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Long-term decline of 137Cs concentration in honey in the second decade after the Chernobyl accident
Science of the Total Environment 382 (2007) 147 – 152 www.elsevier.com/locate/scitotenv
Long-term decline of 137 Cs concentration in honey in the second decade after the Chernobyl accident Donatella Panatto ⁎, Roberto Gasparini, Piero Lai, Paola Rovatti, Giovanni Gallelli DiSSal-Department of Heath Sciences, University of Genoa, Via Pastore 1, 16132 Genoa, Italy Received 18 December 2006; received in revised form 26 March 2007; accepted 29 March 2007 Available online 21 May 2007
Abstract In the years 2001–2004 the 137Cs activity was investigated in a total of 336 samples of different varieties of honey harvested in the Liguria Region of Northern Italy. Our purpose was to define (a) residual radioactive contamination following the Chernobyl accident and 137Cs long-term decline, (b) correlation between 137Cs activity and different honey varieties, and (c) correlation between 137Cs activity and the prevailing geomorphological configuration in the collection areas. The mean 137Cs specific activity was 4.33 ± 5.04 S.D. Bq/kg. Chestnut honey showed higher levels of radioactive contamination, which were ascribed to the extensive, superficial and deep, root apparatus of the tree. Honey samples from acidic argillite soils, which withhold radionuclides after deposition and slowly release them to plants, also showed higher 137Cs activity. Long-term decline was calculated at 456 days, a value lower than those published from different food sources in the years following the accident. The rate of long-term decline decreases with time. © 2007 Elsevier B.V. All rights reserved. Keywords:
137
Cs; Long-term decline; Honey
1. Introduction 1.1. The Chernobyl accident On April 26, 1986, the nuclear accident at Chernobyl released approximately 50–100 MCi of radioactive contaminants into the atmosphere (Floyd, 1986). The Chernobyl Forum (2002) reported a 137Cs contamination of 85 ± 26 1015 Bq. The worldwide distribution of anthropogenic radionuclides depended not only on the severity of the event, but also on the rain and windstorm conditions prevailing in the Chernobyl area on the day of the accident, a circumstance that contributed greatly ⁎ Corresponding author. Tel.: +39 0103538109; fax: +39 0103538109. E-mail address: [email protected] (D. Panatto). 0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.03.040
to the scattering of contaminants. In the days following the accident, contamination reached different areas of Europe. Northern Italy was affected early in May 1986 (Bonazzola et al., 1991). The amount of soil deposition of anthropogenic radionuclides depended on the prevailing weather conditions upon the arrival of contaminants. 1.2. Honey as a biomarker of radioactive contamination Animal and plant species are often employed by current environmental research as biomarkers of physical and chemical contamination. A case in point is the bee, which can be useful for ad hoc surveys because it closely represents the conditions of the surrounding
148
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environment, in consideration of its physiology and ecological behavior (Ford et al., 1988; Tonelli et al., 1990). Honey is produced from the harvest of nectar, honeydew, and pollen of flowers growing within a radius of 1.5 km from the beehive, which usually houses 3000 to 4000 bees. Since each insect can visit up to 1000 flowers during its flights, millions of samples are taken to a single hive every day. Not only does the bee collect the contaminants present in the harvest, but during its food-searching flight it also reaches several water sources and makes contact with particles dispersed in the air. A major consideration is that when the bee deposits its harvest, all the preexisting material in the hive (i.e., honey, wax, pollen) becomes contaminated. As a result, examination of these materials yields precious information on the amount and physical and chemical properties of the contaminants. (Bonazzola et al., 1991; Molzahn and Assmann-Werthmuller, 1993; Carini and Bengtsson, 2001; Mitchel, 2001; Esposito et al., 2002). More than fifteen years after the Chernobyl accident, we utilized an original biological sampling system to evaluate residual radioactive contamination and longterm decline of 137Cs in the Liguria Region of Northern Italy. In this work, the residual 137 Cs contamination and the long-term decline of the isotope were investigated in honey samples from Liguria in 2001–2004. We also
investigated the correlation between concentrations of this pollutant and the diverse varieties of honey sampled. Furthermore, data were broken down by geographical area in order to define the relevance of variables affecting the dispersion of anthropogenic radionuclides, such as the weather conditions prevailing in the area during the days following the accident, and the morphological conformation of the pertinent territory. 2. Materials and methods Between June 2001 and December 2004, a total of 336 samples of honey were analyzed. Sampling criteria aimed at the following: (1) even coverage of a given geographical area; and (2) separation of different honey varieties. A constraint was constituted by the willingness of bee-keepers to participate in the study, in consideration of the fact that honey samples were to be collected at the same hives over a sufficient time-span to yield calculation of the long-term decline of 137Cs. Each bee-keeper was requested to fill in a questionnaire investigating the following: (1) collection area pertaining to the hives; (2) honey production season; (3) prevailing flora in the collection area, in consideration of the feeding pattern of the honeybees; and (4) possible blending of different honey varieties or the production of mixtures of honeys from different areas. The samples were subdivided by area (Fig. 1) and variety.
