Predicting radium availability and uptake from soil properties

Chemosphere 69 (2007) 664–674 www.elsevier.com/locate/chemosphere

Predicting radium availability and uptake from soil properties H. Vandenhove *, M. Van Hees Belgian Nuclear Research Centre, Biosphere Impact Studies, Boeretang 200, B-2400 Mol, Belgium Received 31 October 2006; received in revised form 22 February 2007; accepted 24 February 2007 Available online 16 April 2007

Abstract The results of a potted soil experiment to determine the soil and plant factors ruling radium availability and uptake by ryegrass and clover are described. Nine soils with distinct soil characteristics were spiked with 226Ra. They were thoroughly characterized and the solid liquid partitioning coefficient, Kd, was determined. Kd ranged from 38 l kg1 to 446 l kg1 (average: 188 ± 156 l kg1) and was linearly related to cation exchange capacity (CEC) and organic matter (OM) content. The soil-to-plant transfer factor (TF) was significantly affected by the chemical properties of the soils and ranged from 0.054 kg kg1 to 0.719 kg kg1 for ryegrass and from 0.034 kg kg1 to 1.494 kg kg1 for clover. Overall, no significant difference in TF between ryegrass and clover was observed. TF was related to Kd, to CEC, OM (for ryegrass only when excluding one soil) and the calcium concentration in the soil solution (for both plants if excluding one soil). Radium flux were calculated from the radium concentration in the soil solution and the evapotranspiration, to predict total radium uptake derived from shoot radium concentration and biomass yield. It was found that radium uptake could be predicted from the radium flux (R2 = 0.61 and 0.83 for ryegrass and clover, respectively). Higher predictability (R2 = 0.70 and 0.91 for ryegrass and clover, respectively) was obtained when relating total radium uptake to a radium flow considering competition effects at the root surface by bivalent cations.  2007 Elsevier Ltd. All rights reserved. Keywords: Radium; Solid–liquid distribution; Uptake; Soil-to-plant transfer; Clover; Ryegrass

1. Introduction Considerable interest in the behaviour of the natural radionuclides uranium and thorium and their daughter radionuclides in the terrestrial environment was shown in the past years. Much research was conducted as a consequence of nuclear weapons’ testing during the 1950s (Mortvedt, 1994). Further interest in behaviour of these elements

Abbreviations: AAS, Atomic absorption spectrometry; ANOVA, Analysis of variance; CEC, Cation exchange capacity; DW, Dry weight; FC, Field capacity; ICP-AES, Inductive coupled plasma atomic emission spectrometry; Kd, Solid liquid distribution coefficient; OM, Organic matter; TF, Transfer factor; TIC, Total inorganic carbon; TOC, Total organic carbon. * Corresponding author. Tel.: +32 14 332114; fax: +32 14 321056. E-mail address: [email protected] (H. Vandenhove). 0045-6535/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.02.054

in soils and plants is linked with the potential environmental effects from uranium mining and (geological) disposal activities. Enhanced levels of naturally occurring radionuclides (NOR) in the environment may also be associated with industrial activities extracting and processing materials containing NOR. Apart from the U industry, the likely major wide-spread contributor to environmental radioactive contamination is the phosphate industry. Interest in the environmental behaviour of radium follows from its ubiquitous presence in NORM (naturally occurring radioactive material). Further, the dominant radionuclide in geological disposal is U and its daughter 226Ra may contribute to the human dose at longer time scales. An important pathway for human exposure is food consumption. For assessment of contaminant behaviour in the food chain, it is indispensable to understand the mechanisms and processes underlying contaminant availability

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674

and bio-incorporation. However, many radiological food chain transport models as well as biosphere transport models are based on the traditional transfer factor concept, where soil-plant relationships are described by concentration ratios assuming first order linearity (Gerzabek et al., 1998). Model predictions that form the basis for decision making need to be as precise as possible. In this context, a more realistic description of soil-plant relationships, one that takes into account mechanisms of radionuclide uptake, is required. The availability of an element from soils for plant uptake is controlled by a large array of physical, chemical, biological and climate factors. There are several distinct processes which must take place for plant uptake to occur. The first is the release of the ions from the solid phase of the soil to the soil solution, followed by the movement of these ions to locations where roots prevail. Concepts as availability are helpful in describing the processes and mechanisms that determine the potential of ions for plant uptake and are defined as those belonging to a pool which contribute to uptake by plants grown on soil (Simon and Ibrahim, 1990). The major factors governing availability to plants from soil are the solubility of the element associated with the solid phase. However, the degree of availability in the soil solution is also dependent on the soil-plant interaction. For example, the root system may alter the pH of the soils adjacent to it (Shahandeh et al., 2001). Competitive effects at and exchange of ions onto the root surface, transport across the root membranes and the subsequent translocation into the plant tissue complete the process. From the review of Simon and Ibrahim (1990) and Sheppard et al. (2005) we can learn how few studies were conducted to mechanistically explain observed radium solid–liquid distribution coefficients, Kd (l kg1, defined as the ratio of radium sorbed on the soil (Bq kg1) and radium in the soil solution (Bq l1)), and the soil-to-plant transfer factor, TF (kg kg1, defined as ratio of the concentration of radium in plant (Bq kg1) and in the soil (Bq kg1)), based on soil (and plant) parameters and processes behind. The present study presents soil-to-plant transfer data and explores the dominant physico-chemical soil factors and plant properties ruling Ra availability and uptake by ryegrass and clover. We intend to quantify and predict the Kd and TF observed by mechanistic functions. The study contributes to the common effort to reduce the uncertainty linked with the soil Kd and TF value used in risk assessment models by focusing on the parameters influencing Ra mobility in soil and Ra availability towards plants. 2. Materials and methods 2.1. Soils Nine soils were collected under pasture taking the upper 10 cm after having removed the root mat. The soils were

