technogenic radionuclides in undisturbed bulgarian soils

Journal of Geochemical Exploration xxx (2014) xxx–xxx

GEXPLO-05288; No of Pages 6

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Journal of Geochemical Exploration

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Technogenic radionuclides in undisturbed Bulgarian soils

Ivanka Yordanova ⁎, Donka Staneva, Lidia Misheva, Tsvetanka Bineva, Martin BanovInstitute of Soil Science, Agrotechnology and Plant Protection “N. Poushkarov”, Laboratory of Radioecology and Radioisotopes Research, Shousse Bankya Str. 7, 1080 Sofia, Bulgaria

⁎ Corresponding author.E-mail address: [email protected] (I. Yord

0375-6742/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.gexplo.2014.01.011

Please cite this article as: Yordanova, I., et adx.doi.org/10.1016/j.gexplo.2014.01.011

a b s t r a c t
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Article history:Received 5 November 2012Accepted 13 January 2014Available online xxxx

Keywords:RadionuclidesSoilCesium-137Strontium-90Plutonium

This work presents data for the activity concentration of artificial radionuclides in soils from different regions inBulgaria for the period 1985–2011. The subjects of the studywere undisturbed soils frommountainous, hilly, andplain areas. Special attention has been paid to the contamination with the long-lived technogenic radionuclidescesium-137, strontium-90, plutonium-238, and plutonium-239+ 240. 137Cs was measured by low level gammaspectrometry and 90Sr by radiochemical separation and low level beta counting. The isotopes of plutoniumweremeasured by alpha-spectrometry after a radiochemical procedure of purification and concentration.Cesium-137 and strontium-90 were the main technogenic radionuclides detected in the examined Bulgariansoils few years after the Chernobyl NPP accident. Their content in the soils from high mountain areas is severaltimes higher than that in the soils from the plane areas in the northern part of the country. High heterogeneityin the pollution has been observed. Mean soil activity concentration levels of up to 250 Bq kg−1 for 137Cs and14 Bq kg−1 for 90Sr were found in different years. The maximal values registered in separate soil samples wereabout 1300 Bq kg−1 (137Cs) and 24 Bq kg−1 (90Sr). The activity concentration levels for 239 + 240Pu varied be-tween 0.1 and 3.7 Bq kg−1. The results indicate that the radioactive pollution of the Bulgarian soils with artificialradionuclides is a result of global fallout and Chernobyl nuclear power plant accident.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Technogenic radioactivity is due to radioactivematerials which havebeen produced and released into the environment by human nuclearactivity, including nuclear weapon testing, the operation of nuclearpower plants, research reactors, and nuclear fuel reprocessing. Nucleartest explosions were carried out at a number of sites, mostly located inthe northern hemisphere, between 1945 and 1980. 502 atmospherictests with a total fission and fusion yield of 440 Mt were conducted.The total explosive yield of underground tests is estimated to be 90 Mt(UNSCEAR, 2008).

Nuclear accidents, such as theChernobyl accident, have also releasedlarge amounts of radionuclides into the environment. The accidentin the Chernobyl nuclear power plant (ChNPP) caused the largestuncontrolled radioactive release into the environment dispersed onover the entire northern hemisphere. It was estimated that about85 PBq of 137Cs and about 8 PBq of 90Sr were released from the accident(UNSCEAR, 2008). The radionuclides with long half-life like 137Cs, 90Sr,238Pu, and 239 +240Pu deposited after the accidentwill remain in the en-vironment, mainly in the soil, for decades and will be relevant for envi-ronmental monitoring. Analysis of radionuclide content in soil, plants,and water and knowledge of the behavior of the radionuclides in soil–plant system provides an important part of a data basis for dose estima-tion (UNSCEAR, 1993, 2000).

