1 Introduction

Operation of the Krasnoyarsk Mining and Chemical Combine (KMCC), located approximately 40 km downstream from the city of Krasnoyarsk (Russia), near the Yenisey River, started in August 1958 and continued until 1992. One of its functions was the production of weapon-grade Pu (Sukhorukov et al. 2004). Discharges of radioactive wastes from the KMCC to the river channel have led to artificial radionuclide contamination of river water and sediments. However, no data on parameters related to the actual discharges were available for the early years of operation.

The most complete dataset on KMCC discharges covering the period from 1987 to 2000 was published by Vakulovsky et al. 2006. According to these data, radionuclides released to the river can be arranged in order of descending activity as 60Co—671 GBq > 137Cs—316 GBq > 152Eu—262 GBq. The mean annual discharge of the aforementioned radioisotopes was 111.8, 52.7, and 43.7 GBq, respectively (Table 1).

Table 1 Evaluation of annual and total discharges of 60Co, 137Cs, and 152Eu by the KMCC in 1987–1992 (GBq, Vakulovsky et al. 2006)
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In 1992, after the shutdown of two reactors, the radioisotope inflow to the river channel decreased by an order of magnitude and the total activity of 60Co, 137Cs, and 152Eu released to the Yenisey River in the period from 1993 to 2000 was about 41.8, 29.1, and 5.4 GBq, respectively. There was a significantly greater decrease in 60Co, and especially 152Eu, contributions to the total radionuclide contamination in the late period (from 21 to 7 % of the total activity for 152Eu).

Traces of various medium-lived radionuclides, such as 137Cs, 60Co, 152 Eu, and 154Eu with half-lives of 30.2, 5.3, 13.3, and 8.6 years, respectively, have been registered in alluvial soils even at the beginning of this century (2000 onwards). These radionuclides have been measured at distances of up to 2000 km from the discharge point in the lower reaches of the Yenisey flowing to the Arctic Ocean (Linnik et al. 2013).

Detailed radioecological studies of the Yenisey focusing on the contamination of water and sediments started only in the 1990s and were most frequent in the last decade (Sukhorukov et al. 2000; Bondareva and Bolsunovskii 2008; Bondareva and Artamonova 2011; Sukhorukov et al. 2004, 2006; Linnik et al. 2005, 2006; Vakulovsky et al. 2006, 2014; Bolsunovsky 2011; Kropatcheva et al. 2012; Korobova et al. 2014). The main practical goal of these studies was to evaluate the level and extent of contamination and its radioecological significance. Three radionuclides, i.e., 137Cs, 60Co, and 152 Eu, are of the utmost interest nowadays due to their radioecological importance.

Cs-137 pathways and radioecological effects have been studied intensively worldwide reflecting the radioisotope’s wide distribution and its comparative ease of determination compared to other radionuclides. Its migration in the basins of the Yenisey River and those of the Dnieper and its tributaries, in particular, was investigated in detail after the Chernobyl accident (Vakulovsky et al. 1994; Sansone et al. 1996).

Cobalt-60, a radioisotope which is almost always present in the releases of the enterprises producing weapon-grade Pu and other radiologically hazardous materials, may significantly contribute to doses to human populations and biota, over considerable time periods. For Pu, this may reflect its practically negligible vertical migration in mineral sediments ((1–4) × 10−12 m2 s−1) in the Yenisey coastal floodplain deposits (Nosov 1997).

Cobalt forms highly stable complexes with organic ligands, especially humic and fulvic acids (Glaus et al. 2000). Therefore, soils, or horizons that are rich in organic matter (OM), are able to bind Co more strongly and for longer periods. However, the portion associated with mobile fulvic acids is known to enhance its mobility.

