13 – AFINIDADES GEOQUÍMICAS ENTRE SEDIMENTOS (SOLOS ALUVIAIS) E VEGETAÇÃO (GRAMÍNEAS E CULTIVARES), ALÉM DE CABELO DE RIBEIRINHOS, AO LONGO DAS PRAIAS DE RIOS DA BACIA DO JURUÁ, NO ESTADO DO ACRE E SUA IMPORTÂNCIA AMBIENTAL
Ano 12 (2025) – Número 1 – Acre 2 Artigos
AFINIDADES GEOQUÍMICAS ENTRE SEDIMENTOS (SOLOS ALUVIAIS) E VEGETAÇÃO (GRAMÍNEAS E CULTIVARES), ALÉM DE CABELO DE RIBEIRINHOS, AO LONGO DAS PRAIAS DE RIOS DA BACIA DO JURUÁ, NO ESTADO DO ACRE E SUA IMPORTÂNCIA AMBIENTAL*
10.31419/ISSN.2594-942X.v122025i1a5JARR
Jose de Arimateia Rodrigues do Rego1**
Marcondes Lima da Costa2
1Então, 2005, aluno de pós-graduação do PPGG/UFPA, jr2rego@gmail.com**
2Então professor do PPGG/UFPA, orientador da dissertação de J.A.R. Rego, hoje professor titular voluntário do mesmo, marcondeslc@gmail.com
* Trabalho defendido como dissertação de mestrado pelo primeiro autor em Belém, Pará, em 30 de outubro de 2005, sob a orientação do Prof. Dr. Marcondes Lima da Costa e apoiado por projetos de pesquisa financiados pelo CNPQ (SelenMerAs, Proc. 476874-1 e Geosedintama, Proc. 471109/2003-7) coordenados pelo orientador junto ao Programa de Pós-graduação em Geologia e Geoquímica (PPGG) da Universidade Federal do Pará (UFPA). A dissertação foi avaliada e aprovada pela banca examinadora, constituída pelos professores Dra. Vanda Porpino Lemos (UFPA), Dr. Sambasiva Pachineelam (UFF) e além do orientador, Dr. Marcondes Lima da Costa (UFPA). Nesta publicação manteve-se o conteúdo técnico original, não havendo qualquer alteração, exceto as referências bibliográficas relativas aos trabalhos publicados sobre a dissertação. Mesmo considerando as quase duas décadas decorridas sobre a conclusão do trabalho, acredita-se que este trabalho em sua íntegra é ainda de real importância para as ciências geológicas, para agricultura e para o meio-ambiente, tanto no Acre como além de suas fronteiras. Parte deste conteúdo já foi publicado em Rego et al. (2004, 2018), Costa et al. (2006), entre outros.
**Autor para correspondência
ABSTRACT
The state of Acre is in the extreme southwest of the Brazilian Amazon, occupying an area of 153,149 km2, with a low population density (3.66 inhabitants/km2). The state is crossed by the basins of the Purus and Juruá rivers, which drain silica-clay sediments from the Solimões Formation. During the flood period, they transport large quantities of sediment by traction and in suspension, which are deposited on beaches (point bars) formed along the rivers. During the dry season, the beaches are exposed in their meanders, which in addition to leisure, riverside residents use them for small-cycle agriculture. The beaches are cultivated with corn (Zea-Mays) and beans (Vigna unguiculata (L) Walp) or not. These beaches also have a succession of vegetation with Canarana grass (Costus spicatus) and dew grass (Brachiaria decumbes). This study evaluates the high fertility of beach sediments (soils) and the geochemical affinity between their sediments, cultivars and finally the hair of riverside residents, with the aim of evaluating the cycle of elements to man, as well as the environmental importance of this cycle. For this, the main rivers that make up the Juruá basin were selected, covering the cities of Feijó, Tarauacá, Cruzeiro do Sul and Rodrigues Alves. Along the rivers, 9 stations were established to collect samples of beach sediments, canarana grass leaves, dew grass leaves and grains and bean leaves, in addition to measuring the physical-chemical parameters in the field of river waters. Human hair samples were collected in riverside cities. The mineralogy of the sediments was analyzed by X-ray diffraction (XRD) and chemical characterization (major and trace elements) by ICP-MS. Ca, Fe, K, Na, Ba, Zn, Mo, Co, Cr, Cu, Pb, Hg, As and Se were