Pedogenic fractionation and bioavailability of uranium and thorium in naturally radioactive spodosols

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 U-238 and Th-232. Additional replicate subsamples of fine (< 63 mum) soil material were also extracted with citrate-bicarbonate-dithionite (CBD), ammonium oxalate (AO), sodium pyrophosphate (PP), and Mehlich reagent (M3). Concentrations of U-238 and Th-232 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.