Compared to 137Cs and 60Co, the distribution of 152,154,155Eu has been investigated to a narrower extent and in less detail. The Yenisey river valley downstream from KMCC seems to remain the main site in Russia where Eu migration has been studied because of the considerable levels of contamination associated with this radioisotope in this area. In 1993–2000, the specific activity of 152Eu in Yenisey bottom sediments in the near impact zone attained levels of 2000 Bq kg−1 and the contamination density of alluvium soils due to this radionuclide attained levels, on several islands, of up to 2.6 × 106 Bq m−2. Both 152Eu and 137Cs isotopes were measured 1000 km downstream from their discharge point (Nossov et al. 2002). Mechanisms of 152,154,155Eu migration in the contaminated alluvium are still poorly studied. Vakulovsky et al. (2006) found that in October 5th, 2001, when only 1 out of the 3 reactors initially constructed to produce weapon-grade Pu was still operating, the major portion (61 %) of the total 152Eu discharge (230 Bq m−3) was water soluble and about 39 % was associated with a suspended load.

It has been previously shown that studies of technogenic radionuclide distributions in different granulometric fractions of alluvial soils are important in enabling the reliable reconstruction of the radiation situation for particular landscape-geochemical conditions following serious radiation accidents, such as Chernobyl (Korobova et al. 2008). This is also true for cases of technogenic contamination arising from more routine releases, such as those that occurred in the Yenisey basin (Linnik et al. 2014).

The purpose of this study was to analyze the distribution of 60Co, 137Cs, and 152Eu in alluvial soils downstream from the KMCC and to compare these radionuclides in terms of their ability to associate with different granulometric and organic fractions. The study is based on the data (published and unpublished) of the international project STREAM (Brown et al. 2002; Linnik et al. 2005, 2006).

2 Study area and methods

The study was carried out over several islands and the coastal plain of the Yenisey River, 20 to 250 km downstream from the KMCC (Fig. 1). Regarding radionuclide decay, data concerning radioactive contamination that occurred mainly from the 1960s to the 1980s can now be obtained only by investigation of soil samples and fractions containing measureable quantities of radionuclides. In this publication, we analyze the data obtained from nine soil profiles located at different distances from the KMCC (Fig. 1). The sampling sites were selected at different elevations on the floodplain after a preliminary field radiometric survey indicating areas of maximum radionuclide contamination (Linnik et al. 2005, 2006). Samples were collected to a depth of 40–50 cm and divided into different increments according to the soil master horizons and profile lithology. The samples were air dried and stored under cool conditions.

Fig. 1
figure 1

Study sites and samples’ location (a modified copy from Standring et al. 2009)

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Radionuclide activity in different soil horizons and in their fractions was determined using a Canberra spectrometer equipped with an HPGe detector. The minimum measured activity for 137Cs was less than 0.1 Bq per sample within a counting period of 3600 s. Granulometric analysis was performed for 13 samples with maximum specific activity for 137Cs, 60Co, and 152Eu. In the zone nearer to the KMCC (the Beryozovy Island and B. Balchug), the set of samples included the following profiles and depth horizons (cm): B2-14 (0–5), BP4-1 (8–12), and BP4-45 (5–8). Tolsty Island is represented by samples TOP1-44 (5–10; 35–40); TOP1-48 (4–8; 28–33); the Kazachinskoye site—by KP1-11 (0–5), KP1-28 (5–10); and Ust-Tungusky Island—by UTP-8 (2–12; 19–23), UTP-10 (0–2; 15–20). The resulting numbers of sites with an analytically detectable amount of 137Cs, 60Co, and 152Eu in the different fractions were 13, 9, and 8, respectively.

Granulometric techniques included dry screening and a modified Petelin method (Sval’nov and Alekseeva 2005) with a 1-min ultrasonic pre-treatment by Fritsh Laborette (Germany). Fractions <0.05 mm were collected utilizing a pipette method. The initial weights of the samples were approximately 30 g. Fractionation allowed separation of the following particle size ranges (mm): 1.0–0.5, 0.5–0.25, 0.25–0.125, 0.125–0.1, 0.1–0.063, 0.063–0.05, 0.05–0.01, 0.01–0.005, 0.005–0.001, and <0.001. In this paper, we discuss pelite (<0.01 mm) and aleurite (0.01–0.1 mm according to Puffengoldz 1978) fractions as the most significant for radionuclide fixation (Korobova et al. 2008, 2014).

