l Sediment and pollutant distribution along the Negro River: Patagonia, Argentina

The Negro River extends from the Andes to the Atlantic Ocean supplying water to several cities and to the most important fruit-bearing productive region of Patagonia. Floodplains from the Upper Valley are artificially irrigated with water delivered from the main channel. Pesticides are increasingly used without strict government control. Dams constructed at the tributary rivers retain sediment and therefore the clean water delivered becomes free of sediment and therefore is able to erode certain levees during high-discharge events. Bedload-transport fraction (sand dunes) increases at the Upper Valley and is fixed at the levees of the Lower Valley by riparian vegetation. This riparian vegetation is dominated by Salix humboldtiana and Schoenoplectus californicus while Spartina sp is abundant at the estuarine marsh area. Organochlorine compounds (OCCs) were measured by GC-ECD including dichlorodiphenyl-trichloroethane (DDT), dichlorophenyl-dichloroethylene (DDE) and dichloro-dyphenyl-dichloroethane (DDD) endosulfans, hexachlorocyclo-hexanes (HCH) and chlordanes. OCCs were preferentially accumulated at the Upper Valley in relation to the agricultural activities developed. The dominance of DDE and the parental endosulfan can be explained by their past and current use, respectively. PCBs (different congener groups) showed low levels as a consequence of chronic pollution related to the mud fraction. The high concentration in pollutants at the Upper Valley is significantly reduced to the Middle and Lower Valleys due to filtering effects, assumed to be induced by the riparian vegetation. At the estuary, very low OCCs levels were measured probably due to the dilution induced by mesotidal dynamics. The sedimentation rate of the lower floodplain is of 1 mm/year. The bottom of the main channel is dominated by 2D sand dunes. At the inlet, a sandy gravel bank is composed of megaripples 0.5 m high. Tidal currents guarantee the stability of the inlet, and particularly the asymmetric flood-tidal delta. Wave action induces a net longshore sand transport towards the north.