In the event of a nuclear accident in which radionuclides are released into the biosphere, radioactive contamination of forests can become a significant potential source of public radiation exposure. Cesium 137 and strontium 90 remain in the upper soil layers in forest ecosystems for a long time. These radionuclides get into the wood via the roots and can thus store radioactivity and represent a potential radiation exposure for humans. We provide an overview of our radioactive measurements as well as a literature review on wood, pellets and ash with a focus on cesium 137
Two of these accidents – the Kyshtim accident in the Urals in the USSR (now the Russian Federation) in 1957 and the Chernobyl accident in the USSR (now Ukraine) in 1986 – resulted in significant contamination of thousands of square kilometers of forest land with radionuclide mixtures, including long-lived fission products such as Cs 137 and Sr 90. Measurements and modeling of forest ecosystems after both accidents have shown that after initial contamination, the activity concentration of long-lived radionuclides in wood gradually increases over one to two decades and slowly decreases over the following decades Period. The longevity of contamination is due to the slow migration and persistent bioavailability of radionuclides in the forest soil profile, leading to long-term transfer into wood through the tree root system.
The transfer of cesium-137 from the soil into the wood of trees is relatively low. Wood from Germany is radiobiological harmless to humans. In the ashes, however, the radiocesium activity can be up to 100 times higher than in wood.
Studies of Cs-137 and K-40 in wood from trees.
The following investigations are excerpts from the final report “Radioactivity in Wildlife”, where the radioactivity was also determined by gamma spectrometry from wood, roots, leaves and bark of trees on permanent sample plots (Fielitz 2004). All activity data refer to dry matter!
Aims and methods
Trees play an important role in the elemental balance of forest ecosystems. Therefore, the distribution of radiocesium and potassium-40 in different tree species was investigated on the sample plots. The trees were divided into the following compartments: Leaf organs, branches, bark, wood, roots < 5 mm and roots > 5 mm.
The objective of this study was to determine the extent to which the various tree tissues accumulate radiocesium and the differences in metabolic-physiological behavior between Cs-137 and K-40. Ultimately, this was also intended to answer the question of whether radiocesium is suitable as a tracer for potassium in forest ecosystem research.
Sampling was mainly done in 1992 on permanent sampling plots B1 (Bavaria) and F1 (Lower Saxony). Only a few compartments of some tree species could be examined, and even these only in small numbers. In these cases the statistical evaluation of the measurement results was omitted. Most of the samples could be analyzed for B1 of spruce and for F1 of pine. Transfer factors for Cs-137 were calculated for the individual tree compartments. Since tree species differ in the formation of their horizontal and vertical root systems and thus take up nutrients from different soil depths, the transfer factors were calculated on an area basis. No transfer factors were calculated for branches and bark, because part of the activity present in these compartments still originated from dry and wet deposition from the Chernobyl fallout. For some compartments, the fractions of Cs-137 activity originating from nuclear weapon and Chernobyl fallout were additionally calculated to better understand translocalization processes and long-term behavior of Cs+ ions in tree organs.
Distribution of cesium 137 and potassium 40 in needles, bark, wood, roots and leaves
The results are given in table 1. For the activity data of spruce needles, only the measurement results from the 1st and 3rd needle year were considered, because the individual needle years differed from each other in their biomass and Cs-137 content (younger needles have a lower biomass than older ones). This selection was also made with respect to the stock calculation in order to estimate Cs-137 and K-40 stocks in needle mass as realistically as possible.
The compartments of each tree species differed in their specific Cs-137 and K-40 activities. Both nuclides showed different distribution patterns. Between trees of the same species, Cs-137 activities in the same compartments varied by up to an order of magnitude.
Measured values of radioactivity of beech, birch, spruce and rowan trees
The K-40 measurement data of the individual compartments were partly normally, partly log-normally distributed. For example, the arithmetic mean of specific K-40 activity in bark of spruce (Picea abies) on B1 was 36 Bq/kg, the median 38 Bq/kg, with a minimum value of 14 Bq/kg and a maximum value of 61 Bq/kg. In contrast, the arithmetic mean of needles from these trees was 200 Bq/kg, the median was only 130 Bq/kg, while the measured values ranged from 63 – 620 Bq/kg. B1 ansd B2 are permanent sample plots in Bavaria.
