⁹⁰Sr and ¹³⁷Cs Accumulation in Plants in the Area of Radiation Accidents

Abstract

An investigation of radioactive contamination of soil, crops, vegetables, feed, grass of hayfields and pastures, wild berries, and mushrooms as a result of radioactive contamination due to operation of radiochemical plant, Production Association (PA) “Mayak,” was carried out on the territory of northeastern part of Chelyabinsk region in 2006–2012. ⁹⁰Sr and ¹³⁷Cs content in the agricultural products and wild mushrooms in the whole territory of 30–35 km from PA “Mayak” did not exceed maximum permissible level and was not dangerous for the population. ⁹⁰Sr accumulation in wild berries and grass of hayfield and pastures on the territory of East-Ural radioactive trace did not exceed the radiological sanitary limits which could be dangerous for the population and farm animals. On the rest of the territory, both berries and grass were non-contaminated. The accumulation coefficients of ⁹⁰Sr and ¹³⁷Cs in agricultural products and wild plants and mushrooms vary significantly. The highest values of the accumulation coefficients, sometimes >1, were found for grass of hayfields and pastures. Among agricultural plants, the highest values of the accumulation coefficients were found for carrot, Sudan grass, and corn, whereas the lowest values were found for cabbage, beetroot, and potato. The accumulation coefficients of the radionuclides found for vegetables from private farmers were lower than the ones from large agricultural plant. A content of ⁹⁰Sr and ¹³⁷Cs in soil in an easy accessible form for the plants depends on the soil humidity and may vary significantly. ⁹⁰Sr deposited in soil is transferred into soil solution as ionic species. ¹³⁷Cs in soil solution is present as colloidal species.

1 Introduction

The forecasting of entry of radioactive isotopes into the products of agriculture and forestry in the areas affected by radioactive contamination is an important and urgent problem. Despite increased security measures at nuclear power plants and radiochemical enterprises, radiation accidents accompanied by the release of radioactive substances into the environment are regularly happening on the planet. The study of radionuclides entry into the plants in 50–60 years after the fallout of radioactive substances on the soil surface is necessary for a long-term forecasting of radiation risks for people and the environment.

2 Materials and Methods

The researches have been conducted in 2006–2012 in the territory of northeastern part of Chelyabinsk region that suffered from radioactive contamination caused by the activity of radiochemical industrial complex of the Production Association “Mayak.” The specific activity of ¹³⁷Cs in the soil samples was measured by means of scintillation gamma-ray spectrometer with the software “Progress.” From 2011, the spectrometer facility MKC-01A “MULTIRAD” was used for this purpose. In the low-activity soil samples and in the plant ash, ¹³⁷Cs level was measured using the radiochemical iodine–antimony method after concentrating it on the nickel ferrocyanide. The specific activity of ⁹⁰Sr in the samples was measured by means of the extractive method based on daughter ⁹⁰Y using monoisooctylmethylphosphonate. The ß-activity of the selected radionuclides was measured using low-background radiometric facilities UMF-1500 and UMF-2000 with flame-photometric control of strontium carrier yield. All applied methods and measuring means have state calibration certificates.

3 Results and Discussion

Accumulation of ⁹⁰Sr and ¹³⁷Cs in the cereal crops and vegetable harvests under the conditions of a large agricultural enterprise. The ability of plants to accumulate radioactive substances is shown by the proportionality coefficients (C_p) between the soil contamination level and the level of contamination of agricultures. Also used are the transport coefficient (C_t)—the ratio of the radionuclide-specific activity in the marketable part of the harvest to the soil contamination density—and the accumulation coefficient (C_a)—the ratio of the radionuclide-specific activity in the marketable part of the harvest to the specific activity of the radionuclide in soil. These coefficients and their dependence on various factors are studied by many researchers.

We conducted research in 2007 in the territory of the agricultural enterprise «Sovkhoz Beregovoy» LLC. The territory of this enterprise was situated in the middle part of the East-Ural radioactive trace (EURT) that resulted from the explosion of a waste tank of the Production Association “Mayak” in 1957. The nearest settlements are Beregovoy, Karabolka, and Bulzi. The soil on which the “Sovkhoz Beregovoy” LLC is situated was formed as a gray forest medium to heavy loamy soil. The soil was treated for many years, its agrochemical properties have changed, and the humus horizon was exhausted in some places. Nevertheless, at that stage of the study when calculating the contamination density, we viewed it as a gray forest heavy loamy soil.

In all sampling points, the density of contamination with ¹³⁷Cs and ⁹⁰Sr exceeds the level determined by global fallouts. The average density of contamination with ¹³⁷Cs of a 0- to 40-cm soil layer from eastern irrigated field with vegetable rotation is 6.3 ± 1.1 kBq⁻¹ m² and with ⁹⁰Sr is 8.1 ± 1.5 kBq⁻¹ m². The contamination with ⁹⁰Sr of a 0–40 cm soil layer from the southern part of the farm is 7.4 ± 1.8 k Bq⁻¹ m², and the contamination of a 0–20 cm soil layer with ¹³⁷Cs is 4.4 ± 1.8 kBq⁻¹ m². The contamination of a 0–40 cm soil layer from the northern part with ¹³⁷Cs is 10.4 ± 1.8 kBq⁻¹ m² and with ⁹⁰Sr is 45.9 ± 14.1 kBq⁻¹ m². The highest contamination with ⁹⁰Sr (111.5 ± 28.9 kBq⁻¹ m²) is observed between 6 and 10 km from the Beregovoy village, where goes the axis of EURT.