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selected from a soil collection (Vandenhove et al., 2007) such that they covered a wide range for those parameters hypothesised as being potentially important in determining Ra-availability (CEC, organic matter, clay content, Ca content, pH). The soils were air-dried, sieved (2 mm) and analysed for the more permanent soil characteristics: texture (according the pipette method; Day, 1965), total organic matter (OM) (loss on ignition: 550 C), cation exchange capacity (CEC; Chhabra et al., 1975), bulk density and field capacity (with the saturated paste method). Total P and Fe content was measured from finely ground and calcinated sub-samples after fusion at 1000 C following the borate fusion procedure (Voı¨novitch, 1988). The fusion beads were then dissolved in a 2 M HNO3 solution. After dilution, P was analysed in the solution by colorimetry (Watanabe and Olson, 1965) and Fe with ICP-AES (Perkin Elmer 4300 Dual View). Amorphous Fe was extracted with oxalate (Blackmore et al., 1987). For both analyses, 2 M HNO3 solution was considered as blank. Analysis was done in triplicate for CEC and amorphous Fe. These soil characteristics are presented in Table 1. Soils were also analysed for their nutritional status and a fertiliser advice was made up for ryegrass and clover by the Belgian Soil Science Service (BSSS), Heverlee, Belgium. Determination of the soil nutritional status was done based on four types of analyses (Beyens, BSSS, personal communication): ammonium lactate extraction of the dried soil (1/20 solid/liquid) and determination of K, Ca, Mg, Na (by Atomic Adsorption Spectrometry, AAS) and P (molybdate blue method) in the extract; pH-KCl (1/20 solid/ liquid); total C; and soil texture. The amounts of N, P2O5 and K2O to be added ranged from, respectively, 70–145 kg ha1, 0–150 kg ha1 and 0–230 kg ha1 depending on soil. Amounts were added using pure chemicals considering pot surface and soil density. Soils (2 batches of 3800 ± 600 g soil depending on soil field capacity and bulk density) were brought to field capacity and contaminated with 50 000 Bq 226Ra per kg dry soil (added as RaBr2 dissolved in distilled water), fertilised according to the fertiliser advice and incubated for 4 weeks. Following incubation, soils were analysed for pH-H2O (1/2.5 solid/liquid); exchangeable cations were measured in a 1 M ammonium acetate extract (NH4Ac, 1/18 solid/ liquid, 24 h end-over-end shaking) at pH 7 and analysed by atomic absorption spectrometry (AAS). Available P was measured following 0.5 M sodium bicarbonate extraction (pH 8.5, 1/20 solid/liquid, 24 h end-over-end shaking) and P in the extraction solution was measured by colorimetry [molybdate blue method (Watanabe and Olson, 1965)]. The composition of the soil solution was measured at field capacity (10 kPa). To collect the soil solution, a disposable 60-ml syringe without plunger was lined with a membrane and filled with soil sample. The syringe was transferred to a centrifuge tube and centrifuged (6000 rpm, 30 min) and the extracted soil solution filtered

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Table 1 Cation exchange capacity (CEC), water content at field capacity (FC), organic matter (OM), texture amorphous Fe content, total Fe and P content of the 9 soils studied Soil

A B C D I L N P R

CEC (meq kg1)

Field capacity g H2O/ 100 g DW

Density (g cm3)

87 ± 5 438 ± 15 517 ± 36 51 ± 5 240 ± 1 245 ± 16 85 ± 2 81 ± 4 100 ± 7

15.5 32.8 35.4 12.8 19.9 24.5 20.3 18.9 18.9

1.56 1.15 1.12 1.60 1.50 1.35 1.29 1.44 1.31

Texture (%) >50 (lm)

20–50 (lm)

10–20 (lm)

2–10 (lm)

0–2 (lm)

30.6 38.6 35.3 63.1 56. 9 34.1 78.0 82.6 68.0

43.9 27.0 17.3 7.0 15.9 25.9 6.6 3.7 12.9

7.6 1.8 10.4 0.3 0.02 2.8 1.5 0.8 0.6

0 8.1 6.9 0.9 7.2 9.4 2.1 2.9 2.9

18.0 24.6 30.0 28.8 20.1 27.8 11.7 10.1 15.6

OM (%)

Amorphous Fe (mg kg1)

Fe (mg kg1)

P (mg kg1)

3.0 12.3 14.9 3.3 5.0 7.8 5.9 4.5 4.1

2922 ± 86 20367 ± 1290 13090 ± 270 2176 ± 56 887 ± 40 3527 ± 36 1869 ± 67 1469 ± 45 1998 ± 59

11020 36110 24730 5565 11430 16780 31740 3015 38500

742 1428 1083 762 961 969 1178 891 680

Single measurements except for CEC and amorphous Fe content (n = 3): Average ± Stdev.

through a 0.45-lm membrane filter (Pall Life Science). The concentration of K+, Ca2+ and Mg2+ in the soil solution was measured by AAS (GBC 932AA), the concentration 2   of HPO2 by ion chromatography 4 , SO4 , NO3 and Cl (Dionex DX-320) and the total inorganic carbon with a DC-190 High-Temperature TOC Analyzer (Dohrmann). For analysis with AAS (K, Ca, Mg) and with the molybdate blue method (P), the detection limit was determined as three times the standard deviation following measurement of blanks, standards and samples. For ion chromatography the detection limit is determined via three methods: via measurement of additions to samples or standard solutions and detection limit; calculated as three times the standard deviation; measurement of standard series with each time lower concentrations until the result was not acceptable anymore. As final detection limit the highest value obtained via the different methods is considered. For TIC measurements the detection limit was calculated as three times the standard deviations following measurement of blanks and samples. Reproducibility was checked via duplicate measurements of samples by two operators on two different days. Precision was checked by analysis using similar equipment and via calculation of recoveries for standard added samples. Recoveries were between 98% and 104% for ion chromatography and between 109% and 120% for TIC measurements. The QC program includes the measurement of an independent standard in each measurement series, three-monthly measurement of a reference material and yearly participation in an international ring test. As blanks the respective extractants were considered (ammonium acetate extract for exchangeable bases and sodium bicarbonate extraction for available P) or distilled water for analysis of the soil solution. Exchangeable Ra was determined in the 1 M ammonium acetate extract. The final total radium content was measured from finely ground soil. Total Ra and exchangeable Ra were measured with a Ge-detector directly via its 186 keV gamma band (method MT.KB.001; geometry 20 ml; measuring time: 5000 s). Detection limits are determined based on the approach of Currie (1968) which is

implemented in the gammaspectrometry software with a = b = 0.05. No reference material is used, the spectrometer is calibrated over an energy range including the 186 keV gamma-ray energy for a fixed volume and counting geometry. Corrections for density or material composition can be made when required so there is no specific dependency on the matrix and hence the method is valid for a broad range of material compositions and material density. No 186 keV signal is observed in a background spectrum. Ra was measured in the soil solution following the ASTM procedure Designation D3454 ‘‘Standard Test Method for Radium in Water’’. This method is based on the measurement of 222Rn when equilibrium is attained between 222Rn and 226Ra. The quantitative separation of the gaseous 222Rn from the sample matrix is done via liquid/gas extraction. The measuring unit (Lucas cell) is a ZnS counting cell which contains the emanated mixture of He and Rn. The Lucas cell is transferred to the scintillation counter for measurement after installation of the equilibrium between Rn and daughters. The scintillation cell is put on a photomultiplier and the counting is started after installation of the equilibrium between Rn and daughters. Detection limit was determined following CEA (1983). As blank, soil solution extracted from non-amended soils was considered. Recovery was determined using a NIST standard solution (SRM4965) and was found to be 100%. All above measurements were performed in triplicate. Radium speciation calculations were performed for the average soil solution composition (considering soil solution radium, anionic and cationic composition and pH). The concentration of free Ra2+ soil solution concentrations was calculated using the geochemical computer code The Geochemist’s Workbench database Pro 5.04 (Bethke, 2001) using the NEA thermodynamic data review by Grenthe et al. (1992).

