An Examination of the Environmental Half-time for Radionuclides Deposited on Vegetation

Publisher Website
Google Scholar
Abstract
After radionuclides were deposited on vegetation, environmental removal processes combined with radioactive decay to reduce the quantity of initial contamination. The time in which one-half of the radioactivity was removed from vegetation by environmental processes alone was referred to as the environmental half-time, Tw. For long-lived radionuclides, the dose to man via ingestion of contaminated terrestrial foods may have been directly influenced by values of Tw, provided that environmental removal processes dominated root uptake and time was sufficiently long between initial exposure of vegetation and harvest. Values of Tw reported in the literature for various radionuclides and methods of deposition were examined. Factors affecting the variability of Tw were related to the physicochemical form of the depositing substance, vegetation type and growth form, climate, season and experimental procedure. For growing vegetation, values of Tw were generally lower than those reported for dormant vegetation. Values of Tw for I vapor and I particulate were less than values of Tw reported for particulates of other elements. Values of Tw determined on a per unit vegetation mass basis were 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 had a range from which to select a value of Tw for the purpose of estimating contamination by long-lived radionuclides, the variability in Tw was small in comparison to many other parameters used in environmental radiological assessments.