Residence half-times of 129I in undisturbed surface soils based on measured soil concentration profiles.

Measured 129I concentration profiles in southeastern United States soil are examined by means of a six-compartment model to estimate accumulation and downward migration in undisturbed surface soil. Effective mean residence half-times in the top 30-cm-depth interval averaged 30 +/- 6 y, based on estimated annual 129I deposition rates in the vicinity of a chemical separations facility for nuclear material over a 25-y period. The model demonstrates the need for subdividing the soil into small depth intervals of 5 cm or less near the surface. The results of this study differ significantly with other surface soil residence time studies where a large single depth interval was assumed. The conclusions do, however, compare favorably with distribution coefficient studies in the surface hydrosphere. Application of the results are graphically demonstrated.