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Perturbations anthropiques du réseau hydrographique du bassin de la Garonne, cas des metaux et des nitrates

Abstract : The Garonne's basin, the second largest French basin (52000 km2 at Mas d'Agenais), is subject to anthropogenic pressures dues to agricultural, domestics and industrials humans activities.
A one-year hydrological sampling of the Garonne's and if its tributary's water highlights significant enrichments for a large type of trace metal elements (TME : Pb, Cr, Ni, Cu, Zn...). It appears that most of those metals elements in trace quantity (ppb) have a particular transport in the river. Many arguments speaking in favour for a one major diffuse source pollution origin of those enrichments: the rains. However, some specific enrichments due to local industrial activities are observed (case of Cr in the Agout). The balance flux [enter : rains), out : the outlet of the catchment basin] show a storage of some metals, probably in soil and in the river bed sediments.
The follow-up of majors flux elements (NO3-, Cl-, SO42-, Mg2+, Ca2+, K+, Na+) and especially of nitrate, in a Garonne stretch upstream to downstream of the Toulouse urban area was performed at a summer low water level period. The anthropic contribution to the river water composition from the urban area was estimated and indicated that NO3-, Cl-, K+ and Na+ represent the most important contributions of the anthropic inputs. Some of those elements, nitrates and potassium, are partially eliminated along this stretch. This looses may be attributed to biological activity (grow of biofilms) that appears in the Garonne river bed, especially in this critical period of anoxic water conditions (less oxygen).
The specific study of nitrate behaviour in a the Garonne riverbank wetland (Monbéqui site) pointed up the capacity of those sites to eliminate this nitrogen compound. Those wetland are the scene of the confluence between the Garonne river water and groundwater, favourable to the development of numerous biogeochemical processes. We identify and quantify strong biological consumption of nitrates in the mixing zone between Garonne water and groundwater. It results from the couple effect of organic matter oxidizing, aerobic respiration, denitrification and reduction of Mn and Fe oxides. On the contrary, nitrification of the ammonium that may originate from the depth groundwater and/or the soil may occur in the zone where the groundwater flows out to the Garonne.
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Contributor : Williams Exbrayat <>
Submitted on : Wednesday, January 31, 2007 - 10:43:49 AM
Last modification on : Thursday, October 15, 2020 - 4:07:30 AM
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  • HAL Id : tel-00128208, version 1



David Baque. Perturbations anthropiques du réseau hydrographique du bassin de la Garonne, cas des metaux et des nitrates. Hydrologie. Université Paul Sabatier - Toulouse III, 2006. Français. ⟨tel-00128208⟩



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