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Soil to plant uptake of fallout 137Cs by plants from boreal areas polluted by industrial emissions from smelters
The Science of the Total Environment 234 Ž1999. 213]221
Soil to plant uptake of fallout 137Cs by plants from boreal areas polluted by industrial emissions from smelters K. Bunzl a,U , B.P. Albers a , W. Shimmack a , K. Rissanen b, M. Suomelab, M. Puhakainen b, T. Raholab, E. Steinnes c a
GSF-National Research Center for En¨ ironment and Health, Institute of Radiation Protection, D-85764 Neuherberg, Germany b Radiation and Nuclear Safety Authority (STUK), P.O. Box 14, FIN 00881 Helsinki, Finland c Norwegian Uni¨ ersity of Science and Technology, Department of Chemistry, N-7034 Trondheim, Norway Received 12 March 1999; accepted 15 May 1999
Abstract To study the impact of industrial pollution on the soil-to-plant uptake of fallout-radiocesium in a boreal forest ecosystem, four study sites were selected at distances of 7, 16, 21 and 28 km from the large copper]nickel smelter at Monchegorsk on the Kola Peninsula ŽRussia.. At each site, soil and selected plant species were sampled from five plots and analysed separately for 137Cs and 40 K. The data show that the root-uptake of 137Cs, as characterised by the median aggregated transfer-factor Tag , decreased significantly Ž P- 0.05. with decreasing distance from the smelter for the plants Vaccinium myrtillus Žfrom 0.023 to 0.007 m2 kgy1 . and Empetrum nigrum Žfrom 0.015 to 0.007 m2 kgy1 ., but increased for Deschampsia flexuosa Žfrom 0.013 to 0.031 m2 kgy1 .. For Vaccinium ¨ itis-idaea a significant trend for the Tag was not observed. The median 40 K activity concentrations in these plants also decreased significantly Ž P- 0.001. with decreasing distance from the smelter for Vaccinium myrtillus Žfrom approx. 140 to 20 Bq kgy1 dry wt.., Empetrum nigrum Žfrom approx. 90 to 40 Bq kgy1 dry wt.., and also for Deschampsia flexuosa Žfrom approx. 270 to 40 Bq kgy1 dry wt... For Vaccinium ¨ itis-idaea such a continuous significant trend was not observed. The results for the Cu]Ni polluted soils thus show: Ž1. that the soil-to-plant transfer of radiocesium can be significantly modified; Ž2. that these modifications are quite specific; and Ž3. that modifications of the uptake of potassium do not always correspond to those of radiocesium. Q 1999 Elsevier Science B.V. All rights reserved. Keywords:
137
Cs; Plant; Soil; Transfer; Pollution; Industry; Smelter
U
Corresponding author. Tel.: q49-89-31872203; fax: q49-89-31873323. E-mail address: [email protected] ŽK. Bunzl.
0048-9697r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 9 9 . 0 0 2 6 5 - X
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1. Introduction Information on the soil-to-plant transfer of radionuclides is required to estimate the radiation exposure of man and animals after accidental releases of artificial radionuclides to the biosphere. For this reason, numerous investigations on this subject are available which consider agricultural crops and, to a lesser extent, also seminatural environments such as forests or meadows. However, especially when studying semi-natural environments, one usually deliberately selects those areas which are not polluted by inorganic or organic chemicals released, e.g. by industrial emissions to the atmosphere. Because it is well known that such emissions may have complex effects on the physico-chemical properties of the soil as well as on the soil microbiology Žsee, e.g. Pennanen et al., 1996., it is doubtful to what extent soil-to-plant transfer factors determined for unpolluted areas will also describe realistically the soilrplant relationships present at polluted sites. At present, no systematic study on this subject seems to be available with the exception of an investigation by Sobakin and Molchanova Ž1998., who very recently studied the plant uptake of 238 U, 226 Ra and 232 Th in a polluted mining area of Russia. The purpose of the present investigation is to determine the aggregated soil-to-plant transfer factors of fallout 137 Cs in an area polluted by heavy metals and sulphur, i.e. by chemicals frequently found in industrial emissions. These contaminants have been released for 65 years to the air in large amounts from a large copper]nickel smelter complex at Monchegorsk on the Kola Peninsula ŽRussia., this is why the area was selected for the present study. To examine quantitatively the effect of different pollution levels on the 137Cs plant uptake, several sites at various distances from the smelter were selected. The boreal forests present in this area are typical for many similar ecosystems found in Northern Fennoscandia. Radiocaesium present there is mainly due to the cumulated global fallout of nuclear weapons testing and only to a minor extent due to the fallout from the reactor accident at Chernobyl in 1986 ŽSaxen et al., 1986;
Strand et al., 1998.. Nevertheless, the risk of an additional large-scale radioactive fallout in these areas cannot be excluded due the existence of nuclear power plants, stored liquid and solid nuclear wastes, as well as nuclear powered vessels and submarines in the Kola region.
