**4. Results and discussion**

#### **4.1 Physical and chemical parameters**

Conductivity, redox potentials (Eh) and pH (to a lesser extent) in the Charente River and its tributaries tended to increase from upstream to downstream, probably reflecting local geological characteristics (metamorphic rocks in the upstream sections; limestone formations in the downstream watershed) and land use (vineyards). The dissolved oxygen saturation values were rather similar whatever the sites (Table 1).

#### **4.2 Discharge monitoring**

The spatial variability of the Charente River water discharge is linearly related to its drainage basin area for the section between CHA1 and CHA9, except for CHA8 (Figure 2).

Fig. 2. Relationship between daily water discharges measured at different sites of the Charente River and the corresponding surface areas

The permanence of shoal sedimentation during the Middle and Upper Jurassic is a local feature which explains the development of the La Rochefoucauld karstic system, close to Angouleme city (Figure 1) and the relatively low water discharge measured at CHA8. The Bandiat and the Tardoire Rivers flow over the La Rochefoucauld karst accounting for more than 50% of the outlet discharges (Kurtulus & Razack, 2007). The La Touvre spring, located near of Angouleme city, constitutes an important discharge system of the aquifer varying 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 irrigation (Figure 2).

#### **4.3 SPM measurements**

26 Novel Approaches and Their Applications in Risk Assessment

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

Conductivity, redox potentials (Eh) and pH (to a lesser extent) in the Charente River and its tributaries tended to increase from upstream to downstream, probably reflecting local geological characteristics (metamorphic rocks in the upstream sections; limestone formations in the downstream watershed) and land use (vineyards). The dissolved oxygen

The spatial variability of the Charente River water discharge is linearly related to its drainage basin area for the section between CHA1 and CHA9, except for CHA8 (Figure 2).

**CHA9**

**CHA8**

0 2000 4000 6000 8000 **Area (km²)**

Fig. 2. Relationship between daily water discharges measured at different sites of the

The permanence of shoal sedimentation during the Middle and Upper Jurassic is a local feature which explains the development of the La Rochefoucauld karstic system, close to Angouleme city (Figure 1) and the relatively low water discharge measured at CHA8. The Bandiat and the Tardoire Rivers flow over the La Rochefoucauld karst accounting for more than 50% of the outlet discharges (Kurtulus & Razack, 2007). The La Touvre spring, located near of Angouleme city, constitutes an important discharge system of the aquifer varying

**CHA10**

**3.7 Dissolved and particulate Cd analysis** 

**4. Results and discussion** 

**4.2 Discharge monitoring** 

**4.1 Physical and chemical parameters** 

saturation values were rather similar whatever the sites (Table 1).

**CHA7**

0

Charente River and the corresponding surface areas

20

40

60

**Qj (m3/s)**

80

100

phase.

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 event with Q=350 m3/s)

#### **4.4 Nitrate concentrations**

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

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 versus daily water discharge in the Charente River.

Spatial Cadmium Distribution in the Charente Watershed and

**CHA10**

Fig. 4. Spatial distribution of dissolved Cd concentrations

**CHA10**

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

Fig. 5. Spatial distribution of particulate Cd concentrations in SPM

concentration below which no effect on organisms is expected;