To study radionuclide association with the total (Ctot) and organic fraction of C (Corg), we analyzed correlations between 60Co, 137Cs, and 152Eu activity and the content of Ctot and Corg in 20 bulk samples collected from different horizons to a depth of 40–50 cm at soil profiles BP4-1, BP4-45, KP1-11, KP1-28, and TOP1-48 (Fig. 1). Concentrations of C in the soil horizons were determined using a Hewlett-Packard CHN analyzer Model 185. Carbon concentrations were measured before and after elimination of carbonates to separate organic carbon and carbonates. Organic C (Corg) was C determined after removal of carbonates, while Ccarb was calculated as the difference between the total C (Ctot) determined before carbonate removal and Corg.

The relationships between radionuclide and OM were studied on a sample from a layer at 5–10 cm depth from the soil profile BT2-14 (a duplicate of profile B2-14) located in the upper part of the Beriozovy Island (20 km from KMCC). The sample was subjected to extraction of humic and fulvic acid fractions and subsequent determination of radionuclides in the separated phases and the residue component. Organic fractions were separated by saturation of the air-dry sample with 0.1 M NaOH, with further precipitation of humic acid from an extract brought to pH 1 with 1 M HCl (Varshal 1994; Varshal et al. 1996). The separation resulted in three fractions of fulvic acids, humic acids, and residue containing the denuded mineral phase and the refractory organic residue.

In general, the data were useful in the process of characterizing the sites but were not amenable to statistical analyses. However, we still consider it worthwhile to present these data in the form of a case study. The experimental data may be used for supporting hypotheses pertaining to patterns of artificial radionuclide migration and sedimentation for particular sites (each with their own characteristic granulometry, content of organic and mineral C in alluvium).

3 Results and discussion

3.1 Parameters of KMCC radionuclide discharge behavior

Radionuclides may be discharged in water-soluble (ions, complexes), molecular, colloidal, and suspended particulate forms (Varshal 1994; Novikov 2010).

Once released, these forms could interact with natural mineral and organic suspended particles and colloids. During high water periods, with concomitant elevated suspended sediment loads, contaminated river waters come into contact with soils. The locations where this occurs can provide sedimentation areas for radionuclides. This has been simulated and experimentally proven by Nosov et al. (2010). Apart from physical and chemical sorption, a characteristic deposition of the suspended load, controlled by the prevailing hydrodynamic conditions, contributes to sedimentation of the suspended fraction of radionuclides (Linnik et al. 2006, 2014). It is worth noting that chemical properties of radioisotopes are similar to their stable analogues, but their mass amounts are not comparable and, in general, the behavior of trace elements depends upon the distribution of the major elements defining geochemical conditions (the so-called typomorphic elements, Perel’man 1975).

3.2 Distribution of 60Co, 137Cs, 152Eu, and pelite and aleurite fractions in soil samples

Alluvium composition of the coastal and central parts of the floodplain is often variable in terms of texture. This is believed to reflect differences in the hydrological regime for sedimentation under high water discharge periods.

The distribution of 137Cs-specific activity in different size fractions for a soil layer at a depth of 15–20 cm (sampled in the low-level floodplain of the Beriozovy Island in a dry shallow floodplain lake) is shown in Fig. 2. Here, we have the case of classical radionuclide contamination of floodplain soils due to sedimentation of fines. A considerable increase in specific 137Cs activity was characteristic in three particle size classes belonging to the pelite fraction (<0.01 mm), with no significant difference among them: 20.0, 26.4, and 20.2 Bq g−1, respectively (Fig. 2). A statistically significant higher activity of particles in the 0.01–0.05 mm range (coarse silt or fine sand depending on different classifications) as compared to 0.05–0.100 mm grains (very fine sand, U.S.D.A) was found in the aleurite fraction: 4.6 versus 1.3 Bq g−1.

