Heikkinen, Tarja

Illukka, Eero

Puhakainen, Marketta

Rahola, Tua

English

Helsingin yliopiston digitaalinen arkisto

Boreal environment research 12: 29–35 issn 1239-6095helsinki 22 February 2007 © 2007134cs and 137cs in lichen (Cladonia stellaris) in southern Finlandmarketta Puhakainen, tua rahola, tarja heikkinen and eero illukkaSTUK — Radiation and Nuclear Safety Authority, P.O. Box 14, FI-00881 Helsinki, FinlandReceived 10 May 2006, accepted 23 Oct. 2006 (Editor in charge of this article: Veli-Matti Kerminen)Puhakainen, m., rahola, t., heikkinen, t. & illukka, e. 2007: 134cs and 137cs in lichen (Cladonia stella-ris) in southern Finland. Boreal Env. Res. 12: 29–35.The variation of the amounts of fallout radionuclides in reindeer lichen Cladonia stel-laris (syn. Cladonia alpestris) and the underlying soil below the lichen was investigated in 1986–2004 at three locations in southern Finland. Samples from the lichen carpet were fractionated into three vertical layers and the distribution of radionuclides between the dif-ferent fractions was investigated. The effective half-lives of 137Cs in lichen were almost the same in all three layers and, as a whole, the effective half-life of lichen varied from 2.7 to 3.4 years.IntroductionLichens have been recognised as ideal indicators of airborne radionuclides in various ecosystems. They are long-living organisms composed of symbiotic fungi and algae, obtaining their nutri-ents mainly directly from the air (Mattsson 1975). Lichens effectively accumulate and retain most of the deposited fallout products and thus act as a reservoir of radioactivity.They have no root system and are there-fore unlikely to accumulate significant levels of 137Cs from the substrate (Papastefanous et al. 1989). The uptake of radionuclides by lichens is enhanced by certain of their characteristics, such as the persistence of aerial parts slowing down their growth rates, long life spans and the high surface-to-mass ratio. The uptake of radioactive fallout products by lichens is rapid and nonselec-tive allowing the immediate determination of short-lived isotopes (Ellis and Smith 1987).Their scavenging properties make lichens cost-effective monitoring devices for aerosol reactive radionuclides. In modelling the trans-port of radioactive substances through lichens, it is assumed that the uptake occurs via the accu-mulation of radionuclides from the atmosphere. The removal occurs both via radioactive decay and, by first order, via biological elimination processes. The actual rate of growth does not affect the radionuclide inventory as long as the surface area increases linearly with time. Actual lichen growth alternates with comparatively dor-mant intervals (Smith and Ellis 1990).Reindeer lichen (Cladonia stellaris) is also well known as the main source of intake of radionuclides, such as 137Cs in reindeer. Lichen is the initial link in the food chain lichen–reindeer–man known to efficiently enrich radionuclides. From the point of view of radiation protection, it is especially important to determine the radio-nuclide content in the lichen carpet and the rate at which these radionuclides are eliminated from the carpet.Since the beginning of the 1960s, the con-centrations of 137Cs in reindeer lichens have been 30 Puhakainen et al. • Boreal env. res. vol. 12investigated in Finland. In the 1960s, the highest mean concentration of 137Cs in lichen in Finland was about 2400 Bq kg–1 dry weight (Tuominen and Jaakkola 1973), and in 1985 the 137Cs con-centration was approximately 200 Bq kg–1 (STUK 1987). After the Chernobyl accident, in 1986 and 1987 the 137Cs concentration in the reindeer herd-ing area in northern Finland varied from 200 to 2500 Bq kg–1 dry weight (K. Rissanen pers. comm.). An intensive investigation of the activity concentration of 137Cs in lichen was carried out after the Chernobyl accident in the northern part of Finland because of the risks the radioactivity in lichen presented for the reindeer herding. The radioactive deposition deriving from the Cher-nobyl accident was very unevenly distributed in Finland. Fortunately the amounts of 137Cs depo-sition in the vulnerable northern part of Finland were only about one-tenth of the corresponding amounts in southern