Pedologic Assessment of Radionuclide Distributions: Use of a Radio-Pedogenic Index
- C. V. Evans ,
- L. S. Morton and
- G. Harbottle
Naturally occurring radionuclides in soil profiles can provide a useful pedological tool, if their distribution can be calibrated to pedogenesis. As a first step toward such calibration, this study presents a radio-pedogenic index that compares differences between sola and parent material with respect to distributions and equilibrium relationships of 214Pb and 228Ac activity. The ratios 228Ac/214Pb were used to indicate concurrent secular equilibrium within the parent 232Th and 238U decay series. Among the 12 Typic Haplorthod pedons examined, equilibrium prevailed in C horizons, but not in A, E, Bs, and BC horizons. The C horizon levels of both 214Pb and 228Ac were significantly (P < 0.05) higher than those of E horizons, and considerably higher than those of Bs and BC horizons (P = 0.052 and 0.058, respectively). Nuclide deficiencies were not significantly correlated to podzolization indicators such as the optical density of the oxalate extract (ODOE), melanization, or rubification. Relationships of nuclide deficiencies to KCl pH were strong (r2 > 0.8, P < 0.0001), however. The 228Ac deficiencies were larger than 214Pb deficiencies and more strongly differentiated, particularly in E horizons. Profile distribution of 214Pb was relatively homogenous, compared with that of 228Ac. Analytical methodology ensured equilibrium between 228Ac and 228Ra and between 214Pb and 226Ra. Therefore, deficiencies can be interpreted as a pedogenically directed loss of Ra. Distribution differences, however, are probably due to radiogenic factors, such as half-life differences between the two Ra isotopes or chemistry of decay-chain precursors.
Copyright © .