Long-term studies on transfer of 137Cs from soil to vegetation and to grazing lambs in a mountain area in northern Sweden

Studies were made during 1990-1997 on the transfer of 137Cs from soil to vegetation (herbage) and to grazing lambs on a mountain farm with an uncultivated grazing area of about 10 km2. The farm is situated in an area in Northern Sweden which was contaminated by the Chernobyl fallout in 1986. The mean concentration of 137Cs in the soil to a depth of 10 cm for eight sampling sites observed in the 8-year period was 14.51 kBq/m2, while in the cut herbage the average concentration was 859 Bq/kg d.w. and in lamb meat 682 Bq/kg w.w. A slow vertical migration of 137Cs in the 0-10 cm soil layer was indicated. Although the 137Cs concentration in herbage gradually decreased, the concentration in lamb meat varied from year to year. Soil ingestion by the lambs as a pathway for activity transfer was shown to be negligible, while ingestion of fungi with high concentrations of 137Cs was demonstrated to occur, as large numbers of fungi spores were counted in samples of the lambs' faeces. Fungi ingestion might therefore partly explain the varying mean yearly 137Cs concentrations in lamb muscle. The mean transfer parameters were as follows: for "soil to herbage" 61.3 Bq/kg d.w. herbage per kBq/m2 soil, for "herbage to lamb meat" 0.81 Bq/kg w.w. meat per Bq/kg d.w. herbage, and for "soil to lamb meat" 47.1 Bq/kg w.w. meat per kBq/m2 soil. A trend of decreasing values of the transfer parameter for "soil to herbage" indicated that 137Cs was becoming less available for root-uptake with time. The effective ecological half-life of 137Cs in soil, herbage and lamb meat was calculated to be 19, 7 and 16 years, respectively. It can be concluded that natural areas are vulnerable to 137Cs contamination, resulting in high concentrations in plants, fungi and lamb meat for a long time.     

Distribution of Np and Pu in Swedish lichen samples (Cladonia stellaris) contaminated by atmospheric fallout

The activity concentrations of (237)Np and the two Pu isotopes, (239)Pu and (240)Pu, were determined in lichen samples (Cladonia stellaris) contaminated by fallout from atmospheric nuclear test explosions and the Chernobyl accident. The samples were collected at 18 locations in Sweden, from north to south, between 1986 and 1988 and analysed with high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) and alpha spectrometry. Data on the activity ratios (238)Pu/(239+240)Pu and (134)Cs/(137)Cs measured previously were also included in this study for comparison. The (237)Np activity concentration ranged from 0.08 +/- 0.01 to 2.08 +/- 0.17 MBq kg(-1), depending on the location of the sampling site and time of collection. The (239+240)Pu activity concentration ranged from 0.09 +/- 0.01 to 4.09 +/- 0.15 Bq kg(-1), with the (240)Pu/(239)Pu atomic ratio ranging between 0.16 +/- 0.01 and 0.44 +/- 0.03, the higher ratios indicating a combination of weapons test fallout and Chernobyl fallout. The (237)Np/(239)Pu atomic ratios ranged between 0.06 +/- 0.01 and 0.42 +/- 0.04, the lower ratios indicating combination of weapons test fallout and Chernobyl fallout. At a well-defined sampling site at Lake Rogen (62.32 degrees N, 12.38 degrees E), additional lichen samples were collected between 1987 and 1998 to study the distribution of Np and Pu in different layers. The concentrations of the two elements follow each other quite well in the profile.     

The radiological impact of the Chernobyl debris compared with that from nuclear weapons fallout

Our basic knowledge of the radiological impact of fallout from nuclear accidents is based on the experience gained from the study of nuclear weapons fallout. Radioecologically, the most important radionuclides generated by the Chernobyl accident were ¹³⁷Cs, ¹³⁴Cs, ¹³¹I and, to a lesser degree, ⁹⁰Sr. These nuclides are well known from global fallout, although in different relative amounts to those observed in the Chernobyl debris. Another important difference between the fallout from Chernobyl and that from nuclear weapon tests is that their seasonal and geographical distributions were quite dissimilar. A number of examples show how these differences influenced transfer factors and thus population doses. Special emphasis is paid to the contamination of milk and cereal products.Furthermore, it is shown how the composition of the Chernobyl debris changed with distance from Chernobyl, due to the differences in volatility of the various radionuclides involved. Finally, the paper compares the dose equivalents received from unit releases (1 P Bq ¹³⁷Cs) of global fallout and of Chernobyl accident debris.     