**4.6 Particulate Cd concentrations in SPM** 

**k**

**j**

Potential Risk Assessment for the Marennes Oleron Bay (Southwest France) 29

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

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

Cd (mg/kg) < 0.99 (TEC) 0.99 – 4.98 (PEC) > 4.98

**CHA 4**

**CHA CHA 5 6**

**CHA 7 d e**

**CHA 9**

The TEC (Threshold Effect Concentration; CdP=0.99 mg/kg) is defined as the CdP

**CHA 8**

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

**g h**

Cd (ng/l) 2 – 10 10 – MA > MA

**CHA 4**

**CHA CHA 5 6**

**CHA 7 d e**

**CHA 9**

The particulate Cd concentrations (CdP) were variable from one site to another with values comprised between 1.20 mg/kg and 8.16 mg/kg (Table 1). Most of these values were clearly higher than the average concentration measured in the SPM defined for Word Rivers (MA=1.55 mg/kg; Viers et al., 2009), except for the Upstream Moulde River (Table 1). As observed for CdD in the main hydrological section of the Charente watershed, the CdP concentrations in SPM decreased from upstream to downstream until CHA9; then a significant Cd increase occurred at CHA10 (Figure 3). The spatial evolution of the particulate Cd concentration is not correlated with that of the Charente SPM concentrations (Figure 3). We compared particulate Cd concentrations with consensus-based sediment quality guidelines proposed for freshwater sediments (MacDonald et al., 2000), i.e. concentration classes supporting the assessment of ecotoxicological risk potential (Figure 5).

**CHA 8**

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

**g h**

starts at the CHA5 site, i.e. where the Charente River drains maize areas. Based on this observation, we have determined typical nitrate concentrations for each land use in the Charente System: the mean nitrate concentrations are ~140 µM/l in the areas mainly occupied by pasture, ~350 µM/l for corn/maize production and ~430 µM/l in vineyard areas.

The Charente River faces high nitrate levels (Bry & Hoflack, 2004; EPTB-Charente, 2007). These high levels probably reflect the intensification of agriculture in the central and lower systems and increase the risk of eutrophication. The intensification of crops, particularly corn production, was accompanied by a high water demand for irrigation over the last thirty years. Vineyards are also located in the basin and can generate diffuse water pollution by nitrates. To assess whether the concentrations obtained on the Charente River are comparable to other systems, we compared our results with those recently published for rivers draining the Arcachon basin (SW France), which is affected by eutrophication due to nitrogen transfer from agricultural areas to the river system (Canton et al., 2012). These authors demonstrated that low concentrations (20 to 45 µM/l) occur in watersheds dominated by forest, whereas nitrate concentrations were considered as high in watersheds draining agricultural areas with an average of 140 µM/l (Canton et al., 2012). For these agricultural watersheds, nitrate concentrations increased from 200 to 500 µM/l in winter during high water discharges. Accordingly, our results obtained for the Charente River are similar to those in the rivers studied by Canton et al. (2012) supporting that the high nitrate concentrations may be attributed to agriculture as suggested previously (Bry & Hoflack, 2004; Vernier et al., 2010) and probably result in significant nitrate export to the Marennes-Oleron Bay and the adjacent coastal area.

#### **4.5 Dissolved Cd concentrations**

The dissolved Cd concentrations (CdD) in the Charente watershed ranged from 8 to 31 ng/l, with an average of ~17 ng/l (Table 1). Unlike the spatial nitrate evolution, the dissolved Cd concentrations in the mainstream decreased from upstream to downstream until CHA9; then a slight CdD increase occurred at CHA10 (Figure 3). Based on the CdD levels, a CdD distribution map aims at visualizing the spatial variation in the Charente watershed (Figure 4). The color classes were determined using the detection limit (2 ng/l), the CdD level (10 ng/l) representing "good status" of water quality as proposed by the Ministry of Environment and Sustainable Development and the Water Agencies quality guideline (SEQeau; MEDD and Agences de l'Eau, 2003) and the average CdD in world rivers proposed by Martin & Meybeck (1979) marked MA in Figure 4 (MA=50 ng/l).

In the downstream Charente watershed, the Seugne, Né, Sonnette and Bandiat Rivers showed CdD typical of good water quality (<10 ng/l), as well as the upstream sections of the Moulde, the Argentor and the Son-Sonnette Rivers. In contrast, the CdD concentrations in the upstream section of the Charente River (CHA1; CHA4-CHA6) suggested lower water quality (20-30 ng/l; Table 1). However, even the highest CdD measured in this study were lower than the CdD level defined for the world's major rivers (MA=50 ng/l; Figure 4), with low human influences (e.g. Amazon and Congo Rivers; Martin & Meybeck, 1979).