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Bulgaria, South Bulgaria in particular, was among the Europeancountries with relatively high contamination as a result of the Cherno-byl accident. According to UNSCEAR (1988, Annex D, Table 11), thedeposition of 137Cs on the southern part of Bulgaria is 12 kBq m−2,which is significantly higher than the deposition for countries likeFrance (max 3.2 kBq m−2), Belgium (max 0.84 kBq m−2), Poland(max 5.2 kBq m−2), Hungary (4.8 Bq m−2) and Greece (8 kBq m−2).In the same report (Annex I, Fig. VI) Southern Bulgaria is indicated as aterritory with 137Cs deposition density higher than 5 kBq m−2, whilefor a large part of the European territory it is between 1 and 5 kBq m−2.The activity concentration of man-made radionuclides in theBulgarian soils increased considerably (Kinova, 1997; Pourchet et al.,1997; Tsvetkov et al., 2006; Zhiyanski et al., 2008). Up until now,25 years later, 137Cs and 90Sr are still detected in all soil samples andrepresent a potential danger for the contamination of the plant produc-tion through root feeding. Our work aims the assessment of mean localbackground values for 137Cs and 90Sr in soil on the base of regular radio-logical monitoring of undisturbed soils from high mountainous, hilly,and plain areas in Bulgaria. The areas around Kozloduy NPP are of spe-cial interest. Such data can be used as a reference level from which wecan detect eventual future release of radioactive nuclides.

2. Materials and methods

The sampling strategy chosen for the purposes of the radiologicalcharacterization of the soils in Bulgaria surveying the impact of NPPKozloduy and the evolution of the territory in general corresponds tothe orientated systematic strategy described in ISO, 18589-2 (2007).

s in undisturbed Bulgarian soils, J. Geochem. Explor. (2014), http://

http://dx.doi.org/10.1016/j.gexplo.2014.01.011

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Fig. 1.Map of sampling site locations with site altitude above sea level.

2 I. Yordanova et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

Samples of undisturbed soils are collected annually from one and thesame sampling site. Sampling areas were specified considering windstrength and direction, topography and difference in altitude. In this

Fig. 2. Averaged values for 137Cs activity in soil samples from different

Fig. 3. Averaged values for 90Sr activity in soil samples from different r

Please cite this article as: Yordanova, I., et al., Technogenic radionuclidedx.doi.org/10.1016/j.gexplo.2014.01.011

way regions with higher probability for contamination in case of acci-dent in NPP Kozloduy or cross-border transfer of radionuclides were in-cluded. The location map of the area under study and sampling site

regions [Bq kg−1] (1985–2010 calculated to reference date 1986).

egions [Bq kg−1] (1985–2010 calculated to reference date 1986).




Table 1Average activity concentrations of 137Cs in soils from different parts of Bulgaria [Bq kg−1].

Year 1988 1994 2001 2009 2011

30 km zone around NPP 56 ± 33(60%)

51 ± 15(54%)

25 ± 15(27%)

19 ± 8(42%)

19 ± 13 (69%)min = 7; max = 48

Northern Bulgaria 1 ± 32(64%)

46 ± 28(61%)

27 ± 20(75%)

14 ± 9(64%)

14 ± 10 (53%)min = 4, max = 32

Table 2Average activity concentrations of 90Sr in soils from different parts of Bulgaria [Bq kg−1].

Year 1988 1994 2001 2009 2011

30 km zone around NPP 6 ± 3.6(60%)

5.7 ± 2.6(46%)

2.3 ± 0.54(20%)

3.4 ± 1.1(32%)

2.6 ± 0.5 (20%)min = 1.9; max = 3.0

Northern Bulgaria 5.5 ± 3.7(68%)

5.0 ± 3.0(60%)

2.8 ± 1.4(48%)

2.7 ± 1.1(45%)

2.2 ± 0.9 (41%)min = 0.8, max = 36

3I. Yordanova et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

location is shown in Fig. 1. The sites where sampling in depth was doneare marked additionally with letters— a for the 5–10 cm layer; b for the10–20 cm; and c for the 20–40 cm. According to the altitude of the in-vestigated areas three groups have been defined: plain, hilly andmoun-tainous areas. The plain area includes the sampling sites around theKozloduy NPP and along the Danube river — sampling sites numberedfrom 1 to 32, with altitude between 30 and 340 m (Fig. 1). The hillyareas include the sampling sites from the Sofia valley (numbered from56 to 60 with altitude between 560 and 700 m) and the valleys ofStruma and Mesta rivers (numbered from 49 to 55 with altitude be-tween 840 and 1191 m). The mountainous area includes the Rodopemountains (numbered from 33 to 45 with altitude between 1050 and1926 m). Sampling was done also in the water catchment basin ofdam lake Beli Iskar in Rila mountains (marked as sampling site 61 inFig. 1) for one survey only (conducted in year 1996) and not as a partof the regular sampling strategy.

