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

Dep. of Natural Resources, Univ. of New Hampshire, Durham, NH 03824

G. Harbottle

Brookhaven National Lab., Upton, Long Island, NY 11973

*Corresponding author (cve{at}christa.unh.edu).

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 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.

Scientific Contribution no. 1950 from the New Hampshire Agric. Exp. Stn., Durham.

Received for publication October 14, 1996.

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