Fig. 4. Spatial distribution of dissolved Cd concentrations

#### **4.6 Particulate Cd concentrations in SPM**

28 Novel Approaches and Their Applications in Risk Assessment

starts at the CHA5 site, i.e. where the Charente River drains maize areas. Based on this observation, we have determined typical nitrate concentrations for each land use in the Charente System: the mean nitrate concentrations are ~140 µM/l in the areas mainly occupied by pasture, ~350 µM/l for corn/maize production and ~430 µM/l in vineyard

The Charente River faces high nitrate levels (Bry & Hoflack, 2004; EPTB-Charente, 2007). These high levels probably reflect the intensification of agriculture in the central and lower systems and increase the risk of eutrophication. The intensification of crops, particularly corn production, was accompanied by a high water demand for irrigation over the last thirty years. Vineyards are also located in the basin and can generate diffuse water pollution by nitrates. To assess whether the concentrations obtained on the Charente River are comparable to other systems, we compared our results with those recently published for rivers draining the Arcachon basin (SW France), which is affected by eutrophication due to nitrogen transfer from agricultural areas to the river system (Canton et al., 2012). These authors demonstrated that low concentrations (20 to 45 µM/l) occur in watersheds dominated by forest, whereas nitrate concentrations were considered as high in watersheds draining agricultural areas with an average of 140 µM/l (Canton et al., 2012). For these agricultural watersheds, nitrate concentrations increased from 200 to 500 µM/l in winter during high water discharges. Accordingly, our results obtained for the Charente River are similar to those in the rivers studied by Canton et al. (2012) supporting that the high nitrate concentrations may be attributed to agriculture as suggested previously (Bry & Hoflack, 2004; Vernier et al., 2010) and probably result in significant nitrate export to the Marennes-

The dissolved Cd concentrations (CdD) in the Charente watershed ranged from 8 to 31 ng/l, with an average of ~17 ng/l (Table 1). Unlike the spatial nitrate evolution, the dissolved Cd concentrations in the mainstream decreased from upstream to downstream until CHA9; then a slight CdD increase occurred at CHA10 (Figure 3). Based on the CdD levels, a CdD distribution map aims at visualizing the spatial variation in the Charente watershed (Figure 4). The color classes were determined using the detection limit (2 ng/l), the CdD level (10 ng/l) representing "good status" of water quality as proposed by the Ministry of Environment and Sustainable Development and the Water Agencies quality guideline (SEQeau; MEDD and Agences de l'Eau, 2003) and the average CdD in world rivers proposed by

In the downstream Charente watershed, the Seugne, Né, Sonnette and Bandiat Rivers showed CdD typical of good water quality (<10 ng/l), as well as the upstream sections of the Moulde, the Argentor and the Son-Sonnette Rivers. In contrast, the CdD concentrations in the upstream section of the Charente River (CHA1; CHA4-CHA6) suggested lower water quality (20-30 ng/l; Table 1). However, even the highest CdD measured in this study were lower than the CdD level defined for the world's major rivers (MA=50 ng/l; Figure 4), with low human influences (e.g. Amazon and Congo Rivers; Martin &

Martin & Meybeck (1979) marked MA in Figure 4 (MA=50 ng/l).

areas.

Oleron Bay and the adjacent coastal area.

**4.5 Dissolved Cd concentrations** 

Meybeck, 1979).

The particulate Cd concentrations (CdP) were variable from one site to another with values comprised between 1.20 mg/kg and 8.16 mg/kg (Table 1). Most of these values were clearly higher than the average concentration measured in the SPM defined for Word Rivers (MA=1.55 mg/kg; Viers et al., 2009), except for the Upstream Moulde River (Table 1). As observed for CdD in the main hydrological section of the Charente watershed, the CdP concentrations in SPM decreased from upstream to downstream until CHA9; then a significant Cd increase occurred at CHA10 (Figure 3). The spatial evolution of the particulate Cd concentration is not correlated with that of the Charente SPM concentrations (Figure 3). We compared particulate Cd concentrations with consensus-based sediment quality guidelines proposed for freshwater sediments (MacDonald et al., 2000), i.e. concentration classes supporting the assessment of ecotoxicological risk potential (Figure 5).

Fig. 5. Spatial distribution of particulate Cd concentrations in SPM

 The TEC (Threshold Effect Concentration; CdP=0.99 mg/kg) is defined as the CdP concentration below which no effect on organisms is expected;

Spatial Cadmium Distribution in the Charente Watershed and

ecotoxicological risk at the watershed scale.

