Afinidades geoquímicas entre sedimentos (solos) 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

Abstract

The State of the Acre is located in the southwestern extremity of the Brazilian Amazonia, in an area of 153.149 km2, with a low population density (3,66 inhabitants/km2). The studied area comprehends the basin of the Juruá river between the Feijó and Cruzeiro do Sul cities, including Tarauacá and Rodrigues Alves. In the basin the main rivers are: Juruá, Tarauacá, Envira, Muru and Moa. The same carry in sediments great amounts by traction and suspension from siltic- argillaceous rocks of the Solimões Formation and deposite in beaches formed along the rivers. In drought periods the beaches (point bars) are displayed in its meanders and benefit the riparian that use them to leisure and to short cycle agriculture. The beaches sometimes are used to cultivs of maize (Zea-Mays) and beans (Vigna unguiculata (1) Walp). These beaches also present a vegetation succession of canarana grass (Costus spicatus) and orvalho grass (Brachiaria decumbes). This study evaluates the high fertility of the beaches sediments (soils) and the geochemistry affinity between their sediments and the crops besides riparian hair and the environmental importance of this cycle. So the main rivers that constitute the Juruá basin, enclosing Feijó, Tarauacá, Cruzeiro do Sul and Rodrigues Alves cities were related. Nine stations were established along the rivers and samples of beach sediments, canarana grass leaves, orvalho grass leaves and seeds and beans leaves had been leaves were collected. Besides this, measurement of phisical-chemical parameters of the rivers waters were caried out. Human hair samples were collected in the cities. The sediments samples were analyzed by X-ray difraction (XRD) for mineral determination, are by chemical ICP-MS for characterization (major and trace elements). In attempt to verify the content of Major and Trace Elements, like Ca, Fe, K, Na, Ba, Zn, Mo, Co, Cr, Cu, Pb, Hg, As and Se and their sediment — (solil) — crops transference were camet the chemical compositions analyses were carried out with regal water plus MS-ICP and neutron activation. The studied beaches sediments in the cultivated areas excet the beach of Moa river are essentially fine-grained, ranging from fine sand to silt. And are formed by in decressent order quartz, feldspars clay minerals (smectite, illte and kaolinite) and feldspars. Chemical are constitute binaimes by SiO, (68,0 98,9 % in weight), following by significaty lower values, of Fe>03 (0,13 to 4,37 %), MgO (0,02 to 1,03 %), KO (0,16 to 1,94 %), CaO (0,02 to 1,05 %) and Na,0 (0,02 to 1,03 %) and AlOs, are more concentrated in the sediments of Envira, Tarauacá and Juruá rivers. These values compened chemical with that one of the PAAS and, the Upper Crust, are slightly impoverished in Al, Fe, Mg, K, Ti as well as slightly in Ca and Na, diluted certainly for the high contents of SiO2. In general, the rivers waters of the Juruá basin are dark-colored, as white waters type, due to inorganic material in suspension (suspensates) in high concentration. The criterion of suspensates and phisical-chemical parameters (pH, DO, TDS, temperature, resistivity, electric conductivity and salinity) delimit three geographic zones (sub-basins): a) Envira-Tarauacá, b) Juruá and c) Moa rivers. The first one presents the highest values of all the analyze phisical-chemical parameters, except resistivity, and the area coincides with the more fertile land area State of the Acre. Among the studied vegetables the canarana grass leaves revealed are richerest in K, S, Ca, Mg, P than orvalho grass leaves. Canarana and orvalho grass present chemical similarity in relation to K, P, S, Mg and Ca elements. Only canarana grass, collected in the sediments of the Juruá river tends to become richer in Ca and S. The chemical elements transference sediment (soil)-vegetable presents an absorption for vegetal of the K>P>Ca>P. This order presents the major values for vegetables collected in the sediments of Envira river, emphisizing the importance of Envira river with greater transference potential of macronutrients and being a probable pointer of its superior fertility in relation to the other rivers of Juruá basin. In the sediment-crop-human chain the cycle of Hg shows that in the beach sediments of the Juruá, Envira and Tarauacá rivers the average concentration of Hg (27 ppb) in the sediments below the background range (50 ppb) and in the plants is in according to normal considered range (< 500 ppb) for plants that grow in soils with low contents of Hg; the average contents of Hg in hair in Tarauacá, Cruzeiro do Sul, Rodrigues Alves and Feijó rivers is about 3992 ppb. The lower concentration (average 1680 ppb) was found in Feijó and the higher (6240 ppb) in Cruzeiro do Sul, being normal values and slightly impact indicators, not observed in the region. The slightly anomalous values of Hg in hair is not related to the crops, as well as to the sediments. For hunam other sources of Hg disponibilization must be evaluated as fish or domestic or wild animals meats. The beaches and abrupt declivities (banks) of the white water rivers of the central and western regions of Acre (Juruá basin) are really fertile, adequated to short cycle agriculture and its nutrients and other elements are completely assimilated by vegetables and crops. For the present study the chemistry of the sediments and crops, as well as of human hair, shows that the region does not present geochemistry anomaly of any considerable antropogenic impact.