Tab. 1: K-40 and Cs-137 as well as transfer factors for Cs-137 in compartments of different tree species on B1 and B2, sampling 1992. Activity data in Bq/kg dry matter, as of 1.5.1986.
| Tree compartment (age) | n | Potassium 40 [Bq/kg] | Cesium-137 [Bq/kg] | TF Cs-137 [m/kg] |
| > Test area B1, Bavaria < | Mean±SD | Mean±SD | Mean x 10-2 | |
| Spruce (110 years) | ||||
| Needles | 12 | 200±130 | 1300±820 | 1.30 |
| Branches | 22 | 120±86 | 1200±690 | |
| Bark | 12 | 36±14 | 1000±240 | |
| Wood | 24 | 41±54 | 150±0 | 0.15 |
| Roots <5 mm | 6 | 95±35 | 1800±680 | 1.80 |
| Roots> 5 mm | 6 | 56±13 | 540±220 | 0.55 |
| Beech (80 years) | ||||
| Leaves | 22 | 280±48 | 660±580 | 0.70 |
| Bark | 2 | 74 _ | 1900 _ | |
| Wood | 8 | 46±11 | 57±37 | 0.05 |
| Birch (30 years) | ||||
| Leaves | 2 | 260 _ | 330 _ | 0.35 |
| Bark | 2 | 60 _ | 190 _ | |
| Wood | 2 | 46 _ | 50 _ | 0.05 |
| > Test area B2, Bavaria < | ||||
| Spruce (110 years) | ||||
| Needles | 18 | 190±130 | 1300±540 | 1.60 |
| Branches | 16 | 240±11 | 1400±640 | |
| bark | 2 | 120 _ | 1300 _ | |
| Wood | 3 | _ | 160±33 | 0.20 |
| Beech (100 years) | ||||
| Leaves | 42 | 290±130 | 520±420 | 0.60 |
| Buds | 8 | 370±150 | 770±520 | 0.90 |
| Wood | 1 | 49 _ | 64 _ | 0.10 |
| Mountain ash (60 years) | ||||
| Leaves | 15 | 550±12 | 1800±540 | 2.20 |
| Buds | 6 | 310±86 | 1000±280 | 1.20 |
| Bark | 3 | _ | 1200±93 | _ |
| Wood | 3 | _ | 150±30 | 0.20 |
= K-40 activity was not determined or standard deviation / transfer factor were not calculated
On average, the highest specific K-40 activities in all tree species were found in the leaf organs, the lowest in the wood. Only bark of spruce on B1, with 36 Bq / kg on average, contained less K-40 than wood, with 41 Bq / kg. This difference is not significant (P <0.05).
Spruce, beech, birch and mountain ahs
Beech leaves (Fagus sylvatica) on B1 contained, with an average of 280 Bq / kg, about the same amount of K-40 as those on B2 with 290 Bq / kg, as well as spruce needles with 200 Bq / kg and 190 Bq / kg. On B1, the mean specific K-40 activities in the wood of all tree species examined were roughly the same: spruce 41 Bq / kg, beech 46 Bq / kg and birch (Betula pendula) 46 Bq / kg.
In investigations into the behavior of radiocesium and K-40 on a spruce stand in Rhineland-Palatinate, BLOCK (1993) also found the highest specific K-40 activities in needles and the lowest in wood. ULRICH et al. Came to the same conclusion. (1986) for the distribution of total potassium in different compartments in beech and spruce in the Solling.
The distribution pattern of the K-40 activity is apparently due to the different potassium requirements of the various plant tissues. Leaf tissue in particular has a high metabolism due to its photosynthetic capacity and its cytoplasm contains a relatively large amount of potassium. Wood tissue, on the other hand, consists predominantly of dead cells that are used to strengthen the tree with a correspondingly low potassium content.
