ROOT DISTRIBUTION AND UPTAKE OF SURFACE-APPLIED RADIONUCLIDES BY MAIZE IN FIELD SOILS A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH

4 Abstract Risk assessment models for the soil-to-plant transfer of radionuclides rely on the assumption that soils are homogenously contaminated. However, field soils are characterized by a high degree of heterogeneities mainly caused by macro-structure which can affect the distribution of surface-applied radionuclides and their accessibility by plant roots. The main objective of this thesis was to evaluate the influence of the heterogeneous distribution of surface-applied radionuclides and their spatial relation with roots on the soilto-plant transfer of four radionuclides (Mn, Co, Zn and Cs). The analysis of the distribution of surface-applied radionuclides was carried out using dye tracers which allowed the visualization of the structure-induced water flow paths. Mn, Co, Zn, and Cs have been applied on the surface of two untilled agricultural soils located in Switzerland and maize (Zea mays L. cv. Corso) was used as model plant. Root distribution and the spatial interrelation with the preferential flow paths (PFP) have been carried out by in-situ mapping technique. We questioned the relevance for real field conditions of the standard experimental approach advised by the International Atomic Energy Agency to obtain data for the transfer factor parameter which is used in the mathematical model to quantify the soil-to-plant transfer of radionuclides. Standard experiments carried out in greenhouses with plants grown in sieved and homogeneously contaminated soils. We analyzed the effect of soil macro-structure on the recovery of Mn, Co, Zn and Cs in the aerial part of maize grown in an untilled agricultural soil in comparison to the recovery in the aerial part of maize grown in the greenhouse and on the same soil that was sieved and homogeneously labeled before being repacked in pots. We observed a heterogeneous distribution of the surface-applied radionuclides due to the structure-induced heterogeneous water flow, and a moderate (10 to 15%) occurrence of roots in these areas of radionuclides enrichment. A significantly higher recovery of Co (2-fold) and Cs (10-fold) was observed in the plants grown in the field soil, whereas no differences in the recovery of Mn and Zn between the two experiments was detected. Risk assessment models for the soil-to-plant transfer of radionuclides do not take into account the timedependent variations of radionuclides accessibility by roots occurring at the soil-root interface. We studied the redistribution of Mn, Zn, Co. and Cs in the soil profile and their uptake by maize grown on an untilled agricultural soil during a growing season. Surface-applied radionuclides were concentrated in the preferential flow paths in comparison to the soil matrix due to structure-induced non-uniform water flow. However, the amount and the seasonal distribution of the precipitation promoting the convective vertical displacement of radionuclides and their infiltration into the soil matrix caused an expansion of the PFP and a decrease of radionuclides concentration in these areas with time. Only a small fraction (15-20%) of the root system was located within the preferential flow paths. Results of the study of the time-dependent variation of the recovery of the surface applied radionuclides during an entire maize growth cycle varied