Deposition of 134,137Cs from Chernobyl fallout on Norway spruce and forest soil and its incorporation into spruce twigs

The aims of the present research are to describe the amounts, and the variation with time, of ¹³⁴Cs and ¹³⁷Cs in spruce-twigs (P. abies karst.) and in the soil of a spruce forest in Switzerland following deposition of the Chernobyl fallout. The activity of the twigs was subdivided into 3 compartments: the activity on their surfaces (i.e. the activity which can be removed from the twigs along with their natural wax coating), the activity incorporated into the needles and, finally, the activity incorporated into the wood. These compartments were separately sampled 6 times over a period from 54 to 233 days after the Chernobyl incident. Twigs which sprouted in two successive years (1985, 1986) were sampled and were found to show different behaviours. The activities associated with the 1986 twigs were roughly constant with time, while those of the 1985 twigs decreased exponentially, with half-lives around 150 days. The mean activity associated with 1 g (dry) of 1985 twigs is 724 mBq ¹³⁷Cs g⁻¹, of which 58% is incorporated into the twig wood, 17% into the needles and 25% associated with the adhering aerosol. ¹³⁷Cs on the surface of the needles was found to be water-insoluble. It is believed to be strongly adsorbed on to the soil-derived fraction of the aerosol residing on the needle surface and thus provides a tracer for studying the behaviour of natural aerosols on such surfaces.The same soil profile was measured before and after the Chernobyl incident, allowing direct comparison between nuclear weapons and Chernobyl fallout. The latter is mainly (56%) stored in the litter layer, with only 4% below a depth of 13 cm; it has penetrated into the soil to a much lesser extent than weapons fallout. The forest soil inventory of ¹³⁷Cs showed 2600 Bq m⁻² from nuclear weapons fallout and 6200 Bq m⁻² from Chernobyl.The ¹³⁴Cs/¹³⁷Cs activity ratio of the Chernobyl fallout was found to be 0·58 ± 0·01; the activity ratios in the different compartments investigated prove that incorporation of Cs into spruce occurred exclusively by uptake through the needles. A rough estimate indicates that in a spruce forest the activity stored in the twigs is half that stored in the soil.     

An assessment of cumulative external doses from Chernobyl fallout for a forested area in Russia using the optically stimulated luminescence from quartz inclusions in bricks

Optically stimulated luminescence (OSL) has been used for estimation of the accumulated doses in quartz inclusions obtained from two fired bricks, extracted in July 2004 from a building located in the forested surroundings of the recreational area Novie Bobovichi, the Bryansk Region, Russia. The area was significantly contaminated by Chernobyl fallout with initial (137)Cs ground deposition level of approximately 1.1 MBq m(-2). The accumulated OSL doses in sections of the bricks varied from 141 to 207 mGy, of which between 76 and 146 mGy are attributable to Chernobyl fallout. Using the OSL depth-dose profiles obtained from the exposed bricks and the results from a gamma-ray-survey of the area, the Chernobyl-related cumulative gamma-ray dose for a point detector located in free air at a height of 1m above the ground in the study area was estimated to be ca. 240 mGy for the time period starting on 27 April 1986 and ending on 31 July 2004. This result is in good agreement with the result of deterministic modelling of the cumulative gamma-ray dose in free air above undisturbed ground from the Chernobyl source in the Bryansk Region. Over the same time period, the external Chernobyl-related dose via forest pathway for the most exposed individuals (e.g., forest workers) is estimated to be approximately 39 mSv. Prognosis for the external exposure from 1986 to 2056 is presented and compared with the predictions given by other investigators of the region.     

Formation of radioactivity enriched soils in mountain areas

A field study was carried out in the Mercantour Mountains at 2200 m altitude to investigate the processes of soil enrichment in atmospheric Chernobyl (137)Cs. Soils with high (137)Cs activities have been collected in the pasture areas with frequently measured (137)Cs activity values of the order of 7000 Bq m(-2). At some single spots (about 6% of the studied area), activity in soils reached 300000 Bq m(-2), which represents 44% of the (137)Cs of the total area. Data further showed that spatial distribution of Cs depends widely on its origin: Chernobyl Cs is mainly concentrated in "enriched" soils, whereas older Cs and (241)Am fallout from nuclear weapons tests (NWTs) and natural atmospheric (210)Pb in soils is less heterogeneously distributed.In order to elucidate the processes which have led to the enrichment in Chernobyl (137)Cs in the Alps in May of 1986, we have studied the repartition of atmospheric (7)Be isotope (half-life=53.3 d) in the pasture compartments (soil, litter, grass, and snow). Snow (7)Be data give evidence that fallout enrichment is related to snow accumulation (snow drift). The transfer of beryllium occurs rapidly to the grass and litter, where the strongest pollutant accumulations were measured. However, (7)Be transport to the soil required more than 8 months.     

