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Long-term behaviour of 137Cs in spruce bark in coniferous forests in the Czech Republic
Environmental Pollution 184 (2014) 511e514
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Long-term behaviour of the Czech Republic
137
Cs in spruce bark in coniferous forests in
Petr Rulík a, *,1, Helena Pilátová a,1, Ivan Suchara b, 2, Julie Sucharová b, 2 a b
National Radiation Protection Institute, Bartoskova 28, CZ 140 00 Prague 4, Czech Republic Silva Tarouca Research Institute for Landscape and Ornamental Gardening, Kvetnove namesti 391, CZ 252 43 Pruhonice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2013 Received in revised form 8 October 2013 Accepted 11 October 2013
Activity concentrations of 137Cs were detected in more than 400 outer spruce bark samples collected at sites variably affected by Chernobyl fallout across the Czech Republic in 1995 and 2010. The temporal changes in the 137Cs activities were found. The mean effective half-life (TEF) for 137Cs in spruce bark was 9.6 years, and the mean environmental half-life (TE) was 14 years. The effective half-lives were significantly higher in areas with higher long-term annual precipitation sums. Coefficient a in linear regression y ¼ ax þ b of half-lives on precipitation sums was 0.015 y mm1 for TEF and 0.036 y mm1 for TE. The aggregated transfer factor of 137Cs from soil to bark was determined and the pre-Chernobyl bark contamination related to year 2010 was estimated. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs in spruce bark Effective half-life Environmental half-life Aggregated transfer factor
1. Introduction The outer bark survives on the tree trunk for a long time. The dead tissue of the outer bark consists of remaining cell walls composed mainly of lignin, cellulose, hemicellulose, pectins and suberin. Other organic and inorganic compounds can also be lu et al., 1997). extracted from spruce bark (e.g., Hafizog Airborne pollutants are trapped by bark directly from the atmosphere, when dry deposits are washed from the surfaces of crown compartments and when they are transported through stemflow. Trunks can collect secondary pollution from resuspended deposited pollutants due to wind erosion of humus and soil particles in the surroundings. In the first few years of bark development, a small portion of pollutants caught by leaves or roots can be transported in the living young bark tissue. However, the concentration of pollutants in tree bark correlates tightly with the concentration of these pollutants in the atmosphere. Hence bark analyses have frequently been used for bioindicating anthropogenic atmospheric contamination levels of various pollutants (e.g., Guéguen et al., 2011). Tree bark, as an effective passive sampler, has also been used for determining the levels of environmental contamination due to radionuclides after accidental leaks from a
* Corresponding author. E-mail addresses: [email protected] (P. Rulík), [email protected] (H. Pilátová), [email protected] (I. Suchara), [email protected] (J. Sucharová). 1 Tel.: þ420 226 518 101; fax: þ420 241 410 215. 2 Tel.: þ420 296 528 284. 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.10.012
nuclear power plants or after atmospheric weapon tests (e.g., Belivermis¸ et al., 2010). Following the Chernobyl fallout in 1986, the behaviour of 137Cs has frequently been investigated in forest ecosystems. However, some questions about the retention, migration and cycling of 137Cs in the bark of forest trees have not been satisfactorily answered. In our previous study (Suchara et al., 2011), we detected the distribution of 137Cs activity concentrations in archived spruce bark specimens collected throughout the Czech Republic (CR) in 1995. The goals of our recent investigation were to determine the current 137 Cs activity concentrations in spruce bark that was not growing on trunks at the time when the Chernobyl-derived 137Cs fell (1986) and to assess the environmental half-life and the effective half-life of 137Cs in spruce bark in CR and their dependence on annual precipitation sums and 137Cs activities in bark.
2. Material and methods 2.1. Collecting and processing the bark samples The spruce bark samples were collected at 255 sampling sites distributed throughout CR in 2010 and at 192 sampling sites in 1995. The sampling plots that were used and their site factors are well defined, since these plots have served as long-term monitoring sites for national and European biomonitoring campaigns (e.g., Harmens et al., 2010; Suchara et al., 2011). At each sampling site, bark was collected from six adult Norwegian spruce (Picea abies) trees (typical age 60e90 years). Only the outermost easily separable pieces of bark were gently scraped off around the tree trunks at a height of 1.3 0.1 m. Fallen off pieces of bark about 2e 3 mm in thickness from each tree were stored in individual bags. The sampling procedure was described in detail in Suchara et al. (2011). The collected bark samples were air-dried between sheets of filter papers and pulverized.
512
P. Rulík et al. / Environmental Pollution 184 (2014) 511e514
2.2. Determination and evaluation of
137
Cs activity concentrations in bark
Six bark subsamples (from six trees) from each locality were measured together, as if they were a composite sample. The 137Cs activity concentration was determined using gamma spectrometry in the geometry of cylindrical vessels (6 vessels, each with 200 ml of a sample) held tight around the HPGe detectors. The calibration sources prepared by Czech Metrological Institute were used. The gamma spectrometry measurements were regularly checked by the national authority, and the method of gamma spectrometry was accredited. The activity concentrations (in Bq kg1 of net dry weight) of 137Cs were determined in all samples above the detection limit. The activities were corrected for the density of the sample by multiplication of the measured activity by the ratio of the mean attenuation in the calibration standard and in the bark sample. Mean attenuation was determined mathematically on the base of the theoretical attenuation coefficients (Jaeger et al., 1968) and verified experimentally. The method has been accredited. The minimum detectable activity at the 95% significance level was about 0.6 Bq kg1. The total combined uncertainty was 7e28%, with a mean value of 9%.
