Severe and contrasted polymetallic contamination patterns (1900–2009) in the Loire River sediments (France)

Science of the Total Environment 435–436 (2012) 290–305

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Severe and contrasted polymetallic contamination patterns (1900–2009) in the Loire River sediments (France) C. Grosbois a,⁎, M. Meybeck b, L. Lestel b, I. Lefèvre c, F. Moatar a a b c

Université François Rabelais de Tours, EA 6293 GéHCO, Parc de Grandmont, 37200 Tours, France Université Paris et M. Curie, UMR 7619 CNRS Sisyphe, 4, place Jussieu, 75006 Paris, France Laboratoire des Sciences du Climat et de l'Environnement, UMR CNRS/CEA/UVSQ 1572‐IPSL, 91198 Gif‐sur-Yvette, France

a r t i c l e

i n f o

Article history: Received 22 March 2012 Received in revised form 15 June 2012 Accepted 15 June 2012 Available online 2 August 2012 Keywords: Loire basin Sedimentary archive Polymetallic contamination Contamination history Sensitive element ranking

a b s t r a c t The Loire River basin (117,800km2, France) has been exposed to multiple sources of metals during the last 150years, originating from major mining districts (coal and non-ferrous metals) and their associated industrial activities. Geochemical archives are established here from the analysis of a 4m sediment core in the downstream floodplain and then compared to stream bed sediments from pristine monolithological sub-basins and from bed and bank sediments in impacted tributaries. The contamination is assessed for 55 major and trace elements through their enrichment factors to Al (EF), normalized to the pre-anthropogenic background. Archives from 1900 to 2009 show enrichment (EFb1.3) not only for Ba, Be, Cs, Ga, Rb, REE, Sr, V, and Zr but also for U and Th, despite U mining activities until the 1990s. From 1900 to 1950, the level of contamination is severe for Hg, Au, Ag (10bEFb30), important for Sb and Sn (3bEFb7) and moderate for Cu, Pb and Zn (1.5bEFb3). This state was mostly attributed to coal uses and metal mining. During the period 1950–1980, severe polymetallic contamination is noted for Hg (EF up to 53), Cd (23), Ag (18), Zn (6.2), Cu (6.0), Sn (5.6), Pb(4.8), Sb(4.4) and for new impacted elements as Bi (23.8), As (3.7), Cr (3.4), W (3.1), Mo (2.6), Ni (2.8), Co (1.65) due to mines, smelters, industries and from urban sewers, collected mostly after 1950 (total population of 8.4million people). The limited dilution by detrital material (Loire sediment load about 1.5Mt/year) is an additional cause of such severe contamination. After 1950, river eutrophication is well marked by the general increase of endogenic calcite (EF (Ca)=4), diluting all other elements by 20%. From 1980 to 2009, all contaminants, except Au (EF=100), decrease steadily. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Surveys of metal concentrations in river sediments are very recent, few decades for the longest surveys (i.e. Gibbs, 1973; Forstner and Salomons, 1980; Martin and Meybeck, 1979; Müller and Förstner, 1975; Sinex and Helz, 1981). Most surveys of river metal contamination are therefore based on suspended particulates and on b63 μm bed sediments as metals and most other trace elements (TE) are preferentially carried by the fine particulate matter (Horowitz and Elrick, 1987). Fine sediments, deposited in lakes, reservoirs and floodplains also allow to record and date past levels of contamination. In Western Europe, this type of environmental archives has been used for two decades (Gocht et al., 2001; Heim et al., 2004; Middelkoop, 2000 for the Rhine basin; Winkels et al., 1998 for the Danube basin; Van Den Berg et al., 2001 for the Meuse basin; Baborowski et al., 2007; Brügmann, 1995 for the Elbe basin). It was also developed in French rivers as the Garonne (Grousset et al., 1999), one of the Garonne tributaries, the Lot (Audry et al., 2004a), the Rhone (Arnaud et al., 2005; Charmasson ⁎ Corresponding author. Tel.: +33 247367002; fax: +33 247 367 090. E-mail address: [email protected] (C. Grosbois). 0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2012.06.056

et al., 1998; Miralles et al., 2004; Revel-Rolland et al., 2005) and the Seine basin (Le Cloarec et al., 2011). All these studies have shown that metal contamination has been much pronounced in the past 150 years. In the Loire basin, the Holocene sedimentary evolution has been explored by Garcin et al. (1999) and Négrel et al. (2002, 2004) but the 20th century contamination remained unknown so far, despite several studies of the current composition of river particulates (Négrel, 1997; Négrel and Grosbois, 1999). The Loire River is among the largest French basins (117,800 km2) and it integrates various potential sources of metals as multiple types of mining activities, metal industries and middle-size cities. It is also characterized by a well-marked eutrophication (Lair and Sargos, 1993; Meybeck et al., 1988; Moatar and Meybeck, 2005). The aim of this study is to describe, for the first time in the Loire River basin, a Longue Durée environmental history, combining environmental geochemistry and geographic history. We assess the overall impacts of various human activities on riverine particulates for 35 trace and major elements in comparison with the natural background. Using a systematic indicator, the relative enrichment factor, we describe the contamination patterns at different key periods. This sedimentary archive is essentially based on a core taken in an

