Journal of Environmental Radioactivity 102 (2011) 430e437
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Levels and transfer of
210
Po and
210
Pb in Nordic terrestrial ecosystems
J.E. Brown a, *, R. Gjelsvik a, P. Roos b, J.A. Kålås c, I. Outola d, E. Holm a a
Norwegian Radiation Protection Authority, PO Box 55, N-1332, Østerås, Norway RISØ-DTU P.O. Box 49 DK-4000 Roskilde, Denmark c Norwegian Institute for Nature Research (NINA), Tungasletta 2, 7485 Trondheim, Norway d STUK, Laippatie 4/P.O. BOX 14, 00881 Helsinki, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2010 Received in revised form 22 June 2010 Accepted 23 June 2010 Available online 21 July 2010
Recent developments regarding environmental impact assessment methodologies for radioactivity have precipitated the need for information on levels of naturally occurring radionuclides within and transfer to wild flora and fauna. The objectives of this study were therefore to determine activity concentrations of the main dose forming radionuclides 210Po and 210Pb in biota from terrestrial ecosystems thus providing insight into the behaviour of these radioisotopes. Samples of soil, plants and animals were collected at Dovrefjell, Central Norway and Olkiluoto, Finland. Soil profiles from Dovrefjell exhibited an approximately exponential fall in 210Pb activity concentrations from elevated levels in humus/surface soils to “supported” levels at depth. Activity concentrations of 210Po in fauna (invertebrates, mammals, birds) ranged between 2 and 123 Bq kg1 d.w. and in plants and lichens between 20 and 138 Bq kg1 d.w.. The results showed that soil humus is an important reservoir for 210Po and 210Pb and that fauna in close contact with this media may also exhibit elevated levels of 210Po. Concentration ratios appear to have limited applicability with regards to prediction of activity concentrations of 210Po in invertebrates and vertebrates. Biokinetic models may provide a tool to explore in a more mechanistic way the behaviour of 210 Po in this system. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Po-210 Pb-210 Terrestrial ecosystem Transfer Biokinetic
1. Introduction In order to provide a robust framework for conducting environmental impact assessments (EIAs) for radioactivity, there is a requirement to draw upon information concerning levels of naturally occurring radionuclides within and transfer to wild flora and fauna. By way of example, the ERICA integrated approach (Larsson, 2008) to the assessment and management of environmental risks from ionising radiation provides a number of transfer databases for selected organisms from aquatic (Hosseini et al., 2008) and terrestrial ecosystems (Beresford et al., 2008a) that include natural decay series radionuclides. Such databases enable activity concentrations in plants and animals to be derived from media concentrations in cases where such empirical data are unavailable. Furthermore, the approach utilizes data collated for activity concentrations of naturally occurring radionuclides to derive background dose rates with which calculated exposures, with their provenance in discharges of anthropogenic radioactivity, can be compared and management decisions elaborated.
* Corresponding author. Tel.: þ47 67162663; fax: þ47 67147407. E-mail address:
[email protected] (J.E. Brown). 0265-931X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2010.06.016
Developments within this field of EIA for ionizing radiation have not been restricted to those occurring at a regional level. Indeed, the International Commission on Radiological Protection (ICRP, 2008) has considered that it would be helpful for the decision making process in relation to environmental exposures if information concerning effects on biota was set out in terms of multiples of the natural background dose rates typically experienced by each type of animal or plant (ICRP, 2008). For such a structuring of data to be made, there is clearly a requirement to provide well-characterized activity concentration data for naturally occurring radionuclides in selected categories of wild organism. A number of recent publications have facilitated the process of characterizing environmental background exposure in both terrestrial (Beresford et al., 2008b) and aquatic (Brown et al., 2004) systems. However, from a critical evaluation of these and other published data (Brown et al., 2009), it was evident that information on activity concentrations and transfer is limited for numerous radionuclides from 238U and 232Th decay series and notably, 210Po. Especially in view of the fact that the radioisotopes 210Po and 210Po are known to constitute an important component of internal doses to selected terrestrial organisms (Gómez-Ros et al., 2004), there is clearly a requirement to prioritize the quantification of the levels and variability of these radioisotopes in the process of deriving
J.E. Brown et al. / Journal of Environmental Radioactivity 102 (2011) 430e437
tenable background dose-rate estimates. It is also readily apparent that information on the transfer and fate of 210Po and 210Pb in terrestrial boreal ecosystems is quite limited, most data having been collated for low-land temperate environments. With these considerations in mind, the objectives of this study were to determine activity concentrations of 210Po and 210Pb in biota from terrestrial boreal ecosystems and from this to provide further insight into the environmental behaviour of these radioisotopes in such environments. Furthermore, in the case of releases of naturally occurring radionuclides to the environment, there is a requirement to be able to predict the transfer to selected organism types. In addressing this, a further objective was defined as exploring the efficacy of selected transfer models for 210Po. 1.1.