Fig. 1. Honey harvest areas.
D. Panatto et al. / Science of the Total Environment 382 (2007) 147–152 Table 1 Geomorphological characteristics of the source areas (Giammarino et al., 2002) Area Major characteristics 1 2 3 4 5 6
Lightly acidic, calcareous sedimentary rocks Igneous acidic rocks; lightly acidic sedimentary rocks; argillites Acidic metamorphic rocks; acidic argillites Basic igneous rocks; metamorphic rocks Sedimentary sandy rocks Sedimentary, calcareous rocks; dolomitic rocks
Table 1 shows the major geomorphological characteristics (Giammarino et al., 2002) of the six areas of honey collection. Table 2 shows the rainfall in the Liguria region (source: Liguria Region Weather Annals, 1987), broken down by area of honey collection, in the 48 h following the arrival of the Chernobyl contaminants (Bonazzola et al., 1991). 2.1. Radioactivity measurement Honey samples of about 500 g were used to measure Cs activities using a γ-ray spectrometry system. All samples were counted in Marinelli beakers using a high purity (26% relative efficiency) Ge-detector (Eurisys Mesures, Saint Quentin Yvelines, France). For 137Cs detection, 0.660 MeV energy was selected. The detector was connected to a 4048-channel high-pulse analyzer. This detection system was calibrated for energy and efficiency using a mixed standard γ-emitting source (Isotope Products Laboratories, Valencia, California 91355). Spectra were recorded and analyzed by means of a computer analysis program, which identified energies, assigned them to peaks, carried out background subtraction, integrated the areas under the peaks, and calculated radionuclide activities by comparing them with standard peak areas. The minimum measuring time was fixed at 60,000 s for each sample and 120,000 s for background. This time course entailed a lower limit detection for 137 Cs of 0.5 Bq/kg. 137
149
2.2. Statistics Statistical analysis was performed with StatView IV (abacus Concept) and Microsoft Excel programs. Evaluation of significance of the difference between means for the different factors was performed by means of two-factor (areas and honey varieties) analysis of variance (ANOVA) with repeated measures (time). For internal comparisons the Sheffé test (5% confidence level) was employed. 3. Results and discussion The detection limit for 137Cs was not reached in 49/ 336 (15%) samples. For statistical evaluation, the detection limit of 0.5 Bq/kg was attributed to these samples. The highest specific activity was found in a chestnut honey sample collected in area 3 in 2001. The 137 Cs data were subdivided by (a) collection area; (b) honey variety, and (c) year of harvest. Discussion of results has been treated separately for factors “area” and “honey variety” because ANOVA didn't show significant interaction between these factors. (Factors do not mutually interfere). 3.1. Collection area Table 3 shows 137 Cs specific activity in the six areas of collection. Sheffé's test showed a significantly higher rate of contamination of samples from area 3 than in those from all the remaining areas (p b 0.01). We attribute this difference to the weather conditions prevailing in the area, in particular copious rainfalls, at the time of contamination. In Liguria, radioactive contamination started with dry deposition on May 3 (Bonazzola et al., 1991), but reached its maximum with rain precipitation on May 5, 1986. Rainfall continued on May 6 with a severe downpour. In area 3, rain was more abundant than in the remaining areas on both days (Table 2). Table 3 Mean 137Cs specific activity in the honey samples by area
Table 2 Rainfall in the Liguria Region on May 5 and 6, 1986
Area
Samples (no.)