C. Grosbois et al. / Science of the Total Environment 435–436 (2012) 290–305

island of the lower reach of the Loire. It is complemented by analyses of river bed and bank sediments, taken in the whole basin to define the background and to explore the patterns of contamination from specific sources. The geographic history of potential metal sources is used to propose a first interpretation of the contamination patterns from 1900 to 2009. 2. Methodology and study area characteristics 2.1. Analytical methods After collection, samples were air-dried and sieved through disposable Nylon mesh, either b100 μm, for (i) and (ii) sample sets, or b63μm for core and (iii) sample sets. Grain-size analyses were performed with a laser diffraction microgranulometer Cilas 920 on the b200 μm fraction of 9 core slices, representative of the 3 different units. Representative sub-samples (0.5g of dry b63 μm material) were digested in Teflon beakers on a tunnel oven with LiBO2–Li2B4O7. After samples have been dried, the residues were completely re-dissolved with HNO3 acid. Additional splits of 0.2 g were digested by hot aqua regia (95 °C) for the determination of TE abundances. Total contents of major and minor elements were analyzed by ICP–AES (Jobin-Yvon 70; Govindaraju and Mevelle, 1987), trace elements by ICP–MS (Perkin Elmer 5000, Govindaraju et al., 1994) except for Hg, done by cold vapor AAS (Perkin Elmer 5100). Total carbon (TC), total organic carbon (TOC), with a previous HCl treatment to eliminate carbonates, and total sulfur (TS) were analyzed by O2 flow combustion at 1000 °C using a LECO SC 144 DR. Specific core sediment samples were also analyzed by Rock-Eval pyrolysis (Disnar et al., 2003; Lafargue et al., 1998) in order to analyze inorganic carbon contents and to provide information on the composition of organic matter. All the digestion process and analyses were quality-checked by analyses of sample duplicates and internal reference materials. Accuracy was within 5% of the certified values and analytical errors better than 10% RSD for TE concentrations, at least 30 times higher than detection limits (Table 1a). 2.2. Physical and hydrological characteristics of the Loire River basin The Loire River basin (117,800 km 2) is among the ten largest West-European rivers and the largest French basin. It is characterized by three main geological units (Fig. 1a): (i) old plutonic rocks (granites, gneiss and micaschists, aged between 500 and 300 Ma), covering 40% of the basin area, found in its southern and most western parts (respectively, Central Massif and Armorican Massif); (ii) volcanic areas, from Pliocene to Holocene (last eruption 6700 years ago), present only in the upstream S-E basin (about 3% of the basin); (iii) sedimentary bedrock from Jurassic to Tertiary of the Paris basin unit, mostly in the middle reach of the Loire basin with layers of limestones, chalk and marl (about 30%), detrital siliceous rocks (about 24%) and alluvial deposits (less than 5%). Such a lithological distribution results in various and important mining resources, some of them dating from the pre-Roman era. Iron‐pan formation and Fe mining sites were present in the Tertiary sands in the Middle Loire basin and exploited on-site where iron smelters and metal industries have been then developed. Two major coal districts at the French scale were located in the Upper Loire basin: one in the Furan River basin (St. Etienne coal district) and one in the Bourbince–Arroux basins (Blanzy–Montceau coal district), both in operations since the 18th century up to the late 1990s. Important polymetallic ore deposits, such as Ag–Cu–Pb–Sn–Zn, Ag–Pb–Sb, Sn–W and Au–As–Sb districts, were spread throughout the crystalline Central Massif and Armorican Massif (Carroué, 2010; Fig. 1a) and specific uranium mines in the Vienne basin. The Loire River hydrological regime is a mix of oceanic influence in the middle basin and lower basin and of snow-melt influence in the upper reach of the Loire River and in its major tributary, the Allier (Fig. 1a). The resulting annual hydrological cycle, at the gauging

291

station of Montjean (109,930 km 2) where the sediment core has been sampled (Fig. 1a), is characterized by winter high flows with long and important floods in winter and springs (maximum of averaged monthly discharge equal to 1530m 3/s in February during the 1863–2010 period; www.hydro.eaufrance.fr) and severe summer low flows (minimum of 250 m 3/s in August). At the study station, major floods, occurred in 1910 (6300m3/s), 1977 (6100m3/s) and 1982 (5750m3/s), represent events with large return periods (150years, 40–50years and 90–100years respectively; Duband, 1996). The 1910 flood was the most important hydrological event, lasting 2months and contributing to 55% of the annual water flow in 1910–1911. The present population of the Loire River basin is now 8.4million people upstream of Montjean, and relatively stable from 1900 to 2008 with 65±5people/km2 (Fig. 1b). The biggest city is St. Etienne in the upstream part of the basin (180,400 inhabitants), drained by the Furan River. Six other cities exceed 100,000 inhabitants (Clermont-Ferrand, Limoges, Orleans, Tours, Le Mans and Angers from up- to downstream, the estuary being excluded; Fig. 1a) and they are all associated to industrial activities. In addition, the Loire basin is characterized by a spatial gradient of population and anthropogenic activities, with a higher population density in the western part of the basin and along the main stem of the Loire River (up to 120 people/km2), and a much lower density (20 people/km2) in the upstream part of the basin. 2.3. Site description and sampling methodology 2.3.1. Coring site at Montjean-sur-Loire Floodplain cores chosen for contamination archives are more delicate to use than lakes, reservoirs or costal lagoons cores. Ideally, they correspond to deposits during flood episodes which should be minimally destructive of previous deposited sediments and as fine as possible. Such deposits also integrate information from the upstream basin, in proportions of sediment sources of all tributaries. In the Lower Loire, an appropriate site was selected at Montjean-sur-Loire (47°23′34″N, 0°51′23″E) which integrates the largest draining surface of the Loire basin just upstream of the Loire estuary (110,000 km 2; Fig. 1a). It is located in a grazed alluvial island (named Chalonne) in the middle of the river main stem, with a few settlements and riparian forest vegetation. In the Loire River, such alluvial islands can be considered as potential sedimentary archive sites where finer particles can settle during highest flows, due to the presence of grass and woody vegetation reducing water velocities (Campy and Macaire, 2003; Rodrigues et al., 2006, 2007). This alluvial island elevation varies from 12m to 17m asl today (www.geoportail.fr). Today, it is flooded when the discharge is higher than 2500 m 3/s (334 m high according to the gauging curve; DREAL Survey Department). Long-term geomorphological changes took place in the Loire River basin (Détriché, 2010; Latapie, 2011), when comparing active-channel width adjustments, island and channel positions as established in 1767–1771 (Cassini maps) and with today aerial photos. At Montjean, the active-channel width slowly decreased from 1848 (512m wide) to nowadays (412m in 2002; Latapie, 2011). The number of channels decreased from three at the end of 1955 down to two in 1995. The island width rapidly increased towards the right bank from 223±25 m in 1848 to 301±25m in 1954 and up to 345±25m in 2002 (Latapie, 2011). Hence, the coring site has been part of the island since the late 18th century but the island morphology has changed. In addition, during the 20th century, anthropogenic modifications took place in the main reach such as dykes built to prevent flooding and groins for navigation. Incision was also common in the main Loire channel between the 1970s and the 1990s (about 0.7 m in 125 years in Tours; Latapie, 2011) when intense sand and gravel extraction occurred in the river bed. At the study site, the evolution of the gauging curve also showed incisions of the main channel in the same order of magnitude from 1960 to nowadays (DREAL Survey Department; pers. com.). Considering the evolution of site morphology and of gauging curve, the

292

Depth interval

137

Cs

Age model

Al

TOC Ag

As

Au

2009 2006 2003 2000 1998 1995 1992 1989 1986 1983 1980 1977 1975 1972 1969 1966 1963 1960 1957 1954

