migration of radiocaesium from 1987 to 1995 in undisturbed soil profiles under field conditions, as related to soil type and texture. The sampled sites differ in soil types and textures, i.e. six cultivated or semi-natural mineral soils (Dystrochrept, Haploboroll, Udorthents, Cryorthent and Haplocryod) and two cultivated organic soils (Sulfihemists). The ground deposition of 137Cs ranged from 14 to 184 kBq m-2 (average 82). 137Cs-activities were measured in cm-sliced, 25-cm-deep soil cores and in soil horizons of the entire soil profiles down to 0.7–1.0 m depth. The soil cores were collected on two or three occasions between 1987 and 1995, and migration rates were calculated based on the median depths. Grass samples were taken from the same locations to measure the amount of 137Cs transferred from soil to grass. After eight years, most (50–92%) of the 137Cs fallout was still present in the upper 5 cm (median depth 2.3–5.1 cm), although considerable amounts had migrated to deeper soil layers. Downward migration was most pronounced in the organic soils and in the podzol where 137Cs was found to about 50 cm depth. Migration rates were in the range of 0.5–1.0 cm year-1 for the first year and thereafter 0.2–0.6 cm year-1. The transfer of 137Cs to grass was highest in two gravely sandy loam soils in the mountain region, intermediate in two organic soils, and lowest in soils from the river and coastal areas.

Regional modelling of base cation losses from Swedish forest soils due to whole-tree harvesting

Abstract Concern about the environment has made it desirable to reduce fossil fuel consumption and in Sweden energy production from forest biomass is seen as an attractive alternative. The Swedish forestry industry has so far been practising stem harvesting only with moderate impact on the forest ecosystems. Increased harvesting of forest biomass, i.e. foliage and branches which are comparatively rich in nutrients, will result in increased nutrient export from forest ecostystems. Long-term mineral weathering of base cations was estimated on 1508 plots throughout Sweden, using a regression model. Calcium, Mg and K weathering was compared to stem only and whole-tree accumulation of these nutrients. Base cation inputs from the atmosphere and outputs from leaching were not included in this modelling approach. The balance was mapped for forest land in Sweden. The results indicated that weathering alone can support biomass uptake of Mg and K, but not Ca. If increased biomass utilization, in the form of whole-tree harvesting, is to become a common practice in the future, Ca reserves in soils will become more depleted. For Mg and K, significant areas, especially in south Sweden, will also show depletion in the soil after whole-tree harvesting.

Accumulation of potassium, rubidium and caesium (133Cs and 137Cs in various fractions of soil and fungi in a Swedish forest.

Radiocaesium ((137)Cs) was widely deposited over large areas of forest in Sweden as a result of the Chernobyl accident in 1986 and many people in Sweden eat wild fungi and game obtained from these contaminated forests. In terms of radioisotope accumulation in the food chain, it is well known that fungal sporocarps efficiently accumulate radiocaesium ((137)Cs), as well as the alkali metals potassium (K), rubidium (Rb) and caesium (Cs). The fungi then enhance uptake of these elements into host plants. This study compared the accumulation of these three alkali metals in bulk soil, rhizosphere, soil-root interface, fungal mycelium and sporocarps of mycorrhizal fungi in a Swedish forest. The soil-root interface was found to be distinctly enriched in K and Rb compared with the bulk soil. Potassium concentrations increased in the order: bulk soil<rhizosphere<fungal mycelium<soil-root interface<fungal sporocarps; and Rb concentration in the order: bulk soil<rhizosphere<soil-root interface<fungal mycelium<fungal sporocarps. Caesium was more or less evenly distributed within the bulk soil, rhizosphere and soil-root interface fractions, but was actively accumulated by fungi. Fungi showed a greater preference for Rb and K than Cs, so the uptake of (137)Cs could be prevented by providing additional Rb or K at contaminated sites. The levels of K, Rb, and Cs found in sporocarps were at least one order of magnitude higher than those in fungal mycelium. These results provide new insights into the use of transfer factors or concentration ratios. The final step, the transfer of alkali metals from fungal mycelium to sporocarps, raised some specific questions about possible mechanisms.