Ecological half-life of 137Cs in lichens in an alpine region

About 17 years after the Chernobyl accident, lichen samples were collected in an alpine region in Austria (Bad Gastein), which was heavily contaminated by the Chernobyl fallout. Measured 137Cs activity concentrations in selected lichens (Cetraria islandica, Cetraria cucullata, and Cladonia arbuscula) ranged from 100 to 1100 Bq kg(-1) dry weight, depending on lichen species and sampling site. Ecological half-lives for 137Cs in different lichen samples, obtained by comparison with earlier measurements of the same lichen species at the same site, ranged from 2 to 6 years, with average values between 3 and 4 years. Comparison with earlier studies indicated that ecological half-lives hardly changed during the last 10 years, suggesting that ecological clearance mechanisms (e.g. washout or soil transfer) did not vary substantially at the selected sampling area.     

Short- and long-term patterns of ¹³⁷Cs in fish and other aquatic organisms of small forest lakes in southern Finland since the Chernobyl accident

We summarize the patterns of ¹³⁷Cs activity concentrations and transfer into fish and other biota in four small forest lakes in southern Finland during a twenty-year period following the Chernobyl accident in April 1986. The results from summer 1986 showed fastest accumulation of ¹³⁷Cs into planktivorous fishes, i.e. along the shortest food chains. Since 1987, the highest annual mean values of ¹³⁷Cs have been recorded in fish occupying the highest trophic levels, for perch (Perca fluviatilis) 13,600 Bq/kg (ww) and for pike (Esox lucius) 20,700 Bq/kg (ww). At the same time, activity concentrations of ¹³⁷Cs in crustacean zooplankton and Asellus aquaticus have ranged between 1000 and 19,500 Bq/kg (dw). In 2006, 5-28% of the 1987 ¹³⁷Cs activity concentration levels were still present in perch and pike. Since 1989 their ¹³⁷Cs activity concentrations in oligohumic seepage lakes have remained significantly higher than in polyhumic drainage lakes due to the increased transfer of ¹³⁷Cs into fish in the seepage lakes with lower electrolyte concentrations, longer water retention times and lower sedimentation rate.     

Temporal variation of Cs and Pu in the sea of Japan

Concentrations of the anthropogenic ¹³⁷Cs and ^239,240Pu in surface water of the Sea of Japan were in the ranges of 2·7–3·8 mBq l^-1 and 1·3–8·0 μBq l^-1, respectively, in 1993–1994, which are in the same order of magnitude as those in the North Pacific. The time-series data indicated a marked increase of surface ¹³⁷Cs in 1986 and 1987 after the major radioactive dumping and the Chernobyl fallout in 1986 and then rapidly decreased thereafter. The apparent half-residence time of ¹³⁷Cs in the surface water was estimated to be about 3 years for excess ¹³⁷Cs and 16 years for a rather long time-scale transport, respectively. For ^239,240Pu in the surface water, no systematic temporal variation was observed over the past two decades, which may reflect rapid recycling of deeper Pu. The results revealed that most of the recent radioactivity in water columns of the Sea of Japan was of global fallout origin from atmospheric nuclear testing and partly the Chernobyl fallout. No clear signal about the effect of radioactivity from Russian dumping was observed.     

