The State of the Acre is located in the southwestern ertremity of the Brazilian Amazonia, in an area of 153.149 km(2), with a low population density (3,66 inhabitants/km(2)). The studied area comprehends the basin of the Jurua River between the cities of Feijo and Cruzeiro do Sul, including Tarauaca and Rodrigues Alves.
In the basin, the main rivers are: Jurua, Tarauca, Envira, Muru and Moa. The same carry in great amounts of sediments by traction and suspension from siltic-argillaceous rocks of the Solimoes Formation and deposit it in beaches formed along the rivers. In drought periods the beaches (point bars) are displayed in its meanders and benefit the riparians that use them for leisure and short cycle agriculture. The beaches sometimes are used to cultivate maize (Zea-Mays) and beans (Vigna unguiculata (l) Walp). These beaches also present a vegetational succession of canarana grass (Costus spicatus) and orvalho grass (Brachiaria decumbes).
This study evaluates the high fertility of the beach sediments (soils) and the geochemistry affinity between their sediments, the crops, and also the hair of the riparians to determine the importance of the cycle up to the human being, as well as its environmental impact. So the main rivers that constitute the Jurua basin and affect the cities of Feijo, Tarauaca, Cruzeiro do Sul and Rodrigues Alves cities were chosen and evaluated. Nine stations were established along the rivers and samples of beach sediments, canarana and orvalho grass leaves, together with the seeds and leaves of beans were collected. Besides this, the physical-chemical parameters of the riverwater were established and human hair samples from the riparians were collected.
The mineralogy of the sediment samples was determined by X-ray difraction (XRD) and the chemical characterization of the major and trace elements was determined by ICP-MS. The latter together with neutronic activation, attempted to verify the concentration of themajor and trace elements, like Ca, Fe, K, Na, Ba, Zn, Mo, Co, Cr Cu, Pb, Hg, As and Se and their transference to the sedimentary-soil crops. Meanwhile, with the water samples, a physical-chemical composition analysis was performed together with the quantification of the particles in suspension.
Except for the uncultivated beaches of the Moa River that has quartzitic sands, the studied beach sediments in the cultivated areas, are essentially fine-grained, ranging from fine sand to silt, In decrescent order they areformed by quartz, feldspars, and clay minerals (smectite, illite and kaolinite) that are rich in SiO(2) (68.0 to 98.9% in weight), followed by significantly lower values of Fe(2)O(3) (0.13 to 4.37%), MgO (0.02 to 1.03%), K(2)O (0.16 to 1.94%), CaO (0.02 to 1.05%) and Na(2)O (0.02 to 1.03%). In addition, Al(2)O(3), has a higher concentration in the sediments of the Envira, Tarauaca and Jurua rivers. The chemical composition of these sediments are comparable to those of PAAS (post-arquean Australian schist) that consequently has an upper crust of slightly impoverished Al, Fe, Mg, K, Ti, Ca and Na, which for sure have been diluted by the high content of SiO(2).
In general, the Jurua River Basin waters are muddy, as in while waters, due to the high concentration of suspended inorganic material. The criteria of the material in suspension and the physical-chemical parameters (pH, DO, TDS, temperature, resistivity, electric conductivity and salinity) delimit three geographic zones (sub-basins): a) Envira-Tarauaca, b) Jurua and c) Moa rivers. The first one presents the highest values for all the analyzed physical-chemical parameters, except resistivity, and consequently is the most fertile area in the State of Acre.
Among the studied vegetables the Canarana grass leaves revealed themselves to be richer in K, S, Ca, Mg, P than the Orvalho grass leaves. Both the Canarana and Orvalho grasses present chemical similarity in relation to the K, P S, Mg and Ca elements. Meanwhile, only the Canarana grass, collected in the sediments of the Jurua River tends to be richer in Ca and S. The soil-vegetable transference of the sedimentary chemical elements presents an vegegable-absorption order where K>P>Ca>P This order has the greatest values for vegetables planted and collected in the Envira River sediments, emphasizing the importance of Envira River as the greatest potentialfor macro-nutrient transfer and as such, exhibits superiorfertility in relation to the other rivers of the Jurua Basin.
The Hg cycle in the sediment-crop-human chain shows that in the beach sediments of the Jurua, Envira and Tarauaca Rivers, the average concentration of Hg (27 pph) in the sediments is below the background range (50 ppb), whereas in the plants, it is in accordance with the normal range (< 500 ppb) for plants that grow in low-Hg-content soils the average content of Hg in hair of the ribarians in the cities of Tarauaca, Cruzeiro do Sul, Rodrigues Alves and Feijo is about 3992 ppb.
The lowest concentration of Hg (average 1680 ppb) was found in Feijo and the highest (6240 ppb) in Cruzeiro do Sul. These values are normal and of slight impact for the region. The slightly anomalous Hg values in the hair are not related to the cropsor to the sediments. Other sources for the appearance of Hg in riparian hair must be evaluated, such as fish and domestic or wild animal meals.
The beaches and abrupt declivities (banks) of the white water rivers of the central and western regions of Acre (Jurua basin) are reallyfertile, 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, crops, human hair show that the region does not present geochemical anomaly of any considerable anthropogenic or geological impact.