The specific activity of samples of agricultural products fluctuated widely. The least contaminated were cabbage, beets, and potatoes. And the specific activity of ⁹⁰Sr and ¹³⁷Cs did not exceed 1 Bq kg⁻¹. The maximum permissible level of ¹³⁷Cs in vegetables is 120 Bq kg⁻¹ and ⁹⁰Sr is 40 Bq kg⁻¹ (SanPiN2.2.2.1078-01). This is due to a small contamination of soil in the eastern part and low values of C_a. The accumulation coefficients are shown in the Table 1. C_a of cereal crops is a little higher; in addition, some samples were taken in areas with a high contamination density. Therefore, the specific activity of ³⁷Cs in grain ranged from 0.05 to 3.42 Bq kg⁻¹ and that of ⁹⁰Sr ranged from 0.45 to 28.1 Bq kg⁻¹. The maximum permissible level of ¹³⁷Cs in the food grain is 70 Bq kg⁻¹, and the maximum permissible level of ⁹⁰Sr in the food grain is 40 Bq kg⁻¹ (SanPiN2.2.2.1078-01). The greatest values of C_a were observed in carrots, Sudan grass, and corn. However, these cultures were not grown on the most contaminated territories in the year when the study was conducted, so their specific activity did not exceed the maximum permissible level and ranged in carrots from 0.18 to 4.86 Bq kg⁻¹ for ¹³⁷Cs and from 0.4 to 5.77 Bq kg⁻¹ for ⁹⁰Sr, and in silage cultures from 0.54 to 3.2 Bq kg⁻¹ for ¹³⁷Cs and from 1.2 to 21.8 Bq kg⁻¹ for ⁹⁰Sr. Although in the year when the study was conducted, the permissible levels of radionuclides were not exceeded, we cannot say with certainty that contamination could not be greater in other weather conditions. Therefore, it required an annual radiological control of products grown in the territory of the northern part and in the area of the middle part of the road Bulzi–Beregovoy.

Table 1 Average values of C_a ¹³⁷Cs and ⁹⁰Sr in commodity part of crops for a layer of 0–20 cm of the gray forest soil

It is considered that as far as radionuclides change into bound forms that are less available to plants, their C_p decrease on the average. Thus, during the first year after the formation of EURT, C_t for potatoes (in Bq kg⁻¹/37 kBq⁻¹ m²) was 6.7 (according to our calculations, this corresponds approximately to C_a—0.044). In 1961–1962, C_t for grain was 9.3 (corresponds to C_a—0.060), for potatoes—6.7 (corresponds to C_a—0.044), for cabbage—11 (corresponds to C_a—0.071), for carrots—7.4 (corresponds to C_a—0.048), and for onions—8.4 (corresponds to C_a—0.054). During 1965–1967, according to different authors, C_t for grain was 6.7 (corresponds to C_a—0.044), for potatoes—0.6 (corresponds to C_a—0.004) and 1.0 (corresponds to C_a—0.006), for cabbage—8.9 (corresponds to C_a—0.058), for onions—15 (corresponds to C_a—0.097), and for carrots—17.4 (corresponds to C_a—0.113). In 1997, C_t for potatoes were 1.5 (corresponds to C_a—0.010) EMCRA (2001).

In 1999, in the Guidelines for agro-industry and forestry in the sanitary protection zone of the East-Ural radioactive trace were given summaries of the planned accumulation of ⁹⁰Sr in products that correspond according to our calculations in gray forest soils to the following C_a: wheat—0.038, barley—0.029, oats—0.036, corn (herbage)—0.408, potatoes—0.04, beets—0.062, and cabbage—0.060 (The guide to conducting agro-industrial production and forestry in the territory of a sanitary protection zone East Ural radioactive trace/Chelyabinsk: Administrator. Chernobyl Region 1999).

As can be seen from Table 1, the values of C_a for oats and barley that we obtained in 2007 were higher than values according to the summary data of 1999; for other cultures, they were lower. However, it must be considered that the assortment of agricultures is constantly changing and that even for the same cultivar in the same field at different sampling points, C_a and C_t can vary greatly.

Species and cultivars of agricultural plants vary in their ability to absorb ions selectively. Studies show that varietal differences in the accumulation of ¹³⁷Cs between the cultivars of cereal crops and pulses reach three times (Malikov et al. 1981). In the study of the ability to accumulate ⁹⁰Sr of 54 wheat cultivars, the intervarietal differences in two years amounted to 2.9 and 3.9 times (Korneeva et al. 1976). Four years after the Chernobyl accident were published the data that, when the cultivars of cereals were properly selected, the amount of ¹³⁷Cs in the harvest from the contaminated soils decreased by two to six times and in potatoes, by 13 times (Aleksakhin et al. 1990), whereas C_t are not the same on different soil types, so in 1965–1967, they ranged for grain (in Bq kg⁻¹/37 kBq⁻¹ m⁻²) from 7.4 (on leached black earth) to 13 on the gray forest soil and 18.5 on the soddy-podzolic soil (EMCRA 2001).