Fesenko et al. (2001) reported that the distribution of pine tree roots in a zone of 137Cs soil accumulation and 137Cs bioavailability were the crucial factors controlling the long-term accumulation of 137Cs in trees. More probably, however, outer bark may be contaminated ex post via stemflow that contains 137Cs either from washed-out needle and twig surface dust, or leached from young needles, where 137Cs taken by trees from the soil (a chemical analogue of macroelement potassium) is intensively accumulated or retranslocated from other tree tissues (Cs acts as macronutrient K analogue). 137Cs cycling caused by litter fall and root uptake can keep the 137Cs activities in bark proportional to the original site contamination loads (e.g. Calmon et al., 2009). Whatever the cause may be, the soil-to-spruce bark concentration factor for 137Cs is high. 3.2. Correlations
2.3. Processing the results 137
Cs in bark in 2010 were correlated with the The activity concentrations of Cs atmospheric deposition in 1986. 137 The activity concentrations of Cs found in tree bark were compared for sampling plots at which bark samples were collected both in 1995 and in 2010. The effective and environmental half-lives for 137Cs and spruce bark depending on longterm precipitation sums were estimated from the difference between 137Cs activities in bark in 1995 and in 2010. The long-term (1961e2000) average annual precipitation sums for the bark sampling sites were adopted from the Landscape Atlas of CR (Hrn ciarová et al., 2009). The aggregated transfer factors (Tag) were calculated for all bark sampling plots as the ratio of the measured 137Cs activity concentrations determined in spruce bark (corrected for radioactive decay as of the initial date of collection of the bark samples 8 July 2010) and the respective interpolated 137Cs depositions determined in the national screening in 1986 (Suchara et al., 2011). Both the Chernobyl transfer factor ** were estimated from a weighted Tag* and the pre-Chernobyl transfer factor Tag regression analysis of the 137Cs activity concentrations in spruce bark related to 137Cs deposition after the Chernobyl accident. 137
3. Results and discussion 3.1. Activity concentrations of
137
Cs in bark
The distribution of the activity concentrations of 137Cs in the bark samples collected in 2010 can be well approximated by a lognormal distribution. The determined 137Cs activity concentrations in bark referring to the values in 2010 were in the range between 1.5 and 270 Bq kg1, the geometric mean 9.3 Bq kg1 with geometric standard deviation 2.2 Bq kg1. The lowest activity concentration of 137Cs in bark appeared in the western and eastern parts of CR, which were not affected by showers at the time of the plume fallout in 1986. By contrast, above-average activities were revealed at a few small local “hot spots” located within a wide band running from northeast to southwest through the central part of the country. A very similar relative 137Cs activity distribution pattern was found in spruce bark collected across CR in 1995 (Suchara et al., 2011), indicating the long-term proportional storage of 137Cs in spruce bark. As the spruce bark that was collected in 2010 was not present even in the form of inner bark in 1986, it could not have been contaminated by any direct contact with dry or wet 137Cs depositions. However, 137Cs deposited on the outer bark could have diffused continuously into deeper and younger bark layers and could have kept the current 137Cs activities in the bark more or less proportional to the original local 137Cs bark activities that emerged shortly after the fallout. The 137Cs activities detected in new bark, which are relative to the amounts of 137Cs deposited on the ground in 1986, may also indicate that some other factors have been operating in forests that have new bark contaminated with deposited 137 Cs. The explanation for this may be some constant direct contamination of trees by resuspended local soil or humus particles containing 137Cs. 137Cs can be taken in from the soil solution by spruce roots, and can penetrate into the youngest bark tissue.
The ground contamination by 137Cs determined in 1986 correlated significantly (p < 0.05) with the local precipitation sums, which washed 137Cs from the Chernobyl plumes crossing CR in April/May 1986. The 137Cs activities in spruce bark samples collected in 1995 correlated significantly with the 137Cs activities of the ground in 1986 (Suchara et al., 2011). Similarly, a significant correlation was found for the activity concentrations of 137Cs determined in bark in 2010. Our results confirm that the 137Cs loads deposited on the ground at the time of the fallout are the main site factor significantly governing the current activity of 137Cs in spruce bark in forest stands, even 24 years after the fallout. Taking into account the small differences in the age of the investigated trees, the variability of the 137Cs activity concentration in bark is generally controlled by the variability of the primary atmospheric deposition rates or subsequent 137Cs and secondary tree contamination loads by resuspended ground 137Cs. The variation is due to the different surface retention capacities of the crowns of spruce trees of different ages. However, the contamination mechanism for newly grown bark is not exactly known. 3.3. Effective and environmental half-lives The long-term behaviour of 137Cs in the environment can be well estimated using the effective half-life, which is a combination of the physical half-life (TP) and the environmental half-life (TE) (Zibold and Klemt, 2005; Pröhl et al., 2006). The environmental factors that reduce bark 137Cs activities can include, for example, 137 Cs diffusion, washing out and leaching episodes, and turnover of young bark tissue and falling old bark tissue. Factors acting against the reduction of 137Cs activities in tree bark are 137Cs root uptake from soil, and bark surface contamination caused by resuspended particles containing 137Cs. In any event, the effective and environmental half-lives for 137Cs and spruce bark can be assessed from the difference between the 137Cs activities in bark in 1995 and in 2010. For a further evaluation, we determined the 1995/2010 137Cs bark activity ratios for forest sites repeatedly investigated in 1995 and 2010 that were located not more than 0.5 km apart. A total of 114 paired data items were available. The geometric mean for the 1995/ 2010 137Cs activity ratios at these sites was 3.0. Assuming that 1995 and 2010 the 137Cs activities in bark and the effective half-life (TEF) are governed by the same relationship as for radioactive decay, TEF is given as
TEF ¼
ln2$t ln AA12
(1)
where A1, A2 is the respective 137Cs activities in bark in 1995 and in 2010 and t is the time interval 2010e1995.
