**3.4 Stream sediment sampling**

24 Novel Approaches and Their Applications in Risk Assessment

**CHA1** *Saint Gervais* Charente 40 0.14 X 86.4 7.3 101 98.4 15 112 0.030 3.78 1.69 **CHA2** *Sansac* Charente 44 1.1 X 96.6 7.4 95 92.5 4 170 0.010 2.11 0.73 **CHA3** *Pont de Suris* Charente 110 2.1 Diren 110 7.6 120 91.6 12 163 0.010 4.36 0.70 **CHA4** *Chez Paire* Charente 230 4.5 X 115 7.6 165 90.8 10 162 0.031 8.16 3.06 **CHA5** *Charroux* Charente 346 6.4 Diren 207 7.5 166 86.2 9 223 0.020 4.86 2.63 **CHA6** *Saint-Saviol* Charente 492 8.2 Diren 226 7.6 171 92.6 13 259 0.022 3.51 2.29 **CHA7** *Aunac* Charente 1090 22 X 373 7.8 153 93.6 13 415 0.011 3.07 1.39 **CHA8** *Vindelle* Charente 3750 47 Diren 374 8.0 227 89.7 11 425 0.009 2.33 0.90 **CHA9** *Sireuil* Charente 4070 82 X 378 8.0 205 91.0 12 381 0.008 2.96 2.18 **CHA10** *Chaniers* Charente 7412 91 Diren 461 8.0 197 90.3 9 410 0.010 4.97 2.11 **a** *Massignac* Upstream Moulde 52 0.46 X 98.3 7.4 85 101 6 114 0.009 1.20 0.82 **b** *Chez Boige* Downstream Moulde 54 0.96 X 103 7.5 135 99.8 2 134 0.008 2.72 0.76 **c** *Riou Mort* - - - - - - - - n.d n.d n.d n.d 37.7 **d** *Poursac* Argentor 108 2.9 Diren 426 8.0 175 102 7 335 0.010 3.57 1.20 **e** *Mouton* Son-Sonnette 226 2.7 X 425 8.3 162 115 8 319 0.009 3.95 1.14 **f** *Saint Ciers/Bonnieure* Bonnieure 203 2.7 Diren 262 8.1 224 103 11 218 0.011 2.30 0.89 **g** *Montbron* Upstream Tardoire 389 9.4 Diren 87.5 7.6 94 89.4 9 93 0.013 4.59 1.78 **h** *Feuillade* Bandiat 333 5.2 Diren 176 7.8 189 90.0 9 100 0.009 6.17 1.70 **i** *Coulgens* Downstream Tardoire 1200 8.2 Diren 128 7.2 199 98.1 23 108 0.011 3.59 1.31 **j** *Les Perceptiers* Ne 602 7.4 Diren 651 8.1 202 110 11 477 0.009 1.80 0.72 **k** *La Lijardière* Seugne 902 9.9 Diren 654 7.9 212 97.0 12 470 0.009 2.13 0.72

**m<sup>3</sup> /s**

Table 1. Description of sampling sites, water discharge obtained by the Regional Environment Agency-DIREN or measured in this study (X), physical and chemical parameter values (conductivity, Eh, pH, dissolved oxygen saturation) and SPM, nitrate, dissolved and particulate Cd (in SPM and stream sediments < 63µm) concentrations

This sampling strategy, aiming at estimating instantaneous fluxes, required reliable discharge data for each of the selected observation sites. However, only 12 sites are equipped with permanent gauging stations maintained by the Regional Environment Agency-DIREN (BanqueHydro®). Therefore, for this study, 8 additional river gauging measurements were performed at the other sampling sites (Table 1). Standard instantaneous discharge measurements were made by measuring flow velocities at different depths along vertical profiles, each of them representing a segment of the river cross-section. The crosssectional area of each segment was then multiplied by the corresponding integrated measured velocities to estimate water discharge in the segment. The sum of river discharges in all segments represents the estimated instantaneous water discharge of the river section. The uncertainty on the measurements was estimated between 5 and 10% (Regional

The general physical and chemical parameters (pH, conductivity, Eh and O2) were measured in-situ at each site. Temperature and conductivity were measured using a TetraCon 96® probe (PROFILINE, WTW). Oxygen saturation was determined by an ISY 52® probe. Determinations of pH and Eh were performed using a Sentix® 41 probe (PROFILINE, WTW). At each site, water was sampled manually for SPM, nitrate and Cd concentrations using clean techniques: all materials in contact with the water samples were made of polypropylene (PP), carefully decontaminated as previously detailed in Canton et

**Charente - CHA10 (2006-2007; Dabrin, 2009)**

**3.2 Discharge measurements** 

Environment Agency- DIREN).

al. (2012) for nitrate and in Audry et al. (2004) for Cd.

**3.3 Water sampling** 

**Item River <sup>Q</sup>**

**Area Cond. pH eH O2 SPM Nitrate Dissolved Cd Part. Cd Part. Cd km² µS/cm mV % mg/l µmol/l µg/l SPM mg/kg (<63µm) mg/kg**

mean 0.024 4.89 min 0.011 1.21 max 0.048 9.86

> Stream sediments are commonly used for geochemical prospecting. Collected just after a strong hydrological event, the geochemical composition of these samples corresponds to the maximum particulate transfer to the estuary and coastal zone. Unlike SPM, whose composition can rapidly fluctuate, stream sediments integrate metal contamination (Coynel et al., 2009).