The availability of radionuclides to plants is influenced by their distribution in the soil profile and the agrochemical soil properties. In 1986–1990, in the territory of Russia after the Chernobyl accident were taken protective measures in order to reduce the content of ¹³⁷Cs in plant products (liming of acid soils, use of higher doses of phosphorus–potassium fertilizers). As a result, the contamination of grain and potatoes decreased by 1990 by 20–30 times. [The atlas of modern and expected aspects of consequences of the Chernobyl accident in affected territories of Russia and Belarus (ASPA Russia-Belarus, Moscow-Minsk 2009)].

This diversity is especially expressed in the samples in which specific activity is comparable with the level of sensitivity of the method of radiochemical analysis. Therefore, we cannot say with certainty that in the territory of EURT, C_p continue to decrease in the remote period after the accident.

3.1 Accumulation of ⁹⁰Sr and ¹³⁷Cs in the Harvest of Vegetables from Private Farms

In the private farms, the conditions of entry of radionuclides into plants vary significantly. The farmers usually do not follow the rules of agricultural machinery, use manual labor, and bring into the soil a small amount of mineral fertilizers. As an organic fertilizer, the farmers use manure from animals fed with contaminated feed. In addition in the farms are not usually grown purebred and revitalized plants. Therefore, the content of radionuclides in vegetables from private farms may be different than the content of radionuclides in vegetables from large agricultural enterprises.

For example, in the Bryansk region, after the Chernobyl accident, as a rule, the levels of contamination of products produced on private farms is 1.5–2.0 times higher than in the collective farms. [The atlas of modern and expected aspects of consequences of the Chernobyl accident in affected territories of Russia and Belarus (ASPA Russia-Belarus, Moscow-Minsk 2009).]

For assessing the contamination of settlements in the zone of influence of PA “Mayak” in 2009–2011 were taken samples of garden soil, potatoes, and other vegetables.

During the study period, 83 samples of garden soil were taken. Table 2 presents the average values of specific activity of radionuclides in the soil from different settlements. There is a significant inverse correlation of the average specific activity of ¹³⁷Cs in the soil of 36 settlements with distance from the source of contamination—r = −0.43 (p < 0.01). For the activity of ⁹⁰Sr, the correlation with the distance is not statistically significant and positive r = 0.29. This is due to the fact that the most contaminated with ⁹⁰Sr is the soil in the settlements that are close to the territory of EURT—Karabolka and Allaki and located at a distance of 38–39 km from the source of contamination. The highest coefficient of inverse correlation was obtained between the activity of ¹³⁷Cs in the soil and the angle of deviation from the southeast direction (azimuth 136°), r = −0.49 (p < 0.01). For ⁹⁰Sr, the direction for which the strongest connection was detected corresponds to the direction of axis of EURT (azimuth 30°), r = −0.34 (p < 0.05).

Table 2 Specific activity and C_a ¹³⁷Cs and ⁹⁰Sr in the soil and potatoes on the average on settlements in 2009–2011

It should be noted that the levels of soil contamination with radionuclides within one settlement can vary greatly. Apparently, this is connected with a multi-year application of manure produced by cows fed with contaminated hay. So, in the settlement Karabolka on the household plot under the address Shkolnaya Street, 20, the activity of ⁹⁰Sr in the hay was 56.1 Bq kg⁻¹ and in the soil, it was 279 Bq kg⁻¹. On a plot under the address Shkolnaya Street, 11-1, the activity of ⁹⁰Sr in a sample of hay taken in April 2011 was 2,524 Bq kg⁻¹ and in the sample taken in June 2011, it was 1,709 Bq kg⁻¹. In five soil samples taken from this plot using the envelope method, the activity of ⁹⁰Sr varied from 239 to 820 Bq kg⁻¹. On the neighboring plot under the address Shkolnaya Street, 26, located across the road, are no cows at the moment. The activity of ⁹⁰Sr in the soil is here 134 Bq kg⁻¹.

We can conclude from the results of tests conducted by Urals Research Center for Radiation Medicine earlier that usually about 10 % of the samples of garden soil, potatoes, vegetables, and other products produced in the personal household plots in the studied area are very different in content of radionuclides. The reason for this is that some owners violate the regime of EURT protection zone.

Therefore, for a rough estimation of the radiation situation in the whole territory, we calculated the average values for 90 % of the samples with the lowest values of radionuclide activity.

The average activity of ¹³⁷Cs in 90 % of the soil samples was 58 Bq kg⁻¹, and the maximum activity was 193 Bq kg⁻¹ (in the settlement Kyzylbulyak). The average activity of ⁹⁰Sr was 44.4 Bq kg⁻¹; the maximum activity of ⁹⁰Sr was 820 Bq kg⁻¹ (in the settlement Karabolka).

