**3. Material and methods**

#### **3.1 Sampling campaign**

Tardoire Bandiat

Moulde

Né Seugne

**CHA 1 CHA 2 <sup>b</sup> CHA 3 a**

**CHA 4 c**

**CHA 6 CHA 5**

**<sup>f</sup> <sup>i</sup>**

**g h**

**CHA 7 d e CHA 8**

**CHA 9**

*Angouleme*

Argentor Son-Sonnette

Bonnieure

Charente

22 Novel Approaches and Their Applications in Risk Assessment

estuarine system remains poorly studied in spite of its great hydrological influence on the MOB. The Charente River is the major river discharging directly into the MOB. During summer, 90% of the freshwater inputs into the Bay come from the Charente River (Ravail-Legrand, 1993), which drains an area of 10,549 km² dominated by farming (Bry & Hoflack, 2004). Only few studies have previously assessed the importance of the Charente Estuary to the overall metal contamination of the MOB (Gonzalez et al., 1991; Boutier et al., 2000; Dabrin, 2009). Recently, Dabrin (2009) showed that dissolved and particulate Cd concentrations the outlet of the Charente watershed, i.e. at the entry of the Charente Estuary were similar to those in the Garonne River and contributed up to 60% to total Cd inputs into

In this context, the objective of this study is to (i) characterize the Cd content in water (<0.2 µm; dissolved phase) and in particles (SPM and stream sediments) exported by the Charente sub-watersheds; (ii) assess their level of contamination by comparison with world references and ecotoxicological indices and (iii) identify point sources and/or diffuse sources in the Charente watershed. This first assessment of the sub-watershed contributions to the fluvial Cd export is essential to the control and reduction of Cd contamination in

The Charente watershed (surface area = 10,549 km²; ~500,000 inhabitants) is surrounded by the Massif Central to the East, the Paris sedimentary basin to the North, the Aquitaine sedimentary basin to the South and by the Armorican Massif to the Northwest (Figure 1). It is essentially composed of limestone formations dating from the Secondary: (i) in the North, the Jurassic formations are composed of large limestone beds in various facies and, (ii) in the South, the Cretaceous formations are formed by clay, sand, chalk and decalcification clays. The Primary formations crop out in the most upstream catchment areas of the Charente

> **CHA10 <sup>j</sup> <sup>k</sup>**

 The upstream Charente watershed features metamorphic rocks to the East. Two large reservoir dams, the Lavaud reservoir (400 ha; 10 Mm3) on the Charente River and the

*Saintes*

oysters, i.e. to successful environmental management in this vulnerable region.

watershed, represented by plutonic and metamorphic rocks (BRGM, 2003).

**Oleron**

**Gironde Estuary**

The Charente watershed can be divided into three main domains (Figure 1):

*Marennes Oleron*

the MOB, highlighting the need to precisely identify the origin(s).

**2. Presentation of the study area** 

**France**

*Massif Central*

Fig. 1. Location of sampling sites

**Spain**

*Bay of Biscay* A sampling campaign was conducted from April 6 to April 8 2010. Strategic sites were selected by Geographical Information System (GIS, ArcView ®) for testing different environmental characteristics (e.g. geology, land-use) and evaluating their impacts on Cd concentrations. In total, 20 strategic sites were selected on the Charente River (n=10 sites; notified by CHA; Figure 1; Table 1) and its tributaries (n=10 sites; notified by a letter; Figure 1; Table 1) characterized by contrasting geology, industrial and agricultural activities. An additional site was chosen on a small drain near Riou Mort River (former mining area; "c", Figure 1) for collecting a stream sediment deposit. Note that this Riou Mort River in the Charente watershed is not identical with the well-studied Riou Mort River draining the polluted Decazeville basin, responsible for important historical polymetallic (mainly Cd, Zn, Cu, Pb, Hg, Ag) pollution in the Lot-Garonne river continuum (e.g. Blanc et al., 1999; Schäfer et al., 2002; Audry et al., 2004; Coynel et al., 2009).

Spatial Cadmium Distribution in the Charente Watershed and

grade) and stored at 4°C awaiting analysis.

grain size of suspended sediment (Coynel et al., 2009).

standardized techniques. Dissolved nitrates (ΣNO3-= NO3-

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

volumetric flasks using Milli-Q® water.

**3.4 Stream sediment sampling** 

& Blanc, 2002).

et al., 2009).

**3.5 Nutrient analysis** 

Potential Risk Assessment for the Marennes Oleron Bay (Southwest France) 25

Back in the laboratory, river water samples were homogenized and precise volumes (~500 ml) were filtered through pre-weighed 0.7 µm filters (Durieu®). Then the filters were dried to constant weight (45°C; 12 h) and re-weighed in order to obtain SPM concentrations. For dissolved nitrate and Cd, all river water samples were immediately filtered on-site through 0.2 µm Sartorius® polycarbonate filters. For nitrate, filtrates were collected in 14 ml polypropylene tubes and stored at -80°C until analysis; for dissolved Cd, filtrates were collected in pre-cleaned 30 ml polypropylene bottles, acidified (1/1000; HNO3 suprapur

Suspended particulate matter for Cd analyses was retrieved by pumping up to 80 L of river water (~50 cm from the bank at 10-20 cm depth) using a peristaltic pump with PP-tubing followed by centrifugation (Westfalia, Germany; 12,000 g). This technique is considered a practicable and reliable method for SPM sampling in all hydrological situations (e.g. Schäfer

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

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

The dissolved inorganic compounds were colorimetrically analyzed according to

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

Injection Analysis (FIA) according to Canton et al. (2012). Precision was ±10% for ΣNO3-.

+ NO2-

) were analyzed by Flow


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

#### **3.2 Discharge measurements**

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 Environment Agency- DIREN).

#### **3.3 Water sampling**

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

Back in the laboratory, river water samples were homogenized and precise volumes (~500 ml) were filtered through pre-weighed 0.7 µm filters (Durieu®). Then the filters were dried to constant weight (45°C; 12 h) and re-weighed in order to obtain SPM concentrations. For dissolved nitrate and Cd, all river water samples were immediately filtered on-site through 0.2 µm Sartorius® polycarbonate filters. For nitrate, filtrates were collected in 14 ml polypropylene tubes and stored at -80°C until analysis; for dissolved Cd, filtrates were collected in pre-cleaned 30 ml polypropylene bottles, acidified (1/1000; HNO3 suprapur grade) and stored at 4°C awaiting analysis.

Suspended particulate matter for Cd analyses was retrieved by pumping up to 80 L of river water (~50 cm from the bank at 10-20 cm depth) using a peristaltic pump with PP-tubing followed by centrifugation (Westfalia, Germany; 12,000 g). This technique is considered a practicable and reliable method for SPM sampling in all hydrological situations (e.g. Schäfer & Blanc, 2002).
