Determination of Soil Screening Levels for U and Th in Minas Gerais State, Brazil

Soil screening levels express the levels of contaminant concentration in the soil, which guide the actions to be taken following investigation to confirm contamination. The list of toxic substances or elements under Brazilian legislation includes organics, volatile organics and metals but does not consider radioactive elements. Radioactive elements are all potentially carcinogenic and therefore need to be subject to legal control. The National Nuclear Energy Commission, the federal agency currently responsible for legislation regarding the control of Naturally-Occurring Radioactive Material (NORM) facilities does not establish guiding values for intervention in terms of soil activity concentration in the case of contamination with radioactive elements. In mining, the processing and treating of ores such as cassiterite, uranium, phosphate, niobium, and rare earths contribute to the generation of large amounts of NORM residues. Obviously, the improper disposal of these materials may lead to situations that result in soil and groundwater contamination and unnecessary exposure of the population in general. In order to establish guiding values for soil quality for natural radionuclides in the state of Minas Gerais, the study area included the entire state, which has unique characteristics related to the lithology, genesis, and morphology of the soils. These characteristics have tremendous influence on the petrogeochemistry of elements and radionuclides. A total of 110 soil samples were collected and analyzed in order to determine the activity concentration of Unat and Th. In general, it was possible to verify that the activity concentrations of Unat are higher than those of Th. This fact can be explained by the intense weathering that most of the state's soil has undergone and the chemical and geochemical characteristics of the two elements. The values obtained up to the present are higher than the reference values for soil quality adopted in other parts of Brazil and the world. (author)