Radionuclide transfer factor from soil to plant in tropical and subtropical environments:

Transcription

1 Radionuclide transfer factor from soil to plant in tropical and subtropical environments: statistical analysis for different plant/soil combinations H. Velasco a( * ), J. Juri Ayub a, U. Sansone b a GEA - Instituto de Matemática Aplicada San Luis (IMASL). Universidad Nacional de San Luis, Consejo Nacional de Investigaciones Científicas y Técnicas. Ej. de los Andes 950. D5700HHW San Luis, Argentina. b International Atomic Energy Agency (IAEA), Agency s Laboratories Seibersdorf, A-1400 Vienna, Austria. Abstract In this study, more than 2700 radionuclide soil-to-plant transfer factors obtained for tropical and subtropical environments were statistically analyzed, and the influence of key parameters on different plant and soil combinations were investigated. Four broad soil groups and 13 plant groups were separately investigated. Additionally, different plant compartments were distinguished. The wide variability and uncertainty observed in transfer factors (TF) is considerably reduced when data are independently grouped into clusters of plant/plant part/soil type combinations. For each cluster statistical quantities were determined, and TF probability distribution was examined to improve the use of this empirical parameter in radiological assessment models. In many cases it was found that lognormal probability distribution reasonably represents the data. Although a wide TF variability was found for all radionuclides, when different plant/plant part/soil combinations were considered TF variability was found to be markedly lower. In most plant groups Zn and Sr present the highest transfer values. TF for Cs is lower and minimal transfer was found for Ra, U and Pb. Keywords: Tropical and subtropical environment; Radionuclides, Transfer factor; Root uptake, Soil to plant transfer (*) Corresponding author: address: hvelasco@unsl.edu.ar

2 1. Introduction The tropical zone is the area of the Earth centered on the equator and limited in latitude, respectively, by the Tropics of Cancer (northern hemisphere) and Capricorn (southern hemisphere). In this region ecosystems typically consist of rainforests, dry deciduous forest, spiny forest, or desert. The subtropical region refers to the zones of the Earth immediately north and south of tropic zone including a range of latitudes between 23.5 and approximately 40 degrees. These areas typically have hot summers, and throughout the year the air temperature usually does not go below freezing (0 o C). Subtropical areas don t usually have markedly wet or dry seasons and the rain distribution is fairly regular throughout the year. Investigations regarding radionuclide transfers were mainly limited to temperate regions because when they were carried out scarce data from other climatic zones were available. However, in the previous two decades new data on tropical and subtropical environments have become available. Although many researches demonstrate that there are no systematic differences of soil-to-plant transfer factors (TF) between climatic zones, some extreme values have been reported in individual environments in tropical and subtropical environments (Segalen, 1971, Wasserman, 1998). Probable reasons for these dissimilarities are: a) in tropical environments, almost all organic material that reaches the soil surface decomposes rapidly, and the surface accumulation of soil organic matter is therefore minimal. Consequently, there is rapid recycling of nutrients and contaminants into the vegetation. In temperate zones the decomposition of organic debris is slower, and the accumulation of soil organic matter is usually greater than the rate of decomposition, resulting in highly organic surface soil; b) In the tropics, due to relatively high soil ages and the high mineral weathering rates, clays of low exchange activity such as kaolinite are more common than in temperate zones. This leads to soils that, in spite of having high clay content, can have a low exchange capacity. In this study we analyse more than 2700 TF values from tropical and subtropical environments, with the intent of exploring the variability and dependence on the numerously different soil/crop type combinations. Data were obtained from the updated version of the IAEA - Handbook of Parameter Value for Predicting of Radionuclide transfer in temperate environment (IAEA, 1994), from the IUR Working Group: Soil-to-Plant transfer factors (Frissel 1992), published papers, books, institutional reports, and proceeding conferences. Data were clustered considering the possible combinations of plant group, plant compartment, and soil type, and then a descriptive statistical analysis was carried out. 2. Materials and Methods TF values have been considered in compliance with the definition proposed by the IUR (IUR, 1992), which is: -1 Concentration of radionuclide in plant (Bq kg dry crop mass) TF = (1) -1 Concentration of radionuclide in soil (Bq kg dry soil mass in upper 20 cm)