Fig. 2
figure 2

Specific activity of 137Cs in alluvial soil sampled at the low-level floodplain of the Beriozovy Island (core BP2-14, layer 15–20 cm deep)

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The relationship between 137Cs activity and percent of aleurite and pelite fractions in the bulk alluvial soil samples (down to a depth of 40 cm) collected at study sites at a distance of 250 km downstream from the KMCC (Fig. 1) is shown in Fig. 3.

Fig. 3
figure 3

Cs-137 and percent of aleurite and pelite fractions in the alluvial soil samples collected 20–250 km downstream from the KMCC

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No correlation was found between 137Cs-specific activity and the content of pelite and aleurite fraction of the bulk samples. In contrast to 137Cs, both 60Со and 152Eu exhibited a clear positive exponential correlation with the pelite content in the bulk samples (R 2 = 0.67 and 0.77) and a negative relationship with the aleurite content (R 2 = 0.87 and 0.77, Figs. 4 and 5). Several reasons may be considered to explain this behavior.

Fig. 4
figure 4

Co-60 and percent of aleurite and pelite fraction in the alluvial soil samples collected 20–250 km downstream from the KMCC

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Fig. 5
figure 5

Eu-152 and percent of aleurite and pelite fractions in the alluvial soil samples collected 20–250 km downstream from the KMCC

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The lack of correlation for 137Cs may be explained by the river transport of this radionuclide in water-soluble and suspended forms in almost equal proportion. Moreover, the aggregation and adhesion of the most contaminated clay particles, as shown by Korobova et al. (2014), and a predominance of the low radioactive coarse fractions in the bulk samples, leading to the reduction in the contribution of the accumulating fraction to the total activity of the sample, may account for this observation. According to Sukhorukov et al. (2006), from 3 to 40 % (18 % on the average) of 152,154Eu contained in floodplain and bottom sediments occurred in water-soluble and easily exchangeable forms. The percent of 152Eu in the acid-soluble extract ranged from 79 to 99.2 %, while for 154Eu, this fraction varied from 69 to 96.7 %. The authors suggested that 152,154Eu radioisotopes exist in the Yenisey alluvial soils in the forms of acid-soluble microparticles, organic complexes, and some (unspecified) sorbed state. Taking into account the consideration that the time elapsed following major discharges was sufficient to enable a considerable degree of fixation, it can be inferred that the initially discharged fraction of 60Co and 152Eu was mainly water soluble, but both radionuclides formed mobile organic complexes that were readily adsorbed by the finest particles. As Eu is also a rare-earth element, the possibility of its chemisorption (in valence state II) by carbonates that are present in water and sediments cannot be excluded a priori.

3.3 Distribution of 60Co, 137Cs, 152Eu, and organic carbon in soil samples

Carbon concentration (Corg) in 20 samples from five soil profiles sampled at the Beriozovy, Tolsty, and Kazachinskoye sites varied in a wide range from 0.085 to 4.41 %, with an average value of 1.10 %. Cesium-137 did not show any relation to Corg, while Co and Eu activity appeared higher in samples with larger OM content. In particular, 152Eu demonstrated a distinctly higher affinity to OC than 60Co (Fig. 6). This finding is in agreement with published data on radionuclide behavior (Varshal 1994; Varshal et al. 1996; Novikov 2010). The same order of affinity of these radionuclides to organic substances (152Eu > 60Co) was reported by Bondareva and Bolsunovskii (2008) who studied radionuclide speciation in sediments sampled from the near impact zone by sequential extraction.

Fig. 6
figure 6

Correlation between 137Cs, 60Co, 152,154 Eu activity, and organic carbon (Corg) concentration in layers of floodplain soil samples collected 20–250 km downstream from KMCC

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3.4 Relation of vertical 252Eu distribution in soil profile to mineral and organic forms of C

Recent studies of the Yenisei River bottom sediments, 15 km downstream from the discharge point, showed that 40–60 % of 152Eu is present in the organic fraction and 2–7 % in carbonates (Bondareva and Bolsunovskii 2008). However, the vertical distribution of the radionuclide in soil profiles was not investigated.

