Pedogeochemical models on prognosis and control of radon fluxes from humid tropical landscapes over granite

The principal relationships between radon (²²²Rn and ²²⁰Rn) exhalation intensities and the morpho-dynamic elements of humid tropical landscapes have been established and analyzed with the objective of elaborating Rn exhalation models, which can be used on the scale of small habitational nuclei. The current radioactive processes of generation, emanation and migration of Rn, in relation to its exhalation rates on a landscape slope of granitic rock having normal U and Th contents, were correlated with the latosol-podzolic soil association developed as a product of supergene processes during geological and pedological times and also with the water regime of an overlying phreatic aquifer. This approach, encompassing pedogeochemical models of radionuclide dispersion linked to soil systems and surface geochemistry, was framed within a tridimensional, interdisciplinary and systemic focus, using concepts of nuclear physics, climate and hydrogeology. The characteristic signatures of elementary landscapes include (1) different landforms, namely eluvial, transeluvial and superaqual, (2) residual soils having varying types, thicknesses and perma-porosities and (3) radioanomalies corresponding to different U and Th concentrations, secondary hosting minerals and also to different pedological ages. Considering these factors and their spatial relationships and on the basis of moisture content of soils, it is postulated that the highest Rn exhalation rates, especially of ²²²Rn, are confined to superaqual and transeluvial landscapes. In an eluvial landscape, the Rn fluxes are found to be less important even though gamma anomalies exist mainly due to the presence of resistant minerals containing U and Th. In short, the products of supergene alteration organized in a sloping system can exhibit Rn exhaling intensities higher than that measured on unaltered subjacent rock. Hence, the understanding of U, Th, Ra and Rn behavior on a surface environment is fundamental for the formulation of prognostic Rn exhalation models associated with the elementary humid tropical landscapes. Further, the pedogeochemical information provides important clues supplementing the lithostructural data for delineating preferential sites of Rn fluxes especially on scales compatible to small habitational nuclei or unitary dwellings.