7.06 7.28 6.98 7.32 7.08 7.37 7.33 7.43 7.59 7.53 6.99 8.04 8.12 7.73 7.71 6.41 8.32 8.15 8.35 8.38 7.61 8.08 7.93 8.21 7.63 8.32 8.40 8.32

3.0 n.d. 2.6 n.d. n.d. 2.3 n.d. 2.2 n.d. 2.3 2.3 1.9 2.0 2.1 2.2 2.2 2.9 n.d. 2.7 n.d. 3.6 2.3 2.3 1.8 2.4 1.7 2.2 2.1

27.2 15.4 29.8 213.2 28.7 20.2 28.3 19.7 29.0 35.8 29.6 24.2 28.9 22.6 30.5 21.5 28.2 71.5 31.5 131 35.5 43 43.4 36.8 36.9 31.4 42.2 33.9 33.3 35.2 28.4 40.1 34.6 26.4 40.7 27.2 34.4 29.5 71.3 24.1 43.5 21.6 20.3 19.4 36.3 17.5 27.9 28.2 28.0 26.7 32.4 16.8 30.2 27.9 30.4 27.5

Bi

Cd

Cr

Cu

Hg

Mo

Ni

Pb

Sb

Sn

U

W

Zn

1.0 1.0 1.3 1.6 1.4 1.7 1.6 1.9 6.1 12.0 12.8 10.5 8.5 10.2 8.2 9.7 8.7 5.9 5.6 5.1 4.2 2.0 1.8 1.4 1.6 1.6 1.9 1.9

1.0 1.1 1.4 1.4 1.4 1.8 1.5 1.7 2.0 2.9 4.4 4.1 3.5 4.4 4.3 6.0 4.9 3.9 2.7 3.5 2.3 1.0 1.0 0.8 0.9 0.9 0.6 0.8

103 103 130 144 137 151 144 151 198 239 281 253 212 246 185 192 219 192 192 233 164 123 116 123 116 123 137 130

45.2 49.8 54.0 53.2 51.3 54.4 52.9 51.1 58.2 77.1 99.9 75.3 61.5 63.3 68.9 91.5 99.8 81.0 81.1 91.8 92.5 64.0 57.1 58.6 64.7 55.1 58.4 73.0

0.16 0.17 0.16 0.19 0.19 0.22 0.20 0.23 0.28 0.33 0.39 0.40 0.40 0.39 0.53 0.80 0.75 0.66 0.77 0.91 0.80 0.55 0.56 0.42 0.49 0.46 0.51 0.56

0.5 0.5 0.6 0.7 0.7 0.7 0.6 0.6 0.6 0.7 0.7 0.8 0.8 0.6 0.5 0.4 0.5 0.4 0.4 0.9 0.4 0.3 0.4 0.3 0.4 0.4 0.3 0.4

34.8 37.0 40.5 43.7 42.1 46.7 44.2 47.5 53.6 56.1 65.7 63.1 58.4 59.1 52.5 48.5 58.9 49.6 46.6 60.0 47.1 41.6 36.8 36.2 38.0 36.2 35.7 38.7

46 52 65 85 79 92 83 91 88 92 104 99 88 110 108 126 128 111 115 112 124 91 95 79 91 83 104 104

0.6 0.6 0.7 0.9 0.7 0.7 0.7 0.7 0.8 1.0 0.9 1.0 0.9 0.7 0.8 0.8 1.0 0.9 1.0 1.6 1.6 1.1 1.3 1.3 1.2 1.1 1.0 1.1

11 10 10 13 12 13 13 14 14 18 19 19 20 23 24 21 25 25 24 32 38 21 20 19 18 20 27 28

5.1 4.8 5.1 5.7 5.7 5.8 5.9 6.2 6.5 5.9 5.7 6.0 6.4 5.6 5.8 4.7 5.5 5.9 5.6 6.0 5.1 5.3 5.2 5.6 5.8 6.1 6.2 6.9

4.8 5.0 4.7 5.1 5.8 5.2 5.7 5.8 5.9 6.0 6.3 7.2 6.9 7.0 7.9 6.1 8.2 10.1 11.6 18.6 10.5 9.0 7.7 6.2 6.1 6.2 6.5 6.0

168 185 187 203 200 225 211 225 271 340 481 378 330 381 354 393 419 355 410 440 414 227 228 225 222 206 221 238

a 0–5 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45 45–50 50–55 55–60 60–65 65–70 70–75 75–80 80–85 85–90 90–95 95–100 100–105 108–113 113–118 118–123 123–128 128–138 140–145 145–150

n.d. 11.63 n.d. 23.84 24.96 24.48 24.13 22.29 23.32 16.06 n.d. n.d. 22.70 n.d. 34.14 66.47 73.93 29.69 n.d. 5.25 n.d. n.d. 0.00 n.d. n.d. 0.00 n.d. 0.00

1950 1944

1935 1926

0.7 0.9 1.0 1.1 1.1 1.2 1.1 1.1 1.4 1.8 2.0 1.5 1.3 1.5 1.7 2.0 1.7 1.4 1.5 1.4 1.4 0.8 0.8 0.6 0.7 0.5 0.6 0.7

C. Grosbois et al. / Science of the Total Environment 435–436 (2012) 290–305

Table 1 Major, total organic carbon and trace element concentrations. a — sediment core at the fluvial entrance of the Loire estuary, the studied Montjean station; b — sediments collected in non-impacted and monolithological sub-basins of the Loire River basin (b63 μm fraction, mg/kg except Al and TOC in wt.% and Au in μg/kg, n.d. = not determined).

Depth interval 137

Cs

Age model

Al

TOC

Ag

As

Au

8.39 8.26 8.57 7.73 8.21 8.03 8.45 0.01

2.0 2.1 2.1 1.8 2.2 2.4 2.3 0.1

0.7 0.8 0.7 0.4 0.5 0.6 0.4 0.1

27.1 37.9 33.4 33.7 37.8 28.7 47.6 0.5

Bi

Cu

Hg

Mo

Ni

Pb

Sb

Sn

U

W

0.5 0.7 0.5 0.5 0.5 0.5 0.9 0.1

123 123 116 123 137 123 130 20

62.6 83.3 71.3 57.4 76.5 54.7 55.0 0.1

0.52 0.54 0.49 0.49 0.57 0.69 0.48 0.01

0.4 0.4 0.4 0.3 0.4 0.4 0.4 0.1

37.8 41.2 40.3 34.2 35.2 34.5 41.0 0.1

102 114 90 78 99 99 102 1

1.2 1.2 1.4 1.4 1.6 1.4 2.5 0.1

24 29 24 24 27 31 50 1

6.3 6.3 6.0 8.4 7.5 6.4 5.2 0.1

Zn

Basin bLoire

Sample type

Al

TOC

Ag

As

Au

Bi

Cd

Cr

Cu

Hg

Mo

Ni

Pb

Sb

Sn

U

W

Zn

Associated reference

Montjean core (360–380 cm deep) (n=1) Montjean core (380–400 cm deep) (n=1) Prehistoric sediment in a sand pit (n=1) Micaschist watersheds (n=8) Granitic watersheds (n=10) Siliceous detrital watersheds (n=10) Basaltic watersheds (n=6) Gneissic watershed (n=1)

