ANOMALIAS GEOQUÍMICAS DE U E DE TH, SOLOS RESIDUAIS E GEOFORMAS EM PAISAGENS TROPICAIS ÚMIDAS SOBRE GRANITO
Authors: Rebelo Antonio M. A. , Bittencourt André V. L. , Mantovani Luiz E. 
Published: December 2014

O comportamento do U e do Th foi analisado durante a evolução pedogeoquímica e morfogenética de paisagens tropicais úmidas, sobre rocha granítica com teores normais desses elementos, visando estabelecer relações entre rocha-mãe e anomalias de U e de Th, tipos de solos residuais e geoformas resultantes dessa evolução. Em uma vertente caracterizada pelo sistema de transformação Latossolo-Solo Podzólico, anomalias de U e de Th de grande expressão geográfica e intensidade ocorrem na paisagem eluvial, parte superior da vertente onde se verificam declividades entre 0 e 8%. Tais radioanomalias são de natureza relativa/residual predominante e com baixa proporção de U adsorvido, e situam-se no topo do horizonte B dos Latossolos. As anomalias radioativas mais intensas, porém de menor expressão geográfica, localizam-se na paisagem transeluvial, parte mediana das vertentes com declividades entre 8 e 45%. São em parte residuais e em parte absolutas, resultantes da podzolização dos Latossolos, e associam-se ao topo do horizonte B textural dos Solos Podzólicos. Radioanomalias de menor intensidade, e essencialmente uraníferas, situam-se na paisagem supraqual, parte inferior e com baixas declividades da vertente. São de caráter absoluto, com o U fixado principalmente por adsorção às fases argilo-orgânica e óxido dos horizontes A0 e A1 dos Solos Hidromórficos. Nessa paisagem, em áreas de influência da atual rede de drenagem, pode existir desequilíbrio radioativo entre os radionuclídeos da série do 238 U, com reflexos negativos nas intensidades das radiações gama emitidas. Isto se deve à pouca idade de deposição do U nesses locais durante a podzolização, entre 100.000 e 500.000 anos. Então, no contexto enfocado, a transformação de rochas com teores normais em U e em Th pode resultar em materiais secundários, com concentrações nesses radionuclídeos mais elevadas que as existentes nas rochas inalteradas. A atuação dos processos geoquímicos responsáveis pela redistribuição do U e do Th ocorreu concorrentemente com a dos processos pedogenéticos, morfogenéticos e hidrogeológicos, durante a evolução das paisagens. Os principais tipos de anomalias de U e de Th, paisagens geoquímicas elementares, tipos e horizontes de solos residuais e geoformas estão cronológica e espacialmente relacionados. GEOCHEMICAL ANOMALIES OF U AND TH, RESIDUAL SOILS AND GEOFORMS IN HUMID TROPICAL LANDSCAPES OVER GRANITE Extended Abstract The geochemical behavior of U and Th during the evolution of humid tropical landscapes was analysed, with the objective to establishing relationships between parent rock, and the resulting U and Th anomalies, types of residual soils and geoforms. In one slope over granite with normal contents of U and Th, and characterized by the soil association Latosol-Podzolic, the pedologic horizons providers and receivers of these radionuclides during the processes of latolization and podzolization were evaluated. Studies were carried out on the main characteristics of the radioanomalies that took place in each landscape, mainly the concentration levels and the secondary hosting phases of U and Th, and their impact on the ratio Th/U and Us/Ut (soluble uranium/total uranium), and on their relative gammaradiometric intensities. Multi and interdisciplinary studies were done, making use of the various data and concepts from worldwide scientific literature, specifically in the fields of geochemistry and pedology. In the first case, emphasis was placed on the characteristics of the primary concentrations of U and Th (table 1), and on the geochemical concepts of element abundance, gradients, migration, flows and geochemical barriers, and of elementary landscape geochemistry (figure 1). In the field of pedology, the concept of transformational pedologic system and the cronology of the main pedogenetic phases were addressed. Moreover, concepts and elements of the areas of geomorphology, hydrogeology of the phreatic aquifer system and the nuclear physics of the series of 238 U and 232 Th were used and approached from a tridimensional and systemic perspective. The analyses were carried out in short space-temporal limits, according to the dimensions of the slopes and, regarding time specifics, the pedologic scale was found to be the most relevant. In the context studied, the Latosols occur in the eluvial landscape, associated to older, more stable and higher surfaces of the slope, where declivities between 0 and 8% can be observed. In this landscape, the