137
Arithmetic mean
±S.D.
Geometric mean
C.I.
52 64 57 54 59 50
2.57 4.83 8.45 3.92 2.71 2.62
2.53 4.36 8.83 3.28 2.14 2.09
1.67 3.12 5.37 2.58 1.94 1.93
1.26–2.21 2.49–3.93 4.15–6.96 1.95–3.40 1.55–2.44 1.5–2.49
Area May 5 rainfall (mm) May 6 rainfall (mm) Total rainfall (mm) 1 2 3 4 5 6
1.95 5.85 8.04 5.60 6.60 1.10
20.00 17.80 28.16 4.90 16.10 3.00
21.95 23.65 36.20 10.50 22.70 4.10
1 2 3 4 5 6
Cs (Bq/kg)
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Table 4 Mean specific activity of 137Cs in different varieties of honey Variety
Acacia Chestnut Multi flower Honeydew
Samples (no.)
137
80 90 91 75
1.63 8.42 2.83 3.61
Cs (Bq/kg)
Arithmetic ±S.D. Geometric C.I. mean mean 1.51 7.17 1.87 3.07
1.13 6.27 2.24 2.52
0.93–1.36 5.36–7.35 1.92–2.61 2.04–3.11
Our results are in good agreement with additional studies from different European regions. Several authors – Hamilton et al. (1986), Hohenemser et al. (1986), Papastefanou et al. (1999) and Ji-gen et al. (2006) – report that, besides wind intensity and direction, the amount of rainfall also has a relevant effect on the level of 137Cs contamination. As well as the type of radionuclide and of vegetation prevailing in the area, an additional variable affecting 137 Cs concentration in food sources is the geomorphology of the area, which is related to the persistence of radionuclides in the soil and to their long-term release to plants (Carini, 2001). The isotopes that reach the ground penetrate deeply and are diluted according to soil composition. Sedimentary, sandy terrains allow deep penetration of radionuclides; by contrast, shaly terrains withhold contaminants and release them more slowly. Furthermore, the acidity of the soil affects the adsorption of radionuclides by roots (Al-Oudat et al., 2006; Ji-gen et al., 2006). We believe that the higher contamination in area 3 could also be related to its geomorphological
composition, which is characterized by acidic metamorphic rocks and acidic argillites (pH 4.5–6.5). There are no additional data concerning the relevance of soil composition to 137Cs contamination in Liguria. 3.2. Honey varieties Table 4 shows the mean specific activity of 137Cs in the different varieties of honey considered. Chestnut honey showed significantly (p b 0.001) higher 137Cs specific activity that other varieties of honey. This was probably due to the fact that the chestnut has a large root extension both horizontally and vertically, a condition that facilitates uptake of both the superficial and deep pools of radionuclide (Atkinson and Webster, 2001). The interception factor of radioactive fall-out, defined as the ratio between radioactivity taken up by vegetation and total radioactive deposits, depends on the biology of the plant, with higher values being found in trees than in shrubs and grass (Tonelli et al. 1990; Carini, 2001), an observation in keeping with our data. A final consideration is that acacia and chestnut are almost pure sources for the pertinent varieties of honey; differences in the other varieties considered are more difficult to relate to the uptake of radioactive material because they represent mixtures of nectars from different vegetal sources (Bonazzola et al., 1991; Molzahn and Assmann-Werthmüller, 1993). 3.3. Long-term decline The long-term decline in radiocesium specific activity in the honey varieties considered was calculated
Fig. 2. Mean 137Cs specific activities (Bq/kg) in the different honey varieties by year of harvest. For instance, Eq. (1) describes long-term decline in acacia honey. Cs137 (t) = specific activity of 137Cs at time t; 6.148 = specific activity of 137Cs at time zero; 0.578 = biological decay of 137Cs in acacia honey (biological λ); R2 = significance of the curve.
