Geochemistry of waters in the Kouroumari region, Mali; Al, Si, and Mg in waters concentrated by evaporation: Development of a model

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Abstract
The Kouroumari, an endoreic plain situated in Mali, is composed of deposits from the Niger River. In this confined area, river water are concentrated by exposure to the arid climate. This results in precipitation of calcite and in waters that tend toward alkalinity. The conventional method used for calculating the activity of aqueous species in solution indicates an oversaturation of water with respect to gibbsite, which is lacking in these soils and never precipitates in such areas. Using these results, it was not possible to determine the mechanisms governing the variation of dissolved Si, Al, and Mg molalities with time. By taking into account the aluminium complex Al13O4(OH)28, inherited kaolinite and illite are shown to dissolve. The control of dissolved Si, Al, and Mg molalities is shown to be related to the formation of magnesium montmorillonite.