Uranium and thorium in weathering and pedogenetic profiles developed on granitic rocks from NW Spain.

Uranium and thorium were analyzed in seven weathering and pedogenetic soil profiles developed on granitic rocks from NW Spain. Concentrations were measured by X-ray fluorescence spectrometry (XRF) and the U- and Th-bearing minerals were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Both elements were determined in rock, bulk soil and in different grain-size fractions (sand: 2000-50 microm, silt: 50-2 microm, and clay: <2 microm). U concentrations in the rock varied between 5.3 and 27.7 mg kg(-1) and Th concentrations from 5.5 to 50.7 mg kg(-1). The most alkalic rocks can be considered as U-rich granites. Bulk soil U and Th concentrations are similar to those of the rocks (4.8-29.2 and 7.4-56.7 mg kg(-1), respectively), but in the grain-size fractions both elements show the lowest concentrations in the sand and the highest in the clay. In the latter, concentrations are always higher than those of the rocks, particularly in the C horizons with enrichments up to 4 times for U and 5 times for Th. The concentration profiles and the ratios to the parent rock suggest that U and Th are leached from the surface soil and accumulate in the deeper horizons. Mass balance calculations, using Ti as a reference immobile element, also support U and Th leaching in the solum and supergene enrichment in bottom horizons. Leaching seems to be more intense on horizons with gravel content higher than 20%. The leaching of U and Th in the topmost horizons and the accumulation in the bottom soil horizons can be considered as a natural attenuation of the impact of these radiogenic elements in the environment. But their enrichment in the potentially airborne fraction poses some risk of redistribution in the ecosystems.