D. Panatto et al. / Science of the Total Environment 382 (2007) 147–152
from formula (1) for the radionuclide physical decay constant (Tykva and Sabol, 1995), At ¼ A0 4ekt
ð1Þ
where At is the specific activity at time t, A0 the initial specific activity at time 0, and λ = the physical decay constant (0.693). Fig. 2 shows 137 Cs mean specific activity by honey variety and year of harvest and the exponential equations for the curves approximating these values. In order to calculate the long-term decline (in years), the ratio between physical λ (set at 0.693) and biological λ (depending on the honey type) must be calculated. For instance, acacia honey is associated with a long-term decline of 0.693/0.578 (Fig. 2, Eq. (1), which is equal to 437 days (1.199 years). The calculated decay times range between 577 days (1.58 years) for chestnut honey and 1.08 years (394 days) for honeydew, with a mean of 1.25 years (456 days). Chestnut honey is associated with both higher contamination and a slower decay. In a subsequent study on different food sources, Muck (1997) concluded that the long-term decline of 137 Cs decreased over the years and that values varied with the food source type considered. The highest values were found in tea and cereals (3.72 and 3.4 years, respectively); the lowest in fruit (1.2–1.6 years). In a study of seafood 137Cs concentration, Gallelli et al. (1997) reported a long-term decline of 100–120 days. These lower values were related to the radionuclide dilution in sea water. Amundsen et al. (1996) report an effective ecological half-life of 137Cs in mushrooms of 2–6 years. After a 5year study of the radioactive contamination of mushrooms, Mietelski et al. (1994) concluded that 137Cs contamination of mushrooms decreased very slowly or remained constant for years. 4. Conclusions Chestnut honey appeared to be the best indicator of radioactive contamination among the honey varieties analyzed, as a result of the high transfer of 137 Cs to trees. The values of long-term decline presented in this paper, which were lower than those reported by other authors in studies performed immediately after the Chernobyl accident, support the view that the decline of 137 Cs over time is not constant. As a result, the calculation of the effective dose commitment should consider that the curve of long-term decline varies with time, the rate of 137 Cs removal from the environment being more rapid than originally suggested. We postulate that, in consid-
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eration of the pattern of long-term decline shown in this study, 137Cs will become undetectable by means of standard methods in the Liguria Region before 2010, not only in general foodstuffs, but also in specific indicators of radioactive contamination, such as honey. Acknowledgements The work was supported by grants from the Istituto Zooprofilattico del Piemonte, Liguria e Valle d'Aosta. The authors thank Prof. C. Pesce for his help in revising the manuscript, the Ligurian bee-keepers for sampling and Prof. D. Risso for his help in statistical analysis. References Amundsen I, Gulden G, Strand P. Accumulation and long term behaviour of radiocaesium in Norwegian fungi. Sci Total Environ 1996;184:163–71. Al-Oudat M, Asfary AF, Mukhalallt H, Al-Hamwi A, Kanakri S. Transfer factors of 137Cs and 90Sr from soil to trees in arid regions. J Environ Radioact 2006;90:78–88. Atkinson CJ, Webster AD. The influence of the development of temperate fruit tree species on the potential for their uptake of radionuclides. J Environ Radioact 2001;52:131–46. Bonazzola GC, Ropolo R, Patetta A, Manino A. Distribution of fallout radionuclides in soil, plants and honey. Health Phys 1991;60:575–7. Carini F. Radionuclide transfer from soil to fruit. J Environ Radioact 2001;52:237–79. Carini F, Bengtsson G. Post-deposition transport of radionuclides in fruit. J Environ Radioact 2001;52:215–36. Chernobyl assessment of radiological and health impact. Update of Chernobyl: ten years on 2002. Esposito M, Polic P, Bartolomei P, Benzi V, Martellini M, Cvetkovic O, et al. Survey of natural and anthropogenic radioactivity in environmental samples from Yugoslavia. J Environ Radioact 2002;61:271–82. Floyd PJ. Impact of Chernobyl. International conference on nuclear risks. London; 1986. p. 1–2. Ford BC, Jester WA, Griffith SM, Morse RA, Zall RR, Burgett DM, et al. Cesium-134 and Cesium-137 in honey bees and cheese samples collected in the U.S.A. after the Chernobyl accident. Chemosphere 1988;17:1153–7. Gallelli G, Panatto D, Perdelli F, Pellegrino C. Long-term decline of radiocesium concentration in seafood from the Ligurian Sea (Northern Italy) after Chernobyl. Sci Total Environ 1997;196: 163–70. Giammarino S, Giglia G, Capponi G, Crispini LM, Piazza. Carta Geologica della Liguria (mimeographed material); 2002. Hamilton EI, Zou B, Clifton RJ. The Chernobyl accident— radionuclide fallout in S.W. England. Sci Total Environ 1986;57: 231–51. Hohenemser C, Deicher M, Ernst A, Hofsaess H, Lindner G, Recknagel E. Chernobyl: an early report. Environment 1986;28:6–46. Ji-gen Lu, Yanjun Huang, Fang Li, Lihua Wang, Shuzhen Li, Yuanfu Hsia. The investigation of 137 Cs and 90 Sr background radiation levels in soil and plant around Tianwan NPP, China. J Environ Radioact 2006:1–11. Liguria Region Weather Annals, 23 : 2, 323–324 (1987).
