BIOPROTA: Key issues in biosphere aspects of assessment of the long-term impact of contaminant releases associated with radioactive waste management. Theme 2 Task 1: Model review and comparison for spray irrigation pathway

The objective of Task 1 within Theme 2 was to investigate the modelling of the concentrations of radionuclides on vegetation arising from interception by growing crops using contaminated irrigation water and the consequent contamination of the food consumed by humans. This Task 1 report includes model descriptions provided by participants and the specification and results of model test calculations designed to investigate the significance of the different model assumptions. A significant array of modelling methods has been presented and explained. The equations used and the data adopted have also been presented. This compilation may be useful for future reference. Although the methods vary, most assessment results are within a factor of ten of each other. This is not a large range given the generic nature of the assessment question asked. This is generally due to compensating effects. For example, a high initial assumed retention factor is compensated for when weathering and translocation are then taken into account. That is to say, while different modelling approaches are taken, each conceptual model is internally consistent. Overall, the results lend considerable confidence to the assessment community's ability to assess doses as a result of food contamination due to the direct effects of irrigation. The biggest reasons for discrepancies in results are associated with the treatment of weathering and translocation, post deposition. Values for translocation vary considerably between the participants and there are different interpretations of weathering and translocation data. Another significant reason for differences in models and in results concerns the different irrigation practices used in different areas. These are largely climate driven. Overall it appears that relatively simple models provide very similar results to the more process orientated and data demanding models. Further consideration of the adequacy of the data for assessment purposes will depend on the relative significance of the pathway for contamination considered here with results for the effects of long term accumulation in soil, and exposures due to: root uptake; inhalation of suspended activity; and external irradiation. The first two of these three have been considered in Tasks 2 and 4 of Theme 2 of BIOPROTA, within which correspondingly similar model descriptions and data compilations have been prepared, as well as assessment results based on similar system descriptions. External irradiation is a relatively simple exposure pathway to address, once the activity level in the soil has been evaluated