Regression analysis performed for six successive layers of BP4-1 soil profile, located in the Beriozovy Island, showed that about 94 % of 152Eu variation in fines of the layers could be explained by a linear model with Corg and Ccarb values as independent variables in the form

Eu − 152(Bq g− 1) = 0.34 * Corg(%) + 0.29 * Ccarb(%) − 0.14 (Fig. 7).

Fig. 7
figure 7

Plots of regression analysis showing model fitting the real measurement data in B2-14 soil profile layers

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This relation supports the hypothesis of a chemisorption of Eu by carbonates that may be critical for its migration.

The relevance of both forms of Eu accumulation on C was also supported by a higher correlation found between 152,154Eu activity and TOC, as compared to 152,154Eu and Corg in 20 samples collected in five soil profiles at three sites (BP, TOP, KP; Fig. 1) located 20, 50, and 180 km downstream from the KMCC (Figs. 8 and 6c).

Fig. 8
figure 8

Correlation between 152,154Eu activity and total carbon content (Ctot) in layers of floodplain soil samples collected 20–250 km downstream from the KMCC

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No dependence was found for the corresponding 137Cs values.

3.5 Radionuclide distribution in organic fractions of different mobility

Figures 9 and 10 show distributions of artificial and natural radionuclides in the fractions of fulvic acids, humic acids, and the residue containing the denuded mineral phase and the refractory organic residue. Radionuclides found in the first fraction, characterized by low molecular weight organic compounds, were the most mobile, whereas those in the second fraction were less mobile as they were associated with acid-resistant humus complexes that might be considered to be virtually insoluble in natural waters of the humid zone. The presence of radionuclides in the residue fraction is indicative of a portion strongly fixed onto the mineral matrix.

Fig. 9
figure 9

Distribution of the artificial radionuclides in organic and residue fractions (FA, fulvic acid fraction; HA, humic acid fraction; RES, residue)

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Fig. 10
figure 10

Distribution of the natural radionuclides in organic and residue fractions (FA, fulvic acid fraction; HA, humic acid fraction; RES, residue)

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Radiocesium showed higher affinity to the high molecular mass organic compounds. Eu-152 and 60Co, which are well known to undergo organic complexation, were present in considerable relative amounts in the fraction of the most mobile, low molecular mass fulvic acids (Fig. 9). K-40 and 228Th associated with various minerals dominated in the residue fraction. Cs-137, in a similar way to 238Th and 40K, was mainly associated with the residue mineral fraction, presumably due to exсhangeable sorption and further fixation in K-minerals (Fig. 10).

In general, our analysis of the affinity of radioisotopes to C forms showed that 152Eu > 60Co were likely fixed in soil particles mostly by the organic and carbonate components, while 137Cs association with OM was mainly phenomenological and indicated radiocesium fixation by the top organic soil horizons.

4 Conclusions

Cs-137 discharged from KMCC is mainly associated in floodplain soils and sediments with mineral particles of fine sands, silt, and clay that may form aggregates. Europium radioisotopes are mainly hosted by the pelite fraction.

152Eu > 60Co are fixed in soil particles predominately by the organic and carbonate components, while 137Cs association with OM is mainly phenomenological and indicates radionuclide contamination of the top humic horizons.

A different association of 137Cs, 60Co, and 152 Eu with particulate and organic fractions of river sediments and floodplain soils could be explained by the dominating discharge form (i.e., water soluble or particulate), affinity to organic substances of different mobility, sorption by minerals and their aggregates, and chemisorption.

This study confirms the contention that a comparative analysis of radionuclide distributions in soil fractions of different sizes and C content in soil layers and sediments subjected to radionuclide contamination can give valuable information for evaluation. This relates not only to the general radioecological situation and contamination levels but also to elucidation in terms of expanding the possibilities for interpretation of contamination events and migration patterns.