During the study period were selected 95 samples of potatoes. The average activity of ¹³⁷Cs in 90 % of the samples was 0.44 Bq kg⁻¹, and the maximum activity was 2.43 Bq kg⁻¹. The average activity of ⁹⁰Sr in 90 % of samples was 0.34 Bq kg⁻¹, and the maximum activity was 4.12 Bq kg⁻¹. In all samples of potatoes, the content of ¹³⁷Cs and ⁹⁰Sr is significantly lower than the maximum permissible levels (in potatoes and vegetables, MPL of ¹³⁷Cs is 80 Bq kg⁻¹ and MPL of ⁹⁰Sr is 40 Bq kg⁻¹, SanPiN 2.3.2.1078-01). Specific activities of ¹³⁷Cs and ⁹⁰Sr in potatoes are not connected—r = −0.04.

For all pairs of samples “soil—potatoes” were calculated accumulation for coefficients. The ability of potatoes to accumulate ¹³⁷Cs from soil (C_a) does not correlate with the ability to accumulate ⁹⁰Sr—r = −0.009.

The ranges of fluctuations of C_a ⁹⁰Sr and C_a ¹³⁷Cs are very large. Figure 1a, b shows histograms of distribution of radionuclides C_a in potatoes.

Fig. 1

a The histogram of distribution of C_a ⁹⁰Sr for potatoes. b The histogram of distribution of C_a ¹³⁷Cs for potatoes. c The histogram of distribution of ln(C_a) ⁹⁰Sr for potatoes. d The histogram of distribution of ln(C_a) ¹³⁷Cs for potatoes

The histograms illustrate that the values of C_a of ⁹⁰Sr and ¹³⁷Cs in potatoes from different household plots in the zone of influence of PA “Mayak” can vary considerably. Furthermore, the distribution of C_a is not normal. As can be seen from the Fig. 1c and d, the lognormal distribution suits better for statistical processing of data about C_a. However, as mentioned above, the violation of the protection zone regime by some owners complicates the statistical analysis. In our opinion, the most promising in this case is the application of the principles of fuzzy logic (Kazachonok 2013), but a correct development of the analysis method requires a large amount of experimental data.

Table 2 presents the average values of specific activity of radionuclides in potatoes from different settlements and the values of their C_a.

As can be seen from Table 2, C_a ¹³⁷Cs in potatoes in private farms were similar to C_a ¹³⁷Cs in «Sovkhoz Beregovoy» LLC. C_a ⁹⁰Sr in private farms appeared to be several times lower. As exactly ⁹⁰Sr poses the greatest danger in the territory of EURT, this difference requires further study.

We also analyzed 14 samples of carrots and 12 samples of beets. The average specific activity of ¹³⁷Cs in 90 % of the carrots samples was 0.59 Bq kg⁻¹, and the maximum activity of ¹³⁷Cs in carrots was 3.9 Bq kg⁻¹. The average specific activity of ¹³⁷Cs in 90 % of the beets samples was 0.35 Bq kg⁻¹, and the maximum activity of ¹³⁷Cs in the beets was 1.57 Bq kg⁻¹. For ⁹⁰Sr, the average activity was, respectively, 1.41 Bq kg⁻¹ in carrots and 1.31 Bq kg⁻¹ in beets, the maximum values of ⁹⁰Sr activity were 3.98 Bq kg⁻¹ in carrots and 19.6 Bq kg⁻¹ in beets. We have analyzed one sample of cabbage, one sample of radish and one sample of apples. The activity of ⁹⁰Sr and ¹³⁷Cs did not exceed their 1.0 Bq kg⁻¹.

Radionuclides C_a vary greatly. This complicates very much the prediction of radioactive contamination of agricultural products. In addition, radionuclides C_a in vegetables in farms were lower than in «Sovkhoz Beregovoy» LLC.

3.2 Accumulation of ⁹⁰Sr and ¹³⁷Cs in Grass Biomass in Natural Ecosystems

The study of regularities of radionuclides accumulation in the grass biomass is necessary for predicting the radioactive contamination of feed for farm animals.

In 2009–2011, we took 50 samples of hay from private farms. The average specific activity of ¹³⁷Cs in the hay in 90 % of the samples was 8.9 Bq kg⁻¹, and the maximum specific activity was 70.4 Bq kg⁻¹ (in the village of Bolshoe Taskino). The average specific activity of ⁹⁰Sr in 90 % of samples was 23 Bq kg⁻¹, and the maximum values that exceed the reference level had the hay samples from the village of Allaki (112, 656 and 1,132 Bq kg⁻¹) and from the village of Karabolka (1,709 Bq kg⁻¹ and 2,524 Bq kg⁻¹). The owners of household plots knew that they make hay on the axis of EURT and were aware that it is forbidden to make hay in these plots. The activities of ¹³⁷Cs and ⁹⁰Sr in hay do not correlate, r = −0.07. Therefore, a study of radionuclide accumulation in herbaceous plants is important.

In 2008–2011, we measured the content of ⁹⁰Sr and ¹³⁷Cs in the aboveground biomass of herbaceous plants from the grasslands on the territory within a radius of 30–35 km from the PA “Mayak.”