2. Material and methods 2.1. Site All sampling sites were located on the Kola Peninsula. The main pollution sources there are several Cu]Ni smelters Severonikel at Monchegorsk and Pechenganikel at Nikel. In 1987]1991 the total emissions amounted to approximately 700 000 tonsryear. This amount decreased to 580 000 tonsryear in 1994. Approximately 70% of the emissions are made up by SO 2 . During the period 1987]1991 the Ni-emission was 3700 tons, and that of Cu 2600 tonsryear. In 1996 the corresponding amounts were 1300 and 700 tonsryear, respectively. Most of the Ni and Cu originates from the smelter at Monchegorsk. For further details see Derome Ž1993., Kozlov et al. Ž1993., and Makinen Ž1994.. ¨ To study the effect of the industrial pollution on the 137 Cs plant uptake from the soil, four sites, A, B, C, and D were selected mainly on the basis of observations on visible vegetation damages at various distances south from the Severonikel smelter complex, which was considered as the main release source of the contaminants. The exact locations Žlatitude; longitude. of these sites were: A Ž678519; 328489.; B Ž678469; 328489., C Ž678449; 328519., D Ž678409; 328479.. The corresponding distances of these sites from the smelter complex were 7, 16, 21, and 28 km, respectively. Upon approaching the source, the characteristics of the vegetation gradually change into a final state that may be classified as an industrial desert, where trees can no longer survive. For a detailed description of these damages to the environment see the comprehensive articles by Kashulina et al. Ž1997. and Lukina and Nikonov Ž1996.. By determination of the Cu and Ni concentrations in the different soil layers at these sites, the
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total inventories of these elements in grams per square meter and per soil horizon were obtained, assuming that no deposited Cu or Ni had leached beyond the B-horizon. These values are shown in Fig. 1. and clearly demonstrate the considerable pollution of these sites with the above metals, which decreases continually with increasing distance from the smelter. Note that at each site the relatively high values for the Cu and Ni inventories in the B-horizon Žrelative to the corresponding O- and E-horizons. are due to the greater thickness of this layer Žsee below for these values.. This results in comparatively high values for the Cu and Ni inventories in the B-horizons Žexpressed in grm2 ., even though the concentration of these metals Žin grkg soil. decreases with depth. 2.2. Soil The main rock types are basalts, andesites and intermediate and alkaline volcanic rocks. The rocks are broken by Palaeoproterozoic pyroxenite]gabbronorite]gabbrodiorite layered intrusions. At site A the main type of the underlaying bedrock is leucocratic, at site B amphibol]biotite plagioschists. At site C, the main rock type is
Fig. 1. Inventories of Cu and Ni Žmg my2 . derived from atmospheric deposition in the various soil horizons at four sites located at various distances from the smelter. The values and error limits shown are mean and standard deviation from analysis of five plots separately at each site.
Table 1 Soil texture Ž%., pH and loss on ignition Ž%. for the upper horizons at the sites A through D Horizon
Depth Žcm.
O E B O E B O E B Ol Of E B
0]2 2]3 3]20 0]10 10]15 15]23 0]7 7]10 10]23 0]5 5]6 6]9 9]23
Sand 2]0.2 mm
Fine sand 0.2]0.02 mm
24 33.4
61.2 54.1
20.2 24.9
71.4 67.4
26.7 34.1
64.6 58.9
38.5 40.4
53.9 52.1
Silt 0.02]0.002 mm Site A 12.2 10.1 Site B 7.5 6.4 Site C 7.7 6.5 Site D 6.6 7.1
Clay - 0.002 mm
pH ŽCaCl2 .
Loss on Ignition
0.98 0.43
2.8 3.3 4.1 3.1 3.3 4.1 3.0 3.3 4.0
0.98 0.43
3.0 3.3 4.1
68 11 11 82 5 9 76 8 11 91 60 4 10
2.4 2.4 0.94 1.3
The errors Ž1 S.D.. of these values are approximately 4% for sand, silt and the loss on ignition, and approximately 20% for clay.
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gabbronorite, at site D amphibolites. The predominant soil types in the study area are podzolic Al]Fe soils with a well developed E-horizon ŽFAO system., characterised by high exchangeable acidity and low base saturation. At site A almost all of the trees are dead. The humus layers ŽO-horizons. there are almost totally absent but there is a thick litter layer. The E-layer is very thin. The thickness of the O- and E horizons at the various sites is rather variable. Approximate values are given in Table 1. Further characteristics of these soils, such as texture, pH, and loss on ignition are also shown in Table 1. 2.3. Vegetation All sites selected were open areas in spruce forests Ž Piceetum fruticuloso-hylocomiosum, age ) 100 years.. On the basis of primary productivity parameters there are four significantly different degradation stages of the Piceetum fruticuloso-hylocomiosum subjected to air pollution in the Kola Peninsula: Ž1. Piceetum fruticulosum; Ž2. Piceetum graminoso-fruticulosum; Ž3. sparse Piceetum empetrosum; and Ž4. industrial barrens ŽLukina and Nikonov, 1996.. The main understorey plants growing in the sampling areas were crowberry Ž Empetrum nigrum., blueberry Ž Vaccinium myrtillus ., lingonberry Ž Vaccinium ¨ itis-idaea., and hairy grass Ž Deschampsia flexuosa. According to this classification, site D represents the first stage of degradation. The spatial distribution of the ground vegetation biomass is heterogeneous. This results from a lack of green mosses and from the beginning of a colonisation of the area by Empetrum nigrum as well as Deschampsia flexuosa. The important reason for this is also that the state of the trees, which is responsible for the distribution of the understorey species, already shows symptoms of degradation from defoliating to dead. The sites C and B represent the second stage of degradation: the amount of ground vegetation there is significantly higher than at the site D. The main reason is an increase in biomass of crowberry and hairy grasses. The hairy grasses also formed micro-
groups in which the ratio of fresh green plant material to wilted plant material from the previous years decreased from site D to B, and was minor at site A. Site A is characterised as the third stage of degradation, where trees can no longer survive, i.e. the presence of industrial barrens is increasing substantially. The main species still present are Empetrumnigrum and Deschampsia flexuoasa. Hairy grass forms individual microgroups which contained mainly dead parts of the plant. Because of the heavy pollution the mass and thickness of the litter layer increased from site D to site A. At site A the litter layer was especially thick because the heavy pollution had obviously destroyed plant decomposing microroganisms to a considerable extent. 2.4. Sampling Because the uptake of radiocesium by plants in semi-natural environments usually exhibits a strong seasonal dependence Žsee, e.g. Bunzl and Kracke, 1989., sampling of soil and plant species at sites A]D was performed within 1 week in August 1997. At each site five plots Ž1.5= 1.5 m. were selected at the corners and in the middle of an approximately 10 = 10 m square. The understorey vegetation, Empetrum nigrum Vaccinium myrtillus, Vaccinium ¨ itis-idaea, and Deschampsia flexuosa, from the plots was carefully cut quantitatively with scissors near the soil surface and subsequently separated according to species. At each plot, the soil underlying the litter layer was sampled to a depth of 20]23 cm. From the organic layer 0.5 m2 was taken. The mineral layers were collected from 1m2 area, which decreased stepwise with depth to avoid the contamination of deeper layers with soil from the upper layers. 2.5. Pre-treatment of samples Soil samples were dried at 408C. The samples from the organic layer were milled and sieved to 3 mm. The samples from the mineral soil were sieved to 2 mm. Plant samples were dried at 1058C and homogenised in a mill.