8.37

1.20

0.1

19.8

2.6

0.7

0.3

109

18.6

0.02

0.4

32.6

37

0.5

8

8.8

6.3

90

This study

8.54

1.10

0.2

19.4

2.3

0.6

0.4

89

21.3

0.02

0.3

24.2

33

0.3

7

7.5

5.7

97

This study

7.70

0.30

n.d.

26.0

n.d.

0.5

0.6

105

17.0

0.02

n.d.

40.0

38

2.1

8

7.0

4.5

92

5.7– 10.2 6.2–8.7

n.d.

23–181 n.d.

0.2–0.9

6.7–31

15–40

n.d.

0.2–1.8

4.8–58

n.d.

3–38

n.d.

0.1–0.4

13–74

3.0–21.7

n.d.

6–21

22– 101 31– 151 11–53

13.9– 38.0 16.2

n.d.

30– 199 21.1

15–31

n.d.

94– 875 51

n.d.

n.d.

b0.1– 0.2 2.2

5– 10 5– 23 2– 14 3–5

44

14

10.9

4.1– 16 1.5– 26 1.9– 7.0 1.2– 3.0 12.5

76– 124 26– 129 24–72

n.d.

0.8– 2.0 0.5– 2.0 0.7– 2.4 0.3– 0.6 1.0

3.5–6.4

10.2– 82 2.1– 15.8 2.2– 14.4 88.8

0.02– 0.13 0.03– 0.11 0.01– 0.11 0.03– 0.08 0.05

n.d.

n.d.

0.4– 0.6 0.3– 1.9 0.2– 1.0 0.4– 0.6 0.8

43– 110 12–95

7.73

0.7– 5.6 1.5– 7.4 0.4– 7.2 1.7– 3.8 6.90

Meybeck et al. (in preparation) Meybeck et al. (in preparation) Meybeck et al. (in preparation) Meybeck et al. (in preparation) Meybeck et al. (in preparation) Meybeck et al. (in preparation)

3.30

b1

n.d.

n.d.

n.d.

n.d.

0.2

40

14.0

0.04

n.d.

16.0

20

n.d.

n.d.

n.d.

n.d.

60

5.51

1.70

n.d.

6.0

n.d.

n.d.

0.3

63

17.0

0.04

0.6

21.0

21

0.6

2

2.0

1.2

71

Loire Loire Loire Loire Loire

Samples considered for background comparison Seine Prehistoric sediments in Paris (n=2) Median of European stream sediments (n=845)

1.1–5.7 6.4–8.1

n.d. n.d.

4–22 3.7– 10.0 1.9–4.0

5.9 6.1 6.4 5.6 6.2 5.5 6.5 0.5

99– 180 104

220 230 212 194 223 177 194 1

Meybeck et al. (2004, 2007) www.gtk.fr/publ/ foregsatles

C. Grosbois et al. / Science of the Total Environment 435–436 (2012) 290–305

n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Loire

1.5 1.8 1.4 1.0 1.2 1.2 1.3 0.1

Cr

150–154 154–158 160–167 167–176 176–186 186–198 220–230 Detection limit

Loire

32.3 21.4 13.2 18.7 29.6 28.3 25.9 0.5

Cd

293

294

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water flow level, inundated the studied site, corresponds to a discharge ranging between 2000 and 2500 m 3/s over the 1900–2009 period. With the highest level, the studied site was then flooded during 20days a year for 39years, 45 days a year for 15 years and between 2 and 3 months for 1910 and 1919years (Fig. 1b). This represents at least a total of 89years among the studied 110 years, i.e. more than 80% of the studied time when river deposits contribute to sedimentary archives. The coring site is 200 m upstream of the northern tip of the island, at 5 m from the active channel, on the top of the right bank (~4 m above the low flow level), in a non-cultivated area. The sediment core was sampled in June 2009 using a 10-cm diameter Eijkelamp percussion corer to obtain a 4 m core aiming to reach pre-industrial time. Only the particulate material in the middle of the corer was sampled in 5 cm-slices with a stainless steel knife. Samples were then stored in plastic bags in order to prevent metallic contamination. No living organism and/or bioturbation feature has been clearly observed when slicing. The sediment core is characterized by 3 main units: - From the surface down to 190cm, the upper unit is represented by silty–clayey deposits with a silt percentage around 82% and a median grain-size ranging from 6.1 to 6.5μm with a D99 between 28 and 39μm (Fig. 1c). In the first 50cm, there was no evidence of past agricultural practices. Three major homometric fine-sand episodes were present between 105 and 108cm, 138–140cm and 158–160cm. Another specific episode, between 167 and 176cm, is characterized by a gradual transition with sandy levels interbedded within clayey sediments (Fig. 1c). These alternated fine-sand episodes in silty–clayey sediments are typical of fluvial dynamics variability: sandy episodes are interpreted as more energetic solid transport conditions like floods and they are commonly found in within-channel deposits (Reineck and Singh, 1980). As it will be demonstrated further by the calcium content profile (Sections 3.1 and 4.2), this upper unit is essentially detrital from 190 to 130cm, and gradually enriched with endogenic calcite from 130cm to the surface. Some secondary Fe-oxyhydroxides were observed from place to place at 70–80cm and 115–190cm depth intervals (Fig. 1c) but represented less than 10% of the total sample surface. - From 190 down to 360 cm, this unit is characterized by the dominance of sand levels (Fig. 1c), either well-sorted fine to medium sandy levels, or poorly-sorted coarse sand levels. They are alternated with dark gray silty–clayey levels (median grain-size ranging from 8.5 to 10.4 μm). This sequence could correspond to ancient alluvial sediments of a sand bar. From 230 cm to 190 cm, 5-cm slices were sampled but below 230cm, layers of coarse sand were nearly continuous and have not been considered for chemical analyses. - From 360 to 400 cm, the lower unit is similar to the upper one, with silty–clayey deposits (median grain-size equal to 10.4 μm with a D99 of 86 μm). 2.3.2. Stream bed and river bank sediment samples in tributaries In addition to the core samples, three types of complementary particulates samples were collected by hand in river bank profiles and/or in small streams throughout the Loire River basin: (i) one sample taken 4 m deep in a sand-pit of the Middle Loire basin at Langeais, 2 km downstream of the Cher-Loire confluence (Fig. 1a). Its composition was used to establish the natural background of TE; (ii) stream bed and bank sediment samples (n=35), collected in 2007–2009 in small pristine and monolithologic basins. These pristine monolithologic streams are characterized by basin areas from 10 to 100 km2, by low population densities from 0 to 10 people/km 2, and by forest or alpine grassland land cover. This set was used to determine the geochemical composition of detrital inputs from all rock types of the Loire basin; (iii) bed sediments were also sampled in 2010–2011 (Variqual program under the framework of the CNRS-CYTRIX-EC2CO program and Villerest program under the framework EPL-Feder program) downstream of some anthropogenic sources, historically impacted