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Mietelski JW, Jasinska M, Kubica B, Kozak K, Macharski P. Radioactive contamination of Polish mushrooms. Sci Total Environ 1994;157:217–26. Mitchel NG. Models for radionuclide transfer to fruits and data requirements. J Environ Radioact 2001;52:292–307. Molzahn D, Assmann-Werthmüller U. Caesium radioactivity in several selected species of honey. Sci Total Environ 1993;130/ 131:95–108. Muck K. Long-term effective decrease of cesium concentration in foodstuffs after nuclear fallout. Health Physics 1997;72:659–73.
Papastefanou C, Manolopoulou M, Stoulos S, Ioannidou A, Gerasopoulos E. Soil-to-plant transfer of 137Cs, 40K and 7Be. J Environ Radioact 1999;45:59–65. Tonelli D, Gattavecchia E, Ghini S, Porrini C, Celli G, Mercuri AM. Honey bees and their products as indicators of environmental radioactive pollution. J Radioanal Nucl Chem 1990;141:427–36. Tykva R, Sabol J. Low-level environmental radioactivity. sources and evaluation. Technomic. Pennsylvania USA: Lancaster; 1995.
Long-term decline of 137 Cs concentration in honey in the second decade after the Chernobyl accident Donatella Panatto ⁎, Roberto Gasparini, Piero Lai, Paola Rovatti, Giovanni Gallelli DiSSal-Department of Heath Sciences, University of Genoa, Via Pastore 1, 16132 Genoa, Italy Received 18 December 2006; received in revised form 26 March 2007; accepted 29 March 2007 Available online 21 May 2007
Abstract In the years 2001–2004 the 137Cs activity was investigated in a total of 336 samples of different varieties of honey harvested in the Liguria Region of Northern Italy. Our purpose was to define (a) residual radioactive contamination following the Chernobyl accident and 137Cs long-term decline, (b) correlation between 137Cs activity and different honey varieties, and (c) correlation between 137Cs activity and the prevailing geomorphological configuration in the collection areas. The mean 137Cs specific activity was 4.33 ± 5.04 S.D. Bq/kg. Chestnut honey showed higher levels of radioactive contamination, which were ascribed to the extensive, superficial and deep, root apparatus of the tree. Honey samples from acidic argillite soils, which withhold radionuclides after deposition and slowly release them to plants, also showed higher 137Cs activity. Long-term decline was calculated at 456 days, a value lower than those published from different food sources in the years following the accident. The rate of long-term decline decreases with time. © 2007 Elsevier B.V. All rights reserved. Keywords:
137
Cs; Long-term decline; Honey
1. Introduction 1.1. The Chernobyl accident On April 26, 1986, the nuclear accident at Chernobyl released approximately 50–100 MCi of radioactive contaminants into the atmosphere (Floyd, 1986). The Chernobyl Forum (2002) reported a 137Cs contamination of 85 ± 26 1015 Bq. The worldwide distribution of anthropogenic radionuclides depended not only on the severity of the event, but also on the rain and windstorm conditions prevailing in the Chernobyl area on the day of the accident, a circumstance that contributed greatly ⁎ Corresponding author. Tel.: +39 0103538109; fax: +39 0103538109. E-mail address: [email protected] (D. Panatto). 0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.03.040