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2.6. Measurements Cu and Ni in the soil samples were determined by flame atomic absorption spectrometry. 137 Cs and 40 K in soils and plants were determined by direct gamma-spectrometry using high purity Gedetectors and a multi-channel analyser. For soil samples the counting times were adjusted to provide 1s errors of less than approximately 3% for the above radionuclides. For the plant samples the 1s counting error varied for 137 Cs and 40 K from approximately 4 to 12%, with the exception of some plants for which the sample sizes were very low. In these cases the 1 counting error was 12]48%. All measured radiocesium activities were corrected for radioactive decay to the reference date 1 June 1997. To distinguish in the soil between Chernobyl-derived 137Cs and 137 Cs from the fallout of global weapon testing, 134 Cs was also determined. To calculate the fraction of Chernobyl-derived 137 Cs from the measured 137 Cs and 134 Cs in the samples, a ratio of 137 Cs r 134 Cs s 2.0" 0.3, as observed in October 1986 in reindeer meat in northern Finland, was used ŽRissanen et al., 1987.. However, due to the short half-live of 134 Cs and the rather low radiocesium activity concentrations in the deeper soil layers and in the vegetation, this identification of the source was not always possible.
3. Results and discussion
Fig. 2. Total 137 Cs, 137 Cs from the global fallout of weapons testing, Chernobyl-derived 137 Cs and 40 K in the soil Ž0]23 cm depth, including the litter layer. at four sites A]D, located at various distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
3.1. Soils The inventories Žin kBq my2 . of 137 Cs and 40 K in the soil in the 0]23-cm layer at the five plots at each site A]D are shown in Fig. 2. All radiocesium values are given separately for total 137 Cs, Chernobyl-derived 137 Cs, and 137 Cs from the global fallout of weapons testing. These data show that the total amount of 137 Cs is approximately 1500 Bq my2 at all sites. Only approximately 10% of this value is due to the contribution from the Chernobyl fallout. For each site the variability of the 137 Cs inventories was characterised by caclulating the mean and standard deviation of the individual values observed at the corresponding
five plots. On average, the coefficient of variation of this intra-site variability was approximately 22% for total 137 Cs, and approximately 34% for Chernobyl-derived 137 Cs. Fig. 2. shows that some variability was also present between the median 137 Cs inventories from the five sites. This inter-site variability was somewhat smaller than the intra-site variability. A clear trend of these median values with increasing distance from the smelter was not detectable. A large intra-site variability was also present for 40 K, especially at site D. A continuous trend of the median 40 K inventories from site A to D is not apparent Žsee Fig. 2, bottom., but if all values from sites A and B are pooled and com-
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Fig. 3. Total 137 Cs Žin Bqrkg dry plant material. in four plant species collected at four sites A]D, located at different distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
pared with the pooled values from site C and D, it can be shown that the resulting two medians Ž26 kBq my2 and 37 kBq my2 , respectively. differ by a factor of approximately 1.4 at the 0.05 level by Mann]Whitney U-test. 3.2. Plants The total 137Cs activity concentrations per mass unit in the four plant species collected at each site are shown in Fig. 3. Again, the individual values observed at each of the five plots from each site are given. In general, the activity concentrations for this radionuclide per mass unit are between approximately 10 and 70 Bq kgy1 dry wt. A considerable intra-site variability of these data is apparent. In addition, a clear trend of the 137 Cs plant activity concentration with increasing dis-
tance from the smelter is present for Vaccinium myrtillus and Empetrum nigrum Žpositive trend. or for Deschampsia flexuosa Žnegative trend.. This behaviour will be discussed in more detail in the next section on transfer factors. The corresponding values for 40 K in the four plant species at the four sites are shown in Fig. 4. They clearly demonstrate a continuous positive trend of the values with increasing distance from the smelter for Vaccinium myrtillus, and Empetrum nigrum, and in this case also for Deschampsia flexuosa. These continuous trends for 40 K with distance are also statistically significant at the P- 0.001 level. ŽSpearman correlation test. for Vaccinium myrtillus, Empetrum nigrum, and Deschampsia flexuosa, but not for Vaccinium ¨ itisidaea Ž P) 0.05.. To some extent, the above observations might be due to the somewhat larger 40 K contents in the soils at the more remote sites Žsee above.. Because, however, the 40 K activity concentrations in the plants from sites A and D differ much more Žby a factor of approx. 7 for Vaccinium myrtillus and a factor of 9 for Deschampsia flexuosa., it is more likely that either: Ži. the availability of potassium in the polluted soils closer to the smelter is also reduced; Žii. the root uptake of potassium in the polluted soils is less effective; or Žiii. that, due to the poor health conditions of the plants near the smelter, potassium is partly leached from the brownish part of the plants. At site A, Vaccinium myrtillus had no leaves left. 3.3. Transfer factors The inventories and activity concentrations per mass unit of total 137 Cs in soils and plants can be used to calculate the resulting aggregated transfer factors Tag , defined as Bq 137 Csrkg dry plant material4 divided by Bq 137 Csrm2 soil in the upper 0]23 cm layer4 . These values are shown in Fig. 5 for the four plant species as collected from the five plots at each site. At each site a considerable intra-site variability of the Tag is observable. This variability is, however, to some extent correlated for the different plants. For example, at site D ŽFig. 5. Tag increases or decreases at the five plots according to the same pattern for Vaccinium
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40
Fig. 4. K Žin Bqrkg dry plant material. in four plant species as collected at four sites A]D, located at different distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
myrtillus, Vaccinium ¨ itis-idaea, and Empetrum nigrum. This is most evident for the third plot, where maximum Tag values are observed for each of these species. Similarly, at site B the highest Tag values for Vaccinium myrtillus, Vaccinium ¨ itis-idaea and E mpetrum nigrum are always observed at the first two plots. Another example is site C, where Tag increases rather regularly for the two Vaccinium species when proceeding from the first to the fifth plot. At site A, the Tag at the first plot always exhibits the smallest value for Vaccinium myrtillus, Vaccinium ¨ itis-idaea and Empetrum nigrum. This suggests that if at a given plot of a site the availability for 137 Cs is comparatively high, elevated Tag values are likely to be observed for several plants growing there. However, the observation that this coincident behaviour is predominantly observed between Vaccinium myrtillus, Vaccinium ¨ itis-idaea and Em-
