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Influence of instream geomorphic units on the spatial distribution of pollutants in suburban river sediments: the example of the Chaudanne (France): In urbanized areas, small streams can be greatly damaged by urban inflows and combined sewer overflows (CSO). These polluted inputs can be several times higher than the natural stream flow over short time periods. Sound knowledge of the spatial distribution of the discharged pollutants in sediments is therefore crucial for designing monitoring strategies and suitable remediation operations. This field study combines geomorphic characterization, hydraulic conductivity measurement and pollutant assays in sediments of a small suburban river. The study site consists of two sectors (upstream and downstream a CSO outlet), each divided up into three geomorphic types: riffles, pools and runs. The last two were grouped into one class named "pool-runs" owing to their similar open channel flow hydraulics. Benthic and hyporheic sediments were systematically sampled every 2 m along the reaches. Conventional particulate pollutants (Cr, Pb, Ptot & Corg) were assayed in samples. The main result is that the pollutants were not arbitrarily distributed in the stream sediments, but their location showed statistically significant differences in concentration related to geomorphic units, with a preferential accumulation in the hyporheic zones of riffle units and a lesser one in those of pools. The decrease in hydraulic conductivity was significantly correlated with the increase in pollutant concentration. This occurred mainly at the transition between riffles and pool-run units. Our findings highlight the need to take into account the geomorphological and hydrological functioning of a stream to accurately locate the biogeochemical hotspots to be treated and thereby develop more relevant monitoring and remediation methodologies.
pollution, hotspot, geomorphic unit, riffle, hyporheic zone, urban stream.
Nous remercions l’Agence Nationale de la Recherche française (INVASION ANR-08-CESA-022 coordonnée par Benoit COUNOYER, et EPEC ANR-10-Ecot-007 coordonnée par Jolanda Boisson), l’Observatoire de Terrain en Hydrologie Urbaines (OTHU - www.othu.org/- FED de 4161), le Grand Lyon, l’Agence de l’eau Rhône-Méditerranée-Corse, la ZAEU (Zone atelier environnementale urbaine) - LTER et la région Rhône-Alpes pour leurs soutiens scientifique, administratif et financier.Nous remercions également Guillaume Fantino de AEMGEO pour sa participation à la réalisation des levés géomorphologiques et Stéphanie Petit et Bruno Tilly pour leur aide dans l’organisation et la réalisation des campagnes de prélèvements.
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