Approaches to providing missing transfer parameter values in the ERICA Tool: how well do they work?

A required parameter for the ERICA Tool is the concentration ratio (CR), which is used to describe the transfer from environmental media to a range of organisms. For the original parameterisation of the ERICA Tool, 60% of these values were derived using a variety of extrapolation approaches, including the application of allometric models, the use of values for a similar organism or element with similar biogeochemical behaviour and the use of values from a different ecosystem. Although similar approaches are applied in other assessment systems, there has been little attempt to see how well these approaches perform. In this paper, CR values in the ERICA Tool derived using extrapolation approaches are compared to more recently available empirical data from the IAEA wildlife transfer database. The primary purpose of the default CR database in the ERICA Tool, and other models, is to enable the user to conduct conservative screening assessments. Conservatism was therefore introduced to the analyses by selecting the 95th percentile CR values for subsequent calculations. The extrapolation methodologies are not guaranteed to provide conservative estimates of empirical 95th percentile CRs. For the terrestrial ecosystem, the extrapolation methods provide underpredictions of empirical 95th percentiles as often as they produce overpredictions. In a few cases the underestimation of CR values, when considering all ecosystems, is substantial - by orders of magnitude - which is clearly unacceptable for a screening assessment. Thus, although extrapolation approaches will remain an essential component of screening assessments in the future, because data gaps will always be present, diligence is important in their application. Finally, by synthesizing the results from the current analyses and through other considerations, some recommendations are provided with regards to modifying the original guidance on use of extrapolation approaches in the ERICA Tool.