The assessment of soils contamination from U-238 and U-234 disequilibria data

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Understanding processes involved in the soils formation is important for appropriate assessment of contamination occurring in the superficial horizons. Several tools have been used for such purpose, including the natural radionuclides. Uranium is a lithophile element that is among the main elements contributing to natural terrestrial radioactivity, together with thorium and potassium (40K). The two natural U-isotopes 238U and 235U are precursors of the natural mass number 4n+2 and 4n+3 decay series, respectively. The third natural U-isotope, 234U, is radiogenic and generated in the 238U decay chain according to the sequence: 238U (4.46 Ga, α) → 234Th (24.1 days, β-) → 234Pa (1.18 min, α) → 234U (248 ka, α) →... 206Pb. If radioactive equilibrium occurs within the 238U decay series, all radionuclides activities are equal. In closed systems, not subjected to weathering processes, 234U reaches radioactive equilibrium with 238U and the 234U/238U activity ratio (AR) is unity in the bulk of such systems. 234U needs ~1.2 Ma to reach radioactive equilibrium with 238U in closed systems consisting of many minerals and rocks. Equilibrium and disequilibrium conditions affecting 238U and 234U have been extensively used by the researchers since the 1960s for evaluating the time scale of processes controlling the formation and evolution of soils and weathering profiles. This chapter summarizes a study case focusing the presence of these natural U-isotopes in two soil profiles developed over sandstones from the Paraná sedimentary basin in São Paulo State, Brazil. The reported database has been used in order to evaluate the timescale of the weathering processes taking place there and also for suggesting implications of potential contamination sources into the different soils horizons.



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Soil Contamination: Sources, Assessment and Remediation, p. 107-130.