## 1 Introduction  
Mercury (Hg) is a toxic metal and a global pollutant, historically used in alluvial gold-mining to amalgamate and recover gold. In 2005, small-scale gold mining was estimated to contribute to more than 10% of the annual global anthropic Hg load to the atmosphere [1]. Use of Hg for gold mining was officially banned in French Guiana in 2006. However, \"inherited\" Hg in old gold mined areas is still present and remains a potential source of Hg to the environment. It is estimated that for 1 g of extracted gold, ~1.3 g of Hg are lost, despite its partial recycling [2, 3]. In addition to Hg inputs from gold mining, Amazonian soils naturally contain high Hg concentrations due to the equatorial pedoclimatic situation (on average 10 times higher than temperate soils) [4]. During gold mining and generally during all deforesting operations [5, 6, 7, 8], Hg adsorbed to colloids (clay, organic matter, and iron or aluminum oxides) is exported to the watershed, contaminating downstream ecosystems. In anaerobic environments (stagnant lagoons and sediment/water interface), Hg can be methylated, mainly by sulfate and iron-reducing bacteria, to form monomethylmercury (MMHg) [9, 10]. MMHg is a neurotoxin which readily accumulates in the food chain up to concentrations considered dangerous for human health (around 1 mg kg\({}^{-1}\) fresh weight in carnivorous fish such as Aymara, which is twice the World Health Organization (WHO) recommendation and 10\({}^{5}\)-10\({}^{6}\) times higher than surrounding water concentrations) [11]. In local Amerindian populations, whose diet traditionally consists of freshwater fish, Hg concentrations can reach values clearly exceeding World Health Organization (WHO) safety recommendations. Indeed, concentrations of more than 10 \(\upmu\)g g\({}^{-1}\) hair have been measured in these populations [12]. French Guiana has known several gold rushes towards the end of the 19 th century and especially at the beginning of the 20 th. The gold mining techniques at that time, mainly imported from Anglo-Saxon methods developed in the Rocky Mountains, already used Hg to amalgamate gold. During these periods, gold-mined sites in French Guiana were mainly placers in small riverbeds or lateritic colluvium exploitations. The use of river barges came later, during the second half of the 20 th century. It is considered that the activity at that time was much higher than presently, considering the number of placers and the volume of mineral treated. Evidently, this gold mining activity left traces of Hg contamination both on-site and downstream from the placers in the hydric network and the long-term stability of these Hg stocks is unknown. Many of the old placers are now covered by secondary rain forest, where little erosion occurs. However, these areas are potential sources of Hg pollution and need to be investigated to estimate their potential contribution to Hg dispersion in the Guianese hydric network, especially as many still contain economically relevant amounts of gold and are being re-exploited. Although 35% of the Hg used for goldmining is estimated to be released to the atmosphere, the remaining 65% are dumped on-site as metallic Hg (0) [13]. According to [PERSON] et al. [14], Hg concentrations in Guiana river sediments are generally close to background soil concentrations (100 \(\pm\) 50 \(\upmu\)g kg\({}^{-1}\)) and strongly increase close to gold mining sites. Concentrations up to 12 mg kg\({}^{-1}\) have been measured in the River Approuague sediments and up to 15 mg kg\({}^{-1}\) along the Eddie Creek (unpublished data). Spatial distribution in sediments along French Guiana Rivers indicates that metallic Hg lost by gold miners globally remains in the rejection zones, as contamination \"hot spots\" [14]. At gold-mined sites, Hg in soils either occurs naturally or is left over from past gold mining in the case of reopened old sites [8].