Iodine-129 and caesium-137 in Chernobyl contaminated soil and their chemical fractionation

Soil samples from areas in Belarus, Russia and Sweden contaminated by the Chernobyl accident were analysed for (129)I by radiochemical neutron activation analysis, as well as for 137Cs by gamma-spectrometry. The atomic ratio of (129)I/137Cs in the upper layer of the examined soil cores ranged from 0.10 to 0.30, with an average of 0.18, and no correlation between (129)I/137Cs ratio and the distance from Chernobyl reactor to sampling location was observed. It seems feasible to use the (129)I/137Cs ratio to reconstruct the deposition pattern of 131I in these areas. The association of (129)I and 137Cs in the Chernobyl soil and Irish Sea sediment was investigated by a sequential extraction method. Similar speciation of (129)I in the Chernobyl soil and Irish Sea sediment was found. Approximately 70% of (129)I is bound to oxides and organic matter, and 10-20% is in the readily available phase, while most of the 137Cs (73%) in Chernobyl soil remains in the extraction residue.     

GIS supported calculations of (137)Cs deposition in Sweden based on precipitation data

It is of interest to know the spatial variation and the amount of (137)Cs e.g. in case of an accident with a radioactive discharge. In this study, the spatial distribution of the quarterly (137)Cs deposition over Sweden due to nuclear weapons fallout (NWF) during the period 1962-1966 was determined by relating the measured deposition density at a reference site to the amount of precipitation. Measured quarterly values of (137)Cs deposition density per unit precipitation at three reference sites and quarterly precipitation at 62 weather stations distributed over Sweden were used in the calculations. The reference sites were assumed to represent areas with different quarterly mean precipitation. The extent of these areas was determined from the distribution of the mean measured precipitation between 1961 and 1990 and varied according to seasonal variations in the mean precipitation pattern. Deposition maps were created by interpolation within a geographical information system (GIS). Both integrated (total) and cumulative (decay corrected) deposition densities were calculated. The lowest levels of NWF (137)Cs deposition density were noted in north-eastern and eastern parts of Sweden and the highest levels in the western parts of Sweden. Furthermore the deposition density of (137)Cs, resulting from the Chernobyl accident was determined for an area in western Sweden based on precipitation data. The highest levels of Chernobyl (137)Cs in western Sweden were found in the western parts of the area along the coast and the lowest in the east. The sum of the deposition densities from NWF and Chernobyl in western Sweden was then compared to the total activity measured in soil samples at 27 locations. Comparisons between the predicted values of this study show a good agreement with measured values and other studies.     

Radionuclide contamination of foods imported into Iraq following the Chernobyl nuclear reactor accident

Since early 1986, a monitoring program for radionuclides in imported foods has been carried out by the Iraqi Atomic Energy Commission. After the Chernobyl nuclear reactor accident in the Soviet Union, the program was expanded; our laboratory was officially designated by the Iraqi Government to measure radionuclide activity concentrations in foodstuff imported from countries known to be severely contaminated by Chernobyl radioactive fallout. Gamma-spectrometric analysis was used. Food items such as powdered milk, lamb meat, poultry, cereals and grains imported into Iraq before the Chernobyl accident did not contain any detectable fission products. However, all lamb meat, 81% of the lentil, 44% of the powdered milk and chick-pea, and 17% of the roast beef samples were contaminated with 137Cs or 134Cs and 137Cs. The highest 137Cs contamination levels found were 82, 147, 420, 6 and 4 Bq kg-1, respectively. Contamination by 134Cs was approximately 50% of the values given above.     

Transfer of 137Cs from Chernobyl debris and nuclear weapons fallout to different Swedish population groups

