## 4 Discussion
### Recent Goldmining Activity Increased THg in River Sediment Despite the Hg Ban in 2006  
Overall values of THg in river sediments varied between 40 and 380 ng g\({}^{-1}\) and were similar in magnitude to previous results reported for French Guiana [14]. [PERSON] et al., [25] found 510 ng g\({}^{-1}\), 295 ng g\({}^{-1}\) and 261 ng g\({}^{-1}\) dry sediment at distances of respectively 0, 35, and 50 km downstream from mining areas. [PERSON] et al. [26], measured between 32 and 358 ng g\({}^{-1}\) with significantly higher values in artisanal and small-scale gold-mining (ASGM) areas along the River Oyapock. These results are also close to values of 20 to 53 ng g\({}^{-1}\) reported for river sediments along the Malinoski-Tambopata river system in Peru, a river affected by artisanal and small-scale gold-mining (ASGM) [27]. In another Peruvian river system, the Madre de Dios, also impacted by ASGM [28], authors reported Hg levels in sediments ranging from 9 ng g\({}^{-1}\) upstream up to 95.3 ng g\({}^{-1}\) downstream from the gold mined areas. More recently in Brazil, [29], measured between 19 and 155 ng g\({}^{-1}\) along the River Tapajos. Again, highest values were observed in the vicinity of gold mining areas. However, all these Hg concentrations in goldmining-affected sediments of the Amazonian water basin were clearly lower than those reported for River Gambia in Senegal, where Hg in sediments reached 1160 +/- 800 ng g\({}^{-1}\) in an area affected by ASGM [30].  
In the present study, the highest concentrations of Hg in sediment occurred at points B2, B3, and B4 along the Boulanger Creek. These sites are both located directly downstream from the old tailing ponds, from where Hg-rich sediment can be exported and from the most recently exploited mining area (2011-2012), where bare soils are likely to favor soil erosion. These re-exploited old placers probably are sources of both Hg naturally present in soil and Hg leftovers from old mining practices, which would explain the increase at point B4 between the dry and rainy season (THg doubled from 100 to 200 ng g\({}^{-1}\)) in relation to the onset of the exploitation next to the nearby Fourmi Creek in 2013. Previously, in a study conducted on the Tapajos River in Brazil, [PERSON] et al., [31] concluded that mining operations were at the origin of Hg anomalies, not from the amalgamation process itself, but from the increased erosion during the mining operations. This re-mobilizes the mercury contained naturally in soils and mercury resulting from historical mining activities that then migrates to the river. These results are coherent with our results and are supported by recent research conducted by [PERSON] et al., [26] using an isotopic approach. These authors showed that up to 78% of Hg in sediments downstream from the gold mining site was of anthropogenic origin, of which more than half derived from the release of liquid Hg (0) and the rest from erosion of surrounding Hg-contaminated soils.  
Outside the mining zones, THg concentrations in sediment were of the same order as those measured previously in this area [14]. Accordingly, the relatively low concentrations measured upstream from the mining areas (point B1) were similar to previously reported values for uncontaminated Amazonian sediments [32; 33].  
The decrease in the Hg load in sediments downstream from the Boulanger Creek and especially in the larger rivers such as the Orapu and the Comte is supposedly due to the huge dilution with waters draining the whole watershed and highlight the locally high values of creeks such as the Boulanger, impacted by goldmining.