Abstract

Mass-spectrometric and alpha-spectrometric analyses for the isotopic composition of U238, U235, U234, Th232, and Th230 have been made on several soils and soil profiles in glacial-derived parent material, volcanic rock, and shale. Three deep profiles from Minnesota represent typical well-developed soils on till and loess of Wisconsin age continuously sampled to depths of 6, 7, and 10 feet, respectively. Results on individual soil horizons, in these three profiles, show similar trends in the variation of U234/U238 and Th230/U238 ratios, the predominant features being excess Th230 and deficient U234 compared to U238. The other soils and soil profiles have different variations which can be correlated by use of a model to describe uranium migration.

The data collected provided information for interpreting the causes of uranium migration in soils and for constructing a tentative model to explain the isotopic evolution of uranium and thorium in soil profiles. The proposed model indicates that (1) uranium was leached at depth in the profile, (2) preferential leaching of U234 was continuous in the soil, and (3) upward capillary migration of a fraction of the uranium with above-normal U234/U238 ratio tended to make uranium of high U234 ratio available for isotopic exchange in upper soil horizons and for assimilation in organic complexes in surface soils which are rich in organic matter. Thus some of the organic-rich surface soils, which have had considerable time to develop, contain uranium with excess U234 compared to U238. Interpretations from this model suggest that remixing of radioisotopes by geochemical processes contributes significantly, along with the usual physical processes of radioactive growth and decay, toward producing an isotopic composition near radioactive equilibrium.

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