## 2 Materials and Methods  
### Study Area and Sampling Strategy  
The Comte-Orapu basin covers 3359 km\({}^{2}\). This study focuses on the sub basin of the Boulanger Creek, a tributary of the Orapu River (Figure 1), and downstream rivers including the Comte River, which joins the Orapu, forming the Oyack River. Sampling was carried out along a section of \(\sim\)30 km, from the Boulanger Creek's spring, accessed along the Molokoi path, to the Oyack River, along which water and sediment samples were collected, during a dry season in September 2012 and the following rainy season in March 2013 as described below. During each sampling campaign, 14 water (~15 cm depth) and sediment samples were collected along the Boulanger Creek (Table 1 and Figure 1) and downstream Rivers (Orapu, Comte and Oyack). Suspended Particulate Matter (SPM) were recovered by filtering between 250 and 500 mL water on-site through pre-weighed cellulose nitrate filters (0.45 \(\upmu\)m; \(\O\)47 mm; Sartorius(r) Dourdan, France). The filters alone (blank) and the filters with SPM were transported to the laboratory in closed petrislides, then oven dried (50 \({}^{\circ}\)C, ~36 h) to constant weight. The mass of SPM sampled was determined on a precision balance (0.01 mg scales) from the difference with the initial dry filter weight. The filters were then conserved in sealed polypropylene boxes in the dark until further analysis. Water samples were obtained by filtering 125 mL water (<0.45 \(\upmu\)m, polyvinylidene fluoride filters (PVDF)) into pre-cleaned Teflon fluorinated ethylene propylene (FEP) bottles. Samples were acidified with supraparup HCl 0.5% _v/v_ (Merck, Darmstadt, Germany) and stored in double polyethylene bags until analysis. All materials (PP, Teflon) in contact with samples were acid-washed (5 days in 20% \begin{table}
\begin{tabular}{c c c} \hline \hline
**Water Body** & **Name** & **Description** \\ \hline \multirow{8}{*}{**Boulanger Creek**} & **B1** & Upstream Boulanger Creek \\  & **B2** & Downstream from Old Places \\  & **B3** & Downstream 2011–2012 Places \\  & **B4** & Fourmi Confluance \\  & **B5** & Downstream Leonce \\  & **B6** & Boulanger Creek \\  & **B7** & Boulanger Creek \\  & **B8** & Boulanger Creek \\  & **B9** & Boulanger Creek near the Confluence with the Orapu \\ \hline \multirow{4}{*}{**Orapu river**} & **Or1** & Upstream Boulanger Confluence \\  & **Or2** & Downstream Boulanger Confluence \\  & **Or3** & Orapu Confluence Creek Marguerite \\ \hline \multirow{2}{*}{**Comte**} & **C1** & Uptream from Orapu Confluence \\ \hline \multirow{2}{*}{**Oyack**} & **Oy1** & Oyack Downstream from Comte and Orapu Confluence \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of sampling points. Figure 1: Map of the study area in French Guiana and the sampling points. HNO\({}_{3}\)_v/v_ followed by 5 days in HCl 10% _v/v_), rinsed several times with deionized water (Milli-Q\({}^{\circledR}\), Merck Millipore, Darmstadt, Germany) and dried under a laminar flow hood and stored in sealed polyethylene bags pending sampling. Polyethylene gloves were used for all handling operations. River sediment samples were collected along the river section at the same spots as the water and SPM samples. At each sampling site, sediments were collected from just above the low water line during low tide on river banks where finer sediment tends to accumulate. It is assumed that they provide geochemical information that reflects the previous season's conditions [14]. There was no chemical (saltwater) impact of the tide on the water samples. Sediment samples were stored in polyethylene bottles and sent to the laboratory in Petit Saut (Hydroco, French Guiana, French) for treatment and analysis. During the dry season (September 2012), sediment samples were also collected from the edges of 14 old tailing ponds (Figure 1). Some of these tailing ponds were filled more than 10 years ago and they are now covered by vegetation without having been properly rehabilitated, which makes it too dangerous to walk on the tailing pond's surface.