**4.9 Dissolved and particulate Cd fluxes** 

**Item River**

contribution at strategic sites

Potential Risk Assessment for the Marennes Oleron Bay (Southwest France) 31

phase (e.g. Schleichert, 1975; Dawson & Macklin, 1998; Cobelo-Garcia et al., 2004). In the Charente River system, Dabrin (2009) showed that the dissolved concentrations of some metals such as Cd decreased with water discharges, highlighting the close link between hydrology and geochemistry. As our campaign is representative of "medium to high water" conditions in the Charente River, our evaluation of the SPM quality defined throughout the watershed does not represent the worst geochemical situation. We can therefore assume that CdP may frequently exceed critical thresholds (e.g. PEC) at many sites during low waters and we recommand specific water monitoring during this period in order to evaluate

The instantaneous (daily) dissolved and particulate Cd fluxes were calculated for each sampling site (Table 2). The CdD levels significantly increased between the CHA3 and the CHA4 sites, inducing a clear increase in CdD flux that was not proportional to water discharge, suggesting an additional point source within this section (Figure 7A). The additional Cd point source (1 ~8g/d; Figure 7A) may be linked to the presence of mineral deposits in the metamorphic formation areas and specifically, near the fault of Beaumont Confolent (BRGM, 2003). This is supported by high CdP measured in the sediment collected in the former mining area (site "c"; Figure 1). A second abnormal increase in CdD fluxes (2 ~25g/d; Figure 7A) can be observed between the CHA9 and the CHA10 sites and cannot be explained by the CdD supply via the Ne and Seugne Rivers alone ("j" and "k"; Table 2). Additional campaigns will be needed to identify the source in this section by performing a longitudinal profile along the Charente River. Note that in this geographical area, we have located a manufactury which produces batteries for all types of civil and military aircrafts.

**CHA1** *Saint Gervais* Charente 0.2 0.7 0.4 66% **CHA2** *Sansac* Charente 0.4 0.8 0.9 47% **CHA3** *Pont de Suris* Charente 2.2 10 1.8 85% **CHA4** *Chez Paire* Charente 4.0 32 12 73% **CHA5** *Charroux* Charente 5.0 24 11 69% **CHA6** *Saint-Saviol* Charente 9.1 32 16 67% **CHA7** *Aunac* Charente 25 76 21 78% **CHA8** *Vindelle* Charente 44 102 36 74% **CHA9** *Sireuil* Charente 83 245 56 81% **CHA10** *Chaniers* Charente 71 353 82 81% **a** *Massignac* Upstream Moulde 0.2 0.3 0.3 45% **b** *Chez Boige* Downstream Moulde 0.2 0.5 0.7 40% **c** *Riou Mort* - - -- **d** *Poursac* Argentor 1.8 6.3 2.4 72% **e** *Mouton* Son-Sonnette 1.8 7.1 2.2 77% **f** *Saint Ciers/Bonnieure* Bonnieure 2.5 5.9 2.6 69% **g** *Montbron* Upstream Tardoire 7.2 33 11 76% **h** *Feuillade* Bandiat 3.9 24 4.2 85% **i** *Coulgens* Downstream Tardoire 16 59 8.1 88% **j** *Les Perceptiers* Ne 7.3 13 5.7 70% **k** *La Lijardière* Seugne 11 22 8.0 74%

Table 2. Daily SPM flux and dissolved and particulate Cd flux and particulate Cd flux

**SPM Flux Part Cd flux Diss Cd flux Part Cd flux t/j g/j g/j contribution**

 The PEC (Probable Effect Concentration; CdP =4.98 mg/kg) is defined as the CdP concentration above which effects on organisms are expected.

Most of the CdP in the Charente River SPM were higher than the TEC and at two sites the measured values exceeded the PEC level ("b" and "CHA4" with CdP = 6.17 and 8.16 mg/kg, respectively); two other sites (CHA10; CHA5) have values close to the PEC level (Figure 5). These results suggest that in the Charente River system toxic effects on aquatic organisms due to the presence of Cd (i) cannot be excluded at most of the sites studied and (ii) should be expected locally.

