D9.15 — An evaluation of process-based models and their application in food chain assessments

Vegetation interception and the time of year will largely determine the contamination of foodstuffs immediately following an accidental release. During the transition phase, uptake of radionuclides by vegetation from soil will increase in importance and root uptake will dominate during the longterm rehabilitation phase. Predictions made using radioecological models will be used to make longterm decisions, e.g., with regard to remediation strategies. Models must be sufficiently robust and fit for purpose with uncertainties reduced where practicable. Most radioecological models use empirical transfer factors to estimate soil-to-plant transfer of radionuclides and these do not credibly cope with variation in root uptake caused by variation in soil properties. Consequently, process-based soil-to-plant models were developed to predict radionuclide (predominantly radiocaesium) transfer based upon relatively readily available soil properties. The model originally developed by Absalom et al. (1999) and further improved by Absalom et al. (2001) and Tarsitano et al. (2011) is a typical example of a process-based transfer model. It has been applied to predict radiocaesium transfer under a range of environmental conditions with varying degrees of success. The objective of the CONFIDENCE project’s Work Package 3 (WP3) was to improve the capabilities of radioecological models used to predict activity concentrations in foodstuffs and to better characterise, and where possible, reduce uncertainties. The focus of this deliverable is to consider the use of process based models for post-accident predictions. We begin with assessing the applicability of the ‘Absalom model’ to a range of European soil and plant types that were not included in its initial parameterization. We also demonstrate how the model can be incorporated into the Food Chain and Dose Module for Terrestrial Pathways (FDMT), which is part of the European decision support system, JRODOS. To date, most consideration in the development of process-based transfer models has been focused on radiocaesium. In this deliverable we develop process-based soil-plant transfer models for radiostrontium. Finally, the deliverable reports on end-users views of process-based models. Our assessment of the Absalom model shows that it is a useful tool for predicting radiocaesium transfer to the human food chain. Its predictions for grass and radish (edible root) were mostly within an order of magnitude of the measurements for most of the study soils. We recommend expanding the model database by considering more soils (with different mineralogies) and plant types in its parameterisation. The Absalom model has successfully been incorporated into the FDMT. We have successfully established two process-based models to predict strontium concentrations in a range of crops using relatively few soil parameters and the calcium concentration in crops as inputs. To support these methodologies we have produced a collation of calcium concentrations in crops consumed by humans and farm animals (Chaplow et al., submitted). The approach removes the need for empirical concentration ratios and is able to make predictions for crop types for which no radioecological data exist. Whilst the approaches developed for strontium produced predictions that compared well with measured data, and better than predictions using the commonly used concentration ratio approach, they require further testing against a wider range of soil types and crops.