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Determination of the soil-to-grasstransfer of 137Cs and its relation toseveral soil properties at variouslocations in SerbiaDragana Krstić a , Nenad Stevanović a , Jelena Milivojević b &Dragoslav Nikezić aa University of Kragujevac, Faculty of Science , RadojaDomanovića 12, 34000, Kragujevac, Serbiab Center for Small Grains of Kragujevac , Save Kovačevića 31,34000, Kragujevac, SerbiaPublished online: 27 Apr 2007.

To cite this article: Dragana Krstić , Nenad Stevanović , Jelena Milivojević & Dragoslav Nikezić(2007) Determination of the soil-to-grass transfer of 137Cs and its relation to several soil propertiesat various locations in Serbia, Isotopes in Environmental and Health Studies, 43:1, 65-73, DOI:10.1080/10256010601154171

To link to this article: http://dx.doi.org/10.1080/10256010601154171

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Isotopes in Environmental and Health StudiesVol. 43, No. 1, March 2007, 65–73

Determination of the soil-to-grass transfer of 137Cs and itsrelation to several soil properties at various locations

in Serbia

DRAGANA KRSTIC†, NENAD STEVANOVIC†, JELENA MILIVOJEVIC‡ andDRAGOSLAV NIKEZIC*†

†University of Kragujevac, Faculty of Science, Radoja Domanovica 12, 34000 Kragujevac, Serbia‡Center for Small Grains of Kragujevac, Save Kovacevica 31, 34000 Kragujevac, Serbia

(Received 3 April 2006; in final form 9 October 2006)

Transfer coefficients of 137Cs from soil to grass were determined for the terrain around the city ofKragujevac in central Serbia. Mass activity concentrations of 137Cs in soil and grass samples weredetermined with a high-purity Ge-detector (HPGe). The activity concentration at the depth of 20 cmwas found to be in the range of 14.92–124.05 Bq kg−1, whereas the activity in grass for the samelocation was in the range of 4.60–84.95 Bq kg−1. Transfer factors (TFs) were in the range of 0.07 upto 1.94.

Dependences of TFs on different soil characteristics were presented graphically. Weak dependenceswere determined between them.Absalom’s model was used to predict TFs based on soil characteristics:pH value, contents of clay, exchangeable potassium and humus.

A comparison of measured and predicted values from Absalom’s model is shown graphically. It hasbeen found that Absalom’s model might be carefully used for the prediction of 137Cs in grass forspecific regions.

Keywords: Cesium-137; Grass; Transfer factor; Models; Soil

1. Introduction

Certain amounts of 137Cs were deposited on soil in the period between 1950 and 1970 as aconsequence of nuclear weapon tests performed by some countries. The next major eventwas the Chernobyl accident on April 26th 1986, when a huge quantity of radioactivity,including a significant amount of 137Cs, was ejected in the atmosphere. Other constituentsof radioactivity were also ejected from the destroyed reactor, including other fission products,hot particles, activation isotopes etc. The main deposition mechanism of this radioactivity waswet deposition with rain, the so-called fallout, although dry deposition also took place.

Deposited on the soil, 137Cs migrated due to different processes, like horizontal and verticalmovement with water, diffusion in deeper layers of soil etc. [1–3]. Vertical distribution of

*Corresponding author. Tel.: +381-34-336-223; Fax: +381-34-335-040; Email: nikezic@kg.ac.yu

Isotopes in Environmental and Health StudiesISSN 1025-6016 print/ISSN 1477-2639 online © 2007 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/10256010601154171

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66 Dragana Krstic et al.

137Cs in soil was investigated in refs [4–6], as well as in our previous paper [7]. 137Cs isa biologically active element and can be transferred from soil to plants and subsequentlyin various food chains. 137Cs transfer from soil to plants was investigated in numerouspapers [8–15]. It has been shown that transfer from soil to various plants can be predictedbased on soil characteristics [16–20].

In the presented paper, 137Cs transfer from soil to plants was investigated on the terrainaround the city of Kragujevac.

2. Materials and methods

Soil and grass samples were collected during the spring–summer period of 2001, on a totalof 15 locations around Kragujevac, in central Serbia, 20E55 and 44N01. Sampling was doneon uncultivated soil. One cannot claim that the soil was definitely undisturbed because theprevalent soil type in the investigated area is clay. During dry summers, which are typical forthe abovementioned region, many cracks are created in this soil enabling a certain mixingbetween its vertical layers.

