Environmental behavior of technetium in soil and vegetation : implications for radiological impact assessment

Author

Frank Owen Hoffman

Date of Award

2-1981

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major Professor

Edward E. C. Clebsch

Committee Members

J. Frank McCormick, Robert Bohm, John P. Witherspoon

Abstract

With the exception of recent laboratory experiments on the relationship of technetium in soil and vegetation, data on the environmental behavior of technetium are limited. Significant radiological exposures have been estimated for hypothetical atmospheric releases of Tc-99 from gaseous diffusion facilities when vegetation-to-soil concentration ratios representative of laboratory experiments are substituted for generic default values assumed in current regulatory models. To test the relevancy of these laboratory ratios, field investigations were conducted to obtain measurements of the vegetation-to-soil concentration ratio for Tc-99 in samples collected near operating gaseous diffusion facilities and to observe the dynamic behavior of technetium in soil and vegetation 95m following a single application of a sprayed solution of ^95mTc0^-/₄.

Comparison of observed field concentration ratios and calculated steady-state concentration ratios with ratios obtained from previous laboratory experiments indicates that concentration ratios obtained from field data are one to two orders of magnitude less than those obtained from the laboratory. Furthermore, a substantial accumulation of technetium in soil and vegetation may not occur over long periods of time, since concentrations of technetium in both environmental media were observed to decrease with time subsequent to initial application of ^95mTc0^-/₄. The decrease of technetium concentrations in vegetation is suspected to be the result of leaching by precipitation, particle removal by wind action and dilution by tissue growth.

Although considerable conservatism may exist in other parameters used in radiological assessment models, the difference between field and laboratory concentration ratios would account for a reduction in the predicted dose of at least one order of magnitude over that calculated using a value of the vegetation-to-soil concentration ratio representative of laboratory experiments. Consideration of the dynamic behavior of technetium in soil and vegetation would reduce the predicted dose by more than two orders of magnitude. The possibility is acknowledged that if the technetium in soil is in a chemical form of relatively low mobility, accumulation over long time periods may occur. The amounts of technetium observed in vegetation sampled near the gaseous diffusion facilities could result from root uptake of a residual quantity of otherwise immobile technetium that has become soluble in soil water. The relationship between the Tc-99 in vegetation and the soluble Tc-99 in soil water is similar to the high laboratory vegetation-to-soil concentration ratios that have all been obtained with technetium persisting as a pertechnetate solution in soil.

Measured vegetation concentrations of Tc-99 in samples obtained near the gaseous diffusion facilities are, nevertheless, substantially less than concentrations calculated using laboratory derived vegetation-to-soil concentration ratios. Extrapolation of the results obtained from the present field experiments to assessments of the radiological impact for other locations, vegetation types, and time periods must be made with due caution until the general applicability of these results can be confirmed through future validation experiments and environmental monitoring programs.

Recommended Citation

Hoffman, Frank Owen, "Environmental behavior of technetium in soil and vegetation : implications for radiological impact assessment. " PhD diss., University of Tennessee, 1981.
https://trace.tennessee.edu/utk_graddiss/13443