3 For grass the soil depth considered is 10 cm. In total, 2708 TF values were considered (1353 from tropical environments and 1355 from subtropical environments). Soil types were differentiated by texture, Table 1 shows the adopted criteria defining soil type. The following plant groups were separately considered: cereals, fruits, grasses, herbs, leguminous, leafy and non-leafy vegetables, pasture, rice, root crops, tubers and maize. Statistical analysis For each plant group / plant parts / soil type combination a descriptive statistical analysis was developed using Origin software (OriginLab, 2002). That includes the calculation of mean values (arithmetic (AM) and geometric (GM)) standard deviation (SD), geometric standard deviation (GSD), minimum (Min), maximum (Max) and coefficient of variation (CV=SD/AM). The Shapiro-Wilk Normality Test (Devore, 1998, OriginLab, 2002) was performed to explore the probability distribution of the data in each cluster. Particularly was tested the normality distribution of the logarithm of TF with a significance level of Results and Discussion Radionuclide dependence Table 2 reports TF arithmetic (AM) and geometric (GM) mean values for the principal radionuclides without differentiating plant and soil types. Coefficient of variation (CV) and the differences in order of magnitude (DIF) are given. TF presents a very wide variability. Differences in the order of magnitude of a factor of 2 were found for Co and Zn, but it reaches a factor of 5 for Cs and Ra. CVs are always greater than 1 and the maximum dispersion of the data was found for Cs (CV = 2.6) and Ra (CV = 4.0). Taking into consideration GM values, the following sequence was found: TF Zn > TF Sr > TF Cs TF Co > TF Ra TF U > TF Pb TF I. Fig.1 shows, as box charts, the TF distribution for the principal radionuclides for Tropical and Subtropical environments. In this figure, boxes are determined by the 25th and 75th percentiles. The whiskers are determined by the 5th and 95th percentiles with a coefficient of 1.5 (OriginLab, 2002). Geometric and arithmetic mean values are also reported. In most cases TF data are asymmetrically distributed, and, consequently GM and AM are for the most part different. The Shapiro-Wilk Normality Test (Devore, 1998, OriginLab, 2002) was performed to explore the probability distribution of TF values for each radionuclide. Particularly was tested the normality distribution of the logarithm of TF with a significance level of We found that TF is log-normally distributed. Additionally, considering separately TF values for tropical and subtropical environments, the test indicates differences statistically significant for Cs, Sr, Zn and Co. Influence of plant group

4 The distribution of TF for some plant groups was examined. Fig. 2 shows, as box charts, TF distributions for the main radionuclides for four plant groups. Fig. 3 shows as box charts TF for radiocaesium for all the considered plant groups. When plant groups were separately analyzed, TF variability was markedly lower, with most combinations demonstrating a CV value less than 1. For most plant groups Zn and Sr present the highest TF, with Ra and U presenting the lowest TF. Leguminous and leafy vegetables have the largest TF values. For grains, particularly rice, transfer factor is notably less. For radiocaesium we found that non-leafy and leafy vegetables, pastures, leguminous and herbs present the highest TF values (order of magnitude ranged from 0 to 1). Grasses, fruits and tubers present intermediate TF values (order of magnitude ranged from -1 to 2), while grains (cereals, rice and leguminous vegetables) present minimum TF values (order of magnitude ranged from -2 to 3). Probability distribution of TF values was explored. In many cases for different plant/plant part /soil combinations it was found that a lognormal distribution acceptably fits the point distribution. The Shapiro-Wilk Normality Test (Devore, 1998, OriginLab, 2002) was performed to explore the probability distribution of the data in each cluster. Particularly was tested the normality distribution of the logarithm of TF with a significance level of On the right column of Table 2 is indicated the results of the test (in this table, yes indicates that the logarithm of TF is normal, and, in other words, TF is log-normally distributed). As an example, Fig. 4 shows for radiocaesium the ln(tf) frequency histogram for rice, fruits, and tubers. Normal curves were jointly represented using mean and standard deviation values calculated from the logarithm of the data. In Fig. 5 were represented the values of TF obtained for three plant / plant part groups, separating data from different soil types. For rice, in spite of the TF geometric mean being lower in clay soil, no significant differences in TF distribution were observed for the different soil types. Leafy vegetable TF values from clay soil demonstrate a larger transfer factor than loam and sand soils. In the case of tubers, in spite of TF geometric mean values having the same order of magnitude, the variability is markedly different: clay soil presents the greatest variability (CV=1.2), followed by sand soil (CV=0.7)), and finally loam soil (CV=0.5). 4. Conclusions More than 2700 TF values from tropical and subtropical environments were statistically analyzed with the purpose of exploring the influence of crop types and soil properties on the radionuclide uptake by plants. Various radionuclides have been examined, however most information was found for radioisotopes of Ra, Cs, Sr, U and Co. The foremost results obtained are the following: A wide TF variability was found for all radionuclides when plant and soil groups were not distinguished. CV ranged from 1.6 (Pb and Co) to 4.0 (Ra). The difference in the order of magnitude ranged from 2 (Co and Zn) to 5 (Cs and Ra).