219
Fig. 5. Aggregated transfer factor Tag for four plant species collected at four sites A]D, located at different distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
petrum nigrum but not for Deschampsia flexuosa indicates the 137 Cs uptake for Deschampsia is obviously different from that of the other three species. As is evident from Fig. 5, the median values for the Tag of 137 Cs in the four plant species studied are generally between approximately 0.01 and 0.03 m2 kgy1 . These values are within the relatively large range reported in the literature for the transfer factors of understorey forest vegetation, even though considerably higher values have also been reported Žsee, e.g. Lindner et al., 1994; Wirth et al., 1994; Fawaris and Johanson, 1995; Strebl et al., 1995; McGee et al., 1996; PietrzakFlis et al., 1996; Frissner et al., 1998.. The median 137Cs Tag values found for the plants at each plot indicate a trend with increasing distance from the smelter Žsee Fig. 5.. To examine whether this trend is also statistically significant, the Spearman correlation coefficient and the linear correlation coefficient were calcu-
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lated for the association between the Tag of each plant species and the distance of the site from the smelter. The resulting values are shown in Table 2. They demonstrate that the Tag of 137 Cs increases significantly with increasing distance from the smelter for Vaccinium myrtillus and Empetrum nigrum, but decreases for Deschampsia flexuosa. This result is obtained from the linear correlation coefficient as well as from the Spearman correlation coefficient. For Vaccinium ¨ itisidaea, a positive correlation is indicated, but not significant at the P- 0.05 level Žsee Table 2.. These observations may be compared with the 40 K activity concentrations in these plants presented above Žsee Fig. 4.. For this element, a significant positive trend with distance from the smelter was observed not only for Vaccinium myrtillus and Empetrum nigrum, but also for Deschampsia flexuosa Žsee above. Thus, while Vaccinium myrtillus and Empetrum nigrum react in a similar way with respect to the uptake of 137 Cs and 40 K from the polluted soils, Deschampsia shows the opposite behaviour: at all sites where this plant exhibits high 40 K contents, its 137 Cs activity concentrations are comparatively low. As a result, the 137 Cs r 40 K ratio in plants Žnot shown in a separate Figure. varies rather little with distance from the smelter for Vaccinium myrtillus, Vaccinium ¨ itis-idaea and Empetrum nigrum, but decreases strongly with distance for Deschampsia flexuosa.
4. Conclusions To interpret the above results unambiguously, detailed information on the modifications of the
soil properties and of the plants due to the smelter emissions would be necessary. The observation, however, that at site A, which was very close to the smelter, only a thick litter layer, but hardly any humus layer is present, suggests that the conversion of litter into humic substances is strongly inhibited. As shown by Pennanen et al. Ž1996., especially the fungal part of the microbial biomass in Scandinavian coniferous forests is very sensitive to heavy metals. It is, therefore, not surprising that such drastic changes of biological activities of the soil will also have some effects on the nutrient cycling and availability of potassium or 137 Cs for the plants. In addition, the presence of heavy metal ions in the soil may also have adverse effects on the health of the plants, which may cause modifications of their nutrient balance and also of their radiocesium content. Visual inspection of the plants revealed, for example, that especially the health of Deschampsia flexuosa declined markedly the closer the sites were located to the smelter. The present results show, however, that, depending on the plant species, an increase or a decrease of the plant radiocesium content can occur along industrial pollution gradients like the one studied here. Because only for some and not for all plants potassium and 137 Cs was found to behave in an analogous way, determination of the changes of only potassium in the plants on polluted sites may not necessarily always allow predictions on the radiocesium activity concentrations in these plants. Further investigations which may give some additional information on the modifications of the soil organic matter and on the availability of radiocesium in these polluted soils are in progress.
Table 2 Linear correlation coefficient r and Spearman correlation coefficient r Sp for the association between the transfer factor Tag of 137 Cs in different plants species and the distance of the sampling sites from the smelter complex Plant species
n
r; Ž P .
Vaccinium myrtillus Vaccinium ¨ itis-idaea Empetrum nigrum Deschampsia flexuosa
19 19 20 19
0.5395 Ž0.0171. 0.3549 Ž0.1359. U 0.4741 Ž0.0347. U y0.5905 Ž0.0078.
U
rSp .; Ž P . U
U
0.6697 Ž0.00171. 0.2853 Ž0.2362. U 0.4888 Ž0.02875. U y0.7734 Ž0.0001.
Plants for which a statistically significant correlation Ž P- 0.05. between the Tag for 137 Cs and the distance from the smelter is established. The corresponding probability levels are given in brackets, n denotes the number of samples.
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Acknowledgements This work was carried out as part of the ECproject ‘Effect of industrial pollution on the distribution dynamics of radionuclides in boreal understorey ecosystems’ in the EC-framework ‘Nuclear fission safety’, Contract N8 FI4P-CT960039. References Bunzl K, Kracke W. Seasonal variation of soil-to-plant transfer of K and fallout 134,137 Cs in peatland vegetation. Health Phys 1989;57:593]600. Derome J, editor. Lapland Forest Damage Project. RussianFinnish co-operation report. The Finnish Forest Research Institute. Rovaniemi, Finland: 1993. Fawaris BH, Johanson KJ. A comparative study on radiocesium Ž137 Cs. uptake from coniferous forest soil. J Environ Radioact 1995;28:313]326. Frissner J, Burmann W, Enslin F, Heider R, Klemt E, Miller R, Schick G, Zibold G. Availability of caesium radionuclides to plants}classification of soils and role of mycorrhiza. J Environ Radioact 1998;41:19]32. ¨ ¨ M, Kashulina G, Reimann C, Finne TE, Halleraker JH, Ayras Chekushin VA. The state of the ecosystems in the central Barents Region; scale, factors and mechanisms of disturbance. Sci Total Environ 1997;206:203]224. Kozlov MV, Haukioja E, Yarmisjko VT, editors. Aerial Pollution in Kola Peninsula. Proc. of International Workshop. Apatity, Russia, 1993. Lindner G, Drissner J, Herrmann T, Hund M, Zech W, Zibold G, Zimmerer R. Seasonal and regional variations in the transfer of caesium radionuclides from soil to roe deer and plants in a prealpine forest. Sci Total Environ 1994;157:189]196. Lukina N, Nikonov V. Biochemical cycles in the Northern forest subjected to air pollution. Ed SV Zonn, Apatity, Russian Academy of Science, Kola Science Centre, Parts 1 and 2, 1996.