by former mining and/or industrial sites (Allier, Arroux, Furan, Gartempe, Mayenne, Sioule, Ondaine, and Vienne) and by former major coal mines with associated metal industries (Bourbince and Furan). They are used to assess various geochemical signatures of anthropogenic sources. 2.4. Dating method and chronology Two independent core dating methods have been tested: 210Pb and 137Cs. The radiometric analyses have been performed on about 50 g of b2 mm core material in air-tight plastic boxes for a 24 h gamma-counting. Very low-background detectors, coaxial HP Ge N-type, were used for gamma spectrometry (8000 channels, low back-ground). Efficiencies and backgrounds were periodically controlled with internal soil and sediment standards, pure KCl samples, and IAEA standards (Soils 6, 135 and 375). Measurement of 210Pb in the core samples had not revealed a regular decrease and could not allow an absolute dating. 137Cs was detected with an energy peak at 661 keV, in a spectrum area free of any interference. Activities were corrected to the time of the collection period and the uncertainty on measurements was about 0.5% with a detection limit of 0.3 Bq/kg. The 137Cs artificial radionuclide activity was commonly used in bed sediments as a time calibration, using 3 events: (i) its introduction in 1950 in the atmosphere by the first thermonuclear bomb tests, (ii) a maximum 137Cs atmospheric fallout in 1962/63 and (iii) the 1986 fallout peak following the Chernobyl nuclear accident. We also used the top of the core, between 5 and 0 cm, which corresponds to the last period of flooding in 2007–2009 (2 days in 2009 with a discharge higher than 2500 m 3/s and 10 days in 2007). In river sediments, the construction of the age–depth model is based on the hypothesis that sedimentation is continuous, with minor mechanical erosion and transport of the top sediments. No 137Cs activity was detected below 110cm. The 110–105cm level is therefore attributed to 1950 where 137Cs is first detected (Table 1a). The upward 137Cs profile clearly showed a first sharp peak at 80–85cm depth (73.93Bq/kg), attributed to the 1963 period. A second broad peak was observed between 45cm and 20cm depth. As 137Cs increased from 16.0Bq/kg at 50–45cm up to the maximum 23.3Bq/kg at 45–40cm, the 1986 Chernobyl event was attributed to the 45–40cm level. On the basis of these temporal markers, the following net sedimentation rates have been estimated: (i) 1.9 cm.year −1 from 45 cm to the core top (1986–2009 period); (ii) 1.7 cm.year −1 from 85 to 45 cm (1963–1986 period): and (iii) 1.9 cm.year −1 from 110 to 85 cm (1950–1963 period). Although the sedimentation rate is very site-specific, these figures are consistent with other calculated accretion rates in the Middle Loire basin, ranging from 0.6 to 2 cm.year −1 (Détriché, 2010; Garcin et al., 1999). Below 110 cm, the same sedimentation rate of 1.9 cm.year −1 was applied down to 190 cm as sedimentation characteristics were similar (Fig. 1c). With this sedimentation rate, the limit at 190 cm deep, between the upper clay–silt unit and the sand unit, would correspond to the early 1910 period. This is consistent with flood records of the Loire River: in 1910, the extreme flood, in terms of discharge maximum, annual water flows and flooding period, occurred at Montjean with a maximum discharge of 6300 m 3/s and more than 108 days exceeding 2500 m 3/s (Fig. 1b). This 1910 exceptional flood may have severely eroded previous alluvial deposits in the island, then deposited the coarse layer (360 to 230 cm). Therefore, the bottom unit of the core (400 to 360 cm) could originate well before 1900. Hence, four time intervals will be used based on the 137Cs profile, sedimentary characteristics and hydrological variations as follow: - Early period, probably well before 1900, corresponding to levels 400 to 360 cm. - Period 1900s–1950: levels 230 to 110 cm. - Period 1950–1980: levels from 110 to 50 cm. - Period 1980–2009: levels from 50 to 0 cm.

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Fig. 1. a — Localization of the sediment core (dark triangle at the fluvial entrance of the Loire estuary), of major mining districts and of major towns (>100,000 inhabitants; St E. = Saint Etienne, C.F. = Clermont-Ferrand). b — Temporal evolution of population density and daily river discharge maxima (m3/s) at the studied site since 1900. c — Core sedimentary log with associated grain-size medians and 99-percentiles.

2.5. Data on pollution pressures The survey of metal inputs to rivers from industrial and urban waste waters is relatively recent in France. In the late 1990s, metals were first part of an aggregated water quality indicator, the Metox index, combining As, Cd, Cr, Cu, Hg, Ni, Pb and Zn dissolved concentrations with various weights in industrial waste waters and calculated Metox fluxes were used to collect pollution taxes from industries. Direct analysis of metals in urban and industrial waste waters is only available after 2000 and they are not appropriate to our temporal scale. In the riverbed, regulatory surveys of metal started only in the 1980s with analyses on sediments sieved at b2 mm and aluminum data is lacking. At Montjean, centrifugated suspended matter samples are now taken by the Loire River Basin Authority but they cannot provide a Longue Durée picture. Potential source of metals also originates from multiple mining activities in the Loire basin, some of them already present during the Gallo–Roman era (Négrel et al., 2004; Carroué, 2010). Historical mining data originated from various specific studies for each site (e.g. Périchaud, 1971; Carroué, 2010) but most information can be

found in a national database (Statistiques de l'industrie minérale 1833–1970, sigminesfrance.brgm.fr) except for uranium mining, well documented in the Mimausa database (mimaubdd.irsn.fr). Metal material flow analysis is still under development for the French territory, in comparison to other countries as USA (Brown et al., 2000; Callender, 2003). In France, resource supplies (customs statistics), production and manufacturing are known at various scales from local to national, but never at a basin scale and consumption of metal-containing goods is not available for each metal. Consumption and waste production can be linked to the population density evolution, data available by municipalities (cassini.ehess.fr/cassini and www. insee.fr/fr/ppp/bases-de-donnees/recensement). 3. Results 3.1. Geochemical background in the Loire River sediments 3.1.1. Background samples In the Loire River basin, the pre-industrial status of TE in fine sediments, necessary to assess anthropogenic impacts on sediment