P. Rulík et al. / Environmental Pollution 184 (2014) 511e514 40
The environmental, effective and physical half-lives are related by the equation
Environmental half-life Effective half-life
35
y = 0.036x - 10 R² = 0.83
30
TEF $TP TP TEF
(2)
where TP is the physical half-life and TE is the actual removal environmental half-life of 137Cs from the bark. Solving Eq. (1) and Eq. (2) for the geometric mean A1/A2 ratios of 3.0, the following assessments of the effective and environmental half-lives were obtained: TEF ¼ 9.6 y and TE ¼ 14 y (using TF ¼ 30.2 y and t ¼ 15 y). No half-life values for spruce bark have been found in the literature. However, from the fitted published temporal (1991e 1997) 137Cs activities in Scots pine outer bark in Zhitomir, Ukraine (IAEA, 2002), we estimated TEF ¼ 7.4 y and TE ¼ 9.8 y. This TE value for smoother outer pine bark in the more contaminated environment and more continental climate of the Ukraine is about 70% of the TE value assessed for spruce bark in central Europe. Zibold and Klemt (2005) gathered published data about the temporal reduction of 137Cs activities in various compartments of forest ecosystems in European countries, and stated that TE for spruce bark in the Ukraine was between 9 and 14 years. Pröhl et al. (2006) calculated TE values from temporal changes in 137Cs activities published in the literature, and stated TE values of less than 12 years for various compartments of forest ecosystems in Central Europe. The relationships of the environmental and effective half-life of 137 Cs activity in bark and site long-term (1961e2000) average annual precipitation sums were investigated. The 1995/2010 137Cs bark activity ratios were assigned to nine precipitation amount classes, and the geometric mean of the activity ratios was estimated for each class. Precipitation classes <500 mm and >1200 mm, each with one sample, were omitted from the evaluation. The TEF and TE values were calculated for the geometric mean A1/A2 ratios (Table 1). The results show approximately linear increase of TE and TEF values with increasing precipitation sums (Fig. 1). The effective half-lives were between 7.1 and 16 years, and the environmental half-lives between 9.3 and 34 years. The total combined uncertainty (at level 1s) was determined by the error propagation method (Taylor’s expansion). Coefficient a in the linear regression y ¼ ax þ b was 0.015 y mm1 for TEF and 0.036 y mm1 for TE. The higher TE and TEF in more humid areas may be due to more intensive 137Cs uptake by trees from wet soil, or due to more effective washing out of dry deposits or 137Cs leaching from tree crown structures and transport by stemflow in trunk bark. Seasonal variation in precipitation and other meteorological parameters may control the uptake of 137Cs by root, which is active in the spring and summer seasons. However, the efficiency of the ways of receiving 137 Cs and the mechanism for retaining and transporting 137Cs in spruce bark is not sufficiently known. The approximation based on
Half-life [year]
TE ¼
513
25 20 15 10
y = 0.015x - 0.44 R² = 0.89
5 0 400
500
600
700
800 900 Precipitation [mm]
1000
1100
1200
1300
Fig. 1. Relationships between the effective and environmental half-lives of 137Cs in bark and long-term annual precipitation sums at the bark sampling sites, (R2 ¼ coefficient of determination).
the linear relationship is limited to the investigated range of precipitation sums; from Eq. (2) it is clear that the half-lives cannot be both simultaneously exactly linearly related to precipitation sums. 3.4. Aggregated transfer factor for
137
Cs in bark
The aggregated transfer factor for the transfer of 137Cs from the ground surface into spruce bark Tag (the ratio of the 137Cs activity concentration in bark 2010 and that on the soil surface in 1986) was calculated. The geometric mean value of Tag was 3.9 103 m2 kg1 with the geometric standard deviation 3.2. However, the relationship between the activity concentrations of 137Cs in bark and the accumulated 137Cs activities in the uppermost soil layer variously contaminated by 137Cs deposition after the Chernobyl accident was analysed in greater detail, similarly as in Suchara et al. (2011); see Fig. 2. This regression is well expressed by the linear function
y ¼ a$x þ b
(3)
where y is the activity concentration of 137Cs in bark (Bq kg1) and x is contamination of the soil surface by 137Cs from the Chernobyl fallout (Bq m2). The weighted linear regression coefficients were found to be, for a 5.3 104 m2 kg1 and for b 4.2 Bq kg1. Coefficient a can be interpreted as the aggregated transfer factor (T*ag) of the transfer of 137Cs after 1986 into spruce bark, and intercept b can most likely be interpreted as the present-day significant average background pre-Chernobyl 137Cs activity
Table 1 Geometric mean (GM) of the 1995/2010 bark 137Cs activity concentration ratios and the effective (TEF) and environmental (TE) half-lives of 137Cs activity in bark in areas variously affected by long-term annual precipitation sums, (n ¼ number of samples, s e combined standard uncertainty). Precipitation class (mm)
n
GM
Czech Republic (whole) 500e550 550e600 600e650 650e700 700e800 800e1000 1000e1200
114
3.0
19 21 22 10 7 15 18
3.5 4.3 3.3 2.9 2.5 2.3 1.9
sGM
TEF (year)
sTEF
TE (year)
sTE
(year)
0.1
9.6
0.2
14
1
0.3 0.4 0.3 0.4 0.3 0.1 0.1
8.2 7.1 8.6 10 11 12 16
0.5 0.4 0.6 1 1 1 1
11 9.3 12 15 18 21 34
1 0.8 1 3 3 3 4
(year)
Fig. 2. Weighted linear regression between activity concentrations of 137Cs in spruce bark in 2010 and the 137Cs surface deposition levels in 1986, (R2 ¼ coefficient of determination).