Data from measurements on the body burden of ¹³⁴Cs, ¹³⁷Cs and ⁴⁰K in various Swedish populations between 1959 and 2001 has been compiled into a national database. The compilation is a co-operation between the Departments of Radiation Physics in Malmö and Göteborg, the National Radiation Protection Authority (SSI) and the Swedish Defense Research Agency (FOI). In a previous study the effective ecological half time and the associated effective dose to various Swedish populations due to internal contamination of ¹³⁴Cs and ¹³⁷Cs have been assessed using the database. In this study values of human body burden have been combined with data on the local and regional ground deposition of fallout from nuclear weapons tests (only ¹³⁷Cs) and Chernobyl debris (both ¹³⁴Cs and ¹³⁷Cs), which have enabled estimates of the radioecological transfer in the studied populations.The assessment of the database shows that the transfer of radiocesium from Chernobyl fallout to humans varies considerably between various populations in Sweden. In terms of committed effective dose over a 70 y period from internal contamination per unit activity deposition, the general (predominantly urban) Swedish population obtains 20-30 μSv/kBq m^− 2. Four categories of populations exhibit higher radioecological transfer than the general population; i.) reindeer herders (∼700 μSv/kBq m^− 2), ii.) hunters in the counties dominated by forest vegetation (∼100 μSv/kBq m^− 2), iii.) rural non-farming populations living in sub-arctic areas (40-150 μSv/kBq m^− 2), and iv.) farmers (∼50 μSv/kBq m^− 2). Two important factors determine the aggregate transfer from ground deposition to man; i.) dietary habits (intakes of foodstuff originating from natural and semi-natural ecosystems), and ii.) inclination to follow the recommended food restriction by the authorities. The transfer to the general population is considerably lower (∼a factor of 3) for the Chernobyl fallout than during the 1960s and 70s, which is partly explained by a higher awareness of the pathways of radiocaesium to man both by the public and by the regulating authorities, and by the time-pattern of the nuclear weapons fallout during the growth season in Sweden.     

Application of potassium chloride to a Chernobyl-contaminated lake: modelling the dynamics of radiocaesium in an aquatic ecosystem and decontamination of fish

This study tests a whole-lake experiment to reduce the bioaccumulation of radiocaesium (137Cs) in fish in lakes contaminated by the Chernobyl accident. In many lakes in the Chernobyl contaminated areas, radiocaesium activity concentrations in fish are still significantly higher (up to 100 times in some species) than acceptable limits for human consumption. Estimates of the long-term rate of decline of 137Cs in fish in these regions, in the absence of countermeasures, show that radioactivity in fish in some lakes may remain above acceptable consumption limits for a further 50-100 years from the present date. In February 1998 we applied 15 t of potassium chloride to Lake Svyatoe, Kostiukovichy. The addition of potassium chloride fertilizer to the lake resulted in a decrease in activity concentration of 137Cs to approximately 40% of pre-countermeasure values in a number of different fish species. In contrast to Lake Svyatoe, 137Cs activity concentrations in fish from four control lakes showed no systematic decrease over the study period. Simplified models for transfers of 137Cs in lakes successfully 'blind' predicted the changes in 137Cs in water and fish resulting from this major alteration of the potassium concentration of the lake. The experiment represents the first test of a predictive model for the dynamics of radiocaesium in response to a major perturbation in potassium (its major competitor ion) in a whole lake ecosystem.     

Distribution of radiocaesium in an Austrian forest stand

Within an Austrian spruce stand, vertical distribution of radiocaesium in soil as well as 137Cs concentration in different forest ecosystem compartments including spruce and surface water were investigated 10 years after the Chernobyl accident. The total 137Cs inventory in the forest was estimated to be 46 kBq m-2 (ref. date: 86-05-01). From the collected data annual input rates via litterfall of 0.48% per year and output rates through waterflows of only 0.02% per year were derived. The results identify the high importance of forest soils as a sink for radiocaesium. The estimated ecological residence half-times turned out to be highest in the organic soil horizons (1-3 years per cm), whereas in mineral horizons the values decrease significantly. As a consequence, soil inventory represents more than 95% of the total, whereas only approximately 3.3% of the 137Cs inventory is stored in the living biomass of spruce trees and a further 0.5% in the phytomass of understorey vegetation.     

Long-term trends in the uptake of radiocesium in Rozites caperatus

Measurements of (134)Cs and (137)Cs in Rozites caperatus from 1991 to 2001 showed differences in the uptake pattern of Chernobyl and fallout derived radiocesium over time. The effective half-life of Chernobyl derived (137)Cs was determined to 12.4 years, whereas fallout derived (137)Cs had an effective half-life of 9.8 years. However, the trend for Chernobyl derived (137)Cs was clearly not linear. An effective average half-life of 3.1 years was determined for Chernobyl derived (137)Cs in the period from 1991 to 1994. The corresponding ecological half-life was 3.4 years. In the period from 1994 to 2001 it is possible that the effective half-life was longer than the physical, which indicates that more Chernobyl cesium becomes available than disappears through radioactive decay. The difference may be due to depth distribution of radiocesium, i.e. fallout derived cesium have penetrated deeper than Chernobyl. The changes in the half-lives of Chernobyl derived radiocesium over time together with the half-life of the older fallout derived radiocesium may be valuable in predictions of future radiocesium concentrations in mushrooms and thereby of food chain transfer to man.     

