Effet de la matière organique produite dans les bassins de rétention des eaux pluviales sur la spéciation les éléments traces métalliques

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Retention ponds, which are part of the stormwater drainage system, have specific hydrodynamics. The residence time of the water, higher than in the conduits, favours the sedimentation of particles. During spring and summer, mixing and thermal stratification alternate. A high phytoplankton biomass can develop and produce organic matter which can interact with micropollutants. That is why it is essential to understand the impact of organic matter locally produced on trace metals. For this purpose, we studied the speciation of nickel, copper, zinc and lead in the waters of a retention pond located in derivation of a channelled river in a Paris suburb (France). Three fractions of trace metals (total, dissolved and labile fractions) were sampled during dry weather and analyzed. During dry weather, few labile metals, potentially bioavailable, were present in the pond (less than 50% of the total concentration), less than in the river (40-70%). The chlorophyll a concentration was higher in the pond (90 to 140 mu g/L) than in the river (30 mu g/L): autochthonous dissolved organic matter seems to favour the complexation of dissolved metals. The reduction of nickel and copper trace metal bioavailability in the retention pond compared to the river which feeds the pond, which needs to be confirmed with more measurements, shows the interest to have urban storm waters transit through retention ponds to reduce their toxicity before their release in the natural waters.