Finland. No reindeer herd-ing takes place at the highest fallout locations, but lichen has been transported from these areas to the northern parts for feeding reindeer. To obtain information about the content and distri-bution of radionuclides and the half-life of 137Cs in lichens, samples of reindeer lichen were col-lected annually from 1986 to 2004 at three sites of high fallout deposition in southern and central Finland; Padasjoki, Loviisa and Olkiluoto. On 1 October 1987, the mean 137Cs deposition in these municipalities was 77.7, 22.7 and 19.5 kBq m–2, respectively (Arvela et al. 1990).Material and methodsSamples of reindeer lichen (Cladonia stellaris) and the underlying soil (about 5 cm below lichen) were collected at three locations in southern and central Finland: in Padasjoki, the highest fallout area in Finland after the Chernobyl accident; in Loviisa, Hästholmen, near the nuclear power plant of Loviisa; and in Eurajoki, Olkiluoto, near the nuclear power plant of Olkiluoto. For practi-cal reasons the sampling locations were chosen close to sites where also other studies were car-ried out. In Loviisa and Olkiluoto, the sampling sites were close to the sea in areas with rocks and stones, some barren trees and sparse under-vegetation. Lichen grows on the rocks with very little soil below the lichen layer. The Padasjoki sampling site in central Finland is an area with many lakes of different sizes. Samples were col-lected from 1986 to 2004.A standardised sampling technique was employed using a frame (25 ¥ 25 cm) corre-sponding to an area of 0.0625 m2. The samples were collected from the top of stones having little soil below the lichen. The mean dry weights of the soil below the lichen were 5.3 kg m–2 in Loviisa, 2.8 kg m–2 in Olkiluoto and 2.7 kg m–2 in Padasjoki. Parallel samples were collected when possible. Because reindeers prefer to eat only the fresh green top layer of the lichen carpet, it is important to examine the vertical distribution of the radioactivity in the lichen carpet and underly-ing material in order to estimate the transfer of radionuclides to reindeer. The lichen carpet was fractionated into four layers: fraction A (upper), fraction B (middle), and fraction C (lower) con-sisting mostly of the dead part of the lichen carpet (Fig. 1). The lichen samples were sprayed with water in the laboratory and all litter and foreign matter were removed from the lichen carpet with a pair of tweezers and combined into part D.The samples were dried in an oven at 105 °C and homogenised, and the activities were meas-ured gammaspectrometrically. The total uncer-tainty of the 137Cs analysis (at the confidence level of 68%), including the calibration uncer-tainty of the measuring equipment and the statis-tical uncertainty of the measurement, was below 10%. The average dry weight of the cleaned lichen was 0.9 kg m–2 (Table 1).Fig. 1. Fractionation of lichen Cladonia stellaris.Boreal env. res. vol. 12 • 134Cs and 137Cs in lichen (Cladonia stellaris) in southern Finland 31In Padasjoki four soil samples from a nearby farming area were collected in 2004 to the depth of about 20 cm. The soil horizons were divided into layers, so that the first layer was 2-cm thick, the second layer was 3-cm thick and the other layers were 5-cm thick.ResultsIn the highest fallout area, in 1987 the mean con-centrations of 134Cs and 137Cs in cleaned lichen were 34 000 and 94 000 Bq kg–1 dry weight, respectively, whereas in 2004 the corresponding numbers were 2.5 and 1600 Bq kg–1 dry weight (Table 2). Besides the caesium isotopes, a few other relatively long-lived nuclides, including 106Ru, 110mAg and 125Sb, were detected in the samples taken between 1986 and 1989. Since 1987 the lichen samples were divided into ver-tical layers (Fig. 1). The highest concentration of 110mAg was detected in the topmost layer of lichen and that of 125Sb in the lower layers (Table 3). The distribution of the 137Cs concentrations in the layers of lichen in various years varied between the different locations (Fig. 2).In the studied regions a small part of the 137Cs concentration were the result of atmospheric nuclear weapons tests conducted in the 1950s and 1960s, but most of 137Cs originated from the fallout of the Chernobyl accident