> The coordinates of the sampling locations were recorded with a differential GPS. At each site, representative samples, consisting of the uppermost 1 cm of sediment from several recent depositional pockets were collected with a plastic spatula within a distance of 5-10 meters to enhance representativeness. The preferential accumulation of metals, either of natural or anthropogenic origin, in the fine-grained sediment fractions may induce grain size effects and reduced sample representativeness (Förstner & Wittmann, 1981; Horowitz, 1991; Benoit & Rozan, 1998). Therefore, stream sediment samples were sieved (<63 µm; nylon sieves) to remove coarse material which was obviously not representative of typical grain size of suspended sediment (Coynel et al., 2009).

#### **3.5 Nutrient analysis**

The dissolved inorganic compounds were colorimetrically analyzed according to standardized techniques. Dissolved nitrates (ΣNO3-= NO3- + NO2 - ) were analyzed by Flow Injection Analysis (FIA) according to Canton et al. (2012). Precision was ±10% for ΣNO3- .

#### **3.6 Particulate Cd extraction in SPM and stream sediment**

Representative subsamples (~30 mg of dry, powdered and homogenized material) of SPM or sediment were digested in acid-cleaned closed reactors using 1.5 ml HCl s.p. (12 M), 2 ml HF s.p. (22 M) and 0.75 µl HNO3 s.p. (14 M) at 110°C for 2 h using a temperature-controlled digestion system (DigiPREP MS®, SCP SCIENCE). After evaporation to dryness (10 h at 110°C), the residues were completely re-dissolved in 0.25 ml HNO3 s.p. (14 M) and 5 ml Milli-Q® water on a heating plate (15 min at 60°C) and after cooling brought to 10 ml in volumetric flasks using Milli-Q® water.

Spatial Cadmium Distribution in the Charente Watershed and

irrigation (Figure 2).

**4.3 SPM measurements** 

event with Q=350 m3/s)

0

0 20 40 60 80 100 **Qj (m3/s)**

versus daily water discharge in the Charente River.

nitrate cadmium

**A B**

100 200

300 400

**Nitrate (µM/l)**

500

**4.4 Nitrate concentrations** 

Potential Risk Assessment for the Marennes Oleron Bay (Southwest France) 27

between 2 m3/s and 40 m3/s. During our water monitoring campaign, the La Touvre water discharge was ~19.3 m3/s. The apparent water discharge deficit at CHA10 compared to the general relation between discharge and area drained may reflect the intense agricultural

The SPM concentrations ranged from 2 mg/l in the Moulde River, just after the Mas-Chaban Reservoir to 23 mg/l in the Tardoire River (Table 1). The lowest value can be explained because of hard to erode rock sills combined with the settling of SPM due to the presence of the Mas-Chaban Reservoir. Based on the world river classification of SPM concentration proposed by Meybeck et al. (2003), these values can be considered as either "very low" (<20 mg/l), generally observed for watersheds located downstream of major or numerous lakes (e.g. Alpine Rhone River), or in very flat and humid regions with wetland predominating (e.g. the Central Amazon watershed) or "low" (20-100 mg/l) characteristic of plain watersheds. However, even if our sampling campaign is representative of a moderate to high discharge hydrological situation (Q=~100 m3/s), SPM concentrations probably are much higher during flood events as previously observed by Dabrin (2009), during 2006- 2007, at the outlet of the Charente watershed (Chaniers site; SPM = 200 mg/l during a flood

The nitrate concentrations were 93-477 µM/l with an average ~246 µM/l (Table 1). Nitrate concentrations measured in the main hydrological section of the Charente River increased from upstream (~100 µM/l) to downstream (up to 425 µM/l at CHA8) and were positively correlated to water discharge at the watershed scale (Figure 3). This clear nitrate increase

0.00

Fig. 3. (A) Nitrate and dissolved Cd concentrations versus daily water discharge in the Charente River; (B) Suspended Particulate Matter (SPM) and particulate Cd concentrations

0.01

0.02

**Cd (µg/l)**

**SPM (mg/l)**

0

0 20 40 60 80 100 **Qj (m3/s)**

MES cadmium

0

2 4

6 8

**Cd (mg/kg)**

10

4

8

12

16

0.03

0.04

#### **3.7 Dissolved and particulate Cd analysis**

Dissolved and particulate Cd concentrations were measured using ICP-MS (X7, THERMO) with external calibration under standard conditions. The applied analytical methods were continuously quality checked by analysis of international certified reference sediments (PACS-1, BCR 320, SL-1, SLRS-4). Accuracy was within 5% and 7% of the certified values in the dissolved and particulate fractions, respectively. The analytical error (relative standard deviation) was better than 5% (rsd) for both phases. The detection limit estimated as 3 sigma of method blanks was 2 ng/l for the dissolved phase and 0.04 mg/kg for the particulate phase.