Time trends (1986-2003) of radiocesium transfer to roe deer and wild boar in two Austrian forest regions

Starting shortly after the Chernobyl accident, samples of roe deer and wild boar from two comparatively highly contaminated Austrian forest stands have been regularly analysed for (137)Cs. Until 1995 average (137)Cs concentrations exceeded 1000 Bq kg(-1) in both roe deer and wild boar. Long-term and seasonal trends are similar in both investigation sites. While (137)Cs aggregated transfer factor (T(ag)) values show a significant decreasing trend in roe deer (ecological half-time 8.6 and 7.2 years, respectively), T(ag)-values in wild boar are highly variable, but rather increasing values are observed over the last years. T(ag)-values for roe deer are between 0.04 and 0.008 m(2)kg(-1) fresh weight (1987-2003); values for wild boar are between 0.008 m(2)kg(-1) (1988) and 0.046 m(2)kg(-1) (1996) fresh weight. Seasonal trends for both species are in good agreement with observations from German forests: increased mushroom ingestion leads to higher (137)Cs T(ag)-values for roe deer in the second half of the year (August-December) compared to the first half (January-July). T(ag)-values for wild boar are highest in the first half of the year.     

Altitude dependent 137Cs concentrations in different plant species in alpine agricultural areas

The Gastein valley in the Central Part of the Austrian Alps was one of the regions most heavily affected by fallout of the Chernobyl nuclear catastrophe, depositing (137)Cs inventory up to 70 kBq/m(2) in May 1986. In many studies dealing with the uptake of (137)Cs by vegetation used for farming, a significant correlation between (137)Cs concentration in the plants and altitude a.s.l. has been observed. In order to quantify the influence of the composition of plant communities on the average (137)Cs concentration in vegetation on farmland, plant-specific activity concentrations in plant species have been determined. Alongside a transect from valley sites at 850 m a.s.l. to alpine pastures at 1660 m, the aggregated transfer factors C(ag) (m(2)/kg) have been measured for plant communities and plant species. C(ag) values for mixed vegetation are more or less similar in valley sites, but they increase exponentially with a doubling height of 122+/-22 m above appr. 1200 m altitude a.s.l. On average all species are affected by this increase in a similar way. C(ag) values of ubiquitous plant indicate that the composition of plant communities is of minor importance for the contamination of mixed vegetation.     

Use of 129I and 137Cs in soils for the estimation of 131I deposition in Belarus as a result of the Chernobyl accident

Using radioactivity measurements for 131I and 137Cs and nuclear activation analysis (NAA) or accelerator mass spectrometry (AMS) for 129I, ratios of 131I/137Cs and 129I/137Cs have been determined in soils from Belarus. We find that the pre-Chernobyl ratio of 129I/137Cs in Belarus is significantly larger than expected from nuclear weapons fallout. For the Chernobyl accident, our results support the hypothesis that there was relatively little fractionation of iodine and caesium during migration and deposition of the radioactive cloud. For sites having 137Cs > 300 Bq/kg, 129I can potentially give more reliable retroactive estimates of Chernobyl 131I deposition. However, our results suggest that 137Cs can also give reasonably good (+/-50%) estimates for 131I in Belarus.     

A blind test of the MOIRA lake model for radiocesium for Lake Uruskul, Russia, contaminated by fallout from the Kyshtym accident in 1957

This paper presents results of a model-test carried out within the framework of the COMETES project (EU). The tested model is a new lake model for radiocesium to be used within the MOIRA decision support system (DSS; MOIRA and COMETES are acronyms for EU-projects). This model has previously been validated against independent data from many lakes covering a wide domain of lake characteristics and been demonstrated to yield excellent predictive power (see H?kanson, Modelling Radiocesium in Lakes and Coastal Areas. Kluwer, Dordrecht, 2000, 215 pp). However, the model has not been tested before for cases other than those related to the Chernobyl fallout in 1986, nor for lakes from this part of the world (Southern Urals) and nor for situations with such heavy fallout as this. The aims of this work were: (1) to carry out a blind test of the model for the case of continental Lake Uruskul, heavily contaminated with 90Sr and 137Cs as a result of the Kyshtym radiation accident (29 September 1957) in the Southern Urals, Russia, and (2) if these tests gave satisfactory results to reconstruct the radiocesium dynamics for fish, water and sediments in the lake. Can the model provide meaningful predictions in a situation such as this? The answer is yes, although there are reservations due to the scarcity of reliable empirical data. From the modelling calculations, it may be noted that the maximum levels of 137Cs in fish (here 400 g ww goldfish), water and sediments were about 100,000 Bq/kg ww, 600 Bq/l and 30,000 Bq/kg dw, respectively. The values in fish are comparable to or higher than the levels in fish in the cooling pond of the Chernobyl NPP. The model also predicts an interesting seasonal pattern in 137Cs levels in sediments. There is also a characteristic "three phase" development for the 137Cs levels in fish: first an initial stage when the 137Cs concentrations in fish approach a maximum value, then a phase with relatively short ecological half-lives followed by a final phase with long ecological half-lives more or less corresponding to the physical decay of radiocesium.     