Chernobyl fallout in the uppermost (0-3 cm) humus layer of forest soil in Finland, North East Russia and the Baltic countries in 2000--2003

The situation resulting from the Chernobyl fallout in 1987 was compared to that in 2000--2001 in Finland and NW Russia and that in 2003 in the Baltic countries. 786 humus (0-3 cm layer) samples were collected during 2000--2001 in the Barents Ecogeochemistry Project, and 177 samples in the Baltic countries in 2003. Nuclides emitting gamma-radiation in the 0-3 cm humus layer were measured by the Radiation and Nuclear Safety Authority-STUK in Finland. In 1987 the project area was classified by the European Commission into four different fallout classes. 137Cs inventory Bg/m2 levels measured in 2000--2003 were compared to the EU's class ranges. Fitting over the whole project area was implemented by generalizing the results for samples from the Baltic countries, for which Bq/m2 inventories could be calculated. A rough estimation was made by comparing the mass of organic matter and humus with 137Cs concentrations in these two areas. Changes in 137Cs concentration levels are illustrated in both thematic maps and tables. Radionuclide 137Cs concentrations (Bq/kg d.w.) were detected in the humus layer at all the 988 sampling sites. 134Cs was still present in 198 sites 15 years after the nuclear accident in Chernobyl. No other anthropogenic nuclides emitting gamma-radiation were detected, but low levels of 60Co, 125Sb and 154Eu isotopes were found in 14 sites. Fifteen years after the Chernobyl accident, the radioactive nuclide 137Cs was and still is the most significant fallout radionuclide in the environment and in food chains. The results show that the fallout can still be detected in the uppermost humus layer in North East Europe.     

Migration of fallout radiocaesium in a grassland soil from 1986 to 2001. Part I: activity-depth profiles of (134)Cs and (137)Cs

The temporal changes of the vertical distribution of (134)Cs (deposited by the Chernobyl fallout in 1986) and (137)Cs (deposited by the Chernobyl and the global fallout) in the soil were investigated at an undisturbed Bavarian grassland site in Germany. At ten sampling dates between 1986 and 2001, the activity density of (134)Cs and (137)Cs was determined in various soil layers down to 80 cm depth. In 2001, the small-scale spatial variability of the radiocaesium activity was determined by sampling five plots within 10 m(2) (coefficient of variation about 20% for the upper soil layers). Between 1987 and 1990, substantial changes of the activity-depth profiles were observed. The percentage depth distributions of (134)Cs and (137)Cs were rather similar. The 50%-depth of the accumulated activity increased from 2.4 cm in 1988 to 5.3 cm in 2001 for (134)Cs and from 2.7 to 5.8 cm for (137)Cs. This indicates that at the study site the migration data of Chernobyl-derived (137)Cs can be estimated by those of total (137)Cs. In the second part of this study, the activity-depth profiles will be evaluated by the convection-dispersion model [Schimmack, W, Feria Márquez, F. Migration of fallout radiocaesium in a grassland soil from 1986 to 2001. Part II: Evaluation of the activity-depth profiles by transport models. Sci Total Environ 2006-this issue].     

Iodine-129 in soils from Northern Ukraine and the retrospective dosimetry of the iodine-131 exposure after the Chernobyl accident