2.2. Greenhouse experiment A monocotyledonous (ryegrass: Lolium perenne cv Melvina) and dicotyledonous plant (clover: Trifolium pratense

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674

cv Violetta) were selected. For each plant-soil type combination, about 3800 g dry soil was brought to field capacity with a solution containing 226Ra (see above). After 4 weeks’ equilibration, about 1200 g of moist soil was transferred to a 1-l pot (4 pots per plant species and soil type); seeds (0.68 g) were sown on top and covered with 60 g of the respective moist contaminated soil. All 78 pots (9 soils, 2 plant species, 4 replicates) were kept under greenhouse conditions. Day and night temperatures were 21–25 C and 13–15 C, respectively, in a 12 h/12 h cycle. Light intensity at canopy height was 300 ± 30 lmol of photons m2 s1. Soil moisture was adjusted with demineralised water by weighing every second day. The amount of water added was recorded to get an idea of the evapotranspiration flux or total water use during the cultivation period. Shoots were harvested after 37 days for ryegrass and after 46 days for clover. Shoot samples were oven-dried (60 C) and dry weight was determined. Radium concentration in ground plant samples was determined with a Ge-detector directly via its 186 keV gamma signal (method MT.KB.001; geometry 20 ml; measuring time: 5000 s) and the transfer factors (Bq kg1 plant/Bq kg1 soil) were calculated. Detection limits were determined following the method of Curie (1983). As blanks, uncontaminated plant samples were analysed. For determination of levels of K, Ca and Mg, dried shoot samples were calcinated at 550 C. The ashes were then dissolved in concentrated hydrochloric acid (Donohue and Friedericks, 1984). Plant digests were analysed for K, Ca and Mg in the digests with AAS. HCl solution was considered as blank.

2.3. Statistical analysis Statistical analysis of data was performed with the statistical software Statistica for Windows (Statsoft, 2004).Tests for normality distribution were performed (Shapiro Wilk’s W test). If even after data transformation (logarithmic, exponential, power) no normal distribution was obtained, data treatment was done with the original data, unless indicated differently. Log-normal distribution was observed only for the radium concentration in the soil solution and the radium concentration in clover shoots and the corresponding TF. Significant differences were considered at p = 0.05, and mean values were ranked by Tukey’s multiple range tests when more than two groups were compared with ANOVA. Single parameter regression analysis was performed with Statsoft (2004). Marked correlations are significant at p < 0.05 level, unless otherwise mentioned. For the interpretation of the radium availability in soil, all data obtained from the analysis of soils destined to grow both plants were considered (since it is the same soil). For the interpretation of radium uptake as function of soil characteristics, the specific soil data for, respectively, ryegrass and clover were considered in the analysis.

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3. Results 3.1. Soil characteristics Some of the more permanent and more variable soil characteristics are presented in Tables 1 and 2, respectively. The CEC varied from 51 to 517 meq kg1, clay content from 10.1% to 30.0% and the OM content from 3.0% to 14.9%. Variation in CEC was well explained by OM (R2 = 0.91, p < 0.05) but less so by clay content (R2 = 0.35, p = 0.09). Soil pH varied from acidic (soil D, pH 4.6) to alkaline (soil I, pH 7.5). Soil solution Ca concentration was 11 meq l1 for soil D and 53 meq l1 for soil C. Exchangeable Ca-content varied 25-fold: from 12 meq kg1 for soil N to 325 meq kg1 for soil I. There was on average a factor 4 difference in soil solution Mg (from 3.4 meq l1 for soil C to 12.3 meq l1 for soil N), but, very high values were recorded for soil R: 71.6 meq l1. Exchangeable Mg-content varied 8-fold from 3.7 meq kg1 for soil D to 27.7 meq kg1 for soil R. Ten-fold and 5-fold differences were observed in soil solution K concentration and exchangeable K content, respectively. There was no relation between soil solution concentrations and exchangeable levels for the respective cations. Soil solution HPO2 content was around 10 lM 4 (10 · 103 mM), except for soil N (100 mM). Olson-P levels were also highest for mentioned soil. Soil solution SO2 4 content was between 0.5 and 1.9 mM, except for soil R (25 mM). The Ra concentration in the soil solution of the 9 soils ranged from 113 Bq l1 (soil N) to 1045 Bq l1 (soil D). There is a significant difference in the Ra concentration in the soil solution between soils as illustrated by the significance levels in Table 2. Soils D and R exhibited the highest Ra levels in the soil solution and soils B, C, I, and N the lowest. Exchangeable radium levels also varied about 10fold (from 2481 to 22272 Bq kg1). There was no relation between soil solution Ra concentrations and exchangeable Ra levels. Ra mobility expressed as Ra concentrations in the soil solution or as the Ra solid/liquid partitioning coefficient Kd were not related to soil pH or soil clay content. Considering all soils, no significant relations were found between the radium concentration in the soil solution (or log-transformed radium concentrations) and any of the soil parameters analysed for. Considering all soils, weak linear correlations were found between Kd and CEC (Kd = 0.71 [CEC]  0.64, R2 = 0.3, p = 0.12) or soil organic matter content ([Kd] = 27 [OM]  27, R2 = 0.4, p = 0.05) or. Omitting the data for soil N, these relations became very significant (R2 of 0.91 and 0.83, respectively, p < 0.01 Fig. 1a and b). 3.2. Plant characteristics Ryegrass dry weight (DW) ranged from 2.9 g pot1 for soil P to 4.5 g pot1 for soil C (all soils: 3.7 ± 0.6 g pot1)

