A new approach to predicting environmental transfer of radionuclides to wildlife: A demonstration for freshwater fish and caesium

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Highlights

  • An alternative approach to estimating radionuclide transfer to wildlife is presented.

  • Analysed a dataset comprising 53 freshwater fish species collected from 67 sites.

  • Residual Maximum Likelihood mixed model regression is used.

  • Model output takes account of the effect of inter-site variation.

  • Successfully predicted 137Cs concentrations in different fish species from 27 lakes.

Abstract

The application of the concentration ratio (CR) to predict radionuclide activity concentrations in wildlife from those in soil or water has become the widely accepted approach for environmental assessments. Recently both the ICRP and IAEA have produced compilations of CR values for application in environmental assessment. However, the CR approach has many limitations, most notably, that the transfer of most radionuclides is largely determined by site-specific factors (e.g. water or soil chemistry). Furthermore, there are few, if any, CR values for many radionuclide-organism combinations. In this paper, we propose an alternative approach and, as an example, demonstrate and test this for caesium and freshwater fish. Using a Residual Maximum Likelihood (REML) mixed-model regression we analysed a dataset comprising 597 entries for 53 freshwater fish species from 67 sites. The REML analysis generated a mean value for each species on a common scale after REML adjustment taking account of the effect of the inter-site variation. Using an independent dataset, we subsequently test the hypothesis that the REML model outputs can be used to predict radionuclide, in this case radiocaesium, activity concentrations in unknown species from the results of a species which has been sampled at a specific site. The outputs of the REML analysis accurately predicted 137Cs activity concentrations in different species of fish from 27 Finnish lakes; these data had not been used in our initial analyses. We recommend that this alternative approach be further investigated for other radionuclides and ecosystems.

Introduction

Over recent years a number of approaches and associated tools have been developed to assess the exposure of wildlife to ionising radiation (e.g. Copplestone et al., 2001, USDOE (United States Department of the Energy), 2002, Brown et al., 2008, International Commission on Radiation Protection (ICRP), 2008, International Commission on Radiological Protection, (ICRP), 2009, Beresford et al., 2008a). These tools use models to predict radionuclide activity concentrations in wildlife to enable internal absorbed dose rates to be estimated. Most commonly this is achieved by using a simple concentration ratio (CRwo-media) which relates the whole organism activity concentration to the activity concentration in the appropriate medium for a given environment (e.g. soil or air for the terrestrial environment, water for aquatic environments) (Beresford et al., 2008a).

Given the large number of organisms and radionuclides that may need to be considered to allow assessment of the many source terms and different environments, it is perhaps not surprising that there are many cases where empirical data to derive CRwo-media are lacking. Where this is the case a variety of extrapolation approaches have been used to enable the estimation of whole organism activity concentrations (Copplestone et al., 2003, Beresford et al., 2008b, Higley et al., 2003, International Commission on Radiological Protection, (ICRP), 2009). Although recent attempts to collate CR values for wildlife have led to improved databases, there are still many gaps in our knowledge (Howard et al., 2013, Copplestone et al., in press, International Commission on Radiological Protection, (ICRP), 2009). Consequently, there is still a need to develop robust extrapolation approaches most especially: (i) to enable initial screening tier assessments for which site-specific data are not available (Brown et al., in press); (ii) for protected species for which it may be impossible to acquire sufficient data (e.g. Copplestone et al., 2003); and (iii) for the International Commission on Radiological Protection's Reference Animals and Plants (RAPs) which are defined specifically at the taxonomic family level but for which there are relatively few data (International Commission on Radiological Protection, (ICRP), 2009, Copplestone et al., in press).

Soil-to-plant transfer of elements of radiological interest has been related to plant evolutionary history, or phylogeny, for Cs (Broadley et al., 1999, Willey et al., 2005), Sr (Willey and Fawcett, 2005a), Ru (Willey and Fawcett, 2006), Cl (Willey and Fawcett, 2005b), Co (Willey and Wilkins, 2008) and U (Willey, 2010). Such phylogenetic relationships present a potential approach to enable predictions of transfer, with some scientific justification, for taxonomic groups for which there are no data either at the generic or site-specific level (Willey, 2010). The potential to derive phylogenetic relationships for organisms other than plants has also been demonstrated by Jeffree et al., 2010, Jeffree et al., in press who suggested that the transfer of a number of radionuclides to marine teleost and chondrichthyan fishes and the amphioxus (fish like chordate) species Branchiostoma lanceolatumis is influenced by phylogeny. However, the work of Jeffree et al. was based upon the results of laboratory studies. Whilst this usefully removes the influences of many confounding factors it is not directly applicable to environmental conditions as foodchain transfer was excluded.

The objective of the work described in this paper was to explore if phylogeny could be used to explain variation in the transfer of radiocaesium to freshwater fish species based on field observations analysed using Residual Maximum Likelihood (REML) mixed-model regression (Willey, 2010) (see Section 2.2).

Section snippets

Data sources

The primary source of data for the analyses was the database on radionuclide transfer to freshwater organisms as described by Yankovich et al. (in press) (see also Copplestone et al., in press). This contains concentration ratios relating the fresh weight (FW) whole organism activity concentration to the activity concentration in water. Where:CRwo-water=ActivityconcentrationinwholeorganismBqkg1FWActivityconcentrationinfilteredwaterBql1.

This database contains 535 CRwo-water entries describing

REML analysis

When all data were considered at the species level, the REML variance component analysis gave a significant (p < 0.001) Wald statistic of 116 with significant variation in REML estimated mean values being explained by hierarchical ANOVA at the order level (ANOVA; p < 0.001) but little additional variation explained by the effects of family within order or genus within family.

The Wald statistic for the analysis at the levels of order, family and genus were 51 (p < 0.001), 54 (p < 0.001) and 107 (p < 

Conclusions

Whilst we have demonstrated differences between the Cs transfer to different taxa of freshwater fish, based upon the data available to us (all species originating from just 10 orders in one class) we cannot describe detailed phylogenetic relationships. Earlier analyses which have suggested phylogenetic relationships for the transfer of radionuclides to plants (see Willey, 2010) and marine fish (Jeffree et al., 2010, Jeffree et al., in press) have included species encompassing much wider

Conflict of interest

No conflict of interest.

Acknowledgements

The authors would like to thank all of those who contributed data to the database initiated by the IAEA under their EMRAS II programme in collaboration with the IUR and ICRP (see Yankovich et al., in press, Copplestone et al., in press, Howard et al., 2013, International Commission on Radiological Protection, (ICRP), 2009). The inputs of N.A. Beresford and M. Muikku to this work were funded under the EURATOM STAR network of excellence in radioecology (www.star-radioecology.org).

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