#### **4.7 Particulate Cd concentrations in stream sediments (<63µm)**

Excluding the Riou Mort site "c" (Cd = 37.7 mg/kg) which drains a former mineral resource deposit, CdP in sieved stream sediments ranged from 0.70 to 3.06 mg/kg (Table 1). The comparison with ecotoxicological indices showed that a majority of sites have CdP concentrations above the TEC, yet without exceeding the PEC level (Figure 6). This suggests that potential toxicity effects on water organisms in the Charente River due to the presence of Cd cannot be excluded.

Fig. 6. Spatial distribution of particulate Cd concentrations in <63 µm sediments

#### **4.8 Comparison with the geochemical monitoring performed during 2006-2007**

The dissolved and particulate Cd concentrations measured at the Chaniers site (CHA10 site) in this study were compared with those obtained during the 2 year-geochemical monitoring achieved in the Inter-Regional project "Défi Cadmium" (Cadmium Challenge, Water Agency Adour-Garonne) performed during 2006 and 2007 (Dabrin, 2009; Table 1). The Chaniers site is considered as the outlet of the fluvial Charente watershed. The CdP in SPM from CHA10 measured in this study was similar to average CdP established during the "Défi Cadmium" project. In contrast, the CdD concentration was lower than the minimum CdD value measured by Dabrin (2009). It has been classically observed in river systems that the lowest metal concentrations in water and in SPM occurred during high flow rates. This phenomenon can be related to dilution by (i) rainwater for the dissolved phase and (ii) coarse particles (coarse silt to sand) with low metal adsorption capacity for the particulate phase (e.g. Schleichert, 1975; Dawson & Macklin, 1998; Cobelo-Garcia et al., 2004). In the Charente River system, Dabrin (2009) showed that the dissolved concentrations of some metals such as Cd decreased with water discharges, highlighting the close link between hydrology and geochemistry. As our campaign is representative of "medium to high water" conditions in the Charente River, our evaluation of the SPM quality defined throughout the watershed does not represent the worst geochemical situation. We can therefore assume that CdP may frequently exceed critical thresholds (e.g. PEC) at many sites during low waters and we recommand specific water monitoring during this period in order to evaluate ecotoxicological risk at the watershed scale.

#### **4.9 Dissolved and particulate Cd fluxes**

30 Novel Approaches and Their Applications in Risk Assessment

The PEC (Probable Effect Concentration; CdP =4.98 mg/kg) is defined as the CdP

Most of the CdP in the Charente River SPM were higher than the TEC and at two sites the measured values exceeded the PEC level ("b" and "CHA4" with CdP = 6.17 and 8.16 mg/kg, respectively); two other sites (CHA10; CHA5) have values close to the PEC level (Figure 5). These results suggest that in the Charente River system toxic effects on aquatic organisms due to the presence of Cd (i) cannot be excluded at most of the sites studied and (ii) should

Excluding the Riou Mort site "c" (Cd = 37.7 mg/kg) which drains a former mineral resource deposit, CdP in sieved stream sediments ranged from 0.70 to 3.06 mg/kg (Table 1). The comparison with ecotoxicological indices showed that a majority of sites have CdP concentrations above the TEC, yet without exceeding the PEC level (Figure 6). This suggests that potential toxicity effects on water organisms in the Charente River due to the presence

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

Cd (mg/kg) < 0.99 (TEC) 0.99 – 4.98 (PEC) > 4.98

**CHA 4**

**CHA CHA 5 6**

**CHA 7 d e**

**CHA 9**

**CHA 8**

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

**g h**

**c**

concentration above which effects on organisms are expected.