5.7 ± 0.3 4.2 ± 0.1 3.2 ± 0.1 4.7 ± 0.2 9.7 ± 0.3 16.8 ± 0.5 8.2 ± 0.2 6.6 ± 0.2 5.7 ± 0.2 8.8 ± 0.4 16.9 ± 0.5 10.1 ± 0.5 3.7 ± 0.1 10.5 ± 0.2 18.7 ± 0.4 10.0 ± 0.2 6.5 ± 0.2 27.7 ± 1.2 37.1 ± 1.6 206.5 ± 5.5 290.2 ± 14.1 11.8 ± 0.5 325.2 ± 7.8 89.1 ± 2.4 33.0 ± 10 43.5 ± 1.8 44.6 ± 1.3 51.5 ± 0.2 70.6 ± 2.9 74.1 ± 5.4 118.5 ± 1.8 101.9 ± 3.2 98.3 ± 4.9 166.3 ± 1.6 106.9 ± 1.5 73.2 ± 0.6 10.3 ± 0.1 12.0 ± 1.0 9.2 ± 0.6 9.0 ± 0.7 12.2 ± 1.2 12.2 ± 1.4 100.8 ± 1.3 8.1 ± 0.5 8.4 ± 0.4 0.77 ± 0.12 1.32 ± 0.19 1.85 ± 0.07 0.53 ± 0.15 9.46 ± 0.60 1.53 ± 0.21 0.68 ± 0.27 0.80 ± 0.30 1.00 ± 0.16 0.61 ± 0.01 1.78 ± 0.01 1.45 ± 0.01 0.50 ± 0.01 1.00 ± 0.00 1.11 ± 0.01 1.86 ± 0.01 1.15 ± 0.01 25.79 ± 0.16 32.6 ± 0.2 17.3 ± 0.1 19.1 ± 0.2 39.2 ± 3.0 21.9 ± 0.0 25.5 ± 0.4 27.8 ± 0.1 32.5 ± 0.1 54.4 ± 0.3 15.27 ± 0.26 3.75 ± 0.10 2.99 ± 0.04 2.97 ± 0.28 0.53 ± 0.01 0.36 ± 0.00 1.82 ± 0.02 1.28 ± 0.00 11.08 ± 0.05 3.2 ± 0.1 0.9 ± 0.0 0.8 ± 0.0 7.7 ± 0.1 3.1 ± 0.2 6.7 ± 0.1 8.8 ± 0.2 8.7 ± 0.2 6.5 ± 0.1 8.2 ± 0.4 6.2 ± 0.1 3.4 ± 0.1 8.7 ± 0.2 3.7 ± 0.3 9.7 ± 0.2 12.3 ± 1.4 10.9 ± 0.9 71.6 ± 1.6 21.3 ± 1.2 44.2 ± 1.0 52.7 ± 1.5 11.3 ± 0.2 34.0 ± 2.5 27.5 ± 0.4 12.7 ± 1.5 28.4 ± 3.5 50.7 ± 5.2 69 ± 6 304 ± 41 391 ± 89 38 ± 1 205 ± 46 91 ± 0 446 ± 100 93 ± 18 53 ± 6 22272 ± 2412 a 16872 ± 1809 bc 13818 ± 1977 bc 9143 ± 2507 d 14184 ± 1908 c 18029 ± 6929 b 2481 ± 2507 f 7710 ± 3648 de 16988 ± 964 b 698 ± 59 ab 225 ± 26 b 185 ± 35 b 1045 ± 24 a 211 ± 53 b 624 ± 17 ab 113 ± 22 b 521 ± 87 ab 954 ± 141 a 48150 ± 636 67700 ± 283 70600 ± 141 40100 ± 849 42150 ± 71 57050 ± 1485 49300 ± 1980 47350 ± 778 50450 ± 636 A B C D I L N P R

pH H2O Exch (meq kg1) Exch (meq kg1) Exch (meq kg1) Olson (mg kg1) SS (lM) SS (mg C l1) SS (mM) SS (mM) SS (mM) SS (meq l1) SS (meq l1) SS (meq l1) Kd (l kg1) SS (Bq l1) Total (Bq kg1)

Exch (Bq kg1)

K+ Mg2+ Ca2+ P HPO2 4 TIC SO2 4 NO 3 Cl K+ Mg2+ Ca2+ Ra Ra Ra Ra Soil

Table 2 Values for the soil characteristics obtained after 4 weeks’ incubation

Total radium, radium in soil solution (SS) and exchangeable (Exch) radium; solid–liquid distribution coefficient (Kd), cations and ions in soil solution (SS), Olson P, exchangeable cations (Exch) and pH-H2O. Average ± Stdev (n = 3). Different letter annotations for certain parameter values denote significant differences between soil types.

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674 5.29 ± 0.05 5.89 ± 0.03 6.40 ± 0.01 4.63 ± 0.00 7.52 ± 0.00 5.45 ± 0.02 5.28 ± 0.01 5.71 ± 0.03 5.93 ± 0.07

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(Table 3) and DW production is significantly affected by soil type. For clover, effect of soil type on DW was even more pronounced with lowest DW observed on soil P (0.5 g pot1) and highest on soil L (5.0 g pot1) (all soils: 2.6 ± 1.6 g pot1). Overall, total DW production for clover was lower than for ryegrass. The concentration of K in ryegrass ranged between 16.8 and 43.4 mg g1 (all soils: 32.3 ± 8.7 mg g1). The concentrations of K are, on average, 50% larger than the reported average concentration of K in the ryegrass (22.1 mg g1, Spedding and Diekmahns, 1972). The Ca and Mg concentrations in the shoots ranged between 6.7 and 15.8 mg g1 (10.0 ± 3.4 mg g1) and between 1.0 and 4.9 mg g1 (2.6 ± 1.1 mg g1), respectively. These values are in agreement with the ranges of 4–10 mg g1 for Ca and 1.7–3.4 mg g1 for Mg as reported by Spedding and Diekmahns (1972). The concentration of K in clover ranged between 22.0 and 38.1 mg g1 (29.1 ± 6.5 mg g1), in agreement with the reported average concentration of K in clover (25.9 (15.4–38.0) mg g1, Spedding and Diekmahns, 1972). The Ca and Mg concentrations in the shoots ranged between 10.1 and 28.1 mg g1 (19.7 ± 5.7 mg g1) and between 2.4 and 11.7 mg g1 (5.7 ± 2.9 mg g1), respectively. These values are in agreement with the ranges of 12–21 mg g1 for Ca but are high compared to reported Mg levels (1.7– 2.9 mg g1) (Spedding and Diekmahns, 1972). Overall, there is no significant difference in K and Mg content between ryegrass and clover. For clover, a significantly higher shoot Ca content than for ryegrass was observed. There was a clear influence of soil type on the levels of K, Ca and Mg in the plant samples (Table 3). However, the levels of plant cations were not significantly correlated with the levels of respective cations in soil solution or with the respective exchangeable fraction. The radium concentrations in ryegrass shoots differed about 10-fold from 2650 Bq kg1 (soil N) to 28 850 Bq kg1 (soil D) (all soils: 14 600 ± 7970 Bq kg1). For clover observed radium concentrations ranged from 2400 Bq kg1 (soil C) to 59 930 Bq kg1 (soil D) (all soils: 17 300 ± 18 550 Bq kg1) (Table 3). Corresponding soilto-plant transfer factors were 0.054 kg kg1 (soil N) to 0.72 kg kg1 (soil D) for ryegrass and 0.034 kg kg1 (soil C) to 1.50 kg kg1 (soil D) for clover (Fig. 2). TF (and radium concentrations in shoots) are calculated as arithmetic means for each plant species per soil group. TF ranged from 0.034 to 1.494 g g1 for clover and 0.054 to 0.719 g g1 for ryegrass (Fig. 2). The original TF data for ryegrass and clover were not normally distributed. Transforming the ryegrass TF data (logarithmic, exponential, power) did not result in a normal distribution. With rather small data sets it is often found that tests for normality are negative (K. Wouters, University Hasselt Statistics’ Department, Pers. Communications). For the clover TF data a normal distribution was obtained after log transformation. Therefore, and also because it is commonly accepted that TFs are log-normally distributed (Sheppard