determined in the cultivars by ICP-MS and neutron activation with the aim of determining the concentrations of these elements as well as their transfer of sediment (soil) – cultivar, measurements of physical-chemical parameters and quantification of suspensions were carried out in the water samples. The beaches studied are essentially made up of fine-grained sediments to silt. Only the uncultivated beaches of the Moa River are sandy quartz. The sediments of these beaches are formed in descending order by quartz, feldspar, and clay minerals (smectite, illite and kaolinite) and are therefore rich in SiO2 (68.0 to 98.9% by weight), with low levels of Al2O3 (0. 41 to 11.9%), Fe2O3 (0.13 to 4.375), MgO (0.02 to 1.03%), K2O (0.16 to 1.94%), Cao (0.02 to 1.05 %) and Na2O (0.02 to 1.03%).
The chemical composition of these sediments can be compared with that of the PAAS (Australian p6s-Archaean shales) and therefore with the Upper Crust, although slightly depleted in Al, Fe, Mg, K, Ti and even more in Ca and Na, certainly diluted by high levels of SiO2. In general, the waters of the rivers in the Juruá basin are muddy, white-water type due to the high concentration of inorganic material in suspension (suspended). The suspension criteria and physical-chemical parameters (pH, DO, TDS, temperature, resistivity, electrical conductivity, and salinity) delimit three geographic zones (sub-basins): Envira-Tarauacá, Juruá and Moa rivers. The Envira Tarauacá sub-basin presents the highest values of all physical-chemical parameters analyzed, except resistivity. The area of the Envira-Tarauacá sub-basin coincides with the most fertile land area in the State of Acre. Of the vegetables studied here, canarana grass leaves are more enriched in K, S, Ca, Mg, P than dew grass leaves. Canarana grass and dew grass show chemical similarity in relation to the elements K, P, S, Mg and Ca, only the canarana grass collected in the sediments of the Juruá river tends to be more enriched in Ca and S. The transfer of chemical elements to the sediment (soil)-vegetable presents the following order of absorption by the plant K>P>Ca>P, this order presents the highest values for vegetables collected in the sediments of the Envira river, highlighting the importance of this river as the one with the greatest potential for transferring macronutrients and possibly being an indicator of its superior fertility to other rivers in the Juruá basin. The Hg cycle in the sediment-cultivar-human chain shows that in the beach sediments of the Juruá, Envira and Tarauacá rivers the average concentration of this element in the sediments (27 ppb) is below the background range (50 ppb) and in plants they are in the range considered normal (< 500 ppb) for plants that grow in soils with low concentrations of Hg; in hair the average concentration of Hg in the cities of Tarauacá, Cruzeiro do Sul, Rodrigues Alves and Feijó is 3992 ppb. The lowest concentration (average: 1680 ppb) was found in Feijó and the highest (6240 ppb) in Cruzeiro do Sul. These are normal values to slightly indicative of impact, not observed in the region. The slightly abnormal Hg values in hairs should not be related to the cultivars and therefore also in the sediments. Other sources of Hg availability for humans must be evaluated, whether fish or meat from domesticated or wild animals. The beaches and banks of the white-water rivers in the central and western region of Acre (Juruá basin) are in fact fertile, suitable for small-cycle agriculture, and their nutrients and other elements are fully assimilated by vegetables and cultivars. The chemistry of the sediments and cultivars, as well as human hair, shows that the region does not present geochemical anomalies suggestive of anthropogenic impact, not even geological.