Pedologic Assessment of Radionuclide Distributions: Use of a Radio‐Pedogenic Index

Abstract 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 214 Pb and 228 Ac activity. The ratios 228 Ac/ 214 Pb were used to indicate concurrent secular equilibrium within the parent 232 Th and 238 U 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 214 Pb and 228 Ac 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 ( r 2 > 0.8, P < 0.0001), however. The 228 Ac deficiencies were larger than 214 Pb deficiencies and more strongly differentiated, particularly in E horizons. Profile distribution of 214 Pb was relatively homogenous, compared with that of 228 Ac. Analytical methodology ensured equilibrium between 228 Ac and 228 Ra and between 214 Pb and 226 Ra. 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.