The farmers in the Chelyabinsk region make hay and graze cows on natural grasslands. Therefore, we did not classify the samples by the species of plant, but collected and analyzed the entire biomass of grass at a distance of not more than 1 m from the soil sampling point. All values of radionuclides content in the grass and C_a are calculated for the dry weight of grass. The reference levels (RL) of radionuclides in the feed according to the “instructions on the radiological control of feed quality” (The instruction about radiological quality control of forages. RL of the content of radionuclides of caesium-134, -137 and strontium-90 in sterns and feed additives)/(it is approved by the Ministry of Agriculture of the Russian Federation 01.12.1994 N 13-7-2/216)) are presented in Table 3.

Table 3 RL of radionuclides in a stern Bq kg⁻¹

In 2008, we measured the content of ⁹⁰Sr and ¹³⁷Cs in grass in two transects that cross EURT perpendicular (Tables 4 and 5). The first transect was located 20 km away from the PA “Mayak” in the grassland at the top of a slight slope to the shore of Lake Alabuga (Table 4). The first soil and grass sampling point was located at a distance of 20 m from the skirt of the birch forest, and the subsequent points were separated by a distance of 20 m from the previous points. Point 11 was at a distance of 40 m from the shore of Lake Alabuga.

Table 4 The contents ⁹⁰Sr and ¹³⁷Cs in a grass on a haying site on the first transect

Table 5 The contents ⁹⁰Sr and ¹³⁷Cs in a grass on the second transect

During the study period, the density of soil contamination with ⁹⁰Sr in this area ranged from 161 to 350 kBq⁻¹ m² and the specific activity of ⁹⁰Sr in grass samples in all sampling points was many times higher than RL. The values of C_a ⁹⁰Sr in grass are high, in some cases they exceed 1.0. Despite the fact that all sampling points were located within an ecosystem that has homogeneous species composition, the range of C_a values is quite large.

The grass contamination with ¹³⁷Cs in this area did not exceed RL, and C_a ¹³⁷Cs is significantly lower than C_a ⁹⁰Sr. The connection between C_a ⁹⁰Sr and C_a ¹³⁷Cs was not found, the correlation coefficient—0.50. The correlation between the specific activity of ⁹⁰Sr and ¹³⁷Cs in grass is even lower—0.15. The dependence of C_a and C_t on the average activity of ⁹⁰Sr and ¹³⁷Cs in soil and on soil contamination density with ⁹⁰Sr and ¹³⁷Cs was not detected.

The second transect was located 30 km away from the PA “Mayak.” The soil and grass sampling points were marked according to the distance from Chelyabinsk city. Table 5 presents data on the content of ⁹⁰Sr and ¹³⁷Cs in the grass on this transect. In this area, the sampling points were located in different ecosystems: birch forest, pine forest, grassland, hayfield, and neglected field. In six grass samples of 12, the activity of ⁹⁰Sr exceeded 100 Bq kg⁻¹, and at one point, it amounted to 1,400 Bq kg⁻¹. The values of C_a are very high too, especially in meadows and fields. The spread of C_a values is also very large; especially large is the spread of C_a ¹³⁷Cs. We assume that this can be explained by the fact that sampling was performed at the end of the growing season, when the accumulation of mineral elements in plant biomass is maximal. In winter, overwintering plants and perennials, the potassium content in the cytoplasm increases while preparation for the winter. Parallel can be accumulated its biochemical analogue—¹³⁷Cs.

In 2009–2012 was studied the contamination of grass in the territory within a radius of 30–35 km from PA “Mayak.” Grass on the entire territory of study contained a relatively small amount of ¹³⁷Cs. Even in the EURT, the maximum activity of ¹³⁷Cs was 62.6 Bq kg⁻¹, and out of EURT, it was from 2.5 to 14.1 Bq kg⁻¹. C_a ¹³⁷Cs during the study period was 0.06–0.08, so we can assume that the excess of RL of ¹³⁷Cs accumulation in grass in the areas of settlements is hardly probable. The accumulation of ⁹⁰Sr in the grass outside the EURT area is also small—from 6.9 to 31.5 Bq kg⁻¹. However, C_a ⁹⁰Sr is significantly higher than C_a ¹³⁷Cs, and on the average, in different years of study, it was 0.55–0.78, and in some samples, it exceeded 1.0. The accumulation of ⁹⁰Sr increases apparently at the end of vegetation to a greater extent than the accumulation of ¹³⁷Cs. Grass selected on July 15, 2011 near the village of Bolshoy Kuyash contained 3.1 Bq kg⁻¹ of ¹³⁷Cs and 13.5 Bq kg⁻¹ of ⁹⁰Sr. Grass selected in the same point on September 27, 2011 contained 4.7 Bq kg⁻¹ of ¹³⁷Cs and 30.9 Bq kg⁻¹ of ⁹⁰Sr. In September 2008, the coefficient of accumulation of ⁹⁰Sr in the grass averaged 2.14.

In the areas of settlements located near the axis of EURT (Karabolka, Allaki, Bolshoy Kuyash), the specific activity of ⁹⁰Sr in the grass can exceed RL. Next to the Lake Alabuga located near the axis of EURT in all grass samples, the content of ⁹⁰Sr exceeded by several times the reference level and reached 2,630 Bq kg⁻¹ in 2011.