Forty-eight soil profiles down to a depth of 40 cm were taken in Russia and Ukraine in 1995 and 1997, respectively, in order to investigate the feasibility of retrospective dosimetry of the 131I exposure after the Chernobyl accident via the long-lived 129I. The sampling sites covered areas almost not affected by fallout from the Chernobyl accident such as Moscow/Russia and the Zhitomir district in Ukraine as well as the highly contaminated Korosten and Narodici districts in Ukraine. 129I was analyzed by radiochemical neutron activation analysis (RNAA) and accelerator mass spectrometry (AMS). 127I was measured for some profiles by RNAA or ion chromatography (IC). The results for 127I demonstrated large differences in the capabilities of the soils to store iodine over long time spans. The depth profiles of 129I and of 137Cs showed large differences in the migration behavior between the two nuclides but also for each nuclide among the different sampling sites. Though it cannot be quantified how much 129I and 137Cs was lost out of the soil columns into deeper depths, the inventories in the columns were taken as proxies for the total inventories. For 129I, these inventories were at least three orders of magnitude higher than a pre-nuclear value of 0.084+/-0.017 mBq m(-2) derived from a soil profile taken in 1939 in Lutovinovo/Russia. From the samples from Moscow and Zhitomir, a pre-Chernobyl 129I inventory of (44+/-24) mBq m(-2) was determined, limiting the feasibility of 129I retrospective dosimetry to areas where the 129I inventories exceed 100 mBq m(-2). Higher average 129I inventories in the Korosten and Narodici districts of 130 and 848 mBq m(-2), respectively, allowed determination of the 129I fallout due to the Chernobyl accident. Based on the total 129I inventories and on literature data for the atomic ratio of 129I/131I=13.6+/-2.8 for the Chernobyl emissions and on aggregated dose coefficients for 131I, the thyroid exposure due to 131I after the Chernobyl accident was estimated for the inhabitants of four villages in the Korosten and of three villages in the Narodici districts. The limitations and uncertainties of the 129I retrospective dosimetry are discussed.     

Efficiency of measures aimed at decreasing the contamination of agricultural products in areas contaminated by the Chernobyl NPP accident.

The sequence of agricultural countermeasures taken in the Polessye area of the Ukraine contaminated by radionuclides as a result of the Chernobyl NPP accident is discussed. The efficiency of these countermeasures has been analysed. The concentration of 137Cs in crops has been shown to vary 10-100-fold, depending on the biological features of the plant species and on soil properties. Liming and mineral fertilizing of meadows increase the yield of grass and reduce 137Cs transfer to cows' milk up to 3.5-fold.     

Long-term decline of 137Cs concentration in honey in the second decade after the Chernobyl accident

In the years 2001-2004 the (137)Cs activity was investigated in a total of 336 samples of different varieties of honey harvested in the Liguria Region of Northern Italy. Our purpose was to define (a) residual radioactive contamination following the Chernobyl accident and (137)Cs long-term decline, (b) correlation between (137)Cs activity and different honey varieties, and (c) correlation between (137)Cs activity and the prevailing geomorphological configuration in the collection areas. The mean (137)Cs specific activity was 4.33+/-5.04 S.D. Bq/kg. Chestnut honey showed higher levels of radioactive contamination, which were ascribed to the extensive, superficial and deep, root apparatus of the tree. Honey samples from acidic argillite soils, which withhold radionuclides after deposition and slowly release them to plants, also showed higher (137)Cs activity. Long-term decline was calculated at 456 days, a value lower than those published from different food sources in the years following the accident. The rate of long-term decline decreases with time.     

Monitoring of air radioactivity at the Jungfraujoch research station: test of a new high volume aerosol sampler

The Swiss Federal Office of Public Health (SFOPH) is responsible for the surveillance of environmental radioactivity in Switzerland and for the protection of the public from ionizing and non-ionizing radiation. In order to improve the Swiss radioactivity monitoring network, a new high volume air sampler (DIGITEL DHA-80) was tested in Fribourg and at the Jungfraujoch High Altitude Research Station at 3454 m. The filters are analyzed in the laboratory by a high purity coaxial germanium detector. In the test period natural radio nuclides and traces of 137Cs, due to the Chernobyl accident of April 1986, were measured and are in good agreement with values from Germany and Poland. Albeit, the main purpose of this radioactivity monitoring network is to detect rapidly any input of artificial radioactivity, it is also helpful to understand transport of natural radio nuclides in the atmosphere, as for example the cosmogenic 7Be and the long-lived radon daughter 210Pb. The DIGITEL sampler proved to be sufficiently reliable for aerosol radioactivity monitoring even in extreme meteorological conditions at Jungfraujoch. A detection limit for artificial 137Cs of 2 microBq/m3 is obtained.     

The radioactivity of coloured rain in Thessaloniki, Greece

The radioactivity of coloured rain precipitated over Thessaloniki, Greece, on 4-5 April 1988 was determined. The long-lived fission product radionuclides such as 137Cs, 134Cs, 144Ce, 106Ru and 125Sb were identified in dust originating from the Sahara desert which was precipitated with the rain to produce coloured rain. Caesium-137 concentrations reached 1000 Bq kg-1, resulting in a deposition of 3.03 Bq m-2, which is four orders of magnitude lower than measured on 5-6 May 1986 after the Chernobyl reactor accident.     