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Soil to plant uptake of fallout 137Cs by plants from boreal areas polluted by industrial emissions from smelters K. Bunzl a,U , B.P. Albers a , W. Shimmack a , K. Rissanen b, M. Suomelab, M. Puhakainen b, T. Raholab, E. Steinnes c a
GSF-National Research Center for En¨ ironment and Health, Institute of Radiation Protection, D-85764 Neuherberg, Germany b Radiation and Nuclear Safety Authority (STUK), P.O. Box 14, FIN 00881 Helsinki, Finland c Norwegian Uni¨ ersity of Science and Technology, Department of Chemistry, N-7034 Trondheim, Norway Received 12 March 1999; accepted 15 May 1999
Abstract To study the impact of industrial pollution on the soil-to-plant uptake of fallout-radiocesium in a boreal forest ecosystem, four study sites were selected at distances of 7, 16, 21 and 28 km from the large copper]nickel smelter at Monchegorsk on the Kola Peninsula ŽRussia.. At each site, soil and selected plant species were sampled from five plots and analysed separately for 137Cs and 40 K. The data show that the root-uptake of 137Cs, as characterised by the median aggregated transfer-factor Tag , decreased significantly Ž P- 0.05. with decreasing distance from the smelter for the plants Vaccinium myrtillus Žfrom 0.023 to 0.007 m2 kgy1 . and Empetrum nigrum Žfrom 0.015 to 0.007 m2 kgy1 ., but increased for Deschampsia flexuosa Žfrom 0.013 to 0.031 m2 kgy1 .. For Vaccinium ¨ itis-idaea a significant trend for the Tag was not observed. The median 40 K activity concentrations in these plants also decreased significantly Ž P- 0.001. with decreasing distance from the smelter for Vaccinium myrtillus Žfrom approx. 140 to 20 Bq kgy1 dry wt.., Empetrum nigrum Žfrom approx. 90 to 40 Bq kgy1 dry wt.., and also for Deschampsia flexuosa Žfrom approx. 270 to 40 Bq kgy1 dry wt... For Vaccinium ¨ itis-idaea such a continuous significant trend was not observed. The results for the Cu]Ni polluted soils thus show: Ž1. that the soil-to-plant transfer of radiocesium can be significantly modified; Ž2. that these modifications are quite specific; and Ž3. that modifications of the uptake of potassium do not always correspond to those of radiocesium. Q 1999 Elsevier Science B.V. All rights reserved. Keywords:
137
Cs; Plant; Soil; Transfer; Pollution; Industry; Smelter
U
Corresponding author. Tel.: q49-89-31872203; fax: q49-89-31873323. E-mail address: [email protected] ŽK. Bunzl.
0048-9697r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 9 9 . 0 0 2 6 5 - X
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1. Introduction Information on the soil-to-plant transfer of radionuclides is required to estimate the radiation exposure of man and animals after accidental releases of artificial radionuclides to the biosphere. For this reason, numerous investigations on this subject are available which consider agricultural crops and, to a lesser extent, also seminatural environments such as forests or meadows. However, especially when studying semi-natural environments, one usually deliberately selects those areas which are not polluted by inorganic or organic chemicals released, e.g. by industrial emissions to the atmosphere. Because it is well known that such emissions may have complex effects on the physico-chemical properties of the soil as well as on the soil microbiology Žsee, e.g. Pennanen et al., 1996., it is doubtful to what extent soil-to-plant transfer factors determined for unpolluted areas will also describe realistically the soilrplant relationships present at polluted sites. At present, no systematic study on this subject seems to be available with the exception of an investigation by Sobakin and Molchanova Ž1998., who very recently studied the plant uptake of 238 U, 226 Ra and 232 Th in a polluted mining area of Russia. The purpose of the present investigation is to determine the aggregated soil-to-plant transfer factors of fallout 137 Cs in an area polluted by heavy metals and sulphur, i.e. by chemicals frequently found in industrial emissions. These contaminants have been released for 65 years to the air in large amounts from a large copper]nickel smelter complex at Monchegorsk on the Kola Peninsula ŽRussia., this is why the area was selected for the present study. To examine quantitatively the effect of different pollution levels on the 137Cs plant uptake, several sites at various distances from the smelter were selected. The boreal forests present in this area are typical for many similar ecosystems found in Northern Fennoscandia. Radiocaesium present there is mainly due to the cumulated global fallout of nuclear weapons testing and only to a minor extent due to the fallout from the reactor accident at Chernobyl in 1986 ŽSaxen et al., 1986;
Strand et al., 1998.. Nevertheless, the risk of an additional large-scale radioactive fallout in these areas cannot be excluded due the existence of nuclear power plants, stored liquid and solid nuclear wastes, as well as nuclear powered vessels and submarines in the Kola region.