to the scattering of contaminants. In the days following the accident, contamination reached different areas of Europe. Northern Italy was affected early in May 1986 (Bonazzola et al., 1991). The amount of soil deposition of anthropogenic radionuclides depended on the prevailing weather conditions upon the arrival of contaminants. 1.2. Honey as a biomarker of radioactive contamination Animal and plant species are often employed by current environmental research as biomarkers of physical and chemical contamination. A case in point is the bee, which can be useful for ad hoc surveys because it closely represents the conditions of the surrounding
148
D. Panatto et al. / Science of the Total Environment 382 (2007) 147–152
environment, in consideration of its physiology and ecological behavior (Ford et al., 1988; Tonelli et al., 1990). Honey is produced from the harvest of nectar, honeydew, and pollen of flowers growing within a radius of 1.5 km from the beehive, which usually houses 3000 to 4000 bees. Since each insect can visit up to 1000 flowers during its flights, millions of samples are taken to a single hive every day. Not only does the bee collect the contaminants present in the harvest, but during its food-searching flight it also reaches several water sources and makes contact with particles dispersed in the air. A major consideration is that when the bee deposits its harvest, all the preexisting material in the hive (i.e., honey, wax, pollen) becomes contaminated. As a result, examination of these materials yields precious information on the amount and physical and chemical properties of the contaminants. (Bonazzola et al., 1991; Molzahn and Assmann-Werthmuller, 1993; Carini and Bengtsson, 2001; Mitchel, 2001; Esposito et al., 2002). More than fifteen years after the Chernobyl accident, we utilized an original biological sampling system to evaluate residual radioactive contamination and longterm decline of 137Cs in the Liguria Region of Northern Italy. In this work, the residual 137 Cs contamination and the long-term decline of the isotope were investigated in honey samples from Liguria in 2001–2004. We also
investigated the correlation between concentrations of this pollutant and the diverse varieties of honey sampled. Furthermore, data were broken down by geographical area in order to define the relevance of variables affecting the dispersion of anthropogenic radionuclides, such as the weather conditions prevailing in the area during the days following the accident, and the morphological conformation of the pertinent territory. 2. Materials and methods Between June 2001 and December 2004, a total of 336 samples of honey were analyzed. Sampling criteria aimed at the following: (1) even coverage of a given geographical area; and (2) separation of different honey varieties. A constraint was constituted by the willingness of bee-keepers to participate in the study, in consideration of the fact that honey samples were to be collected at the same hives over a sufficient time-span to yield calculation of the long-term decline of 137Cs. Each bee-keeper was requested to fill in a questionnaire investigating the following: (1) collection area pertaining to the hives; (2) honey production season; (3) prevailing flora in the collection area, in consideration of the feeding pattern of the honeybees; and (4) possible blending of different honey varieties or the production of mixtures of honeys from different areas. The samples were subdivided by area (Fig. 1) and variety.
Fig. 1. Honey harvest areas.