The soil samples were taken according to the procedure defined inthe Bulgarian Governmental Standard BGS 17.4.5.01-85 which con-forms to the procedure described in ISO, 18589-2 (2007) for collectingsamples of undisturbed soil usinguniform approach,with samplingper-formed at depths independent of the natural variations of the soil char-acteristics. The sampling sites are undisturbed, flat with minimumimpact of water and wind erosion. A composite sample of at least 5 in-crement samples is taken from each location. For assessment of spatialdistribution and temporal development of soil radioactivity due to arti-ficial radionuclide samples from the soil layer 0–5 cm were collected.For study of the vertical migration of radionuclides with depth samplesfrom layers at different depths (5–10 cm; 10–20 cm; 20–40 cm) werecollected at some of the sampling areas.

Fig. 4. Averaged activity concentration for 137Cs in soil.


The soil samples were homogenized, dried at 80 °C and sievedthrough a 2 mm mesh before measurement with a gamma-spectrometer. The samples were stored in air-tight containers for aminimum of 28 days to allow 226Ra to come into equilibrium with itsshort-lived progeny. The measurements were done following standardprocedures (ISO, 18589-3, 2007). A Canberra high-purity germaniumdetector with 20% efficiency energy resolution of 1.8 keV for 60Co γ-ray energy line at 1332 keV was used. The detector was calibratedwith standard reference radionuclide source, type MBSS2, containing241Am, 109Cd, 139Ce, 57Co, 60Co, 137Cs, 113Sn, 85Sr, 88Y, 210Pb, and 203Hgsupplied by the Czech Metrological Institute. The measuring systemincluded a multichannel analyzer DSA 1000 (Canberra, USA). Thespectrum was analyzed by GENIE-2000 software with measurementuncertainties less than 10%. Typical counting times were 19–24 h.The 137Cs concentrations in the soil were obtained by measuring theactivity at 661.62 keV. 134Cs was measured at two gamma-energypeaks: 604.7 keV and 795.8 keV, applying correction coefficients forcoincidence-summing: 1.078 and 1.088 respectively.

The 90Sr activity concentrations were obtained by radiochemicalprocedure by extraction with TBP (tributyl phosphate) and precipita-tion of yttrium with oxalic acid as yttrium oxalate. The precipitationwas done on a hot plate, stirring continuously and neutralizing withNH3 to pH 1.5. The precipitate was filtered after cooling (Maier andScholl, 1982; Tarpanova and Naydenov, 1992). 90Sr was determinedby its progeny 90Y. The measurement was done on low-backgroundalpha/beta counter MPC-9300 with gas filled radiation proportionaldetector.

Plutonium isotopes were determined quantitatively by extractionwith TOPO/cyclohexan and back extraction with HCl/ascorbic acid.

Fig. 5. Averaged activity concentration for 90Sr in soil.




Table 3Content of 137Cs and 90Sr in highmountainous soils from thewater catchment basin of BeliIskar dam lake in the Rila mountain, 1996 (in Bq kg−1 layers 0–5 cm).

Site 137Cs [Bq kg−1] 90Sr [Bq kg−1]

Allinitsa 1208 ± 20 100 ± 5Allinitsa 2 298 ± 6 –

Lake Mousalla shore 164 ± 20 –

Near Mousalla hut 1123 ± 15 –

Borovets 1105 ± 15 49 ± 4

Fig. 6. Distribution of 137Cs and 90Sr along the soil profile for haplic cambisol, averagedfrom sampling sites 6, 15, and 28 from Fig. 1.


The plutonium fraction then was purified radio chemically with LaF3

and anion exchange procedures. The concentration of the alpha-emitters 238Pu and 239 + 240Pu were measured by alpha-spectrometryafter electroplating in oxalate/HCl medium (Pimpl and Schüttelkopf,1986; Schüttelkopf, 1981).

For quality control and assurance the efficiency calibration is doneannually and each month measurements are checked by replicate anal-ysis of reference soils from the IAEA reference materials: IAEA-Soil-6and IAEA-375, Soil. In additionwe participate periodically in proficiencytests and interlaboratory comparisons.

3. Results and discussions

The sampling areas were grouped by locality and altitude as the aimof the study is to specify averaged values of the activity concentrationsof artificial radionuclides in the soil for larger regions in the country.The results are summarized and averaged within the different groupsby years. The data summarized in this way gives information aboutthe average levels of technogenic soil radioactivity in the areas under in-vestigation and provides the regional reference level for future surveys.