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P. Rulík et al. / Environmental Pollution 184 (2014) 511e514
concentration in spruce bark. The pre-Chernobyl bark contamination was a remnant from previous fallout from nuclear tests in the 1960s, and a consequence of the associated 137Cs resuspension. The 137 Cs deposition was reported to be about 5 kBq m2 in the western part of former Czechoslovakia in the 1960s (UNSCEAR, 1982). If spruce bark accumulated pre-Chernobyl 137Cs in the activity concentration of 4.2 Bq kg1, then the new pre-Chernobyl aggregated transfer factor T**ag of 8.4 104 m2 kg1 was derived (the ratio of 4.2 Bq kg1 and 5 103 Bq m2). The aggregated transfer factors in 1995 were higher (Suchara et al., 2011). The reduction of T*ag in 2010 in comparison with T*ag in 1995 can be explained by decreased activity of 137Cs in the soil due to 137Cs uptake by vegetation, percolation of 137Cs deeper into the soil, causing reduced contamination of soil particles resuspended from the soil surface and due to (natural) peeling of the outer bark, which contains most of the 137Cs. The effect of similar site factors can explain changes in the remaining factors. However, individual effects of the site factors have not yet been quantified. Funding This research was funded by institutional support (VUKOZ-IP00027073) and by institutional support, (MV-25972-17/OBVV2010). References lu, S., Kalaycı, G., Pes¸treli, D., 2010. The Belivermis¸, M., Kılıç, Ö., Çotuk, Y., Topcuog usability of tree barks as long term biomonitors of atmospheric radionuclide deposition. Appl. Radiat. Isot. 68, 2433e2437.
Calmon, P., Thiry, Y., Zibold, G., Rontavaara, A., Fesenko, S., 2009. Transfer parameter values in temperate forest ecosystems: a review. J. Environ. Radioact. 100, 757e 766. Fesenko, S.V., Soukhova, N.V., Sanzharova, N.L., Avila, R., Spiridonov, S.I., Klein, D., Lucot, E., Badot, P.M., 2001. Identification of processes governing long-term accumulation of 137Cs by forest trees following the Chernobyl accident. Radiat. Environ. Biophys. 40, 105e113. Guéguen, F., Stille, P., Millet, M., 2011. Air quality assessment by tree bark biomonitoring in urban, industrial and rural environments of the Rhine Valley: PCDD/Fs, PCBs and trace metal evidence. Chemosphere 85, 195e 202. lu, H., Usta, M., Bilgin, Ö., 1997. Wood and bark composition of Picea oriHafizog entalis (L.) Link. Holzforschung 51, 114e118. Harmens, H., Norris, D.A., Steinnes, E., Kubin, E., Piispanen, J., Alber, R., et al., 2010. Mosses as biomonitors of atmospheric heavy metal deposition: spatial and temporal trends in Europe. Environ. Pollut. 158, 3144e3156. Hrn ciarová, T., Mackov cin, P., Zvara, I., et al., 2009. Landscape Atlas of the Czech Republic. Ministry for the Environment, Praha and Silva Tarouca Research Institute for Landscape and Ornamental Gardening, Pruhonice. IAEA, 2002. Modelling the Migration and Accumulation of Radionuclides in Forest Ecosystems. Table III-7. Report of the Forest working group of the biosphere modelling and assessment (BIOMASS) programme, Theme 3. IAEA, Vienna, p. 115. Jaeger, R.G., Blizard, E.P., Chilton, A.B., Grotenhuis, M., Honig, A., Jaeger, Th.A., Eisenlohr, H.H., 1968. Engineering Compendium on Radiation Shielding, vol. I. Springer, Berlin a.o. Pröhl, G., Ehlken, S., Fiedler, I., Kirchner, G., Klemt, E., Zibold, G., 2006. Ecological half-lives of 90Sr and 137Cs in terrestrial and aquatic ecosystems. J. Environ. Radioact. 91, 41e72. Suchara, I., Rulík, P., H ulka, J., Pilátová, H., 2011. Retrospective determination of 137Cs specific activity distribution in spruce bark and bark aggregated transfer factor in forests on the scale of the Czech Republic ten years after the Chernobyl accident. Sci. Total Environ. 409, 1927e1934. UNSCEAR, 1982. Report 1982, Annex E. Exposures Resulting from Nuclear Explosions. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations Sales Publication E.82.IX.8., New York. Zibold, G., Klemt, E., 2005. Ecological half-times of 137Cs and 90Sr in forest and freshwater ecosystems. Radioprotection 40 (Suppl. 1), S497eS502.