Uptake and distribution of 137Cs and 90Sr in Salix viminalis plants

Agricultural areas in middle and northern parts of Sweden were contaminated with radionuclides after the Chernobyl accident in 1986. Alternative crops in these areas are biomass plantations with fast-growing Salix clones for energy purposes. The uptake and internal distribution of 137Cs and 90Sr in Salix viminalis were studied. Plants were grown in microplots under field conditions. The soils in the experimental site had been contaminated in 1961 with 35.7 and 13.4 MBq m(-2) of 137Cs and 90Sr, respectively. The experiment was carried out during three years. The plots were fertilised with 60 kg N ha(-1) and three treatments of K, consisting of 0, 80 and 240 kg K ha(-1) during the first two years. The activity concentration of 137Cs in the different plant parts varied between 140 and 20,000 Bq kg(-1) and was ranked in the following order: lowest in stems < cuttings < leaves < roots. The fine roots (0-1 mm) had the highest 137Cs activity concentration. One-year-old stems had higher 137Cs activity concentrations than two-year-old stems. The activity concentration of 137Cs in the plants was significantly affected by K-supply and was higher in the 0 kg K treatment than in the 80 or 240kg K treatment. Leaves contained more 90Sr than stems and cuttings.

Transfer of radiocaesium in sensitive agricultural environments after the Chernobyl fallout in Sweden. I. County of Gävleborg

In 1986 a large number of farms in the Chernobyl-affected area in the county of Västernorrland in northern Sweden were investigated for radiocaesium transfer to grass and cereal grain. The soil surface layer (0-5 cm) in 1986 and the crop products in 1986-1996 were analysed. The aim was to study the impact of soil and crop rotation on sensitivity of 137Cs transfer in a short and long term perspective. In the fallout year 1986 the transfer to grass was usually much higher than to cereal grain. In this year the transfer to grass was usually much higher in the first cut rather than the second cut. The reduction in transfer with year was large but variable with site and with crop sequence. Ploughing was effective in decreasing the transfer of 137Cs to crops. On arable sites in 1986 the transfer to cereal straw was larger at late stem elongation (LSE) than at the maturing stage. Unexpectedly, there was no clear relationship between transfer of 137Cs to the crops and any of the soil characteristics. In 1986 the transfer of 131I to grass and cereals was also investigated on some of the farms. The results are compared with the transfer of 137Cs, 2 months after the Chernobyl fallout.

Chemical availability of 137Cs and 90Sr in undisturbed lysimeter soils maintained under controlled and close-to-real conditions.

Chemical availability of 137Cs and 90Sr was determined in four undisturbed soils in a lysimeter study three and four years after deposition to the soil surface. The study was part of a larger project on radionuclide soil-plant interactions under well-defined conditions. The soil types were loam, silt loam, sandy loam and loamy sand, and were representatives of important European soil and climatic conditions. The lysimeters were installed in greenhouses with climatic and hydrological control, and were contaminated with 137Cs and 90Sr in an aerosol mixture simulating fallout from a nuclear accident. Soil samples were taken from several depths in each soil in 1997 and 1998 and the samples were sequentially extracted with H2O, NH4Ac, NH2OH.HCl, H2O2 and HNO3. Extractability of 137Cs decreased in the order: HNO3 > R-esidual > or = NH4Ac > H2O2 > or = NH2OH.HCl > or = H2O. More than 80% was found in the acid digestible or residual fractions, and 11-17% in labile fractions. Soil type differences were small. Extractability of 90Sr decreased in the order: NH4Ac > NH2OH.HCl > HNO3 > H2O2 approximately H2O. 31-58% was found in easily available fractions. Differences between soil types were quite small. The results suggest that availability of 137Cs for plant uptake and migration is low, whereas availability of 90Sr is rather high.

Transfer of radiocaesium from soil to vegetation and to grazing lambs in a mountain area in northern Sweden

Abstract Radioecological studies on a mountain farm in the Chernobyl fallout area in the county of Jamtland, Sweden, were carried out in 1990–1993. Activity analyses of soil samples, taken to 10 cm depth at nine sampling sites within a grazing area of about 10km 2 showed a mean deposition of 137 Cs of 15.7 (range 14.1–17.6) kBq/m 2 . The mean ratios of 134 Cs 137 Cs in the upper 0–5 cm and 5–10 cm of the soil layer were 0.523 and 0.521, respectively. The average 137 Cs concentration of the herbage cut at the various sites decreased with time from 1175 to 900 Bq/kg dry weight (when corrected for physical decay to August 1990). This gave an average transfer factor from soil to plant of 0.067 (range 0.074-0.057) m 2 /kg, which corresponds to an average transfer to the cut herbage of 0.86% of deposited 137 Cs. The 137 Cs concentration in single plant species decreased in the order: herbs > grasses > woody plants > trees and shrubs. The average 137 Cs concentration in the abdomen wall muscle of lamb carcasses was 1087, 668, 513 and 597 Bq/kg wet weight in the years 1990–1993, respectively, indicating an effective ecological half-time of 3.4 years or 3.8 years when corrected for physical decay (ecological half-time). All carcasses exceeded the intervention level applied in Sweden, 300 Bq 137 Cs/ kg, and were thus discarded for human consumption. Transfer factors (plant to muscle) decreased from 0.93 kg dry weight/kg wet weight in 1990 to 0.61, 0.56 and 0.71 in the following years. Similarly, the aggregated transfer factor (soil to muscle) decreased over the years, from 0.062 to 0.047, 0.032 and 0.046 m 2 /kg wet weight. The results show high availability and transfer of radiocaesium in the mountain grazing areas of Sweden 4–7 years after the Chernobyl fallout.