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chemistry, has not been defined, except for V, Th and Pb (Négrel et al., 2004). Background levels may be station-specific due to variable contributions of detrital inputs from each lithological unit and they may even vary at a station with grain size and/or with mineral composition. In our study, focused on the whole Loire basin at Montjean, three types of samples are compared to assess the TE background: (i) the core bottom layers, that corresponds to the pre-industrial period (≪1900), (ii) the pre-historic fluvial clay sample from the Langeais sand-pit, integrating 51% of the drainage area of the Loire River basin, and (iii) contemporary stream sediments from small pristine and monolithologic basins (Table 1b). Pre-historic and core bottom samples are very similar, within ±20% or less, for As, Bi, Cd, Cr, Cu, Hg, Pb, Sn, U, W and Zn. The extremely low mercury (~0.02mg/kg) and cadmium (~0.35mg/kg) contents should be noted as these TE are among the most sensible to anthropogenic sources (Gaillardet et al., 2003; Reimann and de Caritat, 2005; Swennen and Van der Sluys, 2002). Two discrepancies are observed for Sb and Ni: their higher contents in the pre-historic sample even when correcting for Al content, could be due to a greater influence of volcanic-derived minerals at this post-glacial period. In addition, pristine stream sediments, clustered by lithologies in the Loire basin (granitic, volcanic, limestone and detrital siliceous bedrock), present very variable TE compositions (Table 1b), in accordance with variations of parent-rock composition (Reimann and de Caritat, 2005). For example, sediments from volcanic basins are characterized by very high Cr and Ni levels as expected when compared to all other lithologies. Maximum contents for As, Bi, Sb, Sn, Pb, U and W are the highest in stream materials from granitic basins and minimum contents are noted in materials from detrital siliceous

and carbonated basins. Concentrations of core bottom samples are enclosed in this TE range, particularly for Cd and Hg, confirming that they were not impacted by anthropogenic metal inputs. These samples were therefore considered as representative of the geochemical background for finer sediments (b63 μm) of the entire Loire basin. This geochemical background of the Loire basin can be compared to the median composition of European stream sediments (Table 1b), considered as barely affected by anthropogenic activities (database of surficial and deeper stream sediments of about 850 stations from the geochemical atlas of Europe; Salminen et al., 2005, available in www. gtk.fi/publ/foregsatles). The European median content was, somewhat, richer in limestone than the Loire background as reflected by the Ca content (1.7±6.2% Ca and 1.6±0.7% Ca respectively) and poorer in silicates (5.5±2.5% Al and 8.0±0.5% Al respectively). Median contents of most TE in these European stream sediments were also lower than the Loire background, due to their higher calcite content, except for Cd and Cu – similar levels – and for Hg – higher level – suggesting a beginning of generalized anthropogenic influence in European streams for these highly sensitive elements. 3.1.2. Enrichment Factor to aluminum Aluminum is used here as the reference element for the particulate material due to (i) its conservative behavior in the core, (ii) its limited solubility in temperate climate, (iii) its characterization of the clay-size fraction which is enriched with most TE and (iv) its very low sensitivity to human activities, as shown on the Seine River sediments impacted by Paris megacity (Meybeck et al., 2007; Thévenot et al., 2007). In this study, aluminum was preferred to Sc,

Fig. 2. a — Major element concentrations (wt.%) in the sediment core and associated SEM images of endogenic calcite in the suspended material of the Middle Loire (Grosbois et al., 2001, 2010). b — Temporal variations of Ca, Mg, P and Cu enrichment factors in the μm sediment core.

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Ti or Zr, strongly linked to specific mineral phases like REE-rich spinels and phosphates, titanomagnetites and rutiles, zircons respectively and which proportions can vary over time and solid transport. Relative enrichment factor, abbreviated here after EF, is calculated for each element as the TE/Al ratio in each core level normalized to the reference background (400–360 cm mean of TE/Al, Table 1b). This aims to (i) take into account grain-size variations in the core samples, remaining in the b63 μm fraction, and the dilution effect by calcite and quartz minerals which are both very depleted in all TE, (ii) to quantify and rank the levels of TE contamination and (iii) to compare their variations at the study site throughout the 20th century. Taking into account analytical uncertainties and the natural variability of background levels, it is assumed here that contamination starts to be suspected when EF>1.3, is considered as well-established when EF>2.0 and as high when EF>5.0.

3.2. Major element profiles Silicium (silicon), aluminum and iron contents were very stable throughout the core (24.5±1.7% Si, 8.0±0.5% Al, and 4.3±0.4% Fe; Fig. 2a) suggesting that the assemblage of dominant minerals assemblage (quartz and aluminosilicates) remained constant. However, in details, Si, Al and Fe contents were slowly decreasing above 130 cm, being 20% less abundant in the core top. This decrease is also observed for all major elements except Ca: it will be attributed to additional calcium, from endogenic calcite linked to eutrophication (see Section 4.2) and for some samples, from smelting activities. Iron profile was quite stable with a constant Fe/Al ratio (0.54±0.02), i.e. with EF (Fe) very close to 1 throughout the whole core. This suggests a conservative behavior at the observation scale (5-cm slices and bulk concentrations) and minimum local post-depositional changes. Magnesium, sodium, potassium and titanium were very stable throughout the core as Mg (0.9±0.1%), Na (0.7±0.1), K (2.1±0.2%) and Ti (0.6±0.1%). They were mainly associated to aluminosilicates and/or originated from crystalline bedrock erosion. They were present in proportions similar to those of 850 European stream sediments (Salminen et al., 2005) with median elemental ratios Mg/Na=1.3, Na/K=0.3, Fe/Ti=7.1, Al/Fe=13.1. The Loire sediments were relatively depleted in Si vs. Al when compared to the European median (respective Si/Al ratios 3.1 and 5.3).