**4.7 Particulate Cd concentrations in stream sediments (<63µm)** 

**CHA10**

**k**

**j**

Fig. 6. Spatial distribution of particulate Cd concentrations in <63 µm sediments

**4.8 Comparison with the geochemical monitoring performed during 2006-2007** 

The dissolved and particulate Cd concentrations measured at the Chaniers site (CHA10 site) in this study were compared with those obtained during the 2 year-geochemical monitoring achieved in the Inter-Regional project "Défi Cadmium" (Cadmium Challenge, Water Agency Adour-Garonne) performed during 2006 and 2007 (Dabrin, 2009; Table 1). The Chaniers site is considered as the outlet of the fluvial Charente watershed. The CdP in SPM from CHA10 measured in this study was similar to average CdP established during the "Défi Cadmium" project. In contrast, the CdD concentration was lower than the minimum CdD value measured by Dabrin (2009). It has been classically observed in river systems that the lowest metal concentrations in water and in SPM occurred during high flow rates. This phenomenon can be related to dilution by (i) rainwater for the dissolved phase and (ii) coarse particles (coarse silt to sand) with low metal adsorption capacity for the particulate

be expected locally.

of Cd cannot be excluded.

The instantaneous (daily) dissolved and particulate Cd fluxes were calculated for each sampling site (Table 2). The CdD levels significantly increased between the CHA3 and the CHA4 sites, inducing a clear increase in CdD flux that was not proportional to water discharge, suggesting an additional point source within this section (Figure 7A). The additional Cd point source (1 ~8g/d; Figure 7A) may be linked to the presence of mineral deposits in the metamorphic formation areas and specifically, near the fault of Beaumont Confolent (BRGM, 2003). This is supported by high CdP measured in the sediment collected in the former mining area (site "c"; Figure 1). A second abnormal increase in CdD fluxes (2 ~25g/d; Figure 7A) can be observed between the CHA9 and the CHA10 sites and cannot be explained by the CdD supply via the Ne and Seugne Rivers alone ("j" and "k"; Table 2). Additional campaigns will be needed to identify the source in this section by performing a longitudinal profile along the Charente River. Note that in this geographical area, we have located a manufactury which produces batteries for all types of civil and military aircrafts.


Table 2. Daily SPM flux and dissolved and particulate Cd flux and particulate Cd flux contribution at strategic sites

Spatial Cadmium Distribution in the Charente Watershed and

MOB.

reaches.

**5. Conclusion** 

during this campaign;

and/or diffuse Cd sources.

Potential Risk Assessment for the Marennes Oleron Bay (Southwest France) 33

related to inadequate sampling frequency may be very high and are likely to result in severe under-estimation of fluxes (Coynel et al., 2004). Based on this extrapolation, annual SPM flux and CdT export at CHA10 were evaluated to ~26,000 t/yr and 159 kg/yr, respectively, corresponding to a specific sediment yield of ~2.45 t/km²/yr and a specific CdT flux ~ 15 g/km²/yr. Accordingly, the specific (potentially under-estimated) Cd flux of the Charente watershed would be 2 times lower than that of the Cd-polluted Garonne watershed in 2006/2007 (39 g/km²/yr; Dabrin, 2009). However, as explained above, this Cd estimation for the Charente River system is probably highly underestimated. Indeed, based on a permanent observation sampling in 2006-2007 with high temporal resolution, the annual specific CdT was evaluated to 54 g/km²/yr for the Charente River, i.e. higher of that reported for the Garonne River (Dabrin, 2009). Furthermore the totality of Cd exported from the Charente watershed via the Charente Estuary directly enters the MOB and may reach the oyster farms therein, whereas only part of the Cd exported from the Gironde system may reach the vulnerable zones of the MOB. Our work intends to demonstrate that the Charente River system should be further studied because of its significant impact on the

This study represents a major advance in the geochemical characterization of water and water-borne particles in the Charente River and contributes to a better understanding of the Cd transfer into the Marennes-Oleron Bay. The main results obtained have allowed to:

 develop a first spatially resolved geochemical database of dissolved and particulate Cd concentrations at the watershed scale, i.e. in three different compartments (water, sediment and SPM) at 21 strategic sites along the Charente River and its tributaries,

 assess the quality of these three geochemical compartments using ecotoxicological criteria or world river references and locate sub-catchment with potential chemical risk; quantify the daily dissolved and particulate Cd fluxes representative of a moderate hydrological situation from the water discharge measurements provided by the Regional Environment Agency-DIREN and complemented by gauging measurements

estimate the Cd contribution from subcatchments and identify major zones with point

