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Résumé :
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The flux of phosphate in the aquatic ecosystems of the Camargue was modelled. The model developed for the hydrological unit, the water basin of the Aube de Bouic, is based on the hydrology of the system, the reaction equations of the adsorption of phosphate onto Fe(OOH) and the solubility product of apatite. The water basin consists of an inlet canal, the Aube de Bouic, ricefields and natural marshes, plus a drainage canal through which the excess water is pumped into the Etang de Vaccares. Phosphate is introduced into ricefields either with the irrigation water, taken from the Rhone, or as fertiliser. Most of the phosphate will be fixed on the ricefield soils either as iron-bound or as calcium-bound phosphate, depending on the pH. The o-phosphate concentration will slowly increase to an extent depending on the pH. From the ricefields water will flow towards the Etang de Vaccares; the amount of phosphate reaching the Etang de Vaccares depends on the pH of the system and the depth of the water outlet of the ricefields. In six different scenarios the amount of phosphate reaching the Etang de Vaccares is calculated depending on the quantity of fertiliser, the depth of the water outlet and the pH. For the Etang de Vaccares the model predicts the present concentration of o-P relatively well and shows that the loading is considerable. This should cause concern; the danger of serious eutrophication is real. Part of the irrigation water is used to refill or keep under water the natural marshes in this hydrological unit, which without irrigation would dry out during summer. Again high P-concentrations are found in the marshes, but mostly in the form of Fe(OOH)approximate to P or CaCO(3)approximate to P, the ratio of which depends again on the pH of the sediments. The o-P concentration has remained low up to now.
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