With Cs-137, too, the lowest activities in the wood were found on average. From spruce trees on B1, most of the samples could be examined, based on all compartments: the wood contained an average of 150 Bqkg-1, the median was 140 Bq / kg. However, the measured values ranged from 33 Bq / kg to 310 Bq / kg, with a coefficient of variation of 47%. Significantly higher activities than wood had roots <5 mm in diameter with 1,800 Bq / kg, needles with 1,300 Bq / kg, branches with 1,200 Bq / kg and bark with 1,000 Bq / kg and roots> 5 mm with 540 Bq / kg. The Cs-137 measured values also varied considerably in each of these compartments.
Distribution of Cs-137 and K-40 in pine and oak
In the Fuhrberg study area, three oaks (Quercus robur) were sampled on F1 and ten pines (Pinus sylvestris) a few hundred meters away. The oaks were 120 years old, the pines 128 and 20 years old, respectively. In order to determine whether the radiocesium activity in wood and bark differs between the base of the trunk and the crown, samples were taken from both areas of the 128-year-old pine. The results are given in Table 2.
Tab. 2: K-40 and Cs-137 as well as transfer factors for Cs-137 in different compartments in pine (Pinus sylvestris) and oak (Quercus robur) on F1, sampling 1992. Activity data in Bq/kg DM, as of 1.5.1986
| Tree compartment (age) | n | Potassium 40 [Bq / kg] | Cesium 137 [Bq / kg] | TF Cs-137 [m/kg] |
| Mean ± SD | Mittelwert±SD | Mittelwert x 10-2 | ||
| Pine (20 years) | ||||
| Needles `92 | 5 | 140±24 | 140±17 | 1,4 |
| Branches | 5 | 77±21 | 100±14 | _ |
| Bark crown | 5 | 64±30 | 170±38 | _ |
| Bark stem base | 5 | 29±9 | 61±19 | _ |
| Wood crown | 5 | 24±5 | 30±8 | 0,3 |
| Wood stem base | 5 | 21±3 | 34±4 | 0,35 |
| Roots <5 mm | 3 | 120±6 | 290±49 | 2,8 |
| Roots >5 mm | 2 | 33± 33 | 140±14 | 1,4 |
| Pine (20 years) | ||||
| Bark | 5 | 31±8 | 23±5 | _ |
| Wood | 5 | 29±10 | 11±3 | 0,1 |
| Oak (120 years) | ||||
| Acorns | 6 | 220±28 | 470±120 | 4,6 |
| Leaves | 26 | 320±110 | 780±250 | 8 |
| Buds | 5 | _ | 540±150 | 5,2 |
| Branches | 5 | 100±17 | 270±79 | _ |
| Bark | 3 | _ | 87±9 | _ |
| Wood | 3 | _ | 15±1 | 0,15 |
= K-40 activity was not determined or standard deviation / transfer factor were not calculated
As with the tree species on B1 and B2, the individual compartments also differed in their mean K-40 and Cs-137 activities in oak and pine on F1. Here, too, wood had little activity, roots <5 mm and needles relatively high.
In the 128-year-old pines, wood from the crown area differed from that from the trunk base neither in the K-40 (24 Bq / kg or 21 Bq / kg) nor in the Cs-137 activity (30 Bq / kg or 34 Bq / kg) significant (P <0.01). ULRICH et al. (1986) found in element inventories of old spruce stands in the Solling that samples for the analysis of trunk wood of an age group can be taken from any height.
However, the wood and bark from the base of the 128-year-old pine were significantly more contaminated with 34 Bq Cs-137 / kg and 61 Bq Cs-137 / kg than in the 20-year-old pine, with 11 Bq / kg and 23, respectively Bq / kg (P <0.01).
Roots contain most of the cesium activity
The roots <5mm from the 128-year-old pines contained an average of 290 Bq / kg, the highest Cs-137 levels compared to the other compartments. Pine needles contained, with 140 Bq / kg, four times more Cs-137 than wood, with 34 Bq / kg.
