Potential sources affecting the activity concentrations of 238U, 235U, 232Th and some decay products in lettuce and wheat samples

Abstract

The activity concentrations of radionuclides within the uranium and thorium series were determined in wheat and lettuce at five sites in France, and in their respective potential sources: crop soils of wheat and crop soils and irrigation waters of lettuce. These data were used to calculate concentration ratios and to enrich the database supported by the technical report series N°472 of the IAEA (2010). For wheat and lettuce, the activity concentrations were in the same range for all radionuclides studied, except for ²¹⁰Pb, which had higher activity concentrations in wheat, ranging between 1.3 and 11 Bq kg⁻¹ (fresh weight) as compared to 0.4 and 0.7 Bq kg⁻¹ (fresh weight) for lettuce. For wheat, the range of activity concentrations (mBq kg⁻¹; fresh weight) decreased as ²¹⁰Pb > ²²⁶Ra (56–1511) ≈ ²²⁸Ra (86–769) > ²²⁸Th (19–176) ≈ ²³⁸U (11–169) ≈ ²³⁴U (12–150) ≈ ²³⁰Th (9.08–197.18) ≈ ²³²Th (8.61–121.45) > ²³⁵U (0.53–7.9). For lettuce, it decreased as ²²⁸Ra (<320–1221) > ²¹⁰Pb (409–746) > ²²⁶Ra (30–599) ≈ ²²⁸Th (<29–347) > ²³⁸U (8–120) ≈ ²³⁴U (8–121) ≈ ²³⁰Th (5.21–134.63) ≈ ²³²Th (5.25–156.99) > ²³⁵U (0.35–5.63). The species differences may reflect different plant physiologies. Through the study of activity ratios of wheat and lettuce in relation with those of the various radionuclide sources it has been possible to highlight the contribution of the main sources of natural radionuclides. Indeed, irrigation water when the uranium concentration is enhanced (>30 mBq L⁻¹) contributed significantly to the activity concentration of uranium in lettuces. Concerning the high activity concentrations of ²¹⁰Pb, it could be explained by atmospheric particle deposition. The effect of soil particles resuspension and their adhesion to the plant surface seemed to be important in some cases. The soil-to-plant transfer factors were calculated for lettuce and wheat. The values were lower in wheat than in lettuce except for ²¹⁰Pb which had similar values in the two species (0.11–0.13 respectively). For both species, ²¹⁰Pb followed by ²²⁸Ra (0.015–0.10) and ²²⁶Ra (0.010–0.051) displayed the highest transfer factor, whereas ²³⁸U had intermediate values (0.0015–0.030) and ²³²Th exhibited the lowest (0.0014–0.013).