ANOMALIAS GEOQUÍMICAS DE U E DE TH, SOLOS RESIDUAIS E GEOFORMAS EM PAISAGENS TROPICAIS ÚMIDAS SOBRE GRANITO

Resumo

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.