210
Po and
210
Pb in terrestrial boreal systems
Deposition of 210Pb, associated with aerosols in the atmosphere, occurs via mesoscale transportation process, sedimentation and precipitation. Early models concerning the atmospheric 210Pb transport were based on the vertical movement of the radioisotope into the troposphere at the equator followed by lateral movement to mid-latitudes and deposition, whereas more recent, refined models have included regional sources of 222Rn to account more robustly for the 210Pb deposition rates observed across the major continents (Macdonald et al., 1996). Atmospheric 210Pb concentrations are positively correlated with the size of underlying landmasses, whereas terrestrial areas covered by ice and snow and marine areas including islands have reduced atmospheric concentrations of 210Pb (El-Daoushy, 1988a,b). Furthermore, the deposition of 210Pb is directly correlated with the level of precipitation (Hill, 1960). The annual deposition varies from a few Bq per m2 such as in the Antarctic (Roos et al., 1994) to several hundred Bq m2 (El-Daoushy, 1988a). The annual deposition in central Sweden was estimated to be about 63 Bq m2 year1 (Persson, 1970) and in Scandinavia, the radionuclide is deposited continuously to earth at a rate of approximately 55 Bq m2 annually (El-Daoushy, 1988a). In mountainous areas dominated by alpine vegetation, lichens appear to play a key role in the introduction of 210Pb into the food chain. Lichens are slow growing, perennial, composite organisms that have high interception potentials for aerosols in precipitation, and therefore contain significantly higher 210Pb concentrations than vascular plants. Animals feeding on lichens, notably reindeer have been shown to have relatively high muscle and organ 210Po and 210Pb activity concentrations (Skuterud et al., 2005). For animals not feeding on lichens, entry of 210Pb and 210Po into the food chain presumably primarily occurs through the ingestion of vegetation (with relatively low concomitant activity concentrations relative to lichens), dust and soil. One field sampling strategy was, therefore, to consider entry into the food-chain through the sampling of lichens and vegetation and, thereafter, to investigate transfer to higher trophic levels by sampling herbivorous and omnivorous animals. Furthermore, in view of the ICRP’s consideration of a defined set of reference animals and plants (ICRP, 2008), efforts were also made to select biota that may have relevance to these organism types, notably small, rodent-like mammals, amphibians and earthworms. 2. Materials and methods 2.1. Field site and sampling A field study was planned and implemented at Dovrefjell, Central part of Norway (62 170 N, 9 360 E). The field study was conducted within a designated landscape-protected area near to Kongsvold adjacent to Dovrefjell-Sunndalsfjella National Park (Fig. 1). This study site, which may be characterised as a semi-natural ecosystem, was selected primarily on the basis that it forms part of the network for
431
Monitoring programme for Terrestrial Ecosystems (TOV) in Norway, led by the Norwegian Institute for Nature Research (NINA), and concerning, in particular, effects of pollution on plants and animals and chemical and biological monitoring. The area was situated in a subalpine region (900e1100 m above sea level). Bedrock consists mostly of phyllitic mica schist and green schist from the Upper Ordovician giving rise to mineral rich soils (Kålås et al., 1994). Eight soil profiles were collected during the period 17e20th June 2007. These profiles were split into an overlying litter/humus layer (i.e. loose organic matter on the soil surface and decaying organic matter directly under this) and thereafter 3 cm (predominantly mineral soil) increments to a depth of 9 cm using a customdesigned soil corer. This was undertaken with a view to enabling analyses of the activity distribution of radionuclides with depth. Three samples of bilberry (Vaccinium myrtillus) and one sample for each of two species of lichens (e.g. Cladonia stellaris and Cladonia arbuscula) were collected by hand during the period 17e20th June 2007. The samples consisted of leaves and new shoots for the bilberry and the entire above ground parts of lichens. Seven samples of two earthworm species (Lumbricus rubellus and Aportectodea caliginosa) were collected in areas of brown earth using a spade. These samples were collected during the period 7e17th June 2007. The samples (consisting of multiple worms per sample) were rinsed in water but the intestinal tract was not removed. Baited traps were used during the period 17e9th June 2007 to collect bank vole (Myodes glareolus) and the common shrew (Sorex araneus). Six transects consisting of 50 traps were set out in the field with 20 m between each trap. A small amount of peanut butter was placed on each trap to attract the animals and a total of 300 traps were set out in the area. The traps were checked twice daily e in the morning and in the evening. Analyses were undertaken for 8 bank vole (4 mature males and 4 mature females, total body wet weight 30.5e40.1 g), and 9 common shrew (4 mature males and 5 mature females, total body wet weight 10.2e14.6 g). Entire animal bodies including the pelt and skeleton (but excluding the gastrointestinal tract and liver) were processed. Five willow grouse (Lagopus lagopus), all males (total body wet weight 525e621 g), shot in the period 10e11th September 2007 were obtained from local hunters. One of the pectoral muscles was removed for further analyses. The study from Dovrefjell was part of a larger project considering levels and transfer of Po in Nordic environments generally (Gjelsvik and Brown, 2009). As part of the broader project, viper (Vipera berus) and frog (Rana temporaria) were also sampled in April 2007 in Olkiluoto, Finland (Fig. 1). Measurements were made for the whole-body of these animals. Soil samples (0e10 cm) were collected in June 2007 from the same location. This is a low lying (