In terms of perspectives, further work should be carried out during low water because of the strong relationship between water quality and hydrology. It would also be necessary to determine more precisely the origins of the observed Cd anomaly, by taking samples from the former mining deposit and agricultural soils. This approach should be completed by a longitudinal sampling profile between CHA9 and CHA10, where significant additional dissolved and particulate Cd fluxes occurred. Our study includes the Charente River upstream from the Chaniers site, which is considered as the outlet of the fluvial system. Furthermore, a more complete mass balance should include fluxes via two other major tributaries, the Boutonne and Arnoult Rivers, entering the Charente Estuary in the tidal

taking into account the geology and land use characteristics;

The evolution of the daily CdP flux in the section of the Charente River was compared to that of the corresponding SPM flux. This comparison revealed a good statistical relationship between both parameters (Figure 7B). Based on this relationship, we can define a CdP average of ~2.90 mg/kg, corresponding to the regression slope and, probably, to the CdP level in agricultural lands which are mechanical eroded during runoff. Based on this, we have detected a small increase in CdP that was not due to the contribution of SPM at the CHA4 site and a strong CdP anomaly at the CHA10 site. Like for CdD, the minor CdP anomaly in CHA4 probably results from the presence of formerly exploited mineral deposits (Ba-Pb-Zn mineralization). Between the CHA9 and CHA10 sites, we have observed a SPM decrease related to sedimentation inducing decreasing SPM fluxes ('loss') in this section. Based on the observed relationship between SPM flux and CdP flux in the Charente River, the expected CdP flux at the Chaniers site for an instantaneous SPM flux of 71 t/d would be ~200 g/d (Figure 7B). The observed CdP flux of 353 g/d being ~1.8 times higher (Table 2) clearly suggests a significant Cd enrichment in SPM due to anthropogenic sources and reflects observations made for CdD.

Fig. 7. Relationship between daily dissolved Cd flux and water discharge (A) and between daily particulate Cd flux and SPM flux (B) at strategic sites in the main channel of the Charente River

The dissolved and particulate flux estimates underline the Cd partitioning (i.e chemical form) in the Charente River system. In the main section of the Charente River, the instantaneous Cd flux was mainly due to SPM transport (reaching 80% at CHA10), except at CHA2 (downstream from the Lavaud Reservoir) and in the Moulde River (upstream and downstream part), where the particulate transport contributed 47%, 45% and 40%, respectively, to total (dissolved + particulate) Cd (CdT) fluxes. These results suggest that the particulate phase is the predominant Cd vector in the Charente River system and demonstrate that efficient reduction of Cd transport into the Marennes-Oleron Bay would imply limitation of mechanical erosion, mainly in the agricultural areas.

By extrapolating daily fluxes to the annual scale, we have obtained a first approximation of annual SPM and Cd exports to the Charente Estuary and in fine to the MOB. These estimates probably represent the orders of magnitude, although errors on flux estimates related to inadequate sampling frequency may be very high and are likely to result in severe under-estimation of fluxes (Coynel et al., 2004). Based on this extrapolation, annual SPM flux and CdT export at CHA10 were evaluated to ~26,000 t/yr and 159 kg/yr, respectively, corresponding to a specific sediment yield of ~2.45 t/km²/yr and a specific CdT flux ~ 15 g/km²/yr. Accordingly, the specific (potentially under-estimated) Cd flux of the Charente watershed would be 2 times lower than that of the Cd-polluted Garonne watershed in 2006/2007 (39 g/km²/yr; Dabrin, 2009). However, as explained above, this Cd estimation for the Charente River system is probably highly underestimated. Indeed, based on a permanent observation sampling in 2006-2007 with high temporal resolution, the annual specific CdT was evaluated to 54 g/km²/yr for the Charente River, i.e. higher of that reported for the Garonne River (Dabrin, 2009). Furthermore the totality of Cd exported from the Charente watershed via the Charente Estuary directly enters the MOB and may reach the oyster farms therein, whereas only part of the Cd exported from the Gironde system may reach the vulnerable zones of the MOB. Our work intends to demonstrate that the Charente River system should be further studied because of its significant impact on the MOB.