2. Material and methods 2.1. Site All sampling sites were located on the Kola Peninsula. The main pollution sources there are several Cu]Ni smelters Severonikel at Monchegorsk and Pechenganikel at Nikel. In 1987]1991 the total emissions amounted to approximately 700 000 tonsryear. This amount decreased to 580 000 tonsryear in 1994. Approximately 70% of the emissions are made up by SO 2 . During the period 1987]1991 the Ni-emission was 3700 tons, and that of Cu 2600 tonsryear. In 1996 the corresponding amounts were 1300 and 700 tonsryear, respectively. Most of the Ni and Cu originates from the smelter at Monchegorsk. For further details see Derome Ž1993., Kozlov et al. Ž1993., and Makinen Ž1994.. ¨ To study the effect of the industrial pollution on the 137 Cs plant uptake from the soil, four sites, A, B, C, and D were selected mainly on the basis of observations on visible vegetation damages at various distances south from the Severonikel smelter complex, which was considered as the main release source of the contaminants. The exact locations Žlatitude; longitude. of these sites were: A Ž678519; 328489.; B Ž678469; 328489., C Ž678449; 328519., D Ž678409; 328479.. The corresponding distances of these sites from the smelter complex were 7, 16, 21, and 28 km, respectively. Upon approaching the source, the characteristics of the vegetation gradually change into a final state that may be classified as an industrial desert, where trees can no longer survive. For a detailed description of these damages to the environment see the comprehensive articles by Kashulina et al. Ž1997. and Lukina and Nikonov Ž1996.. By determination of the Cu and Ni concentrations in the different soil layers at these sites, the
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total inventories of these elements in grams per square meter and per soil horizon were obtained, assuming that no deposited Cu or Ni had leached beyond the B-horizon. These values are shown in Fig. 1. and clearly demonstrate the considerable pollution of these sites with the above metals, which decreases continually with increasing distance from the smelter. Note that at each site the relatively high values for the Cu and Ni inventories in the B-horizon Žrelative to the corresponding O- and E-horizons. are due to the greater thickness of this layer Žsee below for these values.. This results in comparatively high values for the Cu and Ni inventories in the B-horizons Žexpressed in grm2 ., even though the concentration of these metals Žin grkg soil. decreases with depth. 2.2. Soil The main rock types are basalts, andesites and intermediate and alkaline volcanic rocks. The rocks are broken by Palaeoproterozoic pyroxenite]gabbronorite]gabbrodiorite layered intrusions. At site A the main type of the underlaying bedrock is leucocratic, at site B amphibol]biotite plagioschists. At site C, the main rock type is
Fig. 1. Inventories of Cu and Ni Žmg my2 . derived from atmospheric deposition in the various soil horizons at four sites located at various distances from the smelter. The values and error limits shown are mean and standard deviation from analysis of five plots separately at each site.
Table 1 Soil texture Ž%., pH and loss on ignition Ž%. for the upper horizons at the sites A through D Horizon
Depth Žcm.
O E B O E B O E B Ol Of E B
0]2 2]3 3]20 0]10 10]15 15]23 0]7 7]10 10]23 0]5 5]6 6]9 9]23
Sand 2]0.2 mm
Fine sand 0.2]0.02 mm
24 33.4
61.2 54.1
20.2 24.9
71.4 67.4
26.7 34.1
64.6 58.9
38.5 40.4
53.9 52.1
Silt 0.02]0.002 mm Site A 12.2 10.1 Site B 7.5 6.4 Site C 7.7 6.5 Site D 6.6 7.1
Clay - 0.002 mm
pH ŽCaCl2 .
Loss on Ignition
0.98 0.43
2.8 3.3 4.1 3.1 3.3 4.1 3.0 3.3 4.0
0.98 0.43
3.0 3.3 4.1
68 11 11 82 5 9 76 8 11 91 60 4 10
2.4 2.4 0.94 1.3
The errors Ž1 S.D.. of these values are approximately 4% for sand, silt and the loss on ignition, and approximately 20% for clay.
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gabbronorite, at site D amphibolites. The predominant soil types in the study area are podzolic Al]Fe soils with a well developed E-horizon ŽFAO system., characterised by high exchangeable acidity and low base saturation. At site A almost all of the trees are dead. The humus layers ŽO-horizons. there are almost totally absent but there is a thick litter layer. The E-layer is very thin. The thickness of the O- and E horizons at the various sites is rather variable. Approximate values are given in Table 1. Further characteristics of these soils, such as texture, pH, and loss on ignition are also shown in Table 1. 2.3. Vegetation All sites selected were open areas in spruce forests Ž Piceetum fruticuloso-hylocomiosum, age ) 100 years.. On the basis of primary productivity parameters there are four significantly different degradation stages of the Piceetum fruticuloso-hylocomiosum subjected to air pollution in the Kola Peninsula: Ž1. Piceetum fruticulosum; Ž2. Piceetum graminoso-fruticulosum; Ž3. sparse Piceetum empetrosum; and Ž4. industrial barrens ŽLukina and Nikonov, 1996.. The main understorey plants growing in the sampling areas were crowberry Ž Empetrum nigrum., blueberry Ž Vaccinium myrtillus ., lingonberry Ž Vaccinium ¨ itis-idaea., and hairy grass Ž Deschampsia flexuosa. According to this classification, site D represents the first stage of degradation. The spatial distribution of the ground vegetation biomass is heterogeneous. This results from a lack of green mosses and from the beginning of a colonisation of the area by Empetrum nigrum as well as Deschampsia flexuosa. The important reason for this is also that the state of the trees, which is responsible for the distribution of the understorey species, already shows symptoms of degradation from defoliating to dead. The sites C and B represent the second stage of degradation: the amount of ground vegetation there is significantly higher than at the site D. The main reason is an increase in biomass of crowberry and hairy grasses. The hairy grasses also formed micro-
groups in which the ratio of fresh green plant material to wilted plant material from the previous years decreased from site D to B, and was minor at site A. Site A is characterised as the third stage of degradation, where trees can no longer survive, i.e. the presence of industrial barrens is increasing substantially. The main species still present are Empetrumnigrum and Deschampsia flexuoasa. Hairy grass forms individual microgroups which contained mainly dead parts of the plant. Because of the heavy pollution the mass and thickness of the litter layer increased from site D to site A. At site A the litter layer was especially thick because the heavy pollution had obviously destroyed plant decomposing microroganisms to a considerable extent. 2.4. Sampling Because the uptake of radiocesium by plants in semi-natural environments usually exhibits a strong seasonal dependence Žsee, e.g. Bunzl and Kracke, 1989., sampling of soil and plant species at sites A]D was performed within 1 week in August 1997. At each site five plots Ž1.5= 1.5 m. were selected at the corners and in the middle of an approximately 10 = 10 m square. The understorey vegetation, Empetrum nigrum Vaccinium myrtillus, Vaccinium ¨ itis-idaea, and Deschampsia flexuosa, from the plots was carefully cut quantitatively with scissors near the soil surface and subsequently separated according to species. At each plot, the soil underlying the litter layer was sampled to a depth of 20]23 cm. From the organic layer 0.5 m2 was taken. The mineral layers were collected from 1m2 area, which decreased stepwise with depth to avoid the contamination of deeper layers with soil from the upper layers. 2.5. Pre-treatment of samples Soil samples were dried at 408C. The samples from the organic layer were milled and sieved to 3 mm. The samples from the mineral soil were sieved to 2 mm. Plant samples were dried at 1058C and homogenised in a mill.
