A validation test of a model for long-term retention of (129)I in surface soils

A linear compartment model for global transport of iodine that we previously developed predicted that the mean residence time of iodine in the first 1 m of surface soil is about 4,000 y. An independent test of the model prediction is provided by measured depth profiles of (129)I in soil following atmospheric releases from the Savannah River Plant (SRP) in South Carolina and from the Karlsruhe fuel reprocessing plant in Germany. Previous analyses of these data using a linear compartment model for downward transport through soil indicated that the mean residence time in the first 0.3 m is about 40 y at both locations, which suggests that removal of (129)I from surface soil may be considerably more rapid than predicted by the global transport model. In this paper, a diffusion model is used to describe the measured soil profiles of (129)I at Savannah River. The diffusion coefficient obtained from the analysis corresponds to a mean residence time in the first 1 m of surface soil that agrees semi-quantitatively with the prediction of the global model when the concentration of naturally occurring stable iodine in soil and the flux of iodine from the atmosphere onto surface soil at Savannah River, as they differ from globally averaged values, are considered. This paper also discusses (1) the importance of the mean residence time of (129)I in surface soil for estimates of dose to individuals from near-surface land disposal of low-level radioactive wastes and (2) unresolved issues regarding global cycling of iodine.