Kd, l kg-1

a 600 500

Kd = 0.71 x CEC - 0.64

400

R = 0.91

2

300 200

b

600 500

Kd = 27 x OM - 27

Kd, l kg -1

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674

400

R = 0.83

2

300 200 100

100

0

0 0

100

200

300

400

500

0

600

10 OM, %

5

600

Kd, l kg-1

500 400

Kd = 32 (CEC/([Ca]) - 42

300

R = 0.62

2

200 100 0

d

600

Kd, l kg -1

CEC, meq kg-1

c

669

500 400

Kd= 36CEC/([Ca]+[Mg]) - 20

300

R = 0.78

15

20

2

200 100 0

0

2

4

6

8

10

12

0

2

2+

CEC/[Ca ], l kg-1

4 6 8 2+ 2+ CEC/[Ca +Mg ], l kg-1

10

Fig. 1. Relation between the radium solid liquid distribution coefficient, Kd, and (a) cation exchange capacity (CEC), (b) organic matter content (OM), (c) CEC/[Ca2+] and (d) CEC/[Ca2+ + Mg2+], with [Ca2+] and [Ca2++ Mg2+] the Ca2+ or Ca2++ Mg2+ concentration in the soil solution. Values for soil N excluded from the analysis. Error bars depict standard deviations around the mean (n = 3).

and Evenden, 1988), comparison of TFs between soil groups (Fig. 2) was done for log-transformed data. Considering all soils, a significant correlation was found between the ryegrass TF and the radium concentration in the soil solution (R2 = 0.62) (for log transformed data a similar R2 of 0.63 was obtained). TF was related toKd, most significantly through an exponential relation ([TF] = 0.60 e0.006[Kd], R2 = 0.86, Fig. 3a). TF was not related to exchangeable Ra. When omitting the data for soil N, significant exponential correlations were found between TF and hence linear correlations between log-TF and the Ca concentration in the soil solution, [Ca2+] ([log-TF] = 0.016 [Ca2+]  0.006, R2 = 0.76), CEC ([log-TF] = 0.001 [CEC]  0.26, R2 = 0.73) and OM ([log-TF] = 0.005 [OM]  0.20, R2 = 0.70). No relation was found between (log-)TF and the Ca2+ + Mg2+ concentration in the soil solution (even not when excluding soil N from analysis). Ryegrass TF or the shoot radium concentration were not related to the concentration of K, Mg and Ca in the shoots. Since there was a 2-fold difference in yield observed between soils, we also performed correlation and regression analysis with total plant uptake (yield · concentration). However, no significant relationship between the total plant uptake and any of the soil characteristics analysed was observed. For clover the picture was rather similar. The TF was linearly correlated with the radium concentration in the soil solution (R2 = 0.63). TF was related to Kd, most significantly through a power relation (Fig. 3b). Very significant correlation were observed between the log-TF and the radium concentration in the soil solution [Ra] ([log-TF] = 0.0041 [Ra]  1.37, R2 = 0.75), CEC ([log-TF] = 0.003 [CEC]  0.16, R2 = 0.68) and OM ([log-TF] = 0.104 [CEC]  0.008, R2 = 0.63). For CEC and OM, even better correla-

tions were obtained when omitting the data for soil N (R2, respectively 0.86 and 0.68). Omitting the data for soil N, a significant correlation was also found between log-TF and the Ca concentration in the soil solution, [Ca2+] ([logTF] = 0.032 [Ca2+] +0.37, R2 = 0.61) but not with the concentration of Ca2+ + Mg2+ in the soil solution. As was observed for ryegrass, radium shoot concentration was not correlated with the concentration of the cations Ca, Mg or K in shoots. Regression analysis performed with total radium uptake in function of soil parameters did result in less significant (or non-significant) correlations. 4. Discussion 4.1. Predicting radium availability from soil factors The highest concentration of radium in the soil solution observed for soil D is explained by its low CEC, OM and pH, parameters inducing increased radium availability. Soil R also exhibits a high Ra concentration in the soil solution, though for this soil CEC, OM and pH were not specifically low. Moreover its high soluble sulphate content (on average a factor 25 higher than for the other soils) could induce radium precipitation. Soil R, however, has a very high bivalent cation content in the soil solution, 121 meq l1, which would result in increased desorption of Ra from the soil’s exchange sites through action of these competitive cations. The low Ra concentrations in the soil solutions of soils B, C, I and N, can be explained by a high CEC (soils B and C) or a medium CEC and alkaline pH (soil I). Soil N shows the lowest radium concentration in soil solution. Since its CEC, OM and pH are rather low, the only factor which could explain this finding is the low level of bivalent cations in the soil solution (limited

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674 2.0

513 ± 54 465 ± 18 469 ± 6 603 ± 12 512 ± 3 391 ± 15 413 ± 10 501 ± 22 345 ± 2

Water use (l kg1)

670

Clover

1.8

Ryegrass

1.6

TF, kg kg

31.3 ± 1.1 20.6 ± 1.4 16.8 ± 1.4 32.6 ± 0.7 32.8 ± 0.6 43.4 ± 1.6 39.5 ± 0.4 38.2 ± 1.7 35.6 ± 2.5

K (g kg1)

d e f d d a b c dc

-1

1.4 1.2 1.0 0.8 0.6

d e f c h g b e a 2.7 ± 0.0 2.4 ± 0.1 2.1 ± 0.2 2.9 ± 0.1 1.0 ± 0.1 1.7 ± 0.1 3.7 ± 0.0 2.4 ± 0.0 4.9 ± 0.2

0.0

A Ryegrass Clover

10.1 ± 0.3 15.3 ± 0.7 15.8 ± 1.5 6.7 ± 0.3 9.0 ± 0.7 7.9 ± 0.6 8.4 ± 0.2 10.1 ± 0.4 7.1 ± 0.3 22350 ± 720 b 11000 ± 370 d 6800 ± 1130 e 28850 ± 1210 a 10750 ± 520 d 21950 ± 1720 b 2650 ± 130 f 13600 ± 410 c 13450 ± 510 c 494 ± 6 546 ± 13 579 ± 9 526 ± 23 557 ± 25 431 ± 5 474 ± 15 528 ± 31 479 ± 13 33.6 ± 3.5 ab 22.4 ± 1.5 c 22.0 ± 3.0 c 33.6 ± 4.6 ab 28.7 ± 1.5 bc 29.5 ± 2.2 bc 38.1 ± 1.0 a 27.3 ± 11.6 bc 27.7 ± 1.5 bc Average ± Stdev (n = 3).