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2.6. Measurements Cu and Ni in the soil samples were determined by flame atomic absorption spectrometry. 137 Cs and 40 K in soils and plants were determined by direct gamma-spectrometry using high purity Gedetectors and a multi-channel analyser. For soil samples the counting times were adjusted to provide 1s errors of less than approximately 3% for the above radionuclides. For the plant samples the 1s counting error varied for 137 Cs and 40 K from approximately 4 to 12%, with the exception of some plants for which the sample sizes were very low. In these cases the 1 counting error was 12]48%. All measured radiocesium activities were corrected for radioactive decay to the reference date 1 June 1997. To distinguish in the soil between Chernobyl-derived 137Cs and 137 Cs from the fallout of global weapon testing, 134 Cs was also determined. To calculate the fraction of Chernobyl-derived 137 Cs from the measured 137 Cs and 134 Cs in the samples, a ratio of 137 Cs r 134 Cs s 2.0" 0.3, as observed in October 1986 in reindeer meat in northern Finland, was used ŽRissanen et al., 1987.. However, due to the short half-live of 134 Cs and the rather low radiocesium activity concentrations in the deeper soil layers and in the vegetation, this identification of the source was not always possible.
3. Results and discussion
Fig. 2. Total 137 Cs, 137 Cs from the global fallout of weapons testing, Chernobyl-derived 137 Cs and 40 K in the soil Ž0]23 cm depth, including the litter layer. at four sites A]D, located at various distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
3.1. Soils The inventories Žin kBq my2 . of 137 Cs and 40 K in the soil in the 0]23-cm layer at the five plots at each site A]D are shown in Fig. 2. All radiocesium values are given separately for total 137 Cs, Chernobyl-derived 137 Cs, and 137 Cs from the global fallout of weapons testing. These data show that the total amount of 137 Cs is approximately 1500 Bq my2 at all sites. Only approximately 10% of this value is due to the contribution from the Chernobyl fallout. For each site the variability of the 137 Cs inventories was characterised by caclulating the mean and standard deviation of the individual values observed at the corresponding
five plots. On average, the coefficient of variation of this intra-site variability was approximately 22% for total 137 Cs, and approximately 34% for Chernobyl-derived 137 Cs. Fig. 2. shows that some variability was also present between the median 137 Cs inventories from the five sites. This inter-site variability was somewhat smaller than the intra-site variability. A clear trend of these median values with increasing distance from the smelter was not detectable. A large intra-site variability was also present for 40 K, especially at site D. A continuous trend of the median 40 K inventories from site A to D is not apparent Žsee Fig. 2, bottom., but if all values from sites A and B are pooled and com-
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Fig. 3. Total 137 Cs Žin Bqrkg dry plant material. in four plant species collected at four sites A]D, located at different distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
pared with the pooled values from site C and D, it can be shown that the resulting two medians Ž26 kBq my2 and 37 kBq my2 , respectively. differ by a factor of approximately 1.4 at the 0.05 level by Mann]Whitney U-test. 3.2. Plants The total 137Cs activity concentrations per mass unit in the four plant species collected at each site are shown in Fig. 3. Again, the individual values observed at each of the five plots from each site are given. In general, the activity concentrations for this radionuclide per mass unit are between approximately 10 and 70 Bq kgy1 dry wt. A considerable intra-site variability of these data is apparent. In addition, a clear trend of the 137 Cs plant activity concentration with increasing dis-
tance from the smelter is present for Vaccinium myrtillus and Empetrum nigrum Žpositive trend. or for Deschampsia flexuosa Žnegative trend.. This behaviour will be discussed in more detail in the next section on transfer factors. The corresponding values for 40 K in the four plant species at the four sites are shown in Fig. 4. They clearly demonstrate a continuous positive trend of the values with increasing distance from the smelter for Vaccinium myrtillus, and Empetrum nigrum, and in this case also for Deschampsia flexuosa. These continuous trends for 40 K with distance are also statistically significant at the P- 0.001 level. ŽSpearman correlation test. for Vaccinium myrtillus, Empetrum nigrum, and Deschampsia flexuosa, but not for Vaccinium ¨ itisidaea Ž P) 0.05.. To some extent, the above observations might be due to the somewhat larger 40 K contents in the soils at the more remote sites Žsee above.. Because, however, the 40 K activity concentrations in the plants from sites A and D differ much more Žby a factor of approx. 7 for Vaccinium myrtillus and a factor of 9 for Deschampsia flexuosa., it is more likely that either: Ži. the availability of potassium in the polluted soils closer to the smelter is also reduced; Žii. the root uptake of potassium in the polluted soils is less effective; or Žiii. that, due to the poor health conditions of the plants near the smelter, potassium is partly leached from the brownish part of the plants. At site A, Vaccinium myrtillus had no leaves left. 3.3. Transfer factors The inventories and activity concentrations per mass unit of total 137 Cs in soils and plants can be used to calculate the resulting aggregated transfer factors Tag , defined as Bq 137 Csrkg dry plant material4 divided by Bq 137 Csrm2 soil in the upper 0]23 cm layer4 . These values are shown in Fig. 5 for the four plant species as collected from the five plots at each site. At each site a considerable intra-site variability of the Tag is observable. This variability is, however, to some extent correlated for the different plants. For example, at site D ŽFig. 5. Tag increases or decreases at the five plots according to the same pattern for Vaccinium
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40
Fig. 4. K Žin Bqrkg dry plant material. in four plant species as collected at four sites A]D, located at different distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
myrtillus, Vaccinium ¨ itis-idaea, and Empetrum nigrum. This is most evident for the third plot, where maximum Tag values are observed for each of these species. Similarly, at site B the highest Tag values for Vaccinium myrtillus, Vaccinium ¨ itis-idaea and E mpetrum nigrum are always observed at the first two plots. Another example is site C, where Tag increases rather regularly for the two Vaccinium species when proceeding from the first to the fifth plot. At site A, the Tag at the first plot always exhibits the smallest value for Vaccinium myrtillus, Vaccinium ¨ itis-idaea and Empetrum nigrum. This suggests that if at a given plot of a site the availability for 137 Cs is comparatively high, elevated Tag values are likely to be observed for several plants growing there. However, the observation that this coincident behaviour is predominantly observed between Vaccinium myrtillus, Vaccinium ¨ itis-idaea and Em-
219
Fig. 5. Aggregated transfer factor Tag for four plant species collected at four sites A]D, located at different distances from the smelter ŽA. 7 km, ŽB. 16 km, ŽC. 21 km, ŽD. 28 km. The five bars in each group correspond to the individual values determined separately at the five plots at each site. The errors shown are 1s counting errors.