in 1986. The highest activity in the total sample profiles found was 130 000 Bq m–2 in Padasjoki (Fig. 3). Most of the activity was detected in cleaned lichen (layers A, B and C) sampled during 2–3 years after the Chernobyl fallout. Later on, most of the activity was in the underlying soil and the D part (Figs. 3 and 4).Table 1. average dry weights of different layers of lichen (kg m–2).Part loviisa olkiluoto Padasjoki averagea 0.282 0.258 0.258 0.263B 0.257 0.279 0.288 0.277c 0.282 0.343 0.369 0.339Table 2. concentrations of 134cs and 137cs (Bq kg–1 dry weight) in cleaned lichen.Year Padasjoki loviisa olkiluoto    134cs 137cs 134cs 137cs 134cs 137cs1986   10100 20800  1987 34500 94400 8590 22100 3140 77801988 19400 69400 4000 14800 2450 85001989 11900 58600 4200 20300 1330 59601990 5000 34500 2100 14200 800 50901991 3970 36400 1330 12900 455 38801992   434 5440 177 20501993   311 5540 149 24101994 649 14000 171 4100 65 13601995 458 25900 73 2430 50 15401996 267 11200 31 1350 33 14401997   31 1290 20 11601998 58 4670 10 1060 2 9431999 48 5600 2 1520  2000 25 3330   5 7102001 8 1610    2002     0 2692003 4.9 2090 0 813 0 3182004 2.5 1560 0 449 0 308Table 3. concentrations of 106ru, 110mag and 125sb (Bq kg–1 dry weight) in different layers of lichen.Place/date layer of 106ru 110mag 125sb lichenloviisa/29 July 1987 a 1200 150 330 B 1700 89 330 c 1800 82 450loviisa/30 July 1987 a 0 87 0 B 0 0 0 c 0 0 0loviisa/30 July 1987 a 790 94 0 B 540 43 0 c 260 0 140olkiluoto/20 may 1987 a 470 94 0 B 740 0 230 c 820 37 150olkiluoto/26 may 1989 a 0 0 0 B 0 0 0 c 200 0 85Padasjoki/30 oct. 1987 a 0 290 0 B 0 280 1170 c 0 180 1100Padasjoki/30 oct. 1987 a 0 230 0 B 0 260 1400 c 0 230 1300Padasjoki/2 oct. 1988 a 0 0 0 B 0 0 0 c 0 0 1100Padasjoki/1 oct. 1989 a 0 0 0 B 0 0 300 c 0 0 79032 Puhakainen et al. • Boreal env. res. vol. 12In separate soil samples collected in Padas-joki in 2004, 48%–87% of the 137Cs amount were found in the uppermost 5-cm layer and 85%–98% of it was in the uppermost 10-cm layer. Only 1.5%–14% of the 137Cs concentration was in the lower layers.DiscussionLichen is among the most radiation-resistant living plants (Mattsson 1975). In the 1970s the maximum activity concentrations of all radionu-clides, except 125Sb, were found in the top layer of the lichen carpet. According to increasing depth penetration in the lichen carpet, the radionuclides may be arranged into the following sequence: 144Ce ~ 7Be << 95Zr < 137Cs < 106Ru ~ 155Eu << 125Sb. The maximum values for the 125Sb concen-tration were found in the B layer of the lichen carpet, not in the A layer, as was the case with 137Cs and other radionuclides. This same was also observed in our studies conducted in 1987–1989 (Table 3). It seems that already a year after the deposition, 125Sb was not found in the topmost part of lichen but it was transported to the lower parts and to the ground. According to Mattsson (1975), the reason for this was the low fallout rate combined with the very low solubility of 125Sb that does not allow the upward movement with the liquid transport in the lichen plant, as in the case of 137Cs (Mattsson 1975).In Loviisa and Olkiluoto the highest amount of 137Cs was in the topmost layer of lichen col-10010001000010000010000001985 1990 1995 2000 200510010001000010000010000001985 1990 1995 2000 2005137 Cs (Bq kg–1)10010001000010000010000001985 1990 1995 2000 2005YearA B CFig. 2. mean concentra-tion of 137cs (Bq kg–1 dry weight) in different layers in lichen measured in loviisa (top), olkiluoto (middle) and Padasjoki (bottom).Boreal env. res. vol. 12 • 134Cs and 137Cs in lichen (Cladonia stellaris) in southern Finland 33lected during the four years after the Chernobyl fallout. In 1991, the amounts of 137Cs were similar in all three layers. After 1991, the high-est amounts of 137Cs were detected in the lowest layer of lichen. Radiocaesium either moved into the lower layers of lichen or it was in the soil particles contaminating the lower lichen layers (Fig. 5). In Padasjoki, the highest fallout area of Finland, the variation was more erratic.The effective half-life of 137Cs in Cladonia species lichen, as presented in the literature in the 1980s and 1990s, varies from 1 to 10 years (Ellis and Smith 1987, Roos et al. 