Biological transfer of radionuclides in marine environments - identifying and filling knowledge gaps for environmental impact assessments

Methodologies have been developed to allow impacts of ionising radiation on marine ecosystems to be evaluated within selected geographical settings. The stages in the assessment require initial information about unit concentrations of radionuclides in reference media to be collated. Activity concentrations for reference groups of flora and fauna and for representatives of these groups are then derived using an equilibrium concentration factor approach. Following this, dose-rates can be calculated using relevant dose conversion factors. Impacts on the environment are evaluated by comparison with dose-rates at which selected biological effects are known to occur and the natural radiation background. This paper focuses mainly on the transfer part of the assessment drawing attention to gaps in information that have been identified through review and providing an outline of methods that may be used to fill these gaps. Biokinetic models parameterised using allometric relationships have shown their utility in this respect. Initial work looking into the application of such models has met with some success with model predictions comparing favourably with the few empirical data available. Further work will be needed to validate the models for a large range of radionuclides and to consider uncertainty in model estimates through probabilistic analyses. 1. ENVIRONMENTAL IMPACT ASSESSMENT Methodologies to assess the impact of exposure to ionising radiation on flora and fauna have been developed in two recent European collaborative projects [1-3]. The initial stage of the assessment requires the selection of appropriate reference biota and suitable representative organisms (normally defined at the species level) with concomitant collation of life history data sheets. Methods for deriving the transfer and fate of radionuclides are necessary during the assessment procedure as are methods for deriving (weighted or unweighted) dose-rates. Once exposures for reference biota have been derived, they need to be interpreted in terms of biological effects. The analyses cannot be termed a risk assessment at this stage because the probability of specified biological effects occurring has not been adequately considered. 1.1 Reference organisms and Representative species The FASSET definition of “reference organism” is: “a series of entities that provides a basis for the estimation of the radiation dose-rate to a range of organisms that are typical, or representative, of a contaminated environment. These estimates, in turn, would provide a basis for assessing the likelihood and degree of radiation effects.” [3]. For Arctic marine environments in the EPIC project, selection criteria have been applied in order to select a reference organism suite [4]. These were: Article published by EDP Sciences and available at http://www.edpsciences.org/radiopro or http://dx.doi.org/10.1051/radiopro:2005s1-078 S534 RADIOPROTECTION • Ecological niche. This was simply applied as a requirement to have at least one representative from each trophic level. • Intrinsic radiosensitivity. In this case comparison was made between the acute lethal doses expressed by various organism groups. • Radioecological sensitivity, i.e. identification of which organisms are likely to be most exposed either through an expression of relatively high radionuclide bioaccumulation or relatively high activity concentrations in their habitat. • Distribution. Preference was given to those organisms that were year-round residents in the Arctic. • Amenability to research and monitoring. This criterion involved an assessment of whether data sets documenting activity concentrations in various groups of organism were available from monitoring studies and whether future research might be conducted upon the various groups (e.g. exposure experiments etc.). The generic reference organism lists have been used as a basis for deriving appropriate environmental transfer data information and selecting suitable target geometries/phantoms for dosimetric modelling. With respect to these points, it became apparent that the identification of actual species (or in some cases families or classes of organisms) representing each of the broadly defined groups would be helpful in some instances. This was true in the case of deriving food-chain model parameters where detailed information was often required, beyond a generic consideration, with respect to organism characteristics. It was also true in the case of geometry construction where quantitative information on size, shape and density are required and can be derived, simply and transparently, from a consideration of real flora and fauna. Examples of suitable representative species of selected reference organisms were subsequently chosen giving preference to species ubiquitous throughout the specified assessment area, e.g. European Arctic, and the availability of appropriate data. Basic ecological information needs to be collated for each of the selected flora and fauna. The specific organism attributes that should be considered relate directly to the subsequent assessment of exposure. For example, information should be provided on habitat and, where applicable, the fractional occupancy of various organisms in their habitats. This information is important for the weighting of external dose-rates in order to account for the behaviour of the organism. Guidance on the types of ecological information required for reference fauna is provided elsewhere [2, 3]. The resultant initial reference organism list with representative types, in Arctic systems (see [5]), includes examples such as Fucus spp. for macroalgae, Boreogadus saida (polar cod) for Pelagic planktotrophic fish and ‘Seals’ (e.g. Phoca groenlandica) for carnivorous mammal. 1.2 Dose calculation The whole-body absorbed dose-rate is used as a measure of the reference organism exposure to ionising radiation, expressed in units of Gy per unit time, and is the sum of internal and external absorbed dose-rates. It may be appropriate to introduce radiation weighting factors to take account of the differing biological effectiveness of different types of ionising radiation. At the present time such consideration is recommended for alpha particle radiation, and for beta particle radiation with mean particle energies less than 10 keV. Introduction of these weighting factors leads to the weighted absorbed dose-rates, for example: j j j low low j weighted j weighted ext j weighted j weighted total