Chernobyl radioactivity persists in reindeer

Abstract

Transfer of ¹³⁷Cs in the soil–plant/lichen–reindeer food chain was studied in central (Østre Namdal) and southern Norway (Vågå) during 2000–2003. Reindeer from these areas have been continuously subjected to countermeasure application since the 1986 Chernobyl accident. In both areas no decline in ¹³⁷Cs concentrations was detectable in reindeer slaughtered in autumn since 1995, or in reindeer slaughtered in winter since 1998–1999. Seasonal differences in ¹³⁷Cs concentrations in reindeer have been less pronounced in recent years, with ¹³⁷Cs concentrations occasionally higher in autumn than in winter. Soil-to-plant ¹³⁷Cs transfer was significantly higher in Østre Namdal than in Vågå. Climatic influences on lichen growth and abundance, and on soil properties that influence the availability of ¹³⁷Cs for plant uptake, are hypothesized to have a larger impact on long-term transfer of radiocaesium in the soil–plant/lichen–reindeer food chain than has been previously observed.

Introduction

After radioactive fallout, reindeer contain elevated radiocaesium concentrations due to intake of contaminated lichens and other vegetation (Lidén, 1961, Hanson, 1966, Holleman et al., 1971, Gaare and Staaland, 1994, Staaland et al., 1995). Particularly, high concentrations can be observed in reindeer during the winter resulting from a combination of higher radiocaesium intake and lower radiocaesium excretion rate (e.g. Holleman et al., 1971). Contaminated lichens are generally considered to be the main source of radiocaesium intake during winter, and may be the main radiocaesium source during summer (Staaland et al., 1995). Additionally, fungi can be another important source of radiocaesium intake in the autumn (Hove et al., 1990).

Following fallout from the 1986 Chernobyl accident, individual radiocaesium activity concentrations in reindeer meat in the most affected areas in Scandinavia reached 150 kBq kg⁻¹ (fresh weight, FW) (Strand et al., 1992) and declined with effective half-times of 3–5 years (Pedersen et al., 1993, Åhman and Åhman, 1994, Amundsen, 1995, Gaare et al., 2000, Åhman et al., 2001). In comparison, half-times of 6–9 years have been observed in northern Fennoscandia that received less Chernobyl fallout (Skuterud et al., 1999, Åhman et al., 2001, Rissanen et al., 2003). Observations of slower rates of decline in radiocaesium concentrations in several vascular plants compared to lichens (Gaare and Staaland, 1994, Gaare et al., 2000) indicated that the significant seasonal differences in radiocaesium concentrations in reindeer observed during the first years after the Chernobyl accident (e.g. Eikelmann et al., 1990, Pedersen et al., 1993, Åhman and Åhman, 1994) would be less pronounced with time (Gaare and Staaland, 1994). The point in time at which seasonal differences in radiocaesium concentrations become reduced might be dependent on the species composition of the reindeer diet, and may even vary between grazing areas.

About 58% of Norwegian territory is used as pasture by reindeer, a major part of which is used for reindeer herding (Norwegian Reindeer Husbandry Administration, 2004; Danielsen, J. Directorate for Nature Management, Trondheim, personal communication). The probability that any future ¹³⁷Cs contamination event in Norway will affect reindeer grazing areas is therefore great. A thorough understanding of reindeer radioecology is therefore essential in Norwegian nuclear emergency preparedness. However, most terrestrial radioecological studies in Norway were terminated during the 1990s, with the exception of routine monitoring of ¹³⁷Cs concentrations in semi-domestic reindeer in central and southern Norway where a continued requirement existed to ensure that meat complied with the national intervention limit of 3000 Bq kg⁻¹. Furthermore, by the end of the 1990s, studies indicated that rates of decline in ¹³⁷Cs concentrations in other components of the ecosystem were slower than previously reported (Jonsson et al., 1999, Smith et al., 2000). The present study was therefore initiated to obtain new data and more detailed knowledge about the long-term transfer of ¹³⁷Cs in the soil–plant/lichen–reindeer food chain that could help explain the persisting elevated ¹³⁷Cs concentrations in reindeer. The study was conducted during the period 2000–2003 with sampling of soil, vegetation and reindeer tissues in two of the areas in Norway that were most affected by fallout from the Chernobyl accident. Supplementary data on ¹³⁷Cs in reindeer from 1986 onwards were obtained from the Norwegian Reindeer Husbandry Administration.