Radioactive contamination in Nijmegen rainwater after the Chernobyl accident

Following the Chernobyl accident, rainwater was collected in the Nijmegen area during the first three weeks of May 1986. The radionuclides found in the water were 131I, 132Te, 132I, 140La, 134Cs, 137Cs and 103Ru. On the first rainy day, an activity concentration of 9 kBq l-1 was measured, with specific activities of 2.7 kBq l-1 for 131I and 2.3 kBq l-1 for each of 132Te and 132I. The total activity precipitated per square kilometer in this period was approximately 55 GBq. Fresh spinach picked in Nijmegen on 7 May had an activity of 1.5 kBq kg-1.     

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

Radiocaesium (¹³⁷Cs) 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 (¹³⁷Cs), 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 ¹³⁷Cs 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.     

Dynamic behaviour of 137Cs contamination in trees of the Briansk region, Russia

Seven trees were felled in the Briansk region in 1997. The trees were sectioned for sampling, both at different heights and according to year rings, and samples were analysed for content of 137Cs. In general, the specific activity of 137Cs was much higher in the fresh parts of the three (needles, leaves and twigs) than in the core wood. The year ring study showed that 137Cs had penetrated deeply in to the trunk, and no peak was detectable in the year ring corresponding to the Chernobyl accident in 1986. The specific activity in the trunk wood had a maximum at the height corresponding to the growth years at approximately 1986. Neutron activation analysis was used to analyse for stable Cs. The results showed that the relationship between concentrations of 137Cs and stable caesium is much higher in the newer parts of growing trees than in the older parts. Together with a tendency of inward migration this leads to a preliminary conclusion that the 137Cs activity will continue to accumulate in the core wood.     

Fractionation of caesium (Cs) in coniferous forest soil in central Sweden

Sequential extraction procedure (SEP) was applied for fractionation of Chernobyl fallout ¹³⁷Cs bound onto soils of a coniferous forest ecosystem located in central Sweden. Results of sequentially extracted ¹³⁷Cs fractions demonstrated that 8% (mean value) of the total deposited ¹³⁷Cs was water soluble (F1) and 13% was NH₄OAc extractable (F2). Oxidation of F2 residuals by H₂O₂ led to a release of 15% of soil-bound ¹³⁷Cs (F3). Acid digestion of F3 residuals showed a possibility of releasing an extra amount of soil-bound ¹³⁷Cs, 22% of the total soil ¹³⁷Cs inventory (F4). These two fractions (F3 and F4) include strongly bound ¹³⁷Cs that seems to require longer biodegradation processes by soil microflora and microfauna before becoming available for uptake by plants and fungi. More than 37% of the total soil ¹³⁷Cs inventory was bound onto soil residuals in a non-extractable form that includes slowly degradable organic matter and other soil residual compartments. The distribution coefficient (K_d) was rather low and shows an inverse relation with the increase of percentage of soil organic matter, which indicates a week binding of ¹³⁷Cs onto forest soil. In contrast, chemical fractionation of soil bound ¹³⁷Cs showed a substantial fraction of ¹³⁷Cs was strongly bound onto soil as organically bound ¹³⁷Cs. Apparently, the binding processes of radiocaesium onto forest soil seems to be time dependent.     

Mapping of 137Cs deposition over eastern France 16 years after the Chernobyl accident

We observed the relationship between present-day 137Cs surface activity and the rainfall in May 1986 to establish a relevant map of 137Cs deposition over eastern France. This study was performed in an area of the lower Rh?ne valley where clayey soils are assumed to trap caesium efficiently. Based on the rainwater/airborne activity ratio deduced from this relationship, we verified that present-day activities, measured in clayey soils of this type, are relatively representative of the initial depositions. The interlocked contaminated areas of the resulting map can be related to rainfall occurrences, as is the case for all countries affected by the Chernobyl fallout. This map can be reasonably compared with those obtained for countries bordering France in terms of both activity levels and extent of homogeneous activity areas. Lastly, we demonstrate that it would not be possible to produce a coherent map of initial fallout by extending the campaign over the whole of the eastern territory based on present-day soil activity measurements alone. Hence, only the relationship between 137Cs deposition and the rainfall of the first week of May 1986 can be used to construct a map depicting the Chernobyl fallout over eastern France.