The sampling procedure of soil was described in ref. [7]. Standard soil analysis techniqueswere used for pH, humus content, available potassium and clay content determination.The pH value was determined in a water suspension with a soil: solution relation of1:2.5 potentiometrically combining a glass electrode and pH-meter (Digital 870, Dresden,Germany) [21]. Determining the humus content in the soil by the Kotzmann method [22] isbased on the oxidation of organic matter, consisting in soil, using 0.1 N KMnO4 solutions.The air-dried soil sample had to be milled and then sieved through a 0.25 mm resolution sieve.The amount of soil necessary for analysis was 0.1–0.5 g, depending on the humus content.The organic carbon content from humus was measured indirectly from CO2 obtained duringoxidation. The texture soil groups were determined using the American (USDA) scale [23].The soil mechanical composition was determined by a combined method for preparing soilsamples for that analysis using 0.1 M Na3P2O7 · 10H2O solutions. Determining of mechanicalclay fractions (fragments smaller than 0.002 mm diameter) was carried out using the pipettemethod. The amount of easily available potassium was determined by the Egner–Riehmmethod [24]. The soil sample was extracted by an AL extracting solution during 4 hours.The content of potassium was measured in the obtained extract using a photometer.

Grass samples were taken from the same location and at the same time with soil samples,and they were dried in a laboratory at 80 ◦C until the rest mass of the sample was constant.The remaining material was cut in small pieces, and put in Marinelli beakers for gammaspectrometry using a high-purity Ge-detector (HPGe). The soil samples were measuredfor 72,000s, and the grass samples were measured longer, from 100,000s up to 150,000s,depending on 137Cs activity. Spectrum analysis was performed by MAESTRO software. Theconcentration of 137Cs was measured in the examined samples of soil and grass. The transferfactor (TF) of 137Cs from soil to plants (definition is given below in equation 1) was determinedfor all examined samples.

3. Application of Absalom’s model

The model described in refs [17, 18] was intended for determination of the amount of 137Cstransferred from soil to plants, based on soil characteristics like the pH value, exchangeablepotassium, humus and clay contents. The model consisted of a set of equations that describe

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137Cs soil-to-grass transfer 67

the transfer of 137Cs from soil to plants. The TF was defined by equation (1) as follows

TF = Activity in plant (Bq/kg dry weight)

Activity in soil (Bq/kg dry weight)(1)

The activity concentration in plants, Aplant (Bq kg−1) was calculated from the product of theconcentration factor CF (dm3 kg−1) and 137Cs activity concentration in the soil solution Asol

(Bq kg−1) as

Aplant = CF · Asol (2)

Smolders et al. [16] showed that CF for 137Cs uptake by ryegrass may be related to theconcentration of K+ in the solution [mk] (mol dm−3). In ref. [17] and ref. [18] the followingrelation was assumed

log[CF] = k1 − k2 log[mk] (3)

where

mk = Khumusx

√10−k3+k4 pH

khumG (k5 + k6pH) − Khumus

x

(4)

and Khumusx is the content of exchangeable K+ in humus (cmolc kg−1) and can be determined as

Khumusx = Kx

((kclayG CECclayθclay)/khum

G (k5 + k6pH)) + θhumus

(5)

θclay,humus are clay and humus contents in soil in (g/g).The ratio between the 137Cs concentration in the solution and soil is

Asol

Asoil= D

kdl(6)

where D is a function of the time between 137Cs deposition and measurements. In the modelD = 1. kdl is the distribution coefficient of labile 137Cs and it is defined as

kdl = khumusdθ + k

claydθ (7)

where

khumusdθ = 0.01 · θhumus · Khumus

x

mk

(8)

kclaydθ = 10 −k8 θhumus/θclay + k9 · θclay

mk

(9)

The TF can be determined based on the soil characteristics and equations (1)–(9).

TF = 10k1−k2 log(mk)

(10−k8(θhumus/θclay)+k9 · θclay/mk) + (0.01 · θhumus · Khumusx /mk)

(10)

The constants k1, k2, khumG , k

clayG , k8 and k9 were obtained by fitting according to soil

characteristics. The values of remaining constants in this set of equations were fixed andtaken from ref. [18] and they are presented in table 2. Equation (10) was used to calculateTFs using soil characteristics given in table 1, and constants k1, k2, khum

G , kclayG , k8 and k9 from

table 2.

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68 Dragana Krstic et al.

Table 1. Location, soil parameters and types, measured TF.