What is noticeable about the pine is the low content of Cs-137 in the bark from the base of the trunk (61 Bq / kg) compared to the bark from the crown (170 Bq / kg). This could be explained by the fact that the branches in the crown area protrude horizontally or diagonally upwards and provide a relatively large surface for the deposition of radiocesium during the Chernobyl fallout. While comparatively little activity was deposited on the bark of the trunk, which protruded vertically into the air space. In addition, the surface: mass ratio of bark from the crown space is significantly greater than that of bark from the trunk, even if the same Cs activity per area is deposited, a higher specific activity for bark from the crown results.
From Table 12 it can be seen that, as in the case of pine, the mean Cs-137 activity of the compartments decreases in the order of leaves> branches> bark> wood also in oak. It is noticeable that oak leaves contained an average of 55 times more Cs-137 than the wood. Acorn levels were also relatively high, with an average of 470 Bq / kg. In contrast, BLOCK (1993) determined the highest Cs-137 activities in the bark of two oaks (Quercus robur) removed in March 1989 in Rhineland-Palatinate.
Compared to the activity in the compartments of the pine, oak wood contained on average only half as much, but oak leaves contained six times more Cs-137 than pine needles.
Discussion of the results
The results on the distribution of radiocesium and K-40 in trees show that leaves, branches, bark, wood and roots of a tree both accumulate nuclides differently. In all tree species examined, the highest Cs-137 and K-40 contents were found in the leaf organs and roots <5 mm, in wood the lowest. The reason for this distribution pattern is apparently the relatively high potassium requirement of young, still growing or metabolically active plant tissue. Potassium is not incorporated into organic compounds in the plant, there is no storage form, the predominant part is free in the cell sap (STRASBURGER 1978). The low K-40 activities that were determined for wood are therefore obviously due to the high proportion of dead cells that are used to strengthen the tree, while more K-40 is required in the most metabolically active leaf organs, representing total potassium. Apparently radiocesium follows the metabolic pathway of potassium qualitatively, but not quantitatively. For this reason, Cs-137 is not a suitable tracer for potassium in forest ecosystem research.
Transfer factor
Due to the very different Cs-137 area activity of the sample areas (Table 6), the measurement results can best be compared using the values of the transfer factors: the same tree species could only be examined on areas B1 and B2 due to their occurrence. There, the average TF values of beech wood, with 0.05 × 102 m2 / kg and 0.10 × 102 m2 / kg, were significantly lower than those of spruce wood with 0.15 × 102 m2 / kg or 0.20 × 102 m2 / kg. On B2, the Cs-137 transfer in wood was slightly higher for both tree species than that on B1.
Based on all 3 sample areas, the minimum value of the TFPf for birch wood (B1) was found to be 0.05 × 102 m2 / kg, the maximum value of 0.35 × 102 m2 / kg for pine wood (F1). The arithmetic mean (s) of the transfer factors for Cs-137 from all tree species was 0.16 × 102 m2 / kg.
Complex interrelationships
The interpretation of transfer factors is much more complicated for long-lived trees than for annual arable and forest plants. The Cs-137 activity contained in annual plants is continuously absorbed via the roots during the entire biomass formation (the Cs uptake from atmospheric deposition and resuspension of the nuclide on parts of the plant above ground is estimated to be negligible compared to the root uptake). The values of the transfer factors thus indicate the nuclide uptake of the plant for the period of the vegetation period, which is usually only lasts a few months. During this time, the metabolism of the plant has reached a state of equilibrium in relation to the Cs-137 content. The calculation of transfer factors according to equation 7 is only defined for this equilibrium state. In some tree organs, on the other hand, Cs-137 accumulates over several years or decades.
