ABSTRACT
The aim of this paper was study the water of the Madeira River and its tributaries, between the city of Humaitá and its confluence in the Amazon River. There were analyzed: pH, conductivity, turbidity, major ions, trace elements and strontium isotopes during the dry and wet seasons and also, in the transition from the wet to the dry season, throughout 2009 and 2010. The white waters from the Madeira River are bicarbonated, calcic, with pH between 5 and 6 and the ions concentrations are higher than those of the tributaries. The waters of the tributaries are black, are more acidic and are chemically heterogeneous; those from the left margin are chemically similar to the waters of Madeira River, while those from the right margin are enriched in dissolved SiO2. The concentrations of cations, Cl- and NO3- are higher during the wet season due to the contribution of soils, vegetation and the rainwater composition (Cl-), while HCO3-, SO4(2-), Al, Br e P are concentrated in the dry season, and their higher contents may be related to the bedrock. The higher concentrations of SiO2 and rare earth elements in both dry and wet season are related to vegetation, soil and rocks. The interaction of these factors causes the chemical heterogeneity of the water. However, the chemical similarities between the water tributaries of the left margin and the water of the Madeira River, are probably, consequence of the Andean rocks be the sources of the Cenozoic sedimentary rocks percolated by these tributaries while the chemistry water of the tributaries of the right margin are in accordance with the tectonic stability, the intense weathering and the low erosion rate of the Amazon craton.
Este trabalho teve como objetivo estudar as águas do rio Madeira e seus principais tributários entre a cidade de Humaitá e sua foz no rio Amazonas. Foram analisados pH, condutividade, turbidez, íons maiores, elementos traço e isótopos de Sr nos períodos de seca, cheia e transição para a seca entre 2009 e 2010. As águas do Madeira, classificadas com brancas, são bicarbonatadas-cálcicas, têm pH entre 5 e 6 e são mais concentradas que as dos tributários. Estes têm águas de cor preta, mais ácidas e quimicamente heterogêneas, os da margem esquerda são quimicamente mais semelhantes as do Madeira, enquanto os da margem direita têm alta concentração em SiO2. Os cátions, Cl- e NO3- são mais concentrados na cheia o que sugere influência do solo, da vegetação e da composição da água da chuva (Cl-), enquanto HCO3-, SO4(2-), Al, Br e P, com maiores concentrações na seca, devem estar relacionados com a química das rochas. A SiO2 e os elementos terras raras (ETR) com concentrações elevadas na seca e na cheia, estão associados tanto a vegetação e ao solo como as rochas. A interação desses fatores é a causa da heterogeneidade química das águas. Contudo, a semelhança entre as águas dos tributários da margem esquerda e as do Madeira são consequência das rochas dos Andes serem a fonte dos sedimentos cenozóicos percolados por elas, enquanto a química das águas dos tributários da margem direita retrata a estabilidade tectônica, o intenso intemperismo e a baixa taxa de erosão das rochas do cráton Amazônico.
ABSTRACT
In West Africa, particularly in Côte d'Ivoire, the groundwater is contained in hard-rock aquifers and serves as main source of drinking water supplies to the population. To improve access, several studies were conducted in various parts of the country. Most of them use mapping of lineaments related to tectonic fractures to represent corridors of groundwater. In this article, chemistry of major ions was used to highlight quantitatively the axes of groundwater movement, and mixing between different aquifers in Sassandra watershed which is located in the Southwest of the Ivory Coast. The sampling campaigns were accomplished respectively during the dry and wet seasons in the department of Soubré (8 590 km2), located in Sassandra watershed, area where the effects of climate change are observed. The processing of satellite images (optical and active) has produced a map of major lineaments. Geographic positions and technical data of boreholes were integrated into a geographic information system (GIS) to identify point near major lineaments for groundwater sampling and chemical analysis. The waters were collected, and then analyzed by using atomic absorption spectrometer (AAS) and a Varian Vista ICP. The results indicate that groundwater samplings are primarily Ca-HCO3 type or NaKHCO3 and NaK-SO4 types. Calcium and low pH were encountered in the highlands where infiltration of meteoric water occurs relatively quickly through preferential pathways. Chadha diagram has highlighted differences in the chemistry of groundwater between aquifers on one hand, and between systems of surface runoff and deep runoff on the other hand. Most groundwater seems to move relatively quickly. In addition, some groundwaters show a denitrification coupled with pyrite oxidation. These groundwaters have been longer circulating along opened fractures with gentle slopes. The observations and hydrochemical characterization, especially SO4 2-/Cl- ratio, permitted to identify axes of groundwater movements in the study area. However, the major lineaments which are similar to major fractures are not primarily responsible for the groundwater motions. Rather, there are small fractures and topography which control the flow of groundwater in the crystalline hard-rock. Also, the groundwater levels are not always guided by the major lineaments observed. Some small lineaments and topography control fairly the groundwater flow.