A B C D I L N P R

2.2 ± 0.2 2.3 ± 1.8 3.4 ± 1.3 2.1 ± 0.5 4.7 ± 0.2 5.0 ± 0.1 2.0 ± 0.2 0.5 ± 0.3 1.0 ± 0.1

b b ab b a a b c c

25380 ± 3900 b 3870 ± 1320 d 2400 ± 890 d 59930 ± 12530 a 2880 ± 560 d 10830 ± 3240 cd 5080 ± 480 d 26730 ± 3230 b 19060 ± 1970 bc

18.3 ± 1.0 22.1 ± 1.5 22.5 ± 0.3 13.5 ± 3.2 15.8 ± 1.3 10.1 ± 0.3 23.2 ± 0.8 28.2 ± 1.1 23.3 ± 0.6

c b b e d f b a b

5.7 ± 0.2 4.7 ± 0.9 3.9 ± 0.5 6.0 ± 2.5 2.8 ± 0.3 2.4 ± 0.1 8.2 ± 0.3 6.2 ± 0.3 11.7 ± 2.6

b bc bc b c c b b a

K (g kg1) Mg (g kg1) Ca (g kg1) Ra (Bq kg1)

B b b

C e ef

D f f

I a a

L d de

N c c

P g cd

R d b

d b

Fig. 2. Transfer factors (TF, kg kg1) for ryegrass and clover in function of soil type. Error bars depict standard deviations around mean (n = 4). TF with different letter annotations denote significant differences between soils. Test for differences performed on log-transformed data.

3.3 ± 0.1 c 4.3 ± 0.2 ab 4.5 ± 0.2 a 2.9 ± 0.1 c 3.9 ± 0.1 b 4.1 ± 0.2 ab 4.0 ± 0.2 ab 2.9 ± 0. c 3.0 ± 0.3 c

Ca (g kg1)

b a a c b cb cb b c

Mg (g kg1)

0.2

Ra (Bq kg1) Ryegrass

DW (g pot1) DW (g pot1)

Water use (l kg1) Clover Soil

Table 3 Shoot dry weight (DW), shoot radium (Ra), K, Ca and Mg concentration and total water use (l) over the cultivation period per kg DW for clover and ryegrass

0.4

exchange). This soil further shows very high soil solution P and high exchangeable P concentrations compared to the other soils. A negative dependency of radium solubility on soil available phosphate levels was, however, not found in literature. To find out if differences in free Ra2+ soil solution concentrations could explain why soil N performed differently, radium speciation calculations were performed. However, the free Ra2+ concentration did hardly differ from the measured soil solution Ra concentration, including soil N (data not shown). Hence, no better correlations were found when relating soil parameters with the free Ra2+ concentration. Solid liquid distribution coefficients observed in this study range from 38 to 446 l kg1 and are low compared with the earlier reported compendium Kd data by Sheppard and Thibault (1990) (500, 36 000, 9100, 2400 l kg1, for sand, loam, clay and organic soils, respectively) but in agreement with the more recently reported compendium data by Sheppard et al. (2005) (53, 46, 34, 200 l kg1, for sand, loam, clay and organic soils, respectively). In Sheppard et al. (2005) no explanation is given about the reason for this at least 10-fold difference in values. Our lower figures compared with the earlier Sheppard data can perhaps be partially explained by the recent nature of contamination used in our experiments. In soils with radium from natural origin or historically contaminated, radium may be partially associated with the resistant fraction. No relation was found between Kd and pH as found by Titayeva (1967). From the data of Johnston and Gilham (1980) it could be expected that radium would co-precipitate with ferric hydroxides, but in our study we did not observe a correlation between Kd and amorphous iron content. Soil Kd was weakly (negatively) correlated to CEC and OM, yet omitting the data for soil N, these correlations became very significant (Fig. 1a and b). It is unclear why soil N would behave distinct from other soils. Research found organic matter and clays to be the dominant constituents contributing to the sorption of radium

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674

Ryegrass

Clover

-0.006Kd

TF = 0.60e 2

R = 0.86

b

2

TF, kg kg -1

TF, kg kg -1

a

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1.5 1 TF = 76.7 K d

-1.2

2

0.5

R = 0.81

0

0

100

200

300

400

0

500

100

200

300

400

500

600

700

Kd, l kg -1

c 0.8 0.7

d

2

0.6 0.5 0.4 0.3 0.2 0.1 0

TF, kg kg -1

Kd, l kg -1

1.5

TF, kg kg -1

671

2+

2+

TF = 875 *1/([Ca +Mg ]*CEC) + 0.20 2

R = 0.83

2+

2+

TF= 1485* 1/([Ca +Mg ]*CEC) + 0.02 2

R = 0.95

1 0.5 0

0

0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 2+

2+

2 -1

1/([Ca +Mg ]*CEC), meq l kg

0

0.0002 0.0004 0.0006 0.0008

0.001

0.0012

1/([Ca2++Mg 2+]*CEC), meq2 l-1 kg -1

-1

Fig. 3. Transfer factor (TF, kg kg1) for ryegrass (left) or clover (right) in function of solid–liquid distribution coefficient, Kd (a and b) and the inverse of the product of CECx([Ca2+ + Mg2+) (c and d) with CEC, cation exchange capacity and [Ca2+ + Mg2+] concentration of Ca2+ and Mg2+ in the soil solution. For figures c and d values for soil N are excluded from analysis. Error bars depict standard deviations around the mean (n = 4).