Until 1986 the mean values for the specific activities of the abovementioned radionuclides were as follows after Naydenov (1986) andNaydenov and Staneva (1987):

• Northern Bulgaria (around the Kozloduy NPP and along the Danuberiver): 90Sr–4 Bq kg−1 and 137Cs–10 Bq kg−1;

• Southern Bulgaria (the Rodope mountains): 90Sr–4 Bq kg−1 and137Cs–26 Bq kg−1.

These values were received by calculating the arithmetic mean fromthe results for specific activities in soil samples from 32 different sam-pling points in Northern Bulgaria and 16 in Southern Bulgaria. Themean square deviation at this averaging was up to 40%.

The massive deposition of radioactive elements after the accident atthe Chernobyl nuclear power plant has changed radically the radio-ecological status of the soils in Bulgaria concerning the men-made ra-dioactivity. After 1990 only the isotopes of cesium (134 and 137) and90Sr could be established in all the samples. Because of its shorter half-life (2.065 years) negligible activities of 134Cs (2–6 Bq kg−1) weredetected in some soil samples from mountainous areas. Results for thedynamic of cesium-137 and strontium-90 activities in soils fromNorthern Bulgaria and Southern Bulgaria for the period 1985–2010are presented in Figs. 2 and 3 respectively.

Besides the data from 1985 and 1986, results for the period after arealso presented. Cs and Sr activity is calculated to reference date 1986 inorder to discount the decay effect when studying the dynamics of thetechnogenic radioactive pollution of soils. The number of samples for

Table 4Averaged values for the content of 238Pu and 239 + 240Pu in soils from Northern Bulgaria, the R

238Pu [Bq kg−1]

North Bulgaria (7 samples) (1.32 ± 0.35) × 10−2

max. value: 2 × 10−2

The Rhodopes (7 samples) 0.075 ± 0.037max. value: 0.14

Rila (10 samples) 0.07 ± 0.03max. value: 0.31


each group from which the average values were calculated was speci-fied in Section 2. While summing the data and calculating the averagevalues high non-homogeneity was ascertained. The variances were be-tween 20% and 80% for strontium-90 and between 30% and 70% forcesium-137 in the different years and different groups.

To assess the impact of KozloduyNPP activity on the radioactive con-tamination of soils we compared results for soil radioactivity in the30 km zone around the NPP with averaged data for other regions ofNorth Bulgaria by years (Tables 1 and 2). The yearswere randomly cho-sen within the period of investigation. There are no statistically signifi-cant differences between the areas compared.

The same results are obtained when comparing data for areas locat-ed east and west of NPP with data from sampling sites in the town ofKozloduy and just next to the NPP. The location of the sampling sitesis consistent with the direction of the winds in the area. The resultsare shown in Figs. 4 and 5.

Considering this we conclude that there are no statistically signifi-cant changes during the years after the Chernobyl accident. The relativereduction of activity concentrations of 137Cs and 90Sr in surface soil layeris mainly due to their natural decay. The contribution of the vertical mi-gration of the radionuclides to the changes in surface soil layer radioac-tivity cannot be correctly registered and assessed in such a generalizedstudy of large areas as it is hidden within the inhomogeneity of thepollution.

The area around the water catchment basin on the Beli Iskar river inthe Rila mountain was studied in 1996 as an example of high mountainsoils (with an elevation of 1700–2000m altitude). Results are shown inTable 3. These samples have proved a strong heterogeneity of the pollu-tion, namely from 3 to 1700 Bq kg−1 for Cs-137, whereas the highestvalues have been given in Table 3.

Studies for assessing the amount of technogenic alpha-emitting ra-dionuclides became necessary after the change of the radiation statusin 1986. The isotopes of plutonium-238, -239 and -240 are an exampleof rather toxic and long-lived radionuclides of this type. Soil samplesfrom Northern Bulgaria (around the Kozloduy NPP mainly), theRhodopes and Rila mountains were analyzed for determination of Pu.The averaged results are presented in Table 2. The combined uncertainty

odopes and Rila mountains 1996 (in Bq kg−1 layers 0–5 cm).

239 + 240Pu [Bq kg−1] 238Pu/239 +240Pu

0.18 ± 0.06max. value: 0.3

0.09 ± 0.03

1.43 ± 0.71max. value: 2.83

0.05 ± 0.02

0.91 ± 0.62max. value: 3.7

0.08 ± 0.03




Fig. 7.Distribution of 137Cs and 90Sr along the soil profile for epicalcic chernozem, averagedfrom sampling sites 14, 21, and 24 from Fig. 1.