D. Panatto et al. / Science of the Total Environment 382 (2007) 147–152 Table 1 Geomorphological characteristics of the source areas (Giammarino et al., 2002) Area Major characteristics 1 2 3 4 5 6
Lightly acidic, calcareous sedimentary rocks Igneous acidic rocks; lightly acidic sedimentary rocks; argillites Acidic metamorphic rocks; acidic argillites Basic igneous rocks; metamorphic rocks Sedimentary sandy rocks Sedimentary, calcareous rocks; dolomitic rocks
Table 1 shows the major geomorphological characteristics (Giammarino et al., 2002) of the six areas of honey collection. Table 2 shows the rainfall in the Liguria region (source: Liguria Region Weather Annals, 1987), broken down by area of honey collection, in the 48 h following the arrival of the Chernobyl contaminants (Bonazzola et al., 1991). 2.1. Radioactivity measurement Honey samples of about 500 g were used to measure Cs activities using a γ-ray spectrometry system. All samples were counted in Marinelli beakers using a high purity (26% relative efficiency) Ge-detector (Eurisys Mesures, Saint Quentin Yvelines, France). For 137Cs detection, 0.660 MeV energy was selected. The detector was connected to a 4048-channel high-pulse analyzer. This detection system was calibrated for energy and efficiency using a mixed standard γ-emitting source (Isotope Products Laboratories, Valencia, California 91355). Spectra were recorded and analyzed by means of a computer analysis program, which identified energies, assigned them to peaks, carried out background subtraction, integrated the areas under the peaks, and calculated radionuclide activities by comparing them with standard peak areas. The minimum measuring time was fixed at 60,000 s for each sample and 120,000 s for background. This time course entailed a lower limit detection for 137 Cs of 0.5 Bq/kg. 137
149
2.2. Statistics Statistical analysis was performed with StatView IV (abacus Concept) and Microsoft Excel programs. Evaluation of significance of the difference between means for the different factors was performed by means of two-factor (areas and honey varieties) analysis of variance (ANOVA) with repeated measures (time). For internal comparisons the Sheffé test (5% confidence level) was employed. 3. Results and discussion The detection limit for 137Cs was not reached in 49/ 336 (15%) samples. For statistical evaluation, the detection limit of 0.5 Bq/kg was attributed to these samples. The highest specific activity was found in a chestnut honey sample collected in area 3 in 2001. The 137 Cs data were subdivided by (a) collection area; (b) honey variety, and (c) year of harvest. Discussion of results has been treated separately for factors “area” and “honey variety” because ANOVA didn't show significant interaction between these factors. (Factors do not mutually interfere). 3.1. Collection area Table 3 shows 137 Cs specific activity in the six areas of collection. Sheffé's test showed a significantly higher rate of contamination of samples from area 3 than in those from all the remaining areas (p b 0.01). We attribute this difference to the weather conditions prevailing in the area, in particular copious rainfalls, at the time of contamination. In Liguria, radioactive contamination started with dry deposition on May 3 (Bonazzola et al., 1991), but reached its maximum with rain precipitation on May 5, 1986. Rainfall continued on May 6 with a severe downpour. In area 3, rain was more abundant than in the remaining areas on both days (Table 2). Table 3 Mean 137Cs specific activity in the honey samples by area
Table 2 Rainfall in the Liguria Region on May 5 and 6, 1986
Area
Samples (no.)
137
Arithmetic mean
±S.D.
Geometric mean
C.I.
52 64 57 54 59 50
2.57 4.83 8.45 3.92 2.71 2.62
2.53 4.36 8.83 3.28 2.14 2.09
1.67 3.12 5.37 2.58 1.94 1.93
1.26–2.21 2.49–3.93 4.15–6.96 1.95–3.40 1.55–2.44 1.5–2.49
Area May 5 rainfall (mm) May 6 rainfall (mm) Total rainfall (mm) 1 2 3 4 5 6
1.95 5.85 8.04 5.60 6.60 1.10
20.00 17.80 28.16 4.90 16.10 3.00
21.95 23.65 36.20 10.50 22.70 4.10
1 2 3 4 5 6
Cs (Bq/kg)
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Table 4 Mean specific activity of 137Cs in different varieties of honey Variety
Acacia Chestnut Multi flower Honeydew
Samples (no.)
