Hydrologic and geochemical controls on the transport of radionuclides in natural undisturbed arid environments as determined by accelerator mass spectrometry measurements [electronic resource].

Published: Washington, D.C. : United States. Dept. of Energy, 2000.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
Physical Description: 38p : digital, PDF file
Additional Creators: Lawrence Berkeley National Laboratory, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information

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Summary

This project developed techniques for measuring globally distributed radionuclides that occur today in extremely low abundances (''fallout'' from the era of atmospheric nuclear testing), and then applied these techniques to better understand the mechanisms by which radionuclides migrate. The techniques employ accelerator mass spectrometry (AMS), a relatively new analytical tool that permits this work to be conducted for the first time. The goal in this project was to develop AMS analytical techniques for ¹²⁹I (fallout concentration: ≈ 10⁶ atoms/g) ⁹⁹Tc (≈ 10⁹ atoms/g), ⁹°Sr (≈10⁷ atoms/gram soil), and ⁹³Zr (≈ 10⁹ atoms/g), and improved methods for ³⁶Cl (≈ 10⁹ atoms/g). As a demonstration of the analytical techniques, and as an investigation of identified problems associated with characterizing moisture and radionuclide movement in unsaturated desert soils, we developed a vadose zone research site at the Nevada Test Site. Our findings can be summarized as follows: (1) The distribution of chloride and ³⁶Cl at the research site indicates that the widely-used ''chloride accumulation'' method for estimating moisture flux is erroneous; some mechanism for attenuation of chloride exists, violating an assumption of the accumulation method; (2) ¹²⁹I is fractionated into several soil compartments that have varying migration abilities; the two most mobile can be tentatively identified as Fe/Mn oxyhydroxides and organic acids based on our sequential leaching techniques; (3) These most mobile constituents are capable of migrating at a rate greater than that of ³⁶Cl, usually considered the most mobile solute in hydrologic systems; these constituents may be colloidal in character, of neutral surface charge, and therefore conservative in aqueous migration; (4) ⁹⁹Tc is readily measurable by AMS, as we demonstrate by the first AMS ⁹⁹Tc measurements of contaminated waters; extraction of ⁹⁹Tc from silicate soils is difficult, but can be done using the extended methods we have developed; (5 ) Most of the bomb-pulse ⁹⁹Tc observed in the soil profile at the EMSP research site was confined to the upper 50 cm; however, there is some indication that a fraction of the ⁹⁹Tc could move to the same extent as bomb-pulse ³⁶Cl; and (6) The observed AMS sensitivity for ⁹°Sr was ⁹°Sr/Sr=10⁻¹², corresponding to about 20-200 mBq per sample, similar to the sensitivity of traditional decay counting; further work should improve the sensitivity by at least a factor of 10, but even now the AMS ability for sensitive, high throughput, fast turnaround, robust measurements using simple chemistry is an improvement over decay counting methods. Although our results regarding the migration of these radionuclides in arid soils increase the scientific understand of contaminant migration in that environment, we believe the primary benefit of our EMSP research is the development of AMS analytical techniques for low-level radionuclide measurement. The advantages of this include: (1) the ability to conduct migration studies in locations most like those of concern to public health, e.g., a ''far-field'' environment; (2) sites of multiple contamination, e.g., by VOC's, can be avoided; (3) it becomes unnecessary to collect research samples that are themselves radioactive waste and are therefore difficult to handle and dispose of in the laboratory; and (4) since the nuclides are globally distributed, migration research can be conducted in any chosen environment. Our research is therefore designed to make the study of radionuclide migration simpler to do, simpler to interpret, and easier to apply to specific DOE sites. This will lead to a better understanding of the migration potential and ultimate fate of radionuclides, creating much less uncertainty regarding which remediative strategies should be employed. For any project, this will reduce its cost, shorten its schedule, and provide greater confidence in its final result, both for the DOE and for the general public. We feel this is...

Report Numbers

E 1.99:ucrl-id-133321
ucrl-id-133321

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Note

Published through SciTech Connect.
04/01/2000.
"ucrl-id-133321"
McAninch, J; Nimz, G; Caffee, M W.

Funding Information

W-7405-ENG-48

View MARC record | catkey: 14138373

Actions for Hydrologic and geochemical controls on the transport of radionuclides in natural undisturbed arid environments as determined by accelerator mass spectrometry measurements [electronic resource].

Hydrologic and geochemical controls on the transport of radionuclides in natural undisturbed arid environments as determined by accelerator mass spectrometry measurements [electronic resource].

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