Stéphane Audry

Long-term records of cadmium and silver contamination in sediments and oysters from the Gironde fluvial-estuarine continuum - Evidence of changing silver sources

The Gironde fluvial estuarine system is impacted by historic metal pollution (e.g. Cd, Zn, Hg) and oysters (Crassostrea gigas) from the estuary mouth have shown extremely high Cd concentrations for decades. Based on recent work (Chiffoleau et al., 2005) revealing anomalously high Ag concentrations (up to 65 mg kg(-1); dry weight) in Gironde oysters, we compared long-term (~1955-2001) records of Ag and Cd concentrations in reservoir sediment with the respective concentrations in oysters collected between 1979 and 2010 to identify the origin and historical trend of the recently discovered Ag anomaly. Sediment cores from two reservoirs upstream and downstream from the main metal pollution source provided information on (i) geochemical background (upstream; Ag: ~0.3 mg kg(-1); Cd: ~0.8 mg kg(-1)) and (ii) historical trends in Ag and Cd pollution. The results showed parallel concentration-depth profiles of Ag and Cd supporting a common source and transport. Decreasing concentrations since 1986 (Cd: from 300 to 11 mg kg(-1); Ag: from 6.7 to 0.43 mg kg(-1)) reflected the termination of Zn ore treatment in the Decazeville basin followed by remediation actions. Accordingly, Cd concentrations in oysters decreased after 1988 (from 109 to 26 mg kg(-1), dry weight (dw)), while Ag bioaccumulation increased from 38 up to 116 mg kg(-1), dw after 1993. Based on the Cd/Ag ratio (Cd/Ag~2) in oysters sampled before the termination of zinc ore treatment (1981-1985) and assuming that nearly all Cd in oysters originated from the metal point source, we estimated the respective contribution of Ag from this source to Ag concentrations in oysters. The evolution over the past 30 years clearly suggested that the recent, unexplained Ag concentrations in oysters are due to increasing contributions (>70% after 1999) by other sources, such as photography, electronics and emerging Ag applications/materials.

Geochemical background, baseline and origin of contaminants from sediments in the mining-impacted Altiplano and Eastern Cordillera of Oruro, Bolivia

Oruro, located in the Bolivian Altiplano, has been subjected to intense mining and smelting activities since Colonial times (17th Century), yet the current geochemical composition of sediments and trace element behavior is practically unknown. A collection of 91 sediment samples retrieved from five sedimentary cores from Lake Uru Uru (Altiplano) and Cala Cala Lagoon (Eastern Cordillera) subjected to a total digestion technique, with a compilation of a pre-existing database of trace element concentrations in soils and lacustrine sediments obtained from the Oruro Pilot Project (PPO), facilitated the proposal of geochemical backgrounds and a present-time baseline for As, Cd, Cu, Pb, Sb and Zn in sediments from this area. Results obtained by statistics and geographical information system (GIS) analyses showed that the natural geochemical backgrounds of As and Sb are significantly enhanced in comparison to the Upper Continental Crust (UCC) concentrations, world background levels, and those of industrial sites and historical mining sites. The use of a local enrichment factor (EF) normalized by the mean concentration within the Cala Cala lagoon (CCLAC) demonstrated that using UCC concentrations to calculate EFs (EFUCC) is inadequate for this highly mineralized environment and therefore is not supported. Regarding metals and metalloids, the strong multiplicity of sources in this environment makes it difficult to discriminate between natural and anthropogenic inputs into this endorheic drainage basin, although it is suggested that surficial soils are probably impacted by airborne particulates dispersed from the Vinto (Sb-Sn) smelter, while Lake Uru Uru is influenced by mining activities, particularly drainage waste of the San José and Huanuni mines. As a final contribution, a geochemical background and a present-time baseline for Bolivian highlands sediments are provided, which will be helpful for the improvement of environmental legislation and for the future interpretation of geochemistry data in contamination and/or pollution studies in the altiplanic area.