petrum nigrum but not for Deschampsia flexuosa indicates the 137 Cs uptake for Deschampsia is obviously different from that of the other three species. As is evident from Fig. 5, the median values for the Tag of 137 Cs in the four plant species studied are generally between approximately 0.01 and 0.03 m2 kgy1 . These values are within the relatively large range reported in the literature for the transfer factors of understorey forest vegetation, even though considerably higher values have also been reported Žsee, e.g. Lindner et al., 1994; Wirth et al., 1994; Fawaris and Johanson, 1995; Strebl et al., 1995; McGee et al., 1996; PietrzakFlis et al., 1996; Frissner et al., 1998.. The median 137Cs Tag values found for the plants at each plot indicate a trend with increasing distance from the smelter Žsee Fig. 5.. To examine whether this trend is also statistically significant, the Spearman correlation coefficient and the linear correlation coefficient were calcu-
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lated for the association between the Tag of each plant species and the distance of the site from the smelter. The resulting values are shown in Table 2. They demonstrate that the Tag of 137 Cs increases significantly with increasing distance from the smelter for Vaccinium myrtillus and Empetrum nigrum, but decreases for Deschampsia flexuosa. This result is obtained from the linear correlation coefficient as well as from the Spearman correlation coefficient. For Vaccinium ¨ itisidaea, a positive correlation is indicated, but not significant at the P- 0.05 level Žsee Table 2.. These observations may be compared with the 40 K activity concentrations in these plants presented above Žsee Fig. 4.. For this element, a significant positive trend with distance from the smelter was observed not only for Vaccinium myrtillus and Empetrum nigrum, but also for Deschampsia flexuosa Žsee above. Thus, while Vaccinium myrtillus and Empetrum nigrum react in a similar way with respect to the uptake of 137 Cs and 40 K from the polluted soils, Deschampsia shows the opposite behaviour: at all sites where this plant exhibits high 40 K contents, its 137 Cs activity concentrations are comparatively low. As a result, the 137 Cs r 40 K ratio in plants Žnot shown in a separate Figure. varies rather little with distance from the smelter for Vaccinium myrtillus, Vaccinium ¨ itis-idaea and Empetrum nigrum, but decreases strongly with distance for Deschampsia flexuosa.
4. Conclusions To interpret the above results unambiguously, detailed information on the modifications of the
soil properties and of the plants due to the smelter emissions would be necessary. The observation, however, that at site A, which was very close to the smelter, only a thick litter layer, but hardly any humus layer is present, suggests that the conversion of litter into humic substances is strongly inhibited. As shown by Pennanen et al. Ž1996., especially the fungal part of the microbial biomass in Scandinavian coniferous forests is very sensitive to heavy metals. It is, therefore, not surprising that such drastic changes of biological activities of the soil will also have some effects on the nutrient cycling and availability of potassium or 137 Cs for the plants. In addition, the presence of heavy metal ions in the soil may also have adverse effects on the health of the plants, which may cause modifications of their nutrient balance and also of their radiocesium content. Visual inspection of the plants revealed, for example, that especially the health of Deschampsia flexuosa declined markedly the closer the sites were located to the smelter. The present results show, however, that, depending on the plant species, an increase or a decrease of the plant radiocesium content can occur along industrial pollution gradients like the one studied here. Because only for some and not for all plants potassium and 137 Cs was found to behave in an analogous way, determination of the changes of only potassium in the plants on polluted sites may not necessarily always allow predictions on the radiocesium activity concentrations in these plants. Further investigations which may give some additional information on the modifications of the soil organic matter and on the availability of radiocesium in these polluted soils are in progress.
Table 2 Linear correlation coefficient r and Spearman correlation coefficient r Sp for the association between the transfer factor Tag of 137 Cs in different plants species and the distance of the sampling sites from the smelter complex Plant species
n
r; Ž P .
Vaccinium myrtillus Vaccinium ¨ itis-idaea Empetrum nigrum Deschampsia flexuosa
19 19 20 19
0.5395 Ž0.0171. 0.3549 Ž0.1359. U 0.4741 Ž0.0347. U y0.5905 Ž0.0078.
U
rSp .; Ž P . U
U
0.6697 Ž0.00171. 0.2853 Ž0.2362. U 0.4888 Ž0.02875. U y0.7734 Ž0.0001.
Plants for which a statistically significant correlation Ž P- 0.05. between the Tag for 137 Cs and the distance from the smelter is established. The corresponding probability levels are given in brackets, n denotes the number of samples.
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