In French Guiana, gold-mining represents the second largest export activity with more than 50 million Euros in 2006, and began in 1850. Since the 80’s and more recently motivated by the increase of the gold price these last 5 years, this activity has strongly grown in French Guiana, reaching the National Park (“Parc Amazonien de la Guyane”) located in the Oyapock River basin. The gold exploitation has been estimated at 5 to 10 tons of gold per year by the DIREN (Direction régionale de l’industrie, de la recherche et de l’environnement) and WWF (2009) which represents, regarding the gold price, between 120 million and 220 million US$.
Unfortunately, gold price increases in phase with gold-mining activities and associated social impacts. But even if since the 1st of January 2006, the use of Hg is officially prohibited (arrêté préfectoral n°1232/SG du 08 juin 2004), we can estimate with a mean ratio Hg/Au of 1.3, that at least 6.5 to 13 tons of Hg per year are released in the environment, mainly in soils, sediment and surface water. From comparative studies realized in the Bolivian Amazon basin (Maurice B. et al., 2003) and in French Guiana (Laperche V. et al., 2007), it appears that Hg content in suspended sediment increases in exploited areas compared to pristine valleys. This element is mainly transported in rivers adsorbed on fine particles (Maurice B. et al., 2003) but it’s during its transfer in lentic zones, floodplain or reservoir lakes that it can be methylated and bioaccumulated in the food chain (Guimarães et al., 1998; Roulet et al., 2000). The “Office national des forêts » in 2005, estimated that in French Guiana, 1 333 km of rivers were directly impacted by legal and illegal gold-mining activities, contaminating downstream 4 671 km of rivers. Between 2000 and 2005, deforestation by gold-miners increased from 4 000 to 11 500 ha. Only along the Oyapock River, about 15 000 gold-miners are working illegally. But other dramatic consequences of the small-scale gold-mining activities are the increasing of the soil erosion and surface water turbidity, fish and hunt pressure (Charlet and Boudou, 2002) impacting the natural biodiversity of the region. Several organizations are asking for the development of an environmental observatory for Hg. From previous study, it appears that Hg concentrations in human hair of communities along the Oyapock R., the basin we study in the RIMNES project, are not in accordance with the Hg contents in sediment and fish in the same area. Upstream Trois Sauts village, Hg concentrations in sediment and fishes are generally low compared to the Camopi region, while Wayãpis communities are more contaminated upstream than downstream the gold-mining sites (Figure 1 below; Fréry, 2001; Quénel et al., 2007), with concentrations much higher than 10 µg g-1 (WHO limit). This variation can be attributed to differences in their dietary habits, upstream communities eating more freshwater fish. From the analysis in hair of Wayãpis people we realized in 2010 (RIMES-Guyane Project), we observed that 3% of the population living in Camopi (N=29) exceeds the WHO limit as against 18% in Trois Sauts (N=33), the pristine area; children, from 1 to 18 years old are more contaminated than adults.
Figure 1 - Location of the sampling points of sediment, fish and human hair along the Oyapock River, in French Guiana (RIMNES-Guyane ANR Program).
Figure 2. Hair sampling in Amerindian villages around Trois Sauts fall (French Guiana, Oyapock R., 2010).
Our first objective is to understand Hg processes and transformation in the environment using stable isotope signatures as a powerful tool to trace processes and sources of Hg. Our recent studies have demonstrated that Hg isotope signatures in human hair provide qualitative and quantitative information on MMHg sources related to human exposure via diet. A focus on the trophic chain, through bedrock, sediments, biofilms, fish and rice analyses, will permit to understand Hg isotopes fractionation before absorption of MMHg by human.