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The study of the solubilization of manganese in the groundwater pumped from the well field of Capdenac-Gare (riverbank-aquifer system) highlights two different reducing processes of the metal : — The first, permanent, is related to the presence of solid organic matter embedded in the recent sediments, or on the river-bottom of the Lot river, in calm sites where fine sediments were deposited (mud effect). When the pumping, wells are located so that the pumped water percolates through these reduced zones, high concentrations of manganese are observed which lead ineluctably to water treatment. . — The other, seasonal, is found in the presence of dissolved organic matter in the Lot river, provided essentially by the development of algae in summer (eutrophication). The mineralization of the organic matternear the river banks (river bank effect) leads to the consumption of the oxygen and the nitrates accompanied by a decrease of the redox potential. It follows a mobilisation of manganese near the biofiun which envelops the porous media. This mobilisation is favoured by the high temperatures of the Lot river in summer and autumn. When all the bio-degradable organic carbon is consumed, we noticed a reoxygenation of the groundwater by the atmos­ phere, accompanied by the precipitation of soluble manganese. Therefore, one should avoid the implantation of pum­ ping wells in the reduced zone of the groundwater and favour, whenever possible, the reoxygenation of the groundwater through the soil surface.

Effet de berge, effet de vase, deux facteurs différents de mobilisation du manganèse : un exemple dans un champ captant de la vallée du Lot (France)