Fine roots and leaf organs or the youngest needle years rather reflect the current transfer of Cs-137. This transfer consists of the uptake of Cs + ions by the roots and the further transport with the water flow into the leaf organs, as well as the translocation of Cs-137 from internal tree storage tissues. The TFPf values of leaf organs of the spruce were significantly higher at B1 (1.30 × 102 m2 / kg) and B2 (1.60 × 102 m2 / kg) than those of the deciduous beech trees (0.70 × 102 m2 / kg and 0.60 × 102 m2 / kg) and birch (on B1 with 0.35 × 102 m2 / kg). However, for the leaf organs of the mountain ash, the highest TFPf value of all tree species on B1 and B2 was determined with 2.20 × 102 m2 / kg. The reason for the relatively high transfer of Cs-137 could be the low root depth of the mountain ash, which is why there is more root mass in the upper, radiocesium-rich soil layer.
On area F1, the average Cs-137 TFPf for oak leaves, at 8.0 × 102 m2 / kg, was around six times higher than the TFPf for pine needles at 1.4 × 102 m2 / kg. The transfer of Cs-137 in oak leaves was significantly higher than in leaf organs of other tree species on B1 and B2. In contrast, the transfer factors for the leaf organs of conifers agreed well on the three test areas: for spruce needles it was 1.3 × 102 m2 / kg for B1, 1.6 × 102 m2 / kg for B2 and 1.4 × 102 m2 for pine needles / kg calculated.
Annual rings with no correlation to activity
The investigation of individual annual rings in wood was omitted because, according to investigations by KOHNO et al. (1988) and BROWNRIDGE (1984), on different tree species, there is no correlation between the deposition of Cs-137 and the content of the nuclide in the annual rings. In both investigations, Cs-137 activity was found in annual rings that were formed long before the explosion of the first atomic bomb. The variability of the Cs-137 contents, based on the same compartments in each case, was relatively large in trees of the same species. The reasons for this are probably the small-scale inhomogeneity of the Cs-137 activity in the soil, which is passed on to the tree organs via the roots, or the different sociological position of the trees and their individual metabolic rates.
Firewood
How can the radioactivity of wood caused by Cesium 137 be assessed? Table 1 shows measured values for wood. The samples come from Bodenmais, one of the areas in Germany particularly highly contaminated by the Chernobyl fallout. The determined radiocesium contamination of wood should roughly reflect the maximum values for Germany.
Ash
The cesium 1378 activity in the ashes can be up to 100 times higher than in the burned wood. In southern Germany there are some areas with increased cesium-137 soil contamination. The ashes of wood from these affected forests should, if possible, not be scattered in the garden. This eliminates potential cesium 137 contamination of garden products.
Pellets
Pellet heating systems are often used as an alternative to heat production for the fossil fuels oil and gas. Wood pellets are usually burned in a controlled and automated manner in a combustion chamber to generate heat. What about the radioactivity of the wood pellets as a fuel?
Holzpellts are made from wood and / or sawdust and wood shavings. The basic material is wood. Based on the results published to date on the radiation exposure of native wood, wood pellets made from wood grown in Germany should be radiation-biologically harmless. Due to the radionuclides present in forest soils and their transfer factors soil> wood, only cesium 137 is relevant. (possible exception: landfill sites of former uranium production sites).
Wood pellets and imported wood from Eastern Europe
The radiobiological safety for wood pellets should depend on the location of the logging. In 2009, however, wood pellets from Lithuania sold in Italy aroused the media and also the EU Commission. On August 10, 2009, the Greens put a parliamentary question on suspicion of increased cesium 137 contamination. Was asked u. a .:
- When, by whom and by what means was it determined that the wood pellets are radioactive?
- Are the pellets contaminated on the outside or do they contain the radioactivity?
- Which activity inventory do the individual pellets contain, and which activity concentration in Bq / g do the ashes of the individual pellets contain from which radionuclides?
The EU Parliament answered u. a .:
“2. In view of the evidence available, the Commission can confirm that the pellets are not externally contaminated.
3. The Cs-137 content of the pellets is approximately between 10 and 320 Becquerel per kilogram (Bq / kg). An ash sample showed a concentration of about 46,000 Bq / kg.