Transfer of radiocaesium in sensitive agricultural environments after the Chernobyl fallout in Sweden. II. Marginal and seminatural areas in the county of Jämtland

In 1986, two Chernobyl-affected areas in the county of Jamtland, a mountain area and a river valley area, were investigated as to radiocaesium behaviour and transfer to grass. The soil surface layer (0-10 cm) in 1986 and 1989 and grass samples in 1986-1994 were analysed on 9 temporary grassland sites and 8 permanent pasture sites, described individually. The aim of this investigation was to study the sensitivity of different soil types and the influence of normal farming practices, ploughing and K-fertilization on the caesium transfer, in short- and long-term perspectives after the Chernobyl fallout. As expected, the transfer of 137Cs to grass was usually higher on permanent pasture than on temporary grassland. For both types of grassland, however, there was a considerable but different change of transfer with years. The transfer to grass in the year of the fallout, 1986, depended to a large extent on the thickness and interception capacity of the grass sward. In the following years, it also depended on the caesium-fixing capacity to clay minerals, on K-fertilization and the reverse process of K removal by plant uptake. Ploughing down the contaminated surface layer and the mixing of caesium with mineral soil were effective in reducing the transfer. During the period 1986-1994, the transfer was reduced considerably, showing a range of (0.1-177.3 m2/kg d.w.) x 10(-3). The calculated annual reduction halftime, Tar, increased with years after fallout. It is clearly shown that both countermeasures, ploughing and K-fertilization, are of potential value to decrease grass contamination. Where both measures were employed a reduction in the range of 78%-95% was recorded in the year after ploughing.

Uptake and translocation of 109Cd and stable Cd within tobacco plants (Nicotiana sylvestris)

The availability, uptake, and translocation of recently added ((109)Cd) and naturally occurring (stable) soil Cd within tobacco plants were compared. (109)Cd was added to soil in two treatments, A (0.25 MBq kg soil(-1) DW) and B (eight-fold dose): stable Cd was measured in both treatments. Both the added and the stable Cd were higher in leaves and reproductive structures of the plant than in stalks and roots. The uptake of (109)Cd was 5.3 kBq plant(-1) for treatment A and 36.7 kBq plant(-1) for treatment B, and about 26 μg plant(-1) for stable Cd. Leaves of the tobacco plants accumulated 40-45% of the total (109)Cd and about 50% of total stable Cd taken up by the plant. Cadmium concentration in the plant was three times higher than in roots and two times higher than the concentration in soil: the concentration in roots was lower than in the soil.

The distribution of 137Cs, K, Rb and Cs in plants in a Sphagnum-dominated peatland in eastern central Sweden

We record the distribution of (137)Cs, K, Rb and Cs within individual Sphagnum plants (down to 20cm depth) as well as (137)Cs in vascular plants growing on a peatland in eastern central Sweden. In Calluna vulgaris(137)Cs was mainly located within the green parts, whereas Andromeda polifolia, Eriophorum vaginatum and Vaccinium oxycoccos showed higher (137)Cs activity in roots. Carex rostrata and Menyanthes trifoliata showed variable distribution of (137)Cs within the plants. The patterns of (137)Cs activity concentration distribution as well as K, Rb and Cs concentrations within individual Sphagnum plants were rather similar and were usually highest in the capitula and/or in the subapical segments and lowest in the lower dead segments, which suggests continuous relocation of those elements to the actively growing apical part. The (137)Cs and K showed relatively weak correlations, especially in capitula and living green segments (0-10cm) of the plant (r=0.50). The strongest correlations were revealed between (137)Cs and Rb (r=0.89), and between (137)Cs and stable Cs (r=0.84). This suggests similarities between (137)Cs and Rb in uptake and relocation within the Sphagnum, but that (137)Cs differs from K.