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Long-term behaviour of the Czech Republic
137
Cs in spruce bark in coniferous forests in
Petr Rulík a, *,1, Helena Pilátová a,1, Ivan Suchara b, 2, Julie Sucharová b, 2 a b
National Radiation Protection Institute, Bartoskova 28, CZ 140 00 Prague 4, Czech Republic Silva Tarouca Research Institute for Landscape and Ornamental Gardening, Kvetnove namesti 391, CZ 252 43 Pruhonice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2013 Received in revised form 8 October 2013 Accepted 11 October 2013
Activity concentrations of 137Cs were detected in more than 400 outer spruce bark samples collected at sites variably affected by Chernobyl fallout across the Czech Republic in 1995 and 2010. The temporal changes in the 137Cs activities were found. The mean effective half-life (TEF) for 137Cs in spruce bark was 9.6 years, and the mean environmental half-life (TE) was 14 years. The effective half-lives were significantly higher in areas with higher long-term annual precipitation sums. Coefficient a in linear regression y ¼ ax þ b of half-lives on precipitation sums was 0.015 y mm1 for TEF and 0.036 y mm1 for TE. The aggregated transfer factor of 137Cs from soil to bark was determined and the pre-Chernobyl bark contamination related to year 2010 was estimated. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs in spruce bark Effective half-life Environmental half-life Aggregated transfer factor
1. Introduction The outer bark survives on the tree trunk for a long time. The dead tissue of the outer bark consists of remaining cell walls composed mainly of lignin, cellulose, hemicellulose, pectins and suberin. Other organic and inorganic compounds can also be lu et al., 1997). extracted from spruce bark (e.g., Hafizog Airborne pollutants are trapped by bark directly from the atmosphere, when dry deposits are washed from the surfaces of crown compartments and when they are transported through stemflow. Trunks can collect secondary pollution from resuspended deposited pollutants due to wind erosion of humus and soil particles in the surroundings. In the first few years of bark development, a small portion of pollutants caught by leaves or roots can be transported in the living young bark tissue. However, the concentration of pollutants in tree bark correlates tightly with the concentration of these pollutants in the atmosphere. Hence bark analyses have frequently been used for bioindicating anthropogenic atmospheric contamination levels of various pollutants (e.g., Guéguen et al., 2011). Tree bark, as an effective passive sampler, has also been used for determining the levels of environmental contamination due to radionuclides after accidental leaks from a
* Corresponding author. E-mail addresses: [email protected] (P. Rulík), [email protected] (H. Pilátová), [email protected] (I. Suchara), [email protected] (J. Sucharová). 1 Tel.: þ420 226 518 101; fax: þ420 241 410 215. 2 Tel.: þ420 296 528 284. 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.10.012
nuclear power plants or after atmospheric weapon tests (e.g., Belivermis¸ et al., 2010). Following the Chernobyl fallout in 1986, the behaviour of 137Cs has frequently been investigated in forest ecosystems. However, some questions about the retention, migration and cycling of 137Cs in the bark of forest trees have not been satisfactorily answered. In our previous study (Suchara et al., 2011), we detected the distribution of 137Cs activity concentrations in archived spruce bark specimens collected throughout the Czech Republic (CR) in 1995. The goals of our recent investigation were to determine the current 137 Cs activity concentrations in spruce bark that was not growing on trunks at the time when the Chernobyl-derived 137Cs fell (1986) and to assess the environmental half-life and the effective half-life of 137Cs in spruce bark in CR and their dependence on annual precipitation sums and 137Cs activities in bark.
2. Material and methods 2.1. Collecting and processing the bark samples The spruce bark samples were collected at 255 sampling sites distributed throughout CR in 2010 and at 192 sampling sites in 1995. The sampling plots that were used and their site factors are well defined, since these plots have served as long-term monitoring sites for national and European biomonitoring campaigns (e.g., Harmens et al., 2010; Suchara et al., 2011). At each sampling site, bark was collected from six adult Norwegian spruce (Picea abies) trees (typical age 60e90 years). Only the outermost easily separable pieces of bark were gently scraped off around the tree trunks at a height of 1.3 0.1 m. Fallen off pieces of bark about 2e 3 mm in thickness from each tree were stored in individual bags. The sampling procedure was described in detail in Suchara et al. (2011). The collected bark samples were air-dried between sheets of filter papers and pulverized.
512
P. Rulík et al. / Environmental Pollution 184 (2014) 511e514
2.2. Determination and evaluation of
137
Cs activity concentrations in bark
Six bark subsamples (from six trees) from each locality were measured together, as if they were a composite sample. The 137Cs activity concentration was determined using gamma spectrometry in the geometry of cylindrical vessels (6 vessels, each with 200 ml of a sample) held tight around the HPGe detectors. The calibration sources prepared by Czech Metrological Institute were used. The gamma spectrometry measurements were regularly checked by the national authority, and the method of gamma spectrometry was accredited. The activity concentrations (in Bq kg1 of net dry weight) of 137Cs were determined in all samples above the detection limit. The activities were corrected for the density of the sample by multiplication of the measured activity by the ratio of the mean attenuation in the calibration standard and in the bark sample. Mean attenuation was determined mathematically on the base of the theoretical attenuation coefficients (Jaeger et al., 1968) and verified experimentally. The method has been accredited. The minimum detectable activity at the 95% significance level was about 0.6 Bq kg1. The total combined uncertainty was 7e28%, with a mean value of 9%.