4. Italy has implemented Council Directive 96/29 / Euratom (1) laying down the basic safety standards for the protection of the health of workers and the general public against the dangers of ionizing radiation, including levels below which an activity is exempt from the notification requirement can be exempted under the Directive. While the activity concentration in the guideline for Cs-134 and Cs-137 is 10,000 Bq / kg, the Italian legislation specifies a lower value for all radionuclides of 1 Bq / g (or 1,000 Bq / kg) ( see Gazzetta Ufficiale No. 203 of August 31, 2000). It should be noted that the activity concentrations found in the wood pellets by the Italian authorities are well below the national benchmark. However, the activity concentration in the incineration residues exceeds this value. It should be taken into account that the permitted concentration for quantities of material (on the order of one tonne) has been established. It is up to the Italian authorities to assess the exposure pathways resulting from higher concentrations in combustion residues and to determine how to deal with them appropriately and to decide whether the future use of such wood pellets is allowed – either without any restrictions or only under certain conditions.
5. The origin of the radioactivity was not initially determined, but later it turned out that the wood pellets are radioactively contaminated due to the Chernobyl accident. The Chernobyl reactor accident caused contamination with cesium 134/137 in various European countries. When forest areas are contaminated with cesium, radioactive cesium is absorbed into vegetation, including trees. Normally the amount of cesium in wood products is insignificant from the point of view of radiation protection, but if such products are incinerated, cesium accumulates in the combustion residues, which could lead to regulatory concerns. ”
Based on these publications, 10 samples of wood pellets were tested for radioactivity by the pellet industry in 2009. According to Stiftung Warentest, the maximum value determined was 7 Bq / kg. Analyzes carried out in Austria in 2009 showed the maximum value of 4.6 Bq / kg.
It can be assumed that imported wood (and wood pellets produced from it) from the highly contaminated areas of the Chernobyl exclusion zone is heavily contaminated with cesium 137.
Radium 226 and Thorium 232
In the 2015 annual report on environmental radioactivity and radiation exposure, under “Radioactive substances in building materials and industrial products”, measured values for wood wool lightweight panels are also given:
| Ra-226 [Bq/kg] DM | Th-232 [Bq/kg] DM | K-40 [Bq/kg] DM |
| Mean (Range) | Mean (Range) | Mean (Range) |
| 21 (19 – 25) | 12 (11 – 14) | 210 (50 – 360) |
The activity data refer to becquerels per kilogram of dry substance.
Literature
Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit (BMUB), 2016: Umweltradioaktivität und Strahlenbelastung, Jahresbericht 2015. Link zum Download der .pdf
Fielitz U., 1994: Abschlußbericht über das Forschungsvorhaben „Radioaktivität in Wildtieren“. BMU-1994-408. Schriftenreihe „Reaktorsicherheit und Strahlenschutz“.
Kagawa, A. Aoki, T., Okada, N., Katayama Y., 2002: Tree-ring strontium-90 and cesium-137 as potential indicators of radioactive pollution
J. Environ. Qual., 31 (2002), pp. 2001-2007. Link
Ohashi S., Okada N., Tanaka A., Nakai W., Takano S., 2014: Radial and vertical distributions of
radiocesium in tree stems of Pinus densiflora and Quercus serrata 1.5 y after the Fukushima nuclear
disaster. Journal of Environmental Radioactivity, 134, 54-60.
Ohashi S., Kuroda K., Takano T., Suzuki Y., Fujiwara T., Abe H., Kagawa A., Sugiyama M.,Yoshitaka
Kubojima Y., Zhang C., Yamamoto K., 2017: Temporal trends in Cs 137 concentrations in the bark, sapwood, heartwood, and whole wood of four tree species in Japanese forests from 2011 to 2016. Journal of Environmental Radioactivity 178-179, 335-342.
Thüringer Allgemeine – Holzpellets radioaktiv verstrahlt: Stundenlanger Großeinsatz im Eichsfeld