Anthropogenic sources and biogeochemical reactivity of particulate and dissolved Cu isotopes in the turbidity gradient of the Garonne River (France)

High precision MC-ICP-MS Cu isotope measurements were performed on suspended particulate matter (SPM) and filtered water sampled along the turbidity gradient of the fluvial Gironde Estuary (Garonne Branch), following a summer lasting period of low freshwater discharge. High Cu concentrations (up to similar to 200 mu g/g) in particulate organic carbon (POC)-rich (up to similar to 14%) particles upstream from the Maximum Turbidity Zone (MTZ) coincide with the most negative delta Cu-65 values (down to -0.39 parts per thousand). This suggests the preferential uptake of light Cu isotopes by phytoplankton or anthropogenic contamination from CuSO4 fungicides related to intensive wine farming in the area (delta Cu-65 = -0.37 parts per thousand for vineyard soils). Suspended particulate matter samples in the MTZ exhibit lower Cu concentrations (similar to 35 mu g/g) and heavier isotopic compositions (delta Cu-65 = -0.20 parts per thousand in average), as recorded by other samples from the Gironde Watershed. Along the fluvial estuary, dissolved Cu is mostly enriched in heavy isotopes (up to delta Cu-65 = +0.21 parts per thousand), but negative signatures down to delta Cu-65 = -0.66 parts per thousand occur near the city of Bordeaux. Release of dissolved Cu was attributed to the mineralization of organic matter with a total addition of similar to 600 ng/L in the fluvial estuary. Continuously decreasing amounts of dissolved Cu added with distance suggest that the observed Cu release evolved towards completion in the MTZ and contributed to negative values for dissolved Cu isotopes, as modeled by a Rayleigh process. Urban wastewater effluents probably also contribute to both (i) the dissolved Cu addition (10-20%) in the MTZ and (ii) negative Cu isotope signatures. Further work is necessary to assess the respective roles of anthropogenic sources and biogeochemical fractionation processes. Accordingly, the present study provides new insights into the potential of Cu isotopes for fingerprinting sources and mechanisms involved in the biogeochemical cycle of Cu in temperate aquatic environments impacted by anthropogenic activities. (C) 2013 Elsevier B. V. All rights reserved.