Fesenko et al. (2001) reported that the distribution of pine tree roots in a zone of 137Cs soil accumulation and 137Cs bioavailability were the crucial factors controlling the long-term accumulation of 137Cs in trees. More probably, however, outer bark may be contaminated ex post via stemflow that contains 137Cs either from washed-out needle and twig surface dust, or leached from young needles, where 137Cs taken by trees from the soil (a chemical analogue of macroelement potassium) is intensively accumulated or retranslocated from other tree tissues (Cs acts as macronutrient K analogue). 137Cs cycling caused by litter fall and root uptake can keep the 137Cs activities in bark proportional to the original site contamination loads (e.g. Calmon et al., 2009). Whatever the cause may be, the soil-to-spruce bark concentration factor for 137Cs is high. 3.2. Correlations
2.3. Processing the results 137
Cs in bark in 2010 were correlated with the The activity concentrations of Cs atmospheric deposition in 1986. 137 The activity concentrations of Cs found in tree bark were compared for sampling plots at which bark samples were collected both in 1995 and in 2010. The effective and environmental half-lives for 137Cs and spruce bark depending on longterm precipitation sums were estimated from the difference between 137Cs activities in bark in 1995 and in 2010. The long-term (1961e2000) average annual precipitation sums for the bark sampling sites were adopted from the Landscape Atlas of CR (Hrn ciarová et al., 2009). The aggregated transfer factors (Tag) were calculated for all bark sampling plots as the ratio of the measured 137Cs activity concentrations determined in spruce bark (corrected for radioactive decay as of the initial date of collection of the bark samples 8 July 2010) and the respective interpolated 137Cs depositions determined in the national screening in 1986 (Suchara et al., 2011). Both the Chernobyl transfer factor ** were estimated from a weighted Tag* and the pre-Chernobyl transfer factor Tag regression analysis of the 137Cs activity concentrations in spruce bark related to 137Cs deposition after the Chernobyl accident. 137
3. Results and discussion 3.1. Activity concentrations of
137
Cs in bark
The distribution of the activity concentrations of 137Cs in the bark samples collected in 2010 can be well approximated by a lognormal distribution. The determined 137Cs activity concentrations in bark referring to the values in 2010 were in the range between 1.5 and 270 Bq kg1, the geometric mean 9.3 Bq kg1 with geometric standard deviation 2.2 Bq kg1. The lowest activity concentration of 137Cs in bark appeared in the western and eastern parts of CR, which were not affected by showers at the time of the plume fallout in 1986. By contrast, above-average activities were revealed at a few small local “hot spots” located within a wide band running from northeast to southwest through the central part of the country. A very similar relative 137Cs activity distribution pattern was found in spruce bark collected across CR in 1995 (Suchara et al., 2011), indicating the long-term proportional storage of 137Cs in spruce bark. As the spruce bark that was collected in 2010 was not present even in the form of inner bark in 1986, it could not have been contaminated by any direct contact with dry or wet 137Cs depositions. However, 137Cs deposited on the outer bark could have diffused continuously into deeper and younger bark layers and could have kept the current 137Cs activities in the bark more or less proportional to the original local 137Cs bark activities that emerged shortly after the fallout. The 137Cs activities detected in new bark, which are relative to the amounts of 137Cs deposited on the ground in 1986, may also indicate that some other factors have been operating in forests that have new bark contaminated with deposited 137 Cs. The explanation for this may be some constant direct contamination of trees by resuspended local soil or humus particles containing 137Cs. 137Cs can be taken in from the soil solution by spruce roots, and can penetrate into the youngest bark tissue.
The ground contamination by 137Cs determined in 1986 correlated significantly (p < 0.05) with the local precipitation sums, which washed 137Cs from the Chernobyl plumes crossing CR in April/May 1986. The 137Cs activities in spruce bark samples collected in 1995 correlated significantly with the 137Cs activities of the ground in 1986 (Suchara et al., 2011). Similarly, a significant correlation was found for the activity concentrations of 137Cs determined in bark in 2010. Our results confirm that the 137Cs loads deposited on the ground at the time of the fallout are the main site factor significantly governing the current activity of 137Cs in spruce bark in forest stands, even 24 years after the fallout. Taking into account the small differences in the age of the investigated trees, the variability of the 137Cs activity concentration in bark is generally controlled by the variability of the primary atmospheric deposition rates or subsequent 137Cs and secondary tree contamination loads by resuspended ground 137Cs. The variation is due to the different surface retention capacities of the crowns of spruce trees of different ages. However, the contamination mechanism for newly grown bark is not exactly known. 3.3. Effective and environmental half-lives The long-term behaviour of 137Cs in the environment can be well estimated using the effective half-life, which is a combination of the physical half-life (TP) and the environmental half-life (TE) (Zibold and Klemt, 2005; Pröhl et al., 2006). The environmental factors that reduce bark 137Cs activities can include, for example, 137 Cs diffusion, washing out and leaching episodes, and turnover of young bark tissue and falling old bark tissue. Factors acting against the reduction of 137Cs activities in tree bark are 137Cs root uptake from soil, and bark surface contamination caused by resuspended particles containing 137Cs. In any event, the effective and environmental half-lives for 137Cs and spruce bark can be assessed from the difference between the 137Cs activities in bark in 1995 and in 2010. For a further evaluation, we determined the 1995/2010 137Cs bark activity ratios for forest sites repeatedly investigated in 1995 and 2010 that were located not more than 0.5 km apart. A total of 114 paired data items were available. The geometric mean for the 1995/ 2010 137Cs activity ratios at these sites was 3.0. Assuming that 1995 and 2010 the 137Cs activities in bark and the effective half-life (TEF) are governed by the same relationship as for radioactive decay, TEF is given as
TEF ¼
ln2$t ln AA12
(1)
where A1, A2 is the respective 137Cs activities in bark in 1995 and in 2010 and t is the time interval 2010e1995.
