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

Abstract

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.

Introduction

The Gironde Estuary (Fig. 1) is known for its metal pollution since high Cd accumulation (>100 μg g⁻¹, dry weight) in oysters (Crassotreas gigas) was discovered in the end of 1970s (e.g. Claisse, 1989) resulting in prohibition of consumption, production and purification of oysters since 1996 (e.g. Audry et al., 2004a). The main source of metal pollution was coal mining and Zn-ore treatment in the Decazeville watershed. Since the cessation of this metallurgical industry (1986), remediation and pollution management have clearly reduced Cd emission (Audry et al., 2004a). In the Gironde watershed, suspended particles are major vectors for Cd, Ag and other metals (e.g. Audry et al., 2004a, Lanceleur et al., 2011). Part of these particles accumulate in hydroelectric reservoirs, where the pollutants may be recycled (e.g. released to porewaters; Roulier et al., 2010) or stored over decades, providing sedimentary records of past pollutions (Audry et al., 2004b, Castelle et al., 2007, Valette-Silver, 1993). The hydroelectric reservoirs on the Lot River downstream from the Decazeville watershed contain huge sediment stocks with high metal loads (9.47 ± 0.9 × 10⁶ m³ of sediment containing approximately 200 t of Cd), that may be remobilized by floods, dredging and construction work (Audry et al., 2004a, Coynel et al., 2007). Although Cd has long been considered as the major pollutant, many other metals derived from the same source also show important anomalies in both sediment and water (e.g. Hg, Cu, Zn; Castelle et al., 2007, Schäfer et al., 2006). Furthermore, the regional geology and agricultural activities also contribute (e.g. As, Cu, Zn) to the metal/metalloid loads in the Gironde watershed (Masson et al., 2006, Masson et al., 2007). Recent work has revealed remarkably high Ag concentrations (up to 60 mg kg⁻¹ in 2003/2004) in wild oysters at the mouth of the Gironde Estuary (Fig. 1A; i.e. the highest measured in oysters and mussels along the French coast; Chiffoleau et al., 2005), indicating the fluvial–estuarine system is also contaminated by Ag. Both Ag and Cd are highly toxic to aquatic organisms and may be easily bioaccumulated (Eisler, 1996, US EPA, 2001, Vijayavel et al., 2007), especially by oysters (e.g. Gunther et al., 1999, Ivanina et al., 2011, Reinfelder et al., 1997). The measured concentrations in the Gironde oysters clearly exceed safety limits for human consumption for Cd (5 mg kg⁻¹; dw; EC No. 466/2001) and although there is no legal threshold for Ag, consumption of oysters from several sites along the French coast (Chiffoleau et al., 2005) may be incompatible with human maximum Ag daily intake recommended by the EPA (0.005 mg kg⁻¹ d⁻¹; www.epa.gov/iris/subst/0099.htm).

The present work aimed at documenting for the first time the historical Ag records in reservoir sediments upstream and downstream from the Decazeville watershed to (i) establish geochemical background for Ag in the upper Lot River watershed and (ii) quantify the extent of historical Ag contamination by the metal point source (Fig. 1B). Furthermore, we compare and discuss the chronologies of Cd and Ag accumulation in reservoir sediments and oysters from the Gironde watershed to understand (i) their relationships and (ii) changes over time in both concentrations and potential sources.