Weathering processes and dating of soil profiles from São Paulo State, Brazil, by U-isotopes disequilibria.

This study reports the use of the U-series radionuclides 238U and 234U for dating two soil profiles. The soil horizons developed over sandstones from Tatuí and Pirambóia formations at the Paraná sedimentary basin, São Paulo State, Brazil. Chemical data in conjunction with the 234U/238U activity ratios (AR's) of the soil horizons allowed investigating the U-isotopes mobility in the shallow oxidizing environment. Kaolinization and laterization processes are taking place in the profiles sampled, as they are especially common in regions characterized by a wet and dry tropical climate and a water table that is close to the surface. These processes are implied by inverse significant correlations between silica and iron in both soil profiles. Iron oxides were also very important to retain uranium in the two sites investigated, helping on the understanding of the weathering processes acting there. 238U and its progeny 234U permitted evaluating the processes of physical and chemical alteration, allowing the suggestion of a possible timescale corresponding to the Middle Pleistocene for the development of the more superficial soil horizons.

Distribution of 226Ra, 232Th and 40K in soils and sugar cane crops at Corumbataí river basin, São Paulo State, Brazil.

The common use of phosphate fertilizers NPK and amendments in sugar cane crops in Brazilian agriculture may increase the (226)Ra, (232)Th and (40)K activity concentrations in soils and their availability for plants and human food chain. Thus, the main aim of this study was to evaluate the distribution of (226)Ra, (232)Th and (40)K in soils and sugar cane crops in the Corumbataí river basin, São Paulo State, Brazil. The gamma spectrometry was utilized to measure the (226)Ra, (232)Th and (40)K activity concentration in all samples. The soil-to-sugar cane transfer factors (TF) were quantified using the ratio between the radionuclide activity concentration in sugar cane and its activity concentration in soil. The results show that, although radionuclides incorporated in phosphate fertilizers and amendments are annually added in the sugar cane crops, if utilized in accordance with the recommended rates, their use does not lead to hazards levels in soils. The soil-to-sugar cane transfer of radionuclides occurred in the following order (40)K>(226)Ra>(232)Th. Therefore, under these conditions, radionuclides intake through consumption of sugar is not hazardous to human health.