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Calcium profile is very unusual, combining several marked peaks and a general increase between 130 cm deep and the surface (Fig. 2a). From 400 to 130 cm levels, Ca/Al ratios were constant and in the same range than the European median (0.2–0.3) but above 130 cm, a noticeable Ca enrichment (Ca/Al up to 0.6) was observed in the Loire. Two peaks were well-marked, at 128–123 cm depth (from 1.45 to 4.03% Ca) and at 105–100 cm (from 2.3 to 3.4% Ca). From 100 cm to 85cm, Ca contents dropped again near 2.9% Ca, followed by two major peaks at 80–75cm depth (9.8% Ca) and at 55–50cm depth (7.9% Ca), with EF (Ca) equal to 8.2 and 6.0 respectively. From 70 cm to the surface of the core, the Ca level never dropped back to its background value and presented an EF around 4.0. Calcium peaks were very unusual when associated to TE enrichments (see Section 4.2). The generalized Ca enrichment resulted in a significant dilution of all the other major elements (Si, Fe, Al and cations) while their Al-normalized ratios remained constant. Phosphorus profile is also not regular. It was stable in the lower part of the core like other major elements (Fig. 2a) with EF (P) around 1.3, until a sudden EF increase occurred at 105–100 cm. From 105 to 30 cm, EF (P) was significantly higher (1.24bEFb1.69) and then decreased to 1.46 in the surface sediments. This pattern is very similar to the one of calcium. Manganese profile is even more variable than previous behaviors. From 230 to 110 cm, EF (Mn) was stable, suggesting origin and transport similar to those of conservative elements like aluminum. Above 110 cm, both Mn content and EF (Mn) started to increase, showing a peak at 100–95cm (EF=3.2; Fig. 2a). Such heterogeneous profile can be linked to (i) post‐depositional precipitations of Mn-oxyhydroxides, known to rapidly occurred when redox conditions are modified (Aller, 1980; Balzer, 1982) or, more likely, to (ii) industrial impact as this enrichment was accompanied by EF increase of W and Mo (see Sections 3.3 and 4.2). Organic carbon profile (TOC) presents similar characteristics with P and Ca. It was quite stable from the core bottom up to 108 cm deep, ranging between 1.8% and 2.3%. Organic carbon amounts peaked at 105–100 cm (3.6%) like P and Ca and then slightly decreased up to 55cm deep (1.9%). From 50cm deep to the surface, a well-marked increase of TOC was observed up to 3% (5–0 cm; Table 1). Particulate inorganic carbon content was only analyzed on samples presenting the highest Ca enrichment. It was higher than TOC content (9.2% in the 128–123 cm level, 12.3% at 80–75 cm and 11.8% at 55–50cm). 3.3. Temporal trajectories of trace elements A first set of TE does not present any significant enrichment throughout the 4 m-long core like Ba, Be, Co, Cs, REE, Rb, Sr, Th, U, V and Zr. Some other TE present various types of temporal profiles like Ag, As, Au, Bi, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Sn, W and Zn, some of them reaching extreme enrichments, i.e. EF (TE)>20. These TE are termed here “sensitive to human impacts” elements. They have their specific patterns over time, either being enriched throughout the core above 230 cm, or in some levels only. These elements have been clustered here by their EF values for the three periods defined in Section 2.5. Only the major contamination trajectories are presented here and the role of potential contamination sources over time will be discussed in Section 4.2.

Fig. 3. Temporal variations of Ag, Cu, Hg, Pb, Sb, Sn, W and Zn enrichment factors in the sediment core during the period before 1900 to 1945.

3.3.1. Non-impacted elements, from the early 1900s to nowadays Ba (0.9bEFb1.1), Be (0.7bEFb1.1), Co (1.0bEFb1.2), Cs (1.1bEFb1.5), REE (0.9bEF>1.1), Rb (1.0bEFb1.1), Sc (0.8bEFb1.0), Sr (1.0bEFb1.2), Th (0.8bEFb1.2), U (0.6bEFb1.1), V (0.9bEFb1.1), Y (0.8bEFb1.0) and Zr (0.5bEFb1.5). These TEs present very stable profiles with no evidence of enrichment in the core sediments. They are therefore considered as not impacted over the 1900–2009 period throughout the Loire basin. As for major elements, this stability can be related to relatively constant proportions of the main lithological sources. Some

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Table 2 Range of enrichment factors in b63 μm sediment core according to 3 for periods before 1900–1945, 1945–1980 and 1980–2009. Ag

As

Au

Bi

Cr

Cu

Hg

Mo

Ni

Pb

Sb

Sn

U

W

Zn

1.5 2.6

1.3 1.4

2.8 4.0

24.0 36.3

0.8 1.1

1.3 1.4

2.4 3.0

3.7 6.3

3.5 6.7

0.6 1.1

1.0 1.1

2.0 2.5

2.1 3.0

1.4 2.8

1.2 1.4

2.8 4.3

23.4 28.4

0.9 1.3

1.3 1.5

2.4 3.4

2.5 3.5

2.7 4.0

0.7 0.9

1.0 1.1

2.2 2.6

7.6 21.6

2.2 23.8

2.4 22.6

1.3 3.4

3.0 6.1

20.8 52.8

0.9 2.4

1.3 2.8

2.3 4.8

1.9 4.4

2.6 5.6

0.7 0.8

1.1 3.1

2.5 6.2

7.5 101.1

1.8 20.7

3.4 9.3

1.2 2.7

2.7 4.3

9.6 18.5

1.7 2.4

1.5 2.2

1.6 3.0

1.7 2.8

1.5 2.7

0.7 0.9

0.9 1.2

2.2 4.1

Period before 1900 Min 2.7 Max 4.2

1.5 2.4

8.3 12.4

1.7 2.0

Period 1900–1945 Min 3.4 Max 5.5

1.4 2.0

5.3 13.3

Period 1945–1980 Min 4.1 Max 17.6

1.1 3.7

Period 1980–2009 Min 5.6 Max 13.5

1.6 1.8

Cd

enrichment factors are more variable than others and they may even be somewhat inferior to 1.0, i.e. they are less concentrated regards to Al in the top than in the bottom of the core. This corresponds to second‐order variations of detrital fraction proportions (granites/volcanic/siliceous sedimentary) and/or to secondary and/or rare minerals. The ultimate example of such higher natural variability is for Zr, very much linked to zircon grains and for which the EF ranged between 0.52 and 1.12.

