## 3 Results
### Total Hg in River Sediments and Suspended Particulate Matter in Relation to Turbidity  
Total Hg in river sediments (THg) from both sampling campaigns were compared (Figure 3a). Overall, there was little difference between the dry season and the rainy season with THg concentrations varying from 40 to 380 ng g\({}^{-1}\). For both campaigns, the highest concentrations occurred at points B2 and B3 in the sector exploited prior to 2006 (B2) and in the more recently exploited area (B3).  
The lowest THg in river sediment was measured at point B5, where the Boulanger Creek was deviated from its original course. The sediment at this point contained 42 ng kg\({}^{-1}\), close to the lowest value (31 ng g\({}^{-1}\)) found in this area [14]. Moreover, THg concentrations at B5 were even lower in 2013 (34 ng g\({}^{-1}\) dry sediment), when the Creek was positioned to flow through the middle of the flats in a rehabilitated zone.  
Downstream from B5, total Hg concentrations in river sediments increased regularly from 70 to maximum 150 ng g\({}^{-1}\) along the Boulanger Creek to its confluence with the Orapu River (point B9) and were then constant until the Orapu and the Comte Rivers merge and form the Oyack River (points Or1 to Or3).  
Total Hg concentrations in SPM collected during the dry season are comparable to those in the sediment and vary from 80 to 505 ng g\({}^{-1}\) (Figure 3b). However, during the rainy season Hg concentrations in SPM increased by two- to three-fold, except in the most upstream point where values were the same (\(\sim\)150 ng g\({}^{-1}\)) for both seasons. Maximum Hg concentrations in SPM, i.e., \(\sim\)600 ng g\({}^{-1}\) were higher than maximum values in sediments (\(\sim\)380 ng g\({}^{-1}\)).  
Figure 2: Localization of samples taken from old tailing ponds in the Boulanger concession in the dry season of September 2012 and total mercury (THg) contents (ng g\({}^{-1}\)). Points 17–24 are tailing ponds dating before the mercury (Hg) ban for gold mining in 2006, whereas points 25–30 are more recent.  
Figure 3: (**a**) Hg associated to river sediments, (**b**) Hg associated to suspended particulate matter (SPM), (**c**) suspended particulate matter (SPM) and (**d**) Turbidity at all sampling points for the dry season in September 2012 and the rainy season in March 2013. B1 to B9: Boulanger Creek, Or1 to Or3: Orapu River, C1: Comte River and Oy1: Oyack River.  
Although Hg concentrations in SPM increased during the rainy season, the amounts of SPM in river water were globally similar for both sampling campaigns (Figure 3c) and ranged between 2 and 18 mg L\({}^{-1}\). Turbidity on the other hand was 2-9-fold higher during the rainy season with an increase in NTU along the Boulanger Creek to 38 NTU that then dropped in the Orapu and Oyak to around 10-15 NTU (Figure 3d). The lowest value (<5 NTU) corresponded to an isolated spot upstream from the mining district and uninfluenced by human activities.  
Accordingly, amounts of SPM did not follow the same pattern as the turbidity values. However, a linear relation was found between turbidity and SPM concentrations, that differed between the dry and rainy seasons (Figure 4).