5 After different plant group / plant part combinations were considered TF variability was markedly lower. In most combinations CV is less than 1. It was found that for many combinations TF reasonably fit a log-normal probability distribution. In most plant groups Zn and Sr present the highest TF. Ra and U present low TF values, and the transfer factor is minimal for Th. A relatively high transfer factor was found for Ra in grasses. Transfer factors for Cs present an intermediate value. Leguminous and leafy vegetables present the highest TF values, while for grains, particularly rice, transfer factors are lower.. Soil types seem not to have a decisive influence on TF values. Only Cs TF in tubers in sand soil presents a relatively higher value than clay and loam TF values. 5. References Devore, J.L. (1998). Probability and statistics for Engineering and the Sciences, 4th Edition. Brooks/Cole Publishing Company. ISBN Frissel, M. J. (1992). An update of the recommended soil-to-plant transfer factors of 90 Sr, 137 Cs and transuranics. VIIIth Report of the Working Group Soil-to-Plant Transfer Factors, IUR. IAEA (1994). Handbook of parameter values for the prediction of radionuclide transfer in temperate environments. Technical Report Series No.364, IAEA. Origin (2002). Scientific graphing and Analysis Software. Version 7. OriginLab Corporation, Northampton, MA Segalen, P. (1971). Metallic oxides and hydroxides in soils of the warm and humid areas of the world: formation, identification, evolution. In: Soils and Tropical Weathering. UNESCO, Paris (Chapter 2). Wasserman, M.A. (1998). The behaviour of caesium-137 in oxisols and in the Goiania soil. In: Goiania, Ten Years Later. IAEA, Vienna. Acknowledgments This work has been partially financed by the projects PIP 6289 of CONICET and PROICO of the San Luis National University, Argentina. H. Velasco is grateful for the helpful support of Jim Malkowski.

6 Figures captions Figure 1. Box chart representation of radionuclides distribution in tropical and subtropical environments. GM and AM indicate geometric and arithmetic mean values. Figure 2. Box chart representation of TF values for different plant groups. Geometric (GM) and arithmetic (AM) mean values are also represented. Figure 3. Box chart representation of TF for Cs for the different plant groups. GM and AM indicate geometric and arithmetic mean values. Figure 4. Histograms of the logarithm of radiocaesium TF for the different plant / plant part / soil combinations. The curves represent the normal distribution probability with the indicated parameter values Figure 5. Box chart representation of TF values for three plant groups. Different soil types are separately considered. Geometric and arithmetic mean values are represented.

7 FIGURE 1

8 FIGURE 2

9 FIGURE 3 TF GM AM Plant (part) 1 Cereals (grain) 2 Cereals (stem, leaves) 3 Fruits (fruits) 4 Grasses (grain) 5 Grasses (stem, leaves) 6 Herbs (leaves) 7 Leguminous (grain) 8 Leguminous (leaves) 9 Non-Leafy (fruits) 10 Rice (grain) 11 Pasture (stem, leaves) 12 Root crops (roots) 13 Tubers (tubers) 14 Leafy (leaves) Plant groups

10 FIGURE 4. Frequency Plant: Rice Normal curve Part: Grain <x> = Soil: Clay SD = 1.80 Frequency Plant: Fruit plants Normal curve Part: Fruits <x> = Soil: All SD = 2.22 Frequency Plant: Tubers Normal curve Part: Tubers <x> = Soil: All SD = ln TF ln TF ln TF

11 FIGURE Plant: Rice Part: Grain GM AM 2 Plant: Leafy Vegetables Part: Leaves GM AM 1.0 Plant: Tubers Part: Tubers GM AM TF 0.5 TF 1 TF Clay Loam Sand Clay Loam Sand Clay Loam Sand Table 1. Soil groups Hydrolitic acidity Soil groups (ph) % Humus Cation exchange capacity Clay content Sand <20% Loam % Clay >40% Organic Table 2. Principal radionuclides in tropical and subtropical database: Statistical values Rad. Tropical Subtropical N AM GM CV DIF * N AM GM CV DIF * Cs E+00 1,97E E E Sr E E E E Co E E E E Zn E E E E Ra E E U E E Pb E E I E E * Differences in the order of magnitude