onto soil. Organic matter adsorbs about ten times as much radium as clay, which is more adsorptive than other minerals matter (Simon and Ibrahim, 1990). This is confirmed by our study: we observed a significant correlation between Kd and OM but not with clay content. The earlier mentioned compendium data from Sheppard et al. (2005) also suggest that radium sorption is hardly affected by soil granulometry but is by OM content. An increased radium sorption with clay content was found by Nathwani and Phillips (1979). One would expect a reduced radium sorption with increasing concentration of bivalent cations due to competition effects (exchange reaction) at the soil’s exchange sites. The ability of Ca2+ and Mg2+ to replace Ra2+ is, however, considered low, possibly to the difference in ionic size ((Simon and Ibrahim, 1990) and references therein). Yet, on the other hand, Nathwani and Phillips (1979) found that the degree of radium sorption decreased considerably as the soil calcium concentration increased. This opposite effect of CEC and Ca2+ and Mg2+ concentration in the soil solution was applied earlier to assess the sorption of bivalent contaminants present in the soil at trace concentrations. Valcke et al. (1997) predicted the solid liquid distribution coefficient for radiostrontium, K Sr d , based on 2+ CEC the following equation K Sr ¼ , where [Ca ] refers to d ½Ca2þ  the solution concentration of competitive bivalent cation Ca (meq l1). Applying this approach to estimate the radium solid liquid distribution coefficient, no significant relation was observed considering all soils. Excluding the data of soil N from the analysis, a significant (p = 0.56) linear relationship was observed (R2 = 0.62, p = 0.056, Fig. 1c). Considering the sum of the Ca2+ and Mg2+ concentration in the soil solution, a more significant relationship (p = 0.02)was observed (R2 = 0.78, Fig. 1d).

This means that the radium Kd will increase with increased soil sorption capacity (CEC) and decrease with the level of competitive ions in the soil solution. It is not clear why soil N would behave differently from other soils. Ra The predictions for K Sr d were better than for K d (Valcke et al., 1997). Strontium is characterised by a simple and almost reversible ion exchange on the regular exchange sites on clay and organic matter, whereas radium sorption is more complex and due to its large ionic size, Ca and Mg are less able to replace Ra from the exchange complex than they can replace Sr. This may also explain why there was CEC no improvement in prediction power using ½Ca or 2þ  CEC as parameter to assess radium K rather than d ½Ca2þ þMg2þ  CEC. 4.2. Radium transfer to plants The radium transfer factors observed in this study correspond well with literature data, though they may tend to be a bit higher, partially due to the recent nature of the contamination in our experiments. For soils originating from a disused U mining area, Vera Tome´ et al. (2003) reported TFs for prairie grass ranging from 0.10 to 0.50 kg kg1. For sandy soils, TFs ranging from 0.07 to 0.21 kg kg1 were recorded (IUR, 1992) while IAEA (1994) mentioned a TF range to grass from 0.02 to 0.4 kg kg1. Gerzabek et al. (1998) found a TF for ryegrass of 0.15 ± 0.06 kg kg1 following artificial radium contamination. Lower TFs were reported by Chen et al. (2005) for soils contaminated with uranium mine tailings (TF = 0.0021 kg kg1 for ryegrass and 0.034 kg kg1 for clover). Overall, the 226Ra transfer factor showed no difference between ryegrass and clover. This is corroborated with

672

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674

results from Gerzabek et al. (1998) who found no difference in TF between graminaceous and dicotyledonous species. There was no significant correlation between the observed TF and plant dry weight or plant cation contents. Such relation might have elucidated a potential link between Ra uptake and soil parameters ruling cation uptake. Ca content in clover shoots was significantly higher than in ryegrass shoots, yet Ra content in clover and ryegrass was not significantly different. Several authors reported a positive correlation between shoot Ca content and shoot Ra content (Kopp et al., 1989; Linsalata et al., 1989; Million et al., 1994). In agreement with our data, Vasconcellos et al. (1987) failed to detect a significant correlation between plant Ca content and Ra uptake. Ra uptake has been reported to be highly influenced by soil P content (Blanco Rodrı´guez et al., 2002). We found rather weak correlations between the ryegrass and clover TF and total soil P (R2 = 0.46 (p = 0.06) and 0.34 (p = 0.095), respectively). No correlation was found with available P or P concentration in the soil solution. In the study of Million et al. (1994), P nutrition did not affect Ra uptake. We did not observe a significant pH effect on Ra uptake. In lysimeter studies to determine the uptake of 226Ra into agricultural crops, Gerzabek et al. (1998), showed amongst others, significant negative correlations between TF and pH. The pH-effect was explained due to lower radium availability with increasing pH (Hewamanna et al., 1988). Contrary to our findings, Gerzabek et al. (1998) found a negative dependency of TF on the silt and clay content of the top soil. In agreement with our findings, Mortvedt (1994) reported no differences in uptake of radium by several grasses grown on the sand or clay fractions of uranium mine tailings. It has been documented that alkaline earth metals may compete for adsorption binding sites on the surface of roots. In the presence of high soil concentrations of alkaline earth cations the uptake of radium may be suppressed owing to adsorption competition. Several authors found that total soil bivalent cation concentration (Taskaev et al., 1977; Simon and Ibrahim, 1990; Gerzabek et al., 1998) and exchangeable Ca and Mg (Gerzabek et al., 1998) suppressed radium uptake. Vasconcellos et al. (1987) argued that the exchangeable Ca in soils did not seem to influence radium uptake by plants in a defined way. For a greenhouse experiment with three very distinct soils artificially contaminated with radium, Vandenhove et al. (2005) found a significantly negative dependence (power) of the Ra-TF for clover and ryegrass on the concentration of Ca and Mg in soil solution. In present study, a linear or exponential relationship was found between the ryegrass TF and soil solution Ca or Ca2+ + Mg2+ concentration yet only if soil N was excluded from analysis. For clover these relationships were exponential (also soil N excluded). In our study, the most significant correlations, covering all 9 soils, were obtained between the Ra TF or log(TF)