Lake fish as the main contributor of internal dose to lakeshore residents in the Chernobyl contaminated area

Two field expeditions in 1996 studied 137Cs intake patterns and its content in the bodies of adult residents from the village Kozhany in the Bryansk region, Russia, located on the shore of a drainless peat lake in an area subjected to significant radioactive contamination after the 1986 Chernobyl accident. The 137Cs contents in lake water and fish were two orders of magnitude greater than in local rivers and flow-through lakes, 10 years after Chernobyl radioactive contamination, and remain stable. The 137Cs content in lake fish and a mixture of forest mushrooms was between approximately 10-20 kBq/kg, which exceeded the temporary Russian permissible levels for these products by a factor of 20-40. Consumption of lake fish gave the main contribution to internal doses (40-50%) for Kozhany village inhabitants Simple countermeasures, such as Prussian blue doses for dairy cows and pre-boiling mushrooms and fish before cooking, halved the 137Cs internal dose to inhabitants, even 10 years after the radioactive fallout.     

Tracing anthropogenic nuclear activity with (129)I in lake sediment

This study reports the first data of (129)I fallout in Scandinavia, covering the last 80 years. The investigation is based on sediment sections from a lake in central Sweden. In addition to analysis of (129)I, a combination of several radionuclides ((210)Pb, (137)Cs and (14)C) was used to establish an accurate chronology of the sediment profile. The concentration of (129)I exhibits an increasing trend ( approximately 10(7) to approximately 10(9)atoms/g) during the last 40 years, suggesting a significant atmospheric input from the nuclear reprocessing facilities in Sellafield (UK) and La Hague (France). A peak corresponding to fallout from the Chernobyl accident (1986) is clearly distinguishable, whereas the impact of fallout from the nuclear weapons' tests since the early 1950s is not distinguished.     

Resolving Chernobyl vs. global fallout contributions in soils from Poland using Plutonium atom ratios measured by inductively coupled plasma mass spectrometry

Plutonium in Polish forest soils and the Bór za Lasem peat bog is resolved between Chernobyl and global fallout contributions via inductively coupled plasma mass spectrometric measurements of 240Pu/230Pu and 241Pu/239Pu atom ratios in previously prepared NdF3 alpha spectrometric sources. Compared to global fallout, Chernobyl Pu exhibits higher abundances of 240Pu and 241Pu. The ratios 240Pu/230Pu and 241Pu/239Pu co-vary and range from 0.186 to 0.348 and 0.0029 to 0.0412, respectively, in forest soils (241Pu/239Pu = 0.2407 x [240Pu/239Pu] - 0.0413; r2 = 0.9924). Two-component mixing models are developed to apportion 239+240Pu and 241Pu activities; various estimates of the percentage of Chernobyl-derived 239+240Pu activity in forest soils range from < 10% to > 90% for the sample set. The 240Pu/230Pu - 241Pu/239Pu atom ratio mixing line extrapolates to estimate 241Pu/239Pu and the 241Pu/239+240Pu activity ratio for the Chernobyl source term (0.123 +/- 0.0007; 83 +/- 5; 1 May 1986). Sample 241Pu activities, calculated using existing alpha spectrometric 239+240Pu activities, and the 240Pu/230Pu and 241Pu/239Pu atom ratios, agree relatively well with previous liquid scintillation spectrometry measurements. Chernobyl Pu is most evident in locations from northeastern Poland. The 241Pu activities and/or the 241Pu/239Pu atom ratios are more sensitive than 240Pu/239Pu or 238Pu/239+240Pu activity ratios at detecting small Chernobyl 239+240Pu inputs, found in southern Poland. The mass spectrometric data show that the 241Pu activity is 40-62% Chernobyl-derived in southern Poland, and 58-96% Chernobyl in northeastern Poland. The Bór za Lasem peat bog (49.42 degrees N, 19.75 degrees E), located in the Orawsko-Nowotarska valley of southern Poland, consists of global fallout Pu.     