P. Rulík et al. / Environmental Pollution 184 (2014) 511e514 40
The environmental, effective and physical half-lives are related by the equation
Environmental half-life Effective half-life
35
y = 0.036x - 10 R² = 0.83
30
TEF $TP TP TEF
(2)
where TP is the physical half-life and TE is the actual removal environmental half-life of 137Cs from the bark. Solving Eq. (1) and Eq. (2) for the geometric mean A1/A2 ratios of 3.0, the following assessments of the effective and environmental half-lives were obtained: TEF ¼ 9.6 y and TE ¼ 14 y (using TF ¼ 30.2 y and t ¼ 15 y). No half-life values for spruce bark have been found in the literature. However, from the fitted published temporal (1991e 1997) 137Cs activities in Scots pine outer bark in Zhitomir, Ukraine (IAEA, 2002), we estimated TEF ¼ 7.4 y and TE ¼ 9.8 y. This TE value for smoother outer pine bark in the more contaminated environment and more continental climate of the Ukraine is about 70% of the TE value assessed for spruce bark in central Europe. Zibold and Klemt (2005) gathered published data about the temporal reduction of 137Cs activities in various compartments of forest ecosystems in European countries, and stated that TE for spruce bark in the Ukraine was between 9 and 14 years. Pröhl et al. (2006) calculated TE values from temporal changes in 137Cs activities published in the literature, and stated TE values of less than 12 years for various compartments of forest ecosystems in Central Europe. The relationships of the environmental and effective half-life of 137 Cs activity in bark and site long-term (1961e2000) average annual precipitation sums were investigated. The 1995/2010 137Cs bark activity ratios were assigned to nine precipitation amount classes, and the geometric mean of the activity ratios was estimated for each class. Precipitation classes <500 mm and >1200 mm, each with one sample, were omitted from the evaluation. The TEF and TE values were calculated for the geometric mean A1/A2 ratios (Table 1). The results show approximately linear increase of TE and TEF values with increasing precipitation sums (Fig. 1). The effective half-lives were between 7.1 and 16 years, and the environmental half-lives between 9.3 and 34 years. The total combined uncertainty (at level 1s) was determined by the error propagation method (Taylor’s expansion). Coefficient a in the linear regression y ¼ ax þ b was 0.015 y mm1 for TEF and 0.036 y mm1 for TE. The higher TE and TEF in more humid areas may be due to more intensive 137Cs uptake by trees from wet soil, or due to more effective washing out of dry deposits or 137Cs leaching from tree crown structures and transport by stemflow in trunk bark. Seasonal variation in precipitation and other meteorological parameters may control the uptake of 137Cs by root, which is active in the spring and summer seasons. However, the efficiency of the ways of receiving 137 Cs and the mechanism for retaining and transporting 137Cs in spruce bark is not sufficiently known. The approximation based on
Half-life [year]
TE ¼
513
25 20 15 10
y = 0.015x - 0.44 R² = 0.89
5 0 400
500
600
700
800 900 Precipitation [mm]
1000
1100
1200
1300
Fig. 1. Relationships between the effective and environmental half-lives of 137Cs in bark and long-term annual precipitation sums at the bark sampling sites, (R2 ¼ coefficient of determination).
the linear relationship is limited to the investigated range of precipitation sums; from Eq. (2) it is clear that the half-lives cannot be both simultaneously exactly linearly related to precipitation sums. 3.4. Aggregated transfer factor for
137
Cs in bark
The aggregated transfer factor for the transfer of 137Cs from the ground surface into spruce bark Tag (the ratio of the 137Cs activity concentration in bark 2010 and that on the soil surface in 1986) was calculated. The geometric mean value of Tag was 3.9 103 m2 kg1 with the geometric standard deviation 3.2. However, the relationship between the activity concentrations of 137Cs in bark and the accumulated 137Cs activities in the uppermost soil layer variously contaminated by 137Cs deposition after the Chernobyl accident was analysed in greater detail, similarly as in Suchara et al. (2011); see Fig. 2. This regression is well expressed by the linear function
y ¼ a$x þ b
(3)
where y is the activity concentration of 137Cs in bark (Bq kg1) and x is contamination of the soil surface by 137Cs from the Chernobyl fallout (Bq m2). The weighted linear regression coefficients were found to be, for a 5.3 104 m2 kg1 and for b 4.2 Bq kg1. Coefficient a can be interpreted as the aggregated transfer factor (T*ag) of the transfer of 137Cs after 1986 into spruce bark, and intercept b can most likely be interpreted as the present-day significant average background pre-Chernobyl 137Cs activity
Table 1 Geometric mean (GM) of the 1995/2010 bark 137Cs activity concentration ratios and the effective (TEF) and environmental (TE) half-lives of 137Cs activity in bark in areas variously affected by long-term annual precipitation sums, (n ¼ number of samples, s e combined standard uncertainty). Precipitation class (mm)
n
GM
Czech Republic (whole) 500e550 550e600 600e650 650e700 700e800 800e1000 1000e1200
114
3.0
19 21 22 10 7 15 18
3.5 4.3 3.3 2.9 2.5 2.3 1.9
sGM
TEF (year)
sTEF
TE (year)
sTE
(year)
0.1
9.6
0.2
14
1
0.3 0.4 0.3 0.4 0.3 0.1 0.1
8.2 7.1 8.6 10 11 12 16
0.5 0.4 0.6 1 1 1 1
11 9.3 12 15 18 21 34
1 0.8 1 3 3 3 4
(year)
Fig. 2. Weighted linear regression between activity concentrations of 137Cs in spruce bark in 2010 and the 137Cs surface deposition levels in 1986, (R2 ¼ coefficient of determination).