geochemical environment is oxidizing and the pH is acid. The main concentrations of U and Th are due to resistant minerals containing U and Th, insoluble compounds of Th, and U and Th fixed to Fe and Al oxyhydroxides and clays by adsorption. These anomalies are characterized by low Us/Ut and high Th/U ratios, and intense gamma countings (table 2 and 3). They are relative and residual in nature, and are located on the top of latosolic B horizon (figure 2). The Podzolic Soils are of lesser geographic occurrence than the Latosols. Found in the transeluvial landscape, they are related to younger surfaces in intermediate altitudes of the slope, where the declivities are between 8 and 45 %. In this landscape, a strong control of the local parameters of the environment over the configuration and spacial distribution of the soil profiles and their physical properties has been observed. The geochemical environment is oxidizing and the pH is acid on the superficial horizons, changing to reducing toward the B textural horizon (Bt), where the pH levels are higher. The main concentrations of U and Th, being the result of the reworking of those associated with the Latosols, by podzolization, and located on the top of horizon Bt (figure 2 and table 4), present higher levels and higher proportions of soluble U in relation to the total contents. These anomalies show higher Us/Ut and lower Th/U ratios compared to the same ratios in the Latosols and more intense gamma manifestations (table 2). They are partially relative and partially absolute, depending on the migration of radionuclides, inclusively in colloidal solution, due to the eluviation-illuviation processes which take place during podzolization. The Hydromorphic Soils predominate in superaqual landscape, where the aeration zone of the aquifer is either thinner or inexistent, and the water-table outcrops or is present at a shallow depth. The geochemical environment works as a reducing agent due to the frequent saturation of the soils and the presence of organic matter (OM), and the pH , in general, ranges from neutral to alkaline. Besides OM, clay phases and Fe compounds with a high cationic exchange capacity are frequently encountered, making up favorable factors for the fixation by adsorption of U originated from eluvial and transeluvial landscapes during latolization and podzolization. The main concentrations are uraniferous and absolute, associated to horizons A0 and A1 of the soils (figure 2), resulting in high ratios Us/Ut, and decreases in Th/U. The gamma radiation emissions are moderate in intensity (table 2). In this landscape, radioactive disequilibrium in the decay series of 238 U, with a negative impact for the gamma radiation intensity, may occur in places near the drainage system. This happens due to the young age of 238 U deposition in these areas, mainly during the podzolization of the Latosols, between 100,000 and 500,000 years, insufficient time for the re-establishment of the secular radioactive equilibrium in the referred series. Hence, the transformation by the action of supergenic processes, mainly the pedogeochemical ones, of rocks with normal contents of U and Th, may result in materials with different distributions and concentrations of these radionuclides, considerably higher than the ones in unaffected rocks (table 3). Even the intensively leached soils, such as the ones of the association Latosol-Podzolic, present a relative increase in the U and Th values, due to the concentration of resistant minerals and their association with oxide, clay and carbon phases of the soils. U and Th distribution and levels of concentration reveal the nature of fixation of these radionuclides in their original hosting phases and their real mobilities in the different geochemical landscapes along the slope. In conclusion, in the present study, the landscapes which show the highest probability of containing high anomalies of U and Th are the transeluvial landscape, on the top of the textural B horizon of the Podzolic Soils, and the eluvial, on the top of the latosolic B horizon of the Latosols. In the superaqual landscape, the anomalies are uraniferous and associated with horizons A0 and A1 of the Hydromorphic Soils (figure 2 and table 2). Consequently, in the referred climatic environment, the action of the geochemical processes which lead to the re-distribution of U and Th, took place concomitantly with the pedogenetic, morphogenetic and hydrogeological ones, throughout the evolution of the landscapes. The main types of U and Th anomalies, types and horizons of the residual soils, elementary geochemical landscapes and geoforms are cronologically and spatially related.