1987, Gaare 1990, Roos et al. 1991, Synnott et al. 2000). In a lake district in central Sweden, the effective half-life of nuclear bomb-produced 137Cs in lichen was cal-culated to be about 8–10 years. A shorter half-life (7–8 years) was noticed in the upper fraction of lichen as compared with that in the whole lichen carpet (Roos et al. 1991). In our study the effec-tive half-lives of 137Cs from the Chernobyl fallout were almost the same in all three fractions and the half-lives in the total lichen varied from 2.7 to 3.4 years. No variation in half-lives between the different sampling locations was detected by us. The comparison of results obtained from samples collected before the Chernobyl accident with those collected after the accident is difficult. The share of 137Cs that originated from the nuclear weapons test was in our samples so small that it can be considered negligible.The highest uncertainty in our results is caused by the sampling and division of lichen into layers. If only a few years are taken into account when calculating the effective half-life, occasional variation in sampling can highly influ-ence the results (see Fig. 2) (Sloof and Wolter-beek 1992). Many of the half-life calculations of lichen, as presented in the literature, were made using results that cover only a few years.Padasjoki0200004000060000800001000001200001400001986 1988 1990 1992 1994 1996 1998 2000 2002 2004Yearcleaned lichen underlying soil and D partLoviisa01000020000300004000050000600001986 1988 1990 1992 1994 1996 1998 2000 2002 2004Olkiluoto020004000600080001000012000140001986 1988 1990 1992 1994 1996 1998 2000 2002 2004137 Cs (Bq m–2)Fig. 3. average amounts of 137cs (Bq m–2) in the total profile and cleaned lichen.34 Puhakainen et al. • Boreal env. res. vol. 1202000040000600008000010000012000014000016000030.10.198730.10.198702.10.198801.10.198901.10.198901.10.198908.10.199008.10.199022.10.199129.06.199429.06.199414.06.199524.06.199829.06.200027.06.200126.06.200328.06.2004 137Cs (Bq m–2)ABCDSoil0204060801001987 1989 1991 1993 1995 1997 1999 2001 20030204060801001987 1989 1991 1993 1995 1997 1999 2001 2003Activity (percentage of total inventory)0204060801001987 1989 1991 1993 1995 1997 1999 2001 2003YearCBAFig. 4. amounts of 137cs in different layers of lichen and soil beneath the lichen in Padasjoki. all parallel samples are presented separately.Fig. 5. mean per cent of the total inventory of 137cs in different layers in lichen measured in loviisa (top), olkiluoto (middle) and Padasjoki (bottom).Boreal env. res. vol. 12 • 134Cs and 137Cs in lichen (Cladonia stellaris) in southern Finland 35ReferencesArvela H., Markkanen M. & Lemmelä H. 1990. Mobile survey of environmental gamma radiation and fall-out levels in Finland after the Chernobyl accident. Radiation Protection Dosimetry 32: 177–184.Ellis K.M. & Smith J.N. 1987. Dynamic model for radio-nuclide uptake in lichen. J. Environ. Radioactivity 5: 185–208.Gaare E. 1990. Lichen content of radiocesium after the Chernobyl accident in mountain in southern Norway. In: Transfer of radionuclides in natural and seminatu-ral environments. Proceedings of the workshop held at Udine (IT), 11–15 September 1989, London (UK), pp. 492–501.Mattsson S. 1975. 137Cs in the reindeer lichen Cladonia alpestris. Deposition, retention and internal distribution, 1961–1970. Health Physics 28: 233–248.Papastefanou C., Manolopoulou M. & Sawidis T. 1989. Lichen and mosses: biological monitors of fallout from the Chernobyl reactor accident. J. Environ. 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STUK-Finnish Centre for Radiation and Nuclear Safety, Helsinki, Finland.Synnott H.J., McGee E.J., Rafferty B. & Dawson D.E. 2000 Long-term of radiocesium activity concentrations in vegetation in Irish semi-natural ecosystems. Health Physics 79: 154–161.Tuominen Y. & Jaakkola T. 1973. Absorption and accumula-tion of mineral elements and radioactive nuclides. In: Ahmadjian V. & Hale M.E. (eds.), The lichens, Aca-demic Press, New York, pp. 185–223.

134Cs and 137Cs in lichen (Cladonia stellaris) in southern Finland

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134Cs and 137Cs in lichen (Cladonia stellaris) in southern Finland

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