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Dynamique des apports fluviaux dissous et particulaires au lac Togo : bilans, origines et devenir, mécanismes et facteurs contrôle

Abstract : The Lac Togo Bassin (LTB), located in South of Togo, is a region characterized by various anthropogenic activities including agriculture, industry, phosphate mining, etc. To assess the impact of these activities on the quality of surface waters and sediment of this watershed, 105 river’s and lake water samples and 18 bed sediments were collected for analysis. Major elements, alkalinity, DOC, silica, pH, were measured in the dissolved phase of the samples, as well as trace elements (TE) and REE which were also measured in sediments and suspended matter during the period of 2015 to 2016. The dominant ions in river waters are Cl- (4.51 meq.l-1), Na+ (4.29 meq.l-1), HCO3 - (1.32 meq.l- 1), Mg2+ (1.13 meq.l-1), Ca2+ (0.69 meq.l-1) in average and represent about 90% of the total dissolved salts (TDS). The mean DOC and silica concentrations are respectively 9.17 mg/l and 7.13 mg/l. A comparative analysis of the mean concentrations of dissolved TE measured in the water of the stations studied against the Natural Concentrations of the Rivers of the World (NCRW) shows for Sc, Cr, Co, Ni, Cu, Zn, Sr, Zr, Cd, Sn, Pb, Th, U concentrations higher than NCRW values and WHO drinking water standards (INERIS-DRC, 2006) with a significant increase in lake level and downstream samples. The results obtained from the analysis of water and sediment samples during the hydrological cycle 2015-2016 made it possible to estimate the specific fluxes of dissolved chemical elements transported by Zio and Haho rivers respectively of 9,57 t.km-2.an-1 and 38.24 t.km-2.an-1 for major elements and 7.84 and 17.82 Kg.km-2.an-1 for trace elements. The particulate matter flow at Zio (Togblekope) and Haho (Hahotoe) is estimated respectively at 15.3 t.km-2.an-1 and 59.3 t.km-2.an-1. These fluxes allow estimate to the average rates of physical erosion of 3.6 to 4.3 m/Ma for Zio and 10.6 to 12.5 m/Ma for Haho and chemical alteration of 1.1 to 3.3 m/Ma for Zio and 3 to 4.2 m/Ma for Hahob. The specific flux of CO2 consumed by chemical alteration is about 1.9.108 mole.an-1 of CO2 or 75.103 mole.km-2.an-1 for Zio. This CO2 flux for Haho is 6.9.108 mole.an-1 or 202.6.103 moles.km-2.an-1 of which 109.103 moles.km-2.an-1 and 94.103 moles.km-2.an-1 consumed respectively by the alteration of silicates and the dissolution of carbonates. Mechanical erosion takes precedence over chemical weathering and profile deepening. There is an imbalance in the soils of these two sub-basins and therefore an average reduction in soil thickness. The degree of sediment contamination by TEs is assessed by parameters (Sediment Quality Guidelines, SQGs) such as the enrichment factor (EF), the respective % of TE from natural and anthropogenic, the residual and non-residual fraction % (labile) by using Medje Creek sediment as a reference material. The EF is generally less than 2 (limit value for natural geochemical background noise), in particular for Sn, As, Co, Cu, Zn, Pb, Th and U. However, 10% of the samples have EF between 2 and 6.3 in particular Ni, Cr and Cd. Simple extraction at EDTA shows that Co, Cu, Cd and Pb are relatively bioavailable because they are in labile form respectively at 38%, 32%, 26% and 25%. The rare earth (REE) concentration profiles showed that the geochemical signature of bottom sediments would be related to the TRs in the nonresidual fraction. Comparison of the concentrations of major elements, TEs and TRs in sediments and suspended matter indicates that these sediments do reflect suspended matter levels.
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Akouvi Avumandi. Dynamique des apports fluviaux dissous et particulaires au lac Togo : bilans, origines et devenir, mécanismes et facteurs contrôle. Sciences de la Terre. Institut National Polytechnique de Toulouse, 2019. Français. ⟨tel-02482840⟩

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