Quantifying the impacts of artisanal gold mining on a tropical river system using mercury isotopes

International audienceTotal mercury (Hg) concentrations, speciation and isotopic composition were analyzed in soils and sediments along the Oyapock River (French Guiana), subjected to intense artisanal and small-scale gold-mining (ASGM). As a large part of these mining activities is illegal, amalgamation of gold with liquid Hg(0) is the main exploitation process. Further chemical analyses were performed along the same river, in bedrocks, waters, suspended matter, sediments and soils in both pristine and gold-mining areas. Physico-chemical parameters and Hg isotopic composition measured in the different matrices of the pristine area allowed us to trace the origin and fate of the natural Hg. We find that Hg in tropical topsoils preferentially originates from gaseous Hg(0) uptake by plants rather than from wet deposition. A strong negative mass independent fractionation (MIF) between topsoils and deeper horizons was attributed to pre-anthropogenic atmospheric Hg deposition with low MIF that leached to deeper horizons. Hg isotopic composition in pristine sediments confirmed that Hg in river sediments mainly originates from deep soil erosion and is subjected to aquatic photochemical reduction. Combining a Hg-isotope binary mixing model with a multiple linear regression based on physico-chemical parameters, we determined that the currently mined creek sediments are contaminated in Hg. This excess of Hg, which can represent more than half of the total Hg concentration, mainly originates from liqui