An examination of the environmental half-time for radionuclides deposited on vegetation.

After radionuclides are deposited on vegetation, environmental removal processes combine with radioactive decay to reduce the quantity of initial contamination. The time in which one-half of the radioactivity is removed from vegetation by environmental processes alone is referred to as the environmental half-time, Tw. For long-lived radionuclides, the dose to man via ingestion of contaminated terrestrial foods may be directly influenced by values of Tw, provided that environmental removal processes dominate root uptake and time is sufficiently long between initial exposure of vegetation and harvest. Values of Tw reported in the literature for various radionuclides and methods of deposition are examined. Factors affecting the variability of Tw are related in part to the physicochemical form of the depositing substance, vegetation type and growth form, climate, season, and experimental procedure. For growing vegetation, values of Tw are generally lower than those reported for dormant vegetation. Values of Tw for iodine vapor and iodine particulate are less than values of Tw reported for particulates of other elements. Values of Tw determined on a per unit vegetation mass basis are also less than values of Tw determined on a per unit ground area basis, the differences being attributable to the effects of growth dilution. Although an assessor has a range from which to select a value of Tw for the purpose of estimating contamination by long-lived radionuclides, the variability in Tw is small in comparison to many other parameters used in environmental radiological assessments.