Influence of soil texture on the distribution and availability of 238U, 230Th, and 226Ra in soils

The influence of soil texture on the distribution and availability of 238U, 230Th, and 226Ra in soils was studied in soil samples collected at a rehabilitated uranium mine located in the Extremadura region in south-west Spain. The activity concentration (Bq kg−1) in the soils ranged from 60 to 750 for 238U, from 60 to 260 for 230Th, and from 70 to 330 for 226Ra. The radionuclide distribution was determined in three soil fractions: coarse sand (0.5–2 mm), medium-fine sand (0.067–0.5 mm), and silt and clay (<0.067 mm). The relative mobility of the natural radionuclides in the different fractions was studied by comparison of the activity ratios between radionuclides belonging to the same radioactive series. The lability of these radionuclides in each fraction was also studied through selective extraction from the soils using a one-step sequential extraction scheme. Significant correlations were found for 238U, 230Th, and 226Ra between the activity concentration per fraction and the total activity concentration in the bulk soil. Thus, from the determination of the activity concentration in the bulk soil, one could estimate the activity concentration in each fraction. Correlations were also found for 238U and 226Ra between the labile activity concentration in each fraction and the total activity concentration in bulk soil. Assuming that there is some particle-size fraction that predominates in the process of soil-to-plant transfer, the parameters obtained in this study should be used as correction factors for the transfer factors determined from the bulk soil in previous studies.