Influence of geology and soil particle size on the surface-area/volume activity ratio for natural radionuclides

The radioactive concentrations of 226Ra, 232Th, 235U and 40K in surface soil of the province of Caceres (Spain) were studied as a function of the geology and grain size. The activities of the four radionuclides in granitic and metamorphic soils have normal frequency distributions, with the mean value being significantly higher for the granitic soils than for the metamorphic soils. Sedimentary soils present asymmetric distributions, and their activities lie between the previous two types. The specific activities of the four radionuclides rises as the particle radius decreases. The equation Ae = (P1/R) + P2 describes the dependence of the specific activity Ae on radius R,P1 and P2 being parameters that are related to the surface area and volume activities of the grains. The isotopes 226Ra, 232Th and 235U accumulate with greater intensity on the surface of the grains than 40K. This effect is more pronounced in granitic and sedimentary soils than in metamorphic soils. (author) 18 refs.; 2 figs.; 4 tabs