137
80 90 91 75
1.63 8.42 2.83 3.61
Cs (Bq/kg)
Arithmetic ±S.D. Geometric C.I. mean mean 1.51 7.17 1.87 3.07
1.13 6.27 2.24 2.52
0.93–1.36 5.36–7.35 1.92–2.61 2.04–3.11
Our results are in good agreement with additional studies from different European regions. Several authors – Hamilton et al. (1986), Hohenemser et al. (1986), Papastefanou et al. (1999) and Ji-gen et al. (2006) – report that, besides wind intensity and direction, the amount of rainfall also has a relevant effect on the level of 137Cs contamination. As well as the type of radionuclide and of vegetation prevailing in the area, an additional variable affecting 137 Cs concentration in food sources is the geomorphology of the area, which is related to the persistence of radionuclides in the soil and to their long-term release to plants (Carini, 2001). The isotopes that reach the ground penetrate deeply and are diluted according to soil composition. Sedimentary, sandy terrains allow deep penetration of radionuclides; by contrast, shaly terrains withhold contaminants and release them more slowly. Furthermore, the acidity of the soil affects the adsorption of radionuclides by roots (Al-Oudat et al., 2006; Ji-gen et al., 2006). We believe that the higher contamination in area 3 could also be related to its geomorphological
composition, which is characterized by acidic metamorphic rocks and acidic argillites (pH 4.5–6.5). There are no additional data concerning the relevance of soil composition to 137Cs contamination in Liguria. 3.2. Honey varieties Table 4 shows the mean specific activity of 137Cs in the different varieties of honey considered. Chestnut honey showed significantly (p b 0.001) higher 137Cs specific activity that other varieties of honey. This was probably due to the fact that the chestnut has a large root extension both horizontally and vertically, a condition that facilitates uptake of both the superficial and deep pools of radionuclide (Atkinson and Webster, 2001). The interception factor of radioactive fall-out, defined as the ratio between radioactivity taken up by vegetation and total radioactive deposits, depends on the biology of the plant, with higher values being found in trees than in shrubs and grass (Tonelli et al. 1990; Carini, 2001), an observation in keeping with our data. A final consideration is that acacia and chestnut are almost pure sources for the pertinent varieties of honey; differences in the other varieties considered are more difficult to relate to the uptake of radioactive material because they represent mixtures of nectars from different vegetal sources (Bonazzola et al., 1991; Molzahn and Assmann-Werthmüller, 1993). 3.3. Long-term decline The long-term decline in radiocesium specific activity in the honey varieties considered was calculated
Fig. 2. Mean 137Cs specific activities (Bq/kg) in the different honey varieties by year of harvest. For instance, Eq. (1) describes long-term decline in acacia honey. Cs137 (t) = specific activity of 137Cs at time t; 6.148 = specific activity of 137Cs at time zero; 0.578 = biological decay of 137Cs in acacia honey (biological λ); R2 = significance of the curve.
D. Panatto et al. / Science of the Total Environment 382 (2007) 147–152
from formula (1) for the radionuclide physical decay constant (Tykva and Sabol, 1995), At ¼ A0 4ekt
ð1Þ
where At is the specific activity at time t, A0 the initial specific activity at time 0, and λ = the physical decay constant (0.693). Fig. 2 shows 137 Cs mean specific activity by honey variety and year of harvest and the exponential equations for the curves approximating these values. In order to calculate the long-term decline (in years), the ratio between physical λ (set at 0.693) and biological λ (depending on the honey type) must be calculated. For instance, acacia honey is associated with a long-term decline of 0.693/0.578 (Fig. 2, Eq. (1), which is equal to 437 days (1.199 years). The calculated decay times range between 577 days (1.58 years) for chestnut honey and 1.08 years (394 days) for honeydew, with a mean of 1.25 years (456 days). Chestnut honey is associated with both higher contamination and a slower decay. In a subsequent study on different food sources, Muck (1997) concluded that the long-term decline of 137 Cs decreased over the years and that values varied with the food source type considered. The highest values were found in tea and cereals (3.72 and 3.4 years, respectively); the lowest in fruit (1.2–1.6 years). In a study of seafood 137Cs concentration, Gallelli et al. (1997) reported a long-term decline of 100–120 days. These lower values were related to the radionuclide dilution in sea water. Amundsen et al. (1996) report an effective ecological half-life of 137Cs in mushrooms of 2–6 years. After a 5year study of the radioactive contamination of mushrooms, Mietelski et al. (1994) concluded that 137Cs contamination of mushrooms decreased very slowly or remained constant for years. 4. Conclusions Chestnut honey appeared to be the best indicator of radioactive contamination among the honey varieties analyzed, as a result of the high transfer of 137 Cs to trees. The values of long-term decline presented in this paper, which were lower than those reported by other authors in studies performed immediately after the Chernobyl accident, support the view that the decline of 137 Cs over time is not constant. As a result, the calculation of the effective dose commitment should consider that the curve of long-term decline varies with time, the rate of 137 Cs removal from the environment being more rapid than originally suggested. We postulate that, in consid-
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