Protection of non-human biota against radiation in freshwater—Effect of time dependence in tiered exposure assessment
Introduction
So far, protection of the environment against radiation was based on the assumption that non-human biota are protected when exposure levels are protective for humans (e.g. ICRP, 1990, IAEA, 1992). This assumption was however criticized because: (i) non-human biota exists in zones where humans do not; (ii) specific exposure pathways and/or effects can occur for some species, including rare or endangered species. Thus, there is now an international consensus whether a dedicated framework is needed to assess that biota is protected against ionising radiation (Holm, 2004, Janssens, 2004).
Approaches proposed so far or that are under construction (i.e. Jones et al., 2003, Larsson et al., 2004; the ERICA approach (European Commission, 2007a; http://www.erica-project.org)) structure the assessment on the classical steps of environmental risk assessment (ERA), i.e. (1) the problem formulation, (2) the exposure and (3) effect assessment, and (4) the risk characterisation. The scope of this paper is focused on the step (2), that aims at calculating the external and internal exposure levels to biota. The paper is limited to freshwater ecosystems.
The ERICA assessment framework (European Commission, 2007a) is built on a tiered approach, each tier requiring more or less information on the context to be investigated: (i) the Tier 1 assessment is designed to be simple and conservative, requiring a minimum of input data (e.g. maximum releases in the system); (ii) the Tier 2 assessment allows the user to be more interactive and to use ‘personal’ data to perform a more precise evaluation; (iii) Tier 3 is a probabilistic risk assessment in which uncertainties may be determined.
When using such a tiered approach, the time dependency of input data and parameters is a key element for formulating the assessment problem and for defining which tier is actually required for the investigated context. Two types of ‘time dependency’ in the problem formulation can be distinguished:
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Time dependency of input data. Tier 1 assessment generally considers that releases are constant and continuous over the year. Time dependency of the radioactive releases into the ecosystem is therefore neglected and this assumption is considered to be conservative. However, actual releases from NPPs are intermittent. For example, in France, NPPs generally discharge once a month during some hours and releases are constrained by the flow rate of the river (releases being forbidden when the dilution rate in the river is too low or when the consequences of flooding must be avoided). Such industrial and environmental constraints lead to temporal variation in the concentration of radionuclides in the river system (Ciffroy et al., 2005). When conducting a more detailed evaluation by Tiers 2 or 3, it may be justified to consider time-dependent releases into the freshwater system instead of ‘time-averaged’ or ‘time-maximized’ releases.
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Time dependency of transfer parameters. Current approaches are based on two main parameters: (i) the radionuclide distribution coefficient Kd, expressed as the concentration ratio between the particulate phase and the dissolved phase under equilibrium conditions; (ii) the radionuclide Concentration ratio CR, expressed as the concentration ratio between the bioaccumulated concentration and the concentration in dissolved water under equilibrium conditions. In Tier 1, it is generally considered that radionuclides released into the river are immediately equilibrated within all the compartments of the system, especially at the interface between water and particulate suspended matter, and at the interface between water and organisms. However, radionuclides are initially released in a dissolved form and their sorption to particles can be slow (Durrieu et al., 2006). As for suspended particulate matter, it is well known that the accumulation and elimination of radionuclides by organisms (e.g. fish) can be slow (Adam, 1997). Thus, the dynamics of radionuclide uptake/elimination by freshwater organisms can significantly modify the internal exposure, both in terms of concentrations and time scales. Assumptions used in Tier 1 become therefore questionable when conducting evaluation by Tiers 2 or 3. In such cases, it is justified to study the relevance of the “equilibrium” assumption for exposure assessment.
The objective of this paper is to study the effect of ‘time dependency’ when conducting risk assessment in freshwater systems submitted to routine releases from NPPs. For this purpose, two risk assessments were carried out in parallel: the first one followed the ERICA's Tier 1 method and then used equilibrium and conservative assumptions; the second one could be relevant for a Tier 2 approach, since it considered time-dependent and site-specific data. Such a comparison will be used to provide some guidance on how time dependency concepts can be incorporated in Tiers 2 and 3 of frameworks like ERICA.
It has to be noted that only external and internal exposures due to radionuclides present in the water column were considered in this paper and that exposure from contaminated sediments to benthic biota were not studied. In order to study the effect of time-dependent input data and transfer parameters and to compare time-dependent approaches with equilibrium ones, 60Co releases into the Loire River, where 14 NPPs operate, was chosen as a case study.
Section snippets
Site study and preliminary results
The Loire River is considered if not the last, at least as one of the last, wild large rivers in Europe. Several initiatives including local to international actions (MEDD, 2004a, MEDD, 2004b) recognize and establish the specific status of the Loire River and its valley, partially registered in the world patrimony (UNESCO, 2000). The biodiversity of the Loire fauna and flora is rare by its richness: 103 plant species of patrimonial interest and 107 animal species nationally protected. The Loire
Methods and models
The main differences between Tier 1 and Tier 2 (as proposed in this paper) are described in Table 2 for each stage of the exposure assessment. Methods used in this paper for implementing each of these tiers are described below.
Models for studying time dependence of input data (i.e. releases in the river)
For each station and each year, it was possible to compare the two approaches described in the previous section: (i) the ‘Tier 1’ approach, providing maximum annual concentrations in the river, based on maximum annual conditions; (ii) the ‘Tier 2’ approach, providing daily concentrations calculated by a time-dependent transport model and averaged over periods relevant for chronic effects. An example of such a comparison is provided in Fig. 3: the results obtained at Montjean for 60Co are
Conclusions
The case study conducted on the Loire River showed that some improvements can be proposed for assessing radiological risks on freshwater biota when passing from a Tier 1 to a Tier 2 approach. Indeed, it was observed that: (i) attention must be paid to the temporal variations in the releases and gaps between actual instantaneous releases and maximum releases on a yearly time scale must be analysed; (ii) the equilibrium assumption at the water-suspended matter interface must be justified and
Acknowledgment
This study was in part funded by the European project ERICA (http://www.erica-project.org)—contract FI6R-CT-2004-508847.
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