Natural radioactivity of some red Mediterranean soils

Seventeen samples of B-horizon of Pleistocene Terra Rossa soils from carbonate rock were collected in different Countries of the Mediterranean regions (Spain, Italy and Southern Turkey).The concentration of naturally occurring radionuclides (238U, 232Th, Knat) was measured in all samples using gamma-ray spectrometry at the Gran Sasso National Laboratory of INFN (Italy). The uranium content in Terra Rossa paleosols ranges from 1 to 5 ppm, for thorium the concentration ranges from 3 ppm up to 30 ppm, whereas for potassium it goes from 0.13% up to 1.3. The results indicated that soils characterised by absence or scarcity of 2:1 clay minerals are poor in uranium whereas soils with illite–smectite as the dominant minerals in clay fraction are noticeably richer. Then it is suggested that uranium occurring in traces in the calcite lattice of carbonate rocks, after it is the released by weathering, migrates in solution as UO22+ and can be adsorbed by the soil exchange complex, mainly by clay minerals of Terra Rossa soils.