The study of radionuclides accumulation in the grass from Techa River floodplain was complicated by the fact that the distribution of ⁹⁰Sr and ¹³⁷Cs in the profile of the floodplain soil is very heterogeneous. In the meadow floodplain soils, the major part of radionuclides remains in the upper soil layer and therefore, C_a ⁹⁰Sr and ¹³⁷Cs can be calculated based on the average content of these radionuclides in the upper soil layer. The calculation of C_t for these soils is also easy. Table 6 shows the values of C_t of anthropogenic and natural radionuclides in grass samples taken from the pasture on the bank of Techa River near the village of Zatechenskoe. When comparing the data in Tables 4, 5, and 6, it can be seen that C_t ⁹⁰Sr and ¹³⁷Cs in floodplain meadow soil and meadow soil near the Lake Alabuga differ only a little.

Table 6 C_t technogenic and natural radionuclides concerning a layer of earth of 0–40 cm (Bq kg⁻¹) (kBq⁻¹ sq. m⁻¹)

While calculating C_a in bog soils, the following factors must be considered:

• 1. Plant roots are short on air and have too much water, so they are situated in the upper soil layer.

• 2. The greatest content of radionuclides may be in the deep layers of the soil.

• 3. The highest concentration of water-soluble forms of radionuclides may be in the upper soil layer due to capillary rise and evaporation of soil water.

• 4. Floodplain soil is composed of a very large number of heterogeneous layers, so it is very difficult to measure the floodplain soil bulk density and to calculate the contamination density.

Table 7 shows the results of the analysis of floodplain grass and soil samples taken on the right bank of the River Techa near the bog Asanovoe at 5 m from the water. It is impossible to choose correctly the soil layer for calculation of C_a. It is necessary for calculation of C_a to measure accurately the bulk density of the soil and the activity of radionuclide in each thin layer. Bog soil is always waterlogged, so it is very difficult to divide it into layers.

Table 7 The contents ⁹⁰Sr and ¹³⁷Cs in the inundated soil and a grass in 2012 on the Asanovy bog, Bq kg⁻¹

3.3 Accumulation of ⁹⁰Sr and ¹³⁷Cs in Forest Products

In 2008–2011, we measured the content of ⁹⁰Sr and ¹³⁷Cs in wood, berries, and mushrooms.

Table 8 shows the values of the specific activity of ⁹⁰Sr and ¹³⁷Cs in young birch before the leaves start to yellow and fall off. The samples were taken on the axis of EURT 30 km away from the PA “Mayak” and 99 km away from Chelyabinsk. ⁹⁰Sr is mostly accumulated in the leaves, and ¹³⁷Cs is mostly accumulated in the branches. In general, C_a of radionuclides in birch and grass sampled at the same place (Table 5) differ only a little.

Table 8 Contamination of ⁹⁰Sr and ¹³⁷Cs wood and birch leaves

On October 31, 2008, we collected the fallen leaves in the birch forest near the Lake Alabuga. The specific activity of ⁹⁰Sr in fallen leaves was 5,904 Bq kg⁻¹, and that of ¹³⁷Cs was 54 Bq kg⁻¹. The annual contribution of yearly litter to the pollution of forest floor with ⁹⁰Sr is there estimated at 2.37 kBq⁻¹ m² or 2.1 % of the floor contamination that existed in the spring 2008. The contribution of litter to the contamination of floor with ¹³⁷Cs was 0.02 kBq⁻¹ m².

In 2008–2011, we have taken samples of mushrooms and berries in the territory within a radius of 30–35 km from the PA “Mayak.” In all the samples of mushrooms, the specific activity of ¹³⁷Cs and ⁹⁰Sr was significantly below the maximum permissible levels (SanPiN2.2.2.1078-01). The activity of ¹³⁷Cs in 2008–2011 was from 0.55 to 39.0 Bq kg⁻¹, and MPL was 500 Bq kg⁻¹ (SanPiN2.2.2.1078-01). The average accumulation coefficient of ¹³⁷Cs in mushrooms in 2011—0.011 (in 2008—0.011, 2009–2010 the samples of mushrooms were taken from a large area and the accumulation coefficient was not calculated). The activity of ⁹⁰Sr was from 0.14 to 9.8 Bq kg⁻¹, and MPL was 50 Bq kg⁻¹ (SanPiN2.2.2.1078-01). The average accumulation coefficient of ⁹⁰Sr in mushrooms in 2011 was 0,007 (in 2008, it was 0.041). C_a and C_t of ⁹⁰Sr in mushrooms are considerably lower than in the grass, and even lower than those in vegetables and cereal crops. C_a and C_t of ¹³⁷Cs in mushrooms are higher than of ⁹⁰Sr, but in the studied area, high levels of soil contamination with ¹³⁷Cs occur only on the floodplain bogs in the Techa River headwaters. We can assume that the mushrooms are not dangerous for the population; their contribution to the dose of internal irradiation is negligible.

