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DIVISION S-5—PEDOLOGY

Pedogenic Fractionation and Bioavailability of Uranium and Thorium in Naturally Radioactive Spodosols

^a Dep. of Plant Biology, Univ. of New Hampshire
^b Brookhaven National Lab

* Corresponding author (evansc{at}uwp.edu)

Plant uptake of radionuclides into the human food chain is one of many vectors used for calculating exposure rates and performing risk assessment. This study provides a pedogenic assessment of soil–plant relationships in terms of their relationships to nuclide bioavailability. The objectives of this study were (i) to identify associations of U and Th with pedogenic fractions and with soil properties, and (ii) to evaluate U and Th bioavailability in terms of soil processes. We collected composite samples of leaves, stems, AE and Bs horizons from 10 plots containing a Spodosol with naturally high levels of ²³⁸U and ²³²Th. Additional replicate subsamples of fine (<63 µm) soil material were also extracted with citrate–bicarbonate–dithionite (CBD), ammonium oxalate (AO), sodium pyrophosphate (PP), and Mehlich reagent (M3). Concentrations of ²³⁸U and ²³²Th were determined for soils and plants by neutron activation analysis or by gamma spectroscopy. Results indicated that Th was translocated preferentially to plant leaves while U showed little preferential translocation. The CBD extractant removed the most U and Th from soils. The PP extraction ranked second for U, and AO ranked second for Th removal. The M3 extraction was equally inefficient in the removal of both U and Th. Neither Th, nor Th uptake, was closely associated with organic fractions. In contrast, U distribution and plant uptake of U were more closely correlated to organically bound oxide fractions.

Abbreviations: AO, ammonium oxalate • CBD, citrate–bicarbonate–dithionite • CD, sodium citrate–dithionite • M3, Mehlich reagent • PP, sodium pyrophosphate