Location No. Soil type pH (H2O) Kx (cmolc/kg) θclay (g/g) θhumus (g/g) TF Measured TF Calculated

1 Cambisol 5.55 0.352 0.440 0.046 0.86 0.772 Cambisol 6.22 0.658 0.220 0.046 0.21 0.873 Cambisol 6.81 0.679 0.406 0.047 0.07 0.504 Smonitza 5.72 0.263 0.388 0.026 0.42 0.785 Smonitza 7.50 0.212 0.450 0.039 0.48 0.956 Alluvial 6.99 0.339 0.420 0.033 0.12 0.677 Smonitza 6.40 0.233 0.365 0.055 1.11 1.348 Alluvial 6.57 0.382 0.361 0.044 1.59 0.829 Cambisol 5.90 0.594 0.332 0.011 0.15 0.31

10 Cambisol 6.00 0.636 0.525 0.044 1.20 0.4111 Smonitza 6.30 0.245 0.465 0.028 0.76 0.7412 Cambisol 5.63 0.399 0.389 0.045 0.17 0.7613 Cambisol 7.02 0.401 0.365 0.053 0.29 0.8714 Cambisol 7.52 0.400 0.316 0.048 1.94 0.9315 Cambisol 5.60 0.454 0.418 0.053 1.29 0.72

Table 2. Fitted and independent parameters used in the model.

Fitted parameters Independent parameters

k1 k2 kclayG khumus

G k8 k9 k3 k4 k5 k6 kfast kslow Pfast CECclay

1.6 1.7 0.01 0.11 0.009 0.001 3.368 0.16 −34.66 29.72 0.0019 0.00019 0.814 50

4. Results and discussion

TFs, obtained from measurements and equation (1), as a function of the soil depth are shown infigure 1. The TFs slightly increase with the depth, but this does not mean that grass takes more

Depth soil (cm)

0 2 4 6 8 10 12 14 16 18 20 22

Tran

sfer

coe

ffici

ent

0.0

0.5

1.0

1.5

2.0

2.5

Loc 1Loc 2Loc 3Loc 4 Loc 5 Loc 6Loc 7Loc 8Loc 9Loc 10Loc 11Loc 12Loc 13Loc 14Loc 15

Figure 1. TF as a function of the depth in soil.

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137Cs soil-to-grass transfer 69

137Cs from deeper layers; it is the consequence of 137Cs reduction with the depth. The grassroot is mainly in the surface layer of the soil. The activity concentration at the depth of 20 cmwas found to be in the range of 14.92–124.05 Bq kg−1, whereas the activity in grass from thesame location was in the range of 4.60–84.95 Bq kg−1. The corresponding TFs were between0.07 and 1.94.

The dependence of the TF on soil characteristics, exchangeable K+ ion (Kx), pH value,humus and content of clay was investigated. Figure 2 shows the dependence of TF on thecontent of exchangeable K+ ions. The solid line represents TF calculated using equation (10)as a function of Kx, whereas other soil parameters were set as average values: pH = 6.38,θclay = 0.39; θhumus = 0.041. Here, TF decreases very weakly with the K+ content. If theK+ ion content increases from 0.2 to 0.7 cmolc kg−1, TF decreases by a factor of 10. TheSpearman correlation coefficient for all data pairs was R = −0.2143 and the level of sig-nificance was p = 0.4437. Since the p value is not higher than 0.05, the correlation is notsignificant. Frissel et al. [11] explained that, below 0.05 cmolc kg−1 there is a strong influenceof exchangeable K+, but for greater values, other factors have a more significant influenceon the TF value. Zhu and Smolders [25] reported that the uptake of radiocesium depends onthe external concentration of K+ ions. The influence of the concentration of K+ ions is notsignificant for the samples considered in the present work as its concentration exceeds thelimit of 0.05 cmolc kg−1.

Figure 3 presents the dependence of the TF value on the pH. The calculated TF is presentedby a solid line with average values of: Kx = 0.42, θclay = 0.39; θhumus = 0.041. It can be seenthat the TF only slightly decreases with increasing of the pH value. Similar results were shownin ref. [19]. The Spearman correlation coefficient for this group of data pairs was R = −0.0036and p = 0.9922. This means that correlation between the TF and the pH is not detectable.Negative correlation of radiocesium soil-to-grass transfer with soil pH was found in ref. [26].Frissel et al. [11] explained that as in the case of exchangeable K+, there is a threshold value

Kx (cmolc kg-1)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

TF

val

ue

0.0

0.5

1.0

1.5

2.0

2.5TF < 1TF > 1Calculated TF

Figure 2. TF as a function of Kx.