514
P. Rulík et al. / Environmental Pollution 184 (2014) 511e514
concentration in spruce bark. The pre-Chernobyl bark contamination was a remnant from previous fallout from nuclear tests in the 1960s, and a consequence of the associated 137Cs resuspension. The 137 Cs deposition was reported to be about 5 kBq m2 in the western part of former Czechoslovakia in the 1960s (UNSCEAR, 1982). If spruce bark accumulated pre-Chernobyl 137Cs in the activity concentration of 4.2 Bq kg1, then the new pre-Chernobyl aggregated transfer factor T**ag of 8.4 104 m2 kg1 was derived (the ratio of 4.2 Bq kg1 and 5 103 Bq m2). The aggregated transfer factors in 1995 were higher (Suchara et al., 2011). The reduction of T*ag in 2010 in comparison with T*ag in 1995 can be explained by decreased activity of 137Cs in the soil due to 137Cs uptake by vegetation, percolation of 137Cs deeper into the soil, causing reduced contamination of soil particles resuspended from the soil surface and due to (natural) peeling of the outer bark, which contains most of the 137Cs. The effect of similar site factors can explain changes in the remaining factors. However, individual effects of the site factors have not yet been quantified. Funding This research was funded by institutional support (VUKOZ-IP00027073) and by institutional support, (MV-25972-17/OBVV2010). References lu, S., Kalaycı, G., Pes¸treli, D., 2010. The Belivermis¸, M., Kılıç, Ö., Çotuk, Y., Topcuog usability of tree barks as long term biomonitors of atmospheric radionuclide deposition. Appl. Radiat. Isot. 68, 2433e2437.
Calmon, P., Thiry, Y., Zibold, G., Rontavaara, A., Fesenko, S., 2009. Transfer parameter values in temperate forest ecosystems: a review. J. Environ. Radioact. 100, 757e 766. Fesenko, S.V., Soukhova, N.V., Sanzharova, N.L., Avila, R., Spiridonov, S.I., Klein, D., Lucot, E., Badot, P.M., 2001. Identification of processes governing long-term accumulation of 137Cs by forest trees following the Chernobyl accident. Radiat. Environ. Biophys. 40, 105e113. Guéguen, F., Stille, P., Millet, M., 2011. Air quality assessment by tree bark biomonitoring in urban, industrial and rural environments of the Rhine Valley: PCDD/Fs, PCBs and trace metal evidence. Chemosphere 85, 195e 202. lu, H., Usta, M., Bilgin, Ö., 1997. Wood and bark composition of Picea oriHafizog entalis (L.) Link. Holzforschung 51, 114e118. Harmens, H., Norris, D.A., Steinnes, E., Kubin, E., Piispanen, J., Alber, R., et al., 2010. Mosses as biomonitors of atmospheric heavy metal deposition: spatial and temporal trends in Europe. Environ. Pollut. 158, 3144e3156. Hrn ciarová, T., Mackov cin, P., Zvara, I., et al., 2009. Landscape Atlas of the Czech Republic. Ministry for the Environment, Praha and Silva Tarouca Research Institute for Landscape and Ornamental Gardening, Pruhonice. IAEA, 2002. Modelling the Migration and Accumulation of Radionuclides in Forest Ecosystems. Table III-7. Report of the Forest working group of the biosphere modelling and assessment (BIOMASS) programme, Theme 3. IAEA, Vienna, p. 115. Jaeger, R.G., Blizard, E.P., Chilton, A.B., Grotenhuis, M., Honig, A., Jaeger, Th.A., Eisenlohr, H.H., 1968. Engineering Compendium on Radiation Shielding, vol. I. Springer, Berlin a.o. Pröhl, G., Ehlken, S., Fiedler, I., Kirchner, G., Klemt, E., Zibold, G., 2006. Ecological half-lives of 90Sr and 137Cs in terrestrial and aquatic ecosystems. J. Environ. Radioact. 91, 41e72. Suchara, I., Rulík, P., H ulka, J., Pilátová, H., 2011. Retrospective determination of 137Cs specific activity distribution in spruce bark and bark aggregated transfer factor in forests on the scale of the Czech Republic ten years after the Chernobyl accident. Sci. Total Environ. 409, 1927e1934. UNSCEAR, 1982. Report 1982, Annex E. Exposures Resulting from Nuclear Explosions. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations Sales Publication E.82.IX.8., New York. Zibold, G., Klemt, E., 2005. Ecological half-times of 137Cs and 90Sr in forest and freshwater ecosystems. Radioprotection 40 (Suppl. 1), S497eS502.