Silver fluxes to the Gironde Estuary - Eleven years (1999-2009) of monitoring at the watershed scale

摘要

Although Ag has long been recognized as highly toxic to aquatic organisms and as an efficient tracer of urban wastewater inputs, there are very few data available on Ag levels and fluxes in rivers and estuaries. Due to anomalously high Ag concentrations in oysters from the estuary mouth, dissolved (Ag_D) and particulate Ag (Ag_P) concentrations in rivers of the Gironde Estuary watershed were studied. Using daily discharge, Suspended particulate matter (SPM) data and monthly measured Ag_D and Ag_P, the respective fluxes were estimated at the three main entries of the Gironde Estuary and at selected key sites at the outlets of different sub-watersheds for multiple years of the past decade. Dissolved and particulate Ag concentrations were 0.5ng/L (detection limit) to 5.7ng/L and 0.13-13.9mg/kg in all rivers, except for the Riou-Mort River (up to 1260ng/L and 261mg/kg) that has been heavily polluted by former Zn ore treatment and remaining waste tailings. At all sites Ag transport mainly (73-100%) occurred in the particulate phase, i.e. strongly depended on hydrological parameters. Comparing Th-normalised (i.e. grain size independent) Ag_P levels at the different sites including one remote site showed that the regional background is best described by Ag_P/Th_P=0.026. The anthropogenic components in Ag_P levels and fluxes ranged from 24% to 90% at all sites, except for the Riou-Mort River (81-99%). Although this former ore treatment site still is an evident Ag source to the Lot-Garonne fluvial-estuarine system, its contribution to Ag fluxes at the main entry of the Gironde Estuary (i.e. 0.33-2.44t/a at the La Reole site) is <11%. This clearly suggests that other sources such as rock weathering and erosion (0.057-0.817t/a), urban wastewater inputs (0.042-0.057t/a) and cloud-seeding (0.030-0.063t/a) contribute the major parts of the fluvial Ag budget in this watershed. The estimated anthropogenic surface- and population-specific Ag fluxes in the studied system were 1.14-682g/km~2/a and 0.028-6.05g/person/a, respectively. The results obtained suggest that the Gironde fluvial-estuarine system is contaminated by Ag inputs from various sources, but further work is necessary to (i) identify and quantify the major sources and (ii) produce comparable data for other systems, which would be useful for the evaluation of Ag contamination at a larger scale.