and the radium concentration in the soil solution (positive) or Ra Kd(negative). For clover, CEC and OM explained, respectively 68% and 63% of the variation in the TF observed. For ryegrass, these soil parameters did also explain 70% of the variation in TF observed (exponential dependency) yet only if data for soil N were excluded from analysis. Kd was found to be significantly related to the soil CEC. Since soil TF is related to soil Kd, it is straightforward that CEC (and OM which is significantly correlated with CEC) is related to the TF. Gerzabek et al. (1998) did not find a significant relation between TF and CEC or OM. Kirchmann et al. (1966) determined that an inverse relationship existed between the log of the 226Ra content in plants and the amount of sorptive material in soil. Hypothesising that radium TF would be reduced by the soil’s sorption power (CEC) (soil factor) and the concentration of competitive cations in the soil solution (plant factor: competition at root surface for uptake), we tested if TF could be predicted from the inverse product of 1/(CEC Æ [Ca2+ + Mg2+]) with [Ca2+ + Mg2+] the concentration of Ca2+ + Mg2+ in the soil solution. If all soils are included, 49% (p = 0.036) and 63% (p = 0.003) of the variation in TF is explained, for ryegrass and clover, respectively. Excluding soil N from the analysis, 83% and 95% of the variation in the TF of ryegrass or clover is explained (Fig. 3c and d, respectively). Hence the combined parameter 1/(CEC Æ [Ca2+ + Mg2+]) seems to be a better way to predict radium TF than the single parameters CEC and [Ca2+ + Mg2+]. For a more realistic description of the mass transfer of radium from soils to plants, we calculated total radium flows, taking into account radium concentration in the soil solution [Ra, Bq l1], total water use (l pot1), shoot radium concentrations [Rashoot, Bq kg1] and biomass production (DW, kg pot1). The idea behind this simple approach is to allow for recalculation of transfer factors using some additional easily obtainable measurements. Hypothesising that the radium concentration in the soil solution is the readily available pool for uptake and that evaporation is the driving force for uptake, the radium flow (Bq) to the roots can be estimated by multiplying both parameters. We here assume that evaporation from soils is negligible compared to transpiration and not affected by soil type. Baptista et al. (2005) found that for tomatoes grown in greenhouse differences between transpiration and evapotranspiration were very small, which revealed that transpiration could be neglected. This flow can be related with the total radium incorporated in the plant shoot [Bq, total uptake defined as the biomass yield (kg) times the radium concentration in the shoots (Bq kg1)] or Ra-flow ¼ ½Ra  water flow  ½Rashoot   DW ¼ Ra-uptake As demonstrated in Fig. 4 (a1 and b1), the radium flow explained 61–83% of the variation in total radium uptake

H. Vandenhove, M. Van Hees / Chemosphere 69 (2007) 664–674

100

80 60 40

Ra uptake= 0.041 Ra flux + 15.98

20

Ra uptake, Bq

a2 120

100

Ra uptake, Bq

a1 120

R2 = 0.61

0 0

500

1000

1500

80 60 40

Ra uptake = 25.82 Ln(Ra fluxcomp ) - 22.38

20

R2 = 0.70

0 0

2000

20

Ra uptake, Bq

Ra uptake, Bq

b2 150

Ra uptake = 6.96e 0.002 Ra flux R2 = 0.83

100

40

60

80

100

Ra fluxcomp , Bq.l meq -1

Ra flux, Bq

b1 150

673

50

Ra uptake = 8.77e 0.047 Ra fluxc comp R2 = 0.91

100 50

0

0 0

500

1000

1500

0

20

40

60

80

Ra fluxcomp , Bq.l meq -1

Ra flux, Bq

Fig. 4. Total radium uptake for ryegrass (a) and clover (b) in function of the radium flux (1) or Ra fluxcomp, defined as the ratio of the radium flux and the concentration of Ca2+ and Mg2+ in the soil solution (2).

(including all soils). For ryegrass, best fits were obtained with a linear relationship and for clover with an exponential dependency between both parameters and reasons for this different functionality is unclear. Accounting for potential competition effects for uptake induced by the presence of the bivalent cations Ca2+ and Mg2+ in the soil solutions, we estimated the radium uptake by the ratio of the radium flux and the concentrations of those bivalent cations in soil solution, Ra fluxcomp Ra concentration in soil solution (Bq l1) * evapotranspiration water flux (l)/[Ca2+ and Mg2+] (meq l1)]. This resulted in even better estimations of the radium flux both for ryegrass (R2 = 0.70) and clover (R2 = 0.91), with the most significant relationship observed for a logarithmic dependency for ryegrass and an exponential dependency for clover (Fig. 4, a2 and b2). 5. Conclusions The usefulness of a single Kd to predict radium mobility for a certain soil group or of a single TF to predict the concentration in crops from the concentration in soil may be severely limited owing to the large variability which has been reported. We were able to show significant correlations between Kd and the OM content or CEC, a dependency becoming very significant when excluding data from one soil group. The soil to plant transfer factor was significantly related to Kd and excluding data for soil N, OM, and Ca concentration in the soil solution. The universal validity of these relationships and ruling equations should still be demonstrated and validated using an independent data set, which could perhaps also elucidate why certain soil(s) deviate from the general patterns observed. Generally, given the lack of significant correlations between Kd or TF and the soil clay content, as featured

by present data and other literature data, we would be inclined to discourage the prediction or classification of Ra soil Kd based on soil granulometry as is still generally done (e.g. IAEA, 1994). Data calculated using fluxes instead of the traditional transfer factor concept, yielded improved correlations. The potential of the simple flux values cannot be conclusively evaluated at the present time. Here also, the validity of the relationships found should be tested using a larger and independent data set. References Baptista, F.J., Bailey, B.J., Meneses, J.F., 2005. Measuring and modelling transpiration versus evapotranspiration of a tomato crop grown on soil in a mediterranean greenhouse. Acta Hort. (ISHS) 691, 313– 320. Bethke, C.M., 2001. The Geochemist’s Workbench, Release 3.2., a User’s Guide to Rxn, Act2, Tact, React and Gtplot. Hydrogeology Program, University of Illinois, Urbana, IL, USA. Blackmore, L.C., Searle, P.L. Daly, B.K., 1987. Methods for chemical analysis of soils. New Zealand Soil Bureau of Science. Report 80. Soil Bureau, Lower Hutt, New Zealand. Blanco Rodrı´guez, P., Vera Tome´, L., Lozano, J.C., 2002. About the assumption of linearity in soil-to-plant transfer factors for uranium and thorium isotopes and 226Ra. Sci. Total Environ. 284, 167– 175. CEA (Centre d’Etudes Nucle´aires), 1983. Limite de de´tection d’un signal dans un bruit de fond. Application aux mesures de radioactivite´ par comptage. Centre d’Etudes Nucle´aires de Grenoble Report CEA-R5201. Chen, S.B., Zhu, Y.G., Hu, Q.H., 2005. Soil to plant transfer of 238U, 226 Ra and 232Th on a uranium mining-impacted soil from southeastern China. J. Environ. Radioact. 82, 223–236. Chhabra, R., Pleysier, J., Cremers, A., 1975. The measurement of the cation exchange capacity and exchangeable cations in soils: a new method. In: Bailey, S.W. (Ed.), Proceedings of the International Clay Conference. Applied Science Publishers, London, Mexico, pp. 439– 449. Currie, L.A., 1968. Limits for qualitative determination and quantification determination. Anal. Chem. 40, 587–593.

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