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pH value

5.0 5.5 6.0 6.5 7.0 7.5 8.0

TF

val

ue

0.0

0.5

1.0

1.5

2.0

TF < 1TF > 1Calculated TF

Figure 3. TF as a function of the pH value.

for pH in the range of 4–5. The soil pH has a more significant influence on the TF of 137Cs,only if its value is below the threshold value. In the presented paper, the pH values were abovethis limit, i.e., only slightly acid soils were present.

The dependence of TF on the organic matter content (figure 4) is opposite than the abovementioned. With increasing organic matter content from 0.01 to 0.05, TF increases from0.1 to 2. The calculated TF is presented by a solid line with average values of: pH = 6.38,Kx = 0.42 and θclay = 0.39. The Spearman correlation coefficient was R = 0.3214, and thelevel of significance was p = 0.2441. A strong correlation between TF and the organic mattercontent was not found, but figure 4 shows that TF slightly increases with the content of organicmatter. A similar dependence was found in ref. [19].

Clay in soil fixes 137Cs, making it unavailable for plants; so clay decreases the TF.The dependence of TF on the clay content in soil is shown in figure 5, where TF decreases withthe clay content increase. The calculated TF (solid line) was done with pH = 6.38, Kx = 0.42and θhumus = 0.041. The Spearman correlation coefficient was R = 0.019 and p = 0.942.The Absalom model [17, 18] assumed that absorption occurred on both clay and humus,although fixation occurs on clay only.

Measured and calculated (equation 10) values of TF are shown in figure 6. The constantsused in equation (10) are given in table 2. They are different for the observed soil than theones given by the Absalom model [17, 18]. The calculated TFs are in the range from 0.31 to1.33 with the average of 0.76. The experimental average of TF was determined to be 0.71.

To test whether the Absalom model is able to predict TF, the values for the measured and thecalculated TF-values from table 1 were used to calculate the Spearman correlation coefficientR. As a result the following was obtained: R = 0.3807 (significance level p = 0.162). Asp > 0.05, this correlation cannot be considered statistically significant.

The discrepancy between the measured and calculated TF originated from the fact thatvarious soils were investigated in this work. In addition, the plant species has a significant

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137Cs soil-to-grass transfer 71

Organic matter (g /g)

0.00 0.01 0.02 0.03 0.04 0.05 0.06

TF

val

ue

0.0

0.5

1.0

1.5

2.0

2.5TF < 1TF > 1Calculated TF

Figure 4. TF as a function of the organic matter content.

Clay contents (g /g)0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55

TF

val

ue

0.0

0.5

1.0

1.5

2.0

2.5TF < 1TF > 1Calculated TF

Figure 5. TF as a function of clay contents.

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72 Dragana Krstic et al.

Log(TF)

Modelled

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Mea

sure

d

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Figure 6. Measured and predicted log (TF) values of 137Cs for grass.

influence on the TF. TF calculated from the Absalom model [17, 18] is larger than theexperimentally obtained values. Similar results were given in refs [9, 20].

TF values for grass and grassy vegetations can be in a wide range: 0.0022–2.62 [16];0.029–0.066 [20]; 0.012–1.2 [25]; 0.039–44.1 [27]. The empirical parameters used inAbsalom’s model need to be re-evaluated for a variety of different plants for a specific regionand time-dependent variability [13, 25, 26, 28].

5. Summary and conclusion

The TFs soil to grass were determined based on measurements of 137Cs in soil and grass takenfrom the same locations. The Absalom model was applied to calculate TF based on the knownsoil characteristics, pH, K+, clay and organic content. The influence of the soil characteristicson TF was investigated. The values of the significance of Spearman correlation coefficients donot indicate the presence of a strong correlation between soil parameters and TF values for soilsamples. With a larger number of samples a correlation might indeed be detectable. Althoughthe average TF calculated from the Absalom model is somewhat larger than the experimentalaverage TF, it is comparable to the measured values.

According to the previous discussion, this model might be carefully used only for predictionof TF. In order to get more precise results a larger database of soil characteristics is needed.

Acknowledgement

The authors would like to thank the Serbian Ministry of Science and Environment Protection,which supported this work through Project No. 141023.

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137Cs soil-to-grass transfer 73

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