Radionuclide tracing of water masses and processes in the water column and sediment in the Algerian Basin

Caesium-137 and (239,240)PU were analysed in the water column along the Algerian coast. The (137)Cs activity concentration in surface water increased from the west to the east from 1.6 to 3.3 mBq L(-1), documenting a presence of Modified Atlantic Water (MAW) in the region. Higher concentrations observed in deep waters may be due to an intrusion of Levantine Intermediate Water (LIW), which has been carrying higher levels of (137)Cs from Chernobyl accident. The (239,240)Pu sub-surface concentration peaked at about 250 m water depth as a result of biogeochemical processes in the water column. The observed (239,240)Pu/(137)Cs activity ratio at the surface (0.003) was significantly lower than that in global fallout (0.04). This decrease exceeds that expected from radioactive decay of (137)Cs, and confirms that Pu due to its adsorption on sinking particles is more effectively removed from surface layers than is (137)Cs. An increase of the (239,240)Pu/(137)Cs activity ratio with depth suggests that (239,240)Pu, similarly as (137)Cs, should be also transported by advection to maintain the observed ratios in deep waters. An intrusion of LIW may enhance therefore both the (137)Cs and (239,240)Pu concentrations in deep waters. The average (238)Pu/(239+240)Pu activity ratio in seawater was 0.03+/-0.02, confirming a global fallout origin of Pu in the Algerian Basin. Caesium-137 and (239,240)Pu inventories in the water column were estimated to be from 2.7+/-0.5 kBq m(-2) to 3.8+/-0.7 kBq m(-2), and from 13.8+/-2.6 Bq m(-2) to 41+/-7B qm(-2), respectively. The (137)Cs massic activities in surface sediment were almost constant, the average activity was 9.0+/-0.8 Bq kg(-1). Sedimentation rates obtained using the (210)Pb method were from 0.1 to 0.7 cm y(-1), and resulting penetration depths of (137)Cs in the sediment cores were from 15 to over 40 cm. The (137)Cs peaks found in the sediment cores were associated with the Chernobyl accident (1986) and global fallout (1964). The (137)Cs inventories in the sediment were increasing from the west (180 Bq m(-2)) to the east (350 Bq m(-2)).     

137Cs contamination in tea and yerba mate in South America

Gamma-ray spectra from more than 50 samples of food products available in stores of Buenos Aires city were measured using a germanium detector. Activity concentrations of 137Cs up to 10 Bq/kg were found in tea and yerba mate manufactured in Apóstoles, Argentina. Further measurements of tea leaves, yerba mate leaves and soils, all coming from a cultivated area in that region, also show the presence of 137Cs contamination. The results suggest that the area was fertilized with a product that originated in a region affected by the fallout from the Chernobyl nuclear plant accident.     

Measurements and comparisons of gamma radiation doses in a high and a low (137)Cs deposition area in Sweden

Sweden is one of the countries affected by the Chernobyl fallout. The aim of the present study was to investigate the average radiation dose to people living in a high-deposition area (the parish of Hille) in Sweden for comparison with dose rates previously measured in a low-deposition area in western Sweden. Individual measurements (personal and dwelling dose rates) were performed using thermoluminescence dosimeters in 24 randomly chosen individuals. Dwelling and personal dose rates in Hille were 0.12 and 0.11 microSv/h, respectively. The dose rates in Hille were slightly higher than in western Sweden (difference for detached houses=0.024 microSv/h for personal and 0.030 microSv/h for dwelling dose rates), partly because of the higher (137)Cs deposition. In wooden houses, the difference was somewhat greater. Our results indicate a current contribution to personal gamma radiation in this area of about 0.2 mSv per year from the Chernobyl fallout.     

Vertical migration studies of 137Cs from nuclear weapons fallout and the Chernobyl accident

The vertical migration of (137)Cs originating from nuclear weapons fallout (NWF) and the Chernobyl accident has been studied at 33 sampling sites in western Sweden. An attempt to describe the present depth distribution with a solution to the convection-diffusion equation (CDE) with a pulse-like fallout event as the initial condition was made. A sum of two CDEs describing the NWF and Chernobyl debris was fitted to the actual depth profiles measured by soil sampling. The fitted depth profiles were used to correct in situ measurements for the actual depth distribution, showing good agreement with the accumulated activities in soil samples. As expected, the vertical migration was very slow and most caesium was still present in the upper soil layers. The ranges of the apparent convection velocity, v, and apparent diffusion coefficient, D, were between 0 and 0.35 cm/year and 0.06 and 2.63 cm(2)/year, respectively.