Encadeamento geoquímico entre sedimentos (solos) e cultivares em praias, além do cabelo humano de ribeirinhos, ao longo de rios da bacia do Purus, no estado do Acre e a sua importância ambiental

Abstract

The state of Acre is located in southwestern Amazonia and are found two biger important hydrographic basins (Purus and Juruá). The rivers have white water and they show U- shaped valleys in the upper regions and with great floodplains in the lower areas. The water of these rivers is low during the dry season displaying beaches (point bars) and diffs. This situation benefits the population of the region who uses these areas for subsistence agriculture, cultivating mostly maize (Zea mays) and beans (Vigna unguiculata (L) Walp), and complementing the diet with fish and meat of both domestic and wild animals. This work attempts to investigate the high fertility of these beaches and mainly the correlation between the sediments (soils) and crops and between sediments and the riparian (hair samples). The main rivers wich constitute the Purus basin were included in the study as the tonws of Sena Madureira (Iaco and Caeté Rivers), Manuel Urbano (Purus River) and those near Acre river valley (Assis Brazil, Brasiléia, Xapuri, Rio Branco and Porto Acre), where beach sediments, beans leaves and seeds, and maize leaves have been studied in 16 stations. The sediments were analyzed by X-Ray Difraction (X-RD) for mineral identification and by MS-ICP. In attempt to verify the concentrations of elements (major and trace elements) such as Ca, Fe, K, Na, Ba, Zn, Mo, Co, Cr, Cu, Pb, Hg, As and Se and their distribution between sediment (soil) and the crops chemical composition analyses were carried out by “água régia” plus MS-ICP and neutron activation methods. The sediments of the beaches are essentially fine with main mineralogy consisting of quartz, clay minerals (smectite, illite and less frequent kaolinite) and feldspars (K-feldspars and albite). The chemical composition shows high concentrations of SiO2, Al2O3 and Fe2O3, besides K2O, Na2O, CaO. The concentrations found are below of the average of earth’s upper crust (except Si2O), however, they are higher than those of beach sediments, and they are similar to those of high fertility sediments and white water rivers. The chemical results for the crops show that the bean leaves show enrichment in Ca and in K the beans seeds. Ba, Zn, Co and Cr are more concentrated in the leaves than in the bean seeds, while Mo is more concentrated in the seeds. As shows significant concentrations in the crops, especially in bean leaves (average of 338,3 ppb) and this is10 times higher than in the seeds. Mercury does not show variation between bean seed (7,7ppb) and leaf (7,1ppb). Selenium is below the detection limit (20ppb). The maize leaves show enrichment in K (average 1,97%), and Ca (average 0,38%). The correlation between sediments (soil) and crops is analysed by coefficient of biological absorption (CBA) that shows decreasing orders of absorption: for beans leaves (Ca > K > Na > Zn >Ba > Co > Cr > Hg > As); beans seeds (Na > K > Ca > Zn > Co > Hg > As); and maize leaves (K > Ca > Cr > Zn > Cu > Hg > As). The distributions may represent the needs of the crops or even the bioavailability of these elements in the beach environment. The high values of coefficients of biological absorption for these elements in the crops reflect the relatively high values of K, Ca, Mg, Zn, Ba and Cr in the sediments and diffs of the rivers of state of Acre, thus proving their high fertility. The low values of Hg in the sediments (soil) that are also very low in crops and the high values of K, Mg, Ca, Zn and Ba transferred to the crops show the efficiency and strong chemical relation with the soil chemistry. They also show that the beach sediments of staee of Acre do not present geochemistry anomalies of neither natural nor human origin, so we can’t say that there is an environment impact. The high values of Hg found in human hair in Manuel Urbano and in part of Sena Madureira cannot be explained by the consumption of bean seeds and maize. A new source should be investigated as a possibility being the carnivorous or fish diets of population. The present study also confirms the empirical knowledge about the beaches properties for subsistence agriculture that the populations that live near the Acre rivers have known for a long time.