Table 9 shows the values of the specific activity of ⁹⁰Sr and ¹³⁷Cs in mushrooms collected in the territory of EURT. Since mushrooms picked in the area of former village Russkaya Karabolka and the Lake Alabuga, and in areas with contamination density of 140, 700 and 2,400 kBq⁻¹m² have accumulated 8.9, 9.0, and 9.8 Bq kg⁻¹ of ⁹⁰Sr, it is possible that under certain conditions, there may be mushrooms that do not meet sanitary–hygienic standards. In the other territory (outside EURT), wild mushrooms from the areas of now existing settlements can be considered as harmless.

Table 9 Contamination of ⁹⁰Sr and ¹³⁷Cs fresh mushrooms on EURT

In the whole area of study, the specific activity of ¹³⁷Cs in 15 samples of wild berries (Fragaria viridis, Fragaria vesca, cranberries) in 2008–2011 ranged from 0.34 to 5.2 Bq kg⁻¹, on the average, it was 1.58 Bq kg⁻¹, and MPL was 160 Bq kg⁻¹ (SanPiN2.2.2.1078-01). The activity of ⁹⁰Sr in berries outside EURT was also low—from 1.5 to 12 Bq kg⁻¹, on the average, it was 5.5 Bq kg⁻¹, and MPL was 60 Bq kg⁻¹(SanPiN2.2.2.1078-01). The accumulation coefficients of ¹³⁷Cs in berries are almost the same as in mushrooms, and the accumulation coefficients of ⁹⁰Sr are significantly higher than in mushrooms and greater than in vegetables and cereal crops. In the berries of Fragaria vesca near the Lake Alabuga, the activity of ⁹⁰Sr in 2008 was 813 Bq kg⁻¹, and in the bramble, it was 243 Bq kg⁻¹. In the area of the former village of Russkaya Karabolka in a mixture of berries of Fragaria vesca and Fragaria viridis, the ⁹⁰Sr activity was 213 Bq kg⁻¹. It should be considered that wild berries from the territory of EURT pose the greatest danger from the perspective that ⁹⁰Sr enters into food. It is necessary to examine the nature of ⁹⁰Sr accumulation in berries more closely (Table 10).

Table 10 Contamination of ⁹⁰Sr and ¹³⁷Cs of berries on EURT

3.4 Study of the Soil Content of Radionuclides Forms that are Available for Plants

Long-term studies of various authors prove that the soil content of ¹³⁷Cs and ⁹⁰Sr in forms that are available for plants changes over time. Their accumulation coefficients in the marketable part of agricultural products change accordingly. In particular strongly expressed is the decrease in mobility of radionuclides in the first years after the accident.

The studies of the behavior of radionuclides in soils show that a significant part of ⁹⁰Sr and ¹³⁷Cs changes quite rapidly into immobile forms. An immobile form is a form in which the radionuclide is bound in the root systems of plants or soil adsorption complex as a result of adsorption and precipitation (EMCRA 2001). Since the processes of adsorption and precipitation are reversible, the ratio of mobile and immobile forms is determined by the ratio of the speed of bonding and release of radionuclides. The elements bound in the root system and aboveground biomasses are released as far as plant litter decomposes. Elements bound in the soil adsorption complex move into the soil solution as a result of exchange reactions.

We have previously shown that in case of simulation of the washing water regime for a long time, the release of radionuclides from the alluvial soil with gravitational waters ranged from 10.8 to 12.4 % of the contained ⁹⁰Sr and from 0.08 to 0.52 % of ¹³⁷Cs.

In 2008, we measured the content of water-soluble, exchange, and acid-soluble forms of ⁹⁰Sr and ¹³⁷Cs in the gray forest and sod-podzolic soil. We compared the content of bioavailable forms in layers of 0–5 and 5–10 cm in a naturally moist and air-dry gray forest soil, air-dry gray forest soil, and sod-podzolic soil. The results are shown in Tables 11 and 12.

Table 11 The contents in the soil of forms ⁹⁰Sr available to plants in percentage

Table 12 The contents in the soil of forms ¹³⁷Cs available to plants in percentage

We failed to reveal significant differences of content of biologically available forms of strontium in the studied samples of naturally moist and air-dry gray forest soil. There are also practically no differences between layers of 0–5 and 5–10 cm. It was found that in the sod-podzolic soil, the content of exchange form is higher and the content of acid-soluble form is lower than in the gray forest soil.

The content of bioavailable forms of ¹³⁷Cs varies more. In naturally moist gray forest soil in comparison with air-dry soil, the content of exchange form is decreased slightly in a layer of 0–5 cm. In the sod-podzolic soil higher is the content of acid-soluble form of ¹³⁷Cs. However, the greatest differences were observed not between the soil types, but between the layers of 0–5 and 5–10 cm. In all cases, the content of bioavailable forms of ¹³⁷Cs is higher in the 5- to 10-cm layer and the content of strongly bound forms that remain in the solid residue is significantly lower in this layer. Perhaps a high content of mobile forms explains the sharp decline in contamination with ¹³⁷Cs of this layer, with the assumption that in the underlying soil layer goes more ¹³⁷Cs than ⁹⁰Sr. We also compared different modes of extraction of ⁹⁰Sr and ¹³⁷Cs with distilled water from the floodplain soil.

