Processes affecting land-surface dynamics of ¹²⁹I impacted by atmospheric ¹²⁹I releases from a spent nuclear fuel...

Terrestrial environments impacted by atmospheric releases of ¹²⁹I from nuclear plants become contaminated with ¹²⁹I; however, the relative importance of each land-surface ¹²⁹I-transfer pathway in the process of the contamination is not well understood. In this study, transfers of ¹²⁹I in an atmosphere-vegetation-soil system are modeled and incorporated into an existing land-surface model (SOLVEG-II). The model was also applied to the observed transfer of ¹²⁹I at a vegetated field impacted by atmospheric releases of ¹²⁹I (as gaseous I₂ and CH₃I) from the Rokkasho reprocessing plant, Japan, during 2007. Results from the model calculation and inter-comparison of the results with the measured environmental samples provide insights into the relative importance of each ¹²⁹I-transfer pathway in the processes of ¹²⁹I contamination of leaves and soil. The model calculation revealed that contamination of leaves of wild bamboo grasses was mostly caused by foliar adsorption of inorganic ¹²⁹I (81%) following wet deposition of ¹²⁹I. In contrast, accumulation of ¹²⁹I in the leaf due to foliar uptake of atmospheric ¹²⁹I₂ (2%) was lesser. Root uptake of soil ¹²⁹I was low, accounted for 17% of the ¹²⁹I of the leaf. The low root-uptake of ¹²⁹I in spite of the ¹²⁹I contained in the soil was ascribed to the fact that the most fraction (over 90%) of the soil ¹²⁹I existed in "soil-fixed" (not plant-available) form. Regarding the ¹²⁹I-transfer to the soil, wet deposition of ¹²⁹I was ten-fold more effective than dry deposition of atmospheric ¹²⁹I₂; however, the deposition of ¹²⁹I during the year represented only 2% of the model-assumed ¹²⁹I that pre-existed in the soil; indicating the importance of long-term accumulation of ¹²⁹I in terrestrial environments. The model calculation also revealed that root uptake of inorganic ¹²⁹I can be more influential than volatilization by methylation in exportation of ¹²⁹I from soil.