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in the particular analyses was up to 20% for 239 + 240Pu and up to 70% for238Pu where the activities were near the limits of detection.

The global contaminationwith Pu is due to the nuclear test in the 60sas a result of which the average activity concentration ratio 238Pu/239+ 240Pu is about 0.04 in the soils contaminated by the fallout and the de-struction in the stratosphere of the SNAP-9A satellite in 1964(UNSCEAR, 1982). The ratios calculated in this work (Table 4) slightlyexceed the values cited in the literature which is an evidence of falloutcontamination. Having inmind the high uncertainties in the determina-tion of 238Pu and the heterogeneity of the contamination the deviationsof the obtained values from the cited averaged fallout ratio cannot be ac-cepted as statistically authentic.

A characteristic feature of the Chernobyl contamination in Bulgariawas the intensive vertical migration of strontium and cesium in depthalong the soil profile (down to 30 cm) for one month. This is probablydue to the fact that in May 1986 there was a massive deposition of ra-dionuclides for a short time with abundant rainfall.

The distribution of 137Cs and 90Sr in depth is graphically shown inFigs. 6, 7, and 8 for three different soil types: haplic cambisol, epicalcicchernozem, and fluvic cambisol respectively. The values are obtainedby calculating the arithmetic mean of the results from soil samples col-lected in June 1986 from three different sampling sites for each soil type.The samples were taken from soil layer 0–40 cm in four slices (0–5;5–10; 10–20 and 20–40 cm). 137Cs, deposited as a result of the Cherno-byl accident was estimated as a part of the whole detected amount by134Cs using the isotope ratio 134Cs : 137Cs = 1 : 2 established in the ra-dioactive mixture.

Fig. 8.Distribution of 137Cs and 90Sr along the soil profile for fluvisols, averaged from sam-pling sites 2, 12, and 25 from Fig. 1.


For description of the vertical migration down the soil profile an ex-ponential approximation was used, using Microsoft Excel 97. As shownin Figs. 6, 7, and 8 the obtained values for the coefficients of determina-tion (R2 ≈ 1) show that the regression line very well fits the data.

Similar behavior of the two elements can be noticed. The main partof the activity for both isotopes is concentrated in the two upper layers(0–5 and 5–10 cm). The coefficients before x in the exponential function(Figs. 6, 7, and 8) determine the slope of the curves and correlate withthe mobility of the radionuclides. A possible explanation of this experi-mental fact may be the lack of enough time for interaction of the addi-tionally deposited radionuclides (especially cesium-137) with the soilcolloids for stable fixing and clearer manifestation of the chemical dif-ferences between the elements. Another reason for the unusually highmobility of cesiummay be its water-soluble form and the heavy rainfallwith which themassive deposition of radionuclides was combined. It isknown that cesium has worse migration abilities compared to those ofstrontium, as Cs+ ions become fixed in the interlayer (Shimmack andBunzl, 1996). For manifestation of this fixing howevermore time is nec-essary because of the relatively large dimensions of the ions. The differ-ence between the coefficients of the exponential functions for bothradionuclides is significant only for epicalcic chernozem. In our previousstudies (Naydenov et al., 1988a,b)we have pointed as a probable reasonfor this difference the relatively high degree of erosion in the soils atthese sites. Since in eroded soils particles are moving in depth of theprofile we assumed that these processes have accelerated the verticalmigration of 90Sr deposited mainly in poorly soluble forms which char-acterized the fuel particles. Similar behavior of the two radionuclidesdeposited as a result of the Chernobyl accident has been described inUNSCEAR (2000).

4. Conclusions

Regarding the technogenic radionuclides the radiation status of theexamined Bulgarian soils established in the first year after the accidentin the Chernobyl NPP has not changed significantly in the years after.Within the heterogeneity of the existing soil contamination in the coun-try there are no evidences for impacts of the operation of KozloduyNPP.

Quick and similar vertical migration of the two long-lived radionu-clides 137Cs and 90Sr was observed in the first months after the Cherno-byl accident in 1986. For a short timeboth radionuclides reached depthsof 20–40 cm. After that themigration processes are normalized accord-ing to the soil characteristics and the chemical properties of theelements.

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