Geochemical behaviour of dissolved trace elements in a monsoon-dominated tropical river basin, Southwestern India

The study presents a 3-year time series data on dissolved trace elements and rare earth elements (REEs) in a monsoon-dominated river basin, the Nethravati River in tropical Southwestern India. The river basin lies on the metamorphic transition boundary which separates the Peninsular Gneiss and Southern Granulitic province belonging to Archean and Tertiary–Quaternary period (Western Dharwar Craton). The basin lithology is mainly composed of granite gneiss, charnockite and metasediment. This study highlights the importance of time series data for better estimation of metal fluxes and to understand the geochemical behaviour of metals in a river basin. The dissolved trace elements show seasonality in the river water metal concentrations forming two distinct groups of metals. First group is composed of heavy metals and minor elements that show higher concentrations during dry season and lesser concentrations during the monsoon season. Second group is composed of metals belonging to lanthanides and actinides with higher concentration in the monsoon and lower concentrations during the dry season. Although the metal concentration of both the groups appears to be controlled by the discharge, there are important biogeochemical processes affecting their concentration. This includes redox reactions (for Fe, Mn, As, Mo, Ba and Ce) and pH-mediated adsorption/desorption reactions (for Ni, Co, Cr, Cu and REEs). The abundance of Fe and Mn oxyhydroxides as a result of redox processes could be driving the geochemical redistribution of metals in the river water. There is a Ce anomaly (Ce/Ce*) at different time periods, both negative and positive, in case of dissolved phase, whereas there is positive anomaly in the particulate and bed sediments. The Ce anomaly correlates with the variations in the dissolved oxygen indicating the redistribution of Ce between particulate and dissolved phase under acidic to neutral pH and lower concentrations of dissolved organic carbon. Unlike other tropical and major world rivers, the effect of organic complexation on metal variability is negligible in the Nethravati River water.

Synergistic effects of mining and urban effluents on the level and distribution of methylmercury in a shallow aquatic ecosystem of the Bolivian Altiplano

Lake Uru Uru (3686 m a.s.l.) located in the Bolivian Altiplano region receives both mining effluents and urban wastewater discharges originating from the surrounding local cities which are under rapid development. We followed the spatiotemporal distribution of different mercury (Hg) compounds and other metal(oid)s (e.g., Fe, Mn, Sb, Ti and W) in both water and sediments during the wet and dry seasons along a north-south transect of this shallow lake system. Along the transect, the highest Hg and metal(oid) concentrations in both water and sediments were found downstream of the confluences with mining effluents. Although a dilution effect was found for major elements during the wet season, mean Hg and metal(oid) concentrations did not significantly differ from the dry season due to the increase in acid mine drainage (AMD) inputs into the lake from upstream mining areas. In particular, high filtered (<0.45 μm) mono-methylmercury (MMHg) concentrations (0.69 ± 0.47 ng L-1) were measured in surface water representing 49 ± 11% of the total filtered Hg concentrations (THgF) for both seasons. Enhanced MMHg lability in relation with the water alkalinity, coupled with abundant organic ligands and colloids (especially for downstream mining effluents), are likely factors favoring Hg methylation and MMHg preservation while inhibiting MMHg photodegradation. Lake sediments were identified as the major source of MMHg for the shallow water column. During the dry season, diffusive fluxes were estimated to be 227 ng m-2 d-1 for MMHg. This contribution was found to be negligible during the wet season due to a probable shift of the redox front downwards in the sediments. During the wet season, the results obtained suggest that various sources such as mining effluents and benthic or macrophytic biofilms significantly contribute to MMHg inputs in the water column. This work demonstrates the seasonally dependent synergistic effect of AMD and urban effluents on the shallow, productive and evaporative high altitude lake ecosystems which promotes the formation of natural organometallic toxins such as MMHg in the water column.

Circulation de fluide et subduction complète des sédiments de la plaque Cocos (« Leg ODP 170 »)

Abstract During ‘Leg ODP 170’, sediments were recovered from the incoming Cocos Plate, the frontal deformed wedge, the underthrust section and the apron which covers most of the Costa Rica slope. In situ temperature measurements, physical records (LWD) and chemical analyses of the sediment pore waters were also realized. Preliminary results of the Leg show that the regional low heat flow (12 mW·m−2) is certainly due to a sea water cooling of the uppermost crustal rocks. Fluids from deep origin circulate along the decollement and through the deformed sedimentary wedge. 99 % of the incoming Cocos sediments are underthrust and the deformed wedge is made of material of the sedimentary apron that has flowed downslope to the toe of the slope and has been deformed by rapid and long term subduction beneath it.