It follows from Table 13 that the amount of ¹³⁷Cs extracted with distilled water from the floodplain soils when using the normal mode of extraction is lower than from the automorphic soils (gray forest and sod-podzolic soil). Apparently, this is due to the fact that the waterlogged floodplain soil quickly loses water-soluble forms of ¹³⁷Cs. The content of radionuclides in the aqueous extract, as was to be expected, depends on the extraction mode and soil condition. When using dry soil, we observed statistically significant differences in the amount of ¹³⁷Cs extracted in the ratio of 1:15, 1:30, and 1:200. The greatest amount of ¹³⁷Cs is extracted in case of threefold rinsing, although the amount of water in this variation is the least. Apparently, in case of long 3-h mixing takes place a more intensive dispersion of soil particles.

Table 13 Release of water-soluble radionuclides from the inundated soil at different modes of extraction, percentage of the grass

Quantity of ⁹⁰Sr extracted in the ratio of 1:15 and 1:200 does not vary, but it is significantly statistically greater than when extracting in the ratio of 1:30.

When moist soil is used, the amount of ¹³⁷Cs extracted in different modes is also statistically different. The extraction was the best at the ratio of 1:200 and the worst at the ratio of 1:30. It is important to note that from the moist soil in all modes of extraction was released 2–6 times more ¹³⁷Cs than from the dry soil. The differences are statistically significant. For ⁹⁰Sr, this dependence is less expressed, and at the ratio of 1:200, differences between moist and dry soil were not observed at all.

To study the possibility of recovery of content of water-soluble fractions of ¹³⁷Cs and ⁹⁰Sr when moistening the dried soil, we used the same soil sample that was used for the determination of content of water-soluble fractions of ¹³⁷Cs and ⁹⁰Sr. The results of analysis of aqueous extracts are shown in Table 14.

Table 14 The content of radionuclides in the inundated soil and water extracts from the inundated soil after drying and moistening, percentage of the contents in the soil

It follows from Table 14 that when soil is moistened during 1–16 days, there is no recovery of water-soluble ¹³⁷Cs. The quantity of extractable ¹³⁷Cs after moisturization is slightly higher than when the dry soil is used. When soil is moistened during 4–16 days, this difference is statistically significant. When soil is moistened during 1 day, the amount of extractable ⁹⁰Sr is apparently restored. In the future, it reduces. The mechanism of this phenomenon is not entirely clear. We attempted to estimate the amount of ¹³⁷Cs and ⁹⁰Sr that enter into an aqueous solution of soil in case of a longtime continuous extraction.

The soil sample was collected from the 0–20 cm layer of alluvial meadow bog soils in the floodplain of the Techa River. In the moist condition, the soil was rubbed through a 0.5-mm sieve to remove fragments of plants and large mineral particles, thoroughly mixed and divided into two parts.

Part 1 (S—sterilized) was irradiated in the wet condition at a dose of 2,800 Gy (70 h at a dose rate of 0.011 Gy s⁻¹) to kill microorganisms. Part 2 (US—unsterilized) was not irradiated. From each part in the wet condition were made three different samples, 10 g each, and they were put into centrifuge tubes. In the tubes were added 50 ml of distilled water, and then, they were stirred for 60 min on a magnetic stirrer, then centrifuged for 60 min at the velocity of 2,500 rpm, and then poured off and filtered the supernatant fluid. To the rest of soil again were added 50 ml of water, and the cycle was repeated. Three cycles were made during 1 day. After 60 cycles, into the collected filtered supernatant were poured up to 5 l of distilled water, and it was stirred and separated into two parts.

In one part, we precipitated colloids using FeCl₃ (0.2 g l⁻¹) and filtered them in 1 day. Thereafter, we measured the content of ⁹⁰Sr and ¹³⁷Cs in the filtrate after precipitation of colloids (F), in the colloids sediment (C), and in the second part of supernatant liquid with unprecipitated colloids (NPC).

The quantity of ¹³⁷Cs and ⁹⁰Sr that entered into aqueous solution during 60 cycles is shown in Table 15.

Table 15 Relative quantity of ¹³⁷Cs and ⁹⁰Sr arrived in water solution for 60 cycles. Percentage of the initial contents in the soil

From Table 15 follows that when using this method of extraction, about 12–20 % of ⁹⁰Sr and ¹³⁷Cs contained in soil can move into aqueous solution. Differences between the release of radionuclides from sterilized and unsterilized soil were not found. ⁹⁰Sr does not practically precipitate with colloids and, apparently, moves primarily into regular solution in ionic form. ¹³⁷Cs precipitates almost completely, and there are reasons to assume that in the extract it is in the form of a colloidal solution.

4 Conclusion

• 1. The content of ⁹⁰Sr and ¹³⁷Cs in agricultural products and forest mushrooms in the entire territory within a radius of 30–35 km from the PA “Mayak” does not exceed the maximum permissible levels and poses no danger to the population.

• 2. The accumulation of ⁹⁰Sr in wild berries and grass on the territory of EURT may be higher than the radiation-hygienic standards and poses a danger to the population and farm animals.

• 3. The content of ⁹⁰Sr and ¹³⁷Cs in the soil in the form that is available for plants depends on soil moisture and can vary considerably.