3.3.2. The 1900s to 1950 (230 to 110 cm): a period of important contamination for many TE This unit is interpreted as representing the evolution of Loire River sediment quality from the early 1900s to the first nuclear tests in 1950. This period appeared to be already well impacted by anthropogenic activities (Fig. 3). Mercury and gold showed the highest enrichment (EF>10) among all the studied TE during this period (Table 2). They were already extremely enriched at the bottom of this unit, in the early 1900s (230–220 cm) with EF exceeding 30 for Hg (Fig. 3) and 10 for Au. Then, EF slowly decreased and became relatively stable with high EF levels at the end of the 1940s. During this period, mercury was not significantly correlated with any other “sensitive human activities” element (Table 3) except with total organic carbon (r=0.61).

Antimony and tin (3bEFb7) were also already enriched in the early 1900s with EF close to 7. Enrichment factors then decreased progressively during this period to around 3 (Fig. 3). Significant correlations between Sb–As, Sb–Sn, Sb–Fe, Sb–P and Sn–Fe were observed (Table 3). Silver, copper, cadmium, bismuth, lead and zinc (2bEFb6) represent another cluster of trajectories: their EF did not vary much during this period but they all presented two synchronous EF peaks at 158–154 cm and 128–123 cm (EF up to 6 for Ag, 4 for Cu, 3 for Bi, Cd, Pb and Zn). Correlations were noted between Ag, Cu, Bi and Zn (Table 3) and between Cd and Pb. Arsenic (1.5bEFb2.4), nickel (1.3bNib1.4) and chromium (1.3b EFb1.4) were barely enriched during this period. Arsenic was correlated with Sb and Sn, and Ni with Ag (Table 3). Molybdenum (0.9bEFb1.2) and tungsten (1.0bEFb1.1) were not enriched during this period in contrast with the next one, 1945–1980.

3.3.3. The 1950 to 1980 period (110 to 50 cm levels): multiple and severe contaminations This unit represents the post World War II period, well dated with the 137Cs chronology. Many synchronous peaks of severe contamination were observed (Fig. 4) but at different periods according to TE, suggesting different contamination sources. Any trace element was

Table 3 Correlation matrix of Al-normalized trace element contents: upper part for 1945–1980 samples, the lower part for 1900–1945 samples (Gray cells represent significant squared correlation coefficients).

significantly correlated with TOC and therefore, organic matter may

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not play an important role in TE temporal variations. Mercury presents a peculiar pattern. Already highly enriched in the early 1900s, it was even more enriched during this period, with two peaks at 100–95 cm (mid 1950s; EF=46) and at 80–75 cm (mid 1960s; EF=53). These levels corresponded to the maximum contamination observed for Hg, Ag and Cd. Antimony and tin were also the most enriched (105–100 cm, EF respectively 4.5 and 5.6) just before the first Hg peak in the early 1950s. Then, their EF gradually decreased from 100 cm to the core top. Silver, gold, cadmium and bismuth present their highest EF during this period (Table 2). Silver and gold, already enriched during the previous period 1900–1945, were extremely enriched in the mid 1960s at the 80–75cm level like the Hg maximum with EF up to 18 for Ag and up to 21.6 for Au. Cadmium also became very enriched (EF up to 23). Bismuth, previously not enriched, was also well impacted (EF up to 24). All these elements also peaked at 55–50cm (~1980). They were all highly correlated during this period (Table 3), suggesting common sources. Copper, lead and zinc were also more enriched than during period 1900–1945 and they were all highly correlated. They presented three synchronous peaks (Fig. 4) at 105–100 cm (early 1950s) like Sb and Sn, then at 85–80 cm (~1963) and at 55–50 cm (~1980). Their respective maximum EFs were 6, 5 and 6. Chromium, nickel and cobalt became gradually enriched during this period 1945–1980 with EF increasing from 1.2 to 3.4, 1.3 to 2.8 and 1.0 to 1.7 respectively. They all presented synchronous peaks with Cu, Pb and Zn at 80–75cm and 55–50cm levels, suggesting common anthropogenic sources. Arsenic, molybdenum and tungsten, previously not impacted, were now moderately enriched with EF (As) up to 4, up to 2.6 for Mo and up to 3.1 for W. They all presented only one major peak in the mid-1950s (100–95cm level) and were highly correlated (Table 3). Tungsten also presents significant correlation with Hg, Sb and Sn during this period. In conclusion, this period 1945–1980 was characterized by maximum enrichments for most of the TE sensitive to human impacts like Ag, As, Au, Bi, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Sn, W and Zn. Several elemental clusters can be defined, based on inter-element correlations and on their synchronous EF peaks, when normalized to the 1945 level in the early 1950s, mid-1950s, mid-1960s and ~1980 (Fig. 4). The Ag, Cu, Pb and Zn group, already correlated during the first period 1900–1945, was even more significantly correlated during this period 1945–1980. In addition, Cd joined this group with high correlation coefficients with zinc, unlike during the previous period

Fig. 4. Temporal variations of Ag, Cd, Cu, Hg, Pb, Sb, Sn, W and Zn enrichment factors in the sediment core during 1945–1980 and 1980–2009 periods.

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1900–1945 (Fig. 5). During this period, 1945–1980, Cr and Ni also appeared very well correlated to Cu–Pb–Zn when compared to the previous period (Table 3). 3.3.4. The general decontamination since 1980 (50 to 0 cm levels) This unit is time-referenced with the Chernobyl event in 1986 (45–40 cm) and the last flood covering the island in 2009. It is characterized by a general decrease of EF for all the previously enriched TE, except for Au. This decrease was not as well-marked as for all the TE: the maximum contamination drop was noted for Bi (by a factor equal to 13), then for Cd and Hg (factor 7). The slowest drop, between a factor 2 and 4, was for Ag, As, Cr, Cu, Mo, Ni, Pb, Sb, Sn, W and Zn. Some of these contaminants are still around 1.5 nowadays except for Cr, Ni and W which are fully recovering their background level. The current contamination level in 2009 is now much lower for all sensitive elements (Cu, Hg, Pb, Sb, Sn, W, and Zn) or equivalent (Ag and Cd) than its level in 1945, prior to the 1950–1980 maximum contamination (Fig. 4). 4. Discussion The Loire River basin is predominantly crystalline, reflecting higher TE backgrounds compared to other French river basins. It is also characterized by a marked eutrophication after 1950, caused by the increase of nutrients when sewer collection was gradually established during the 1950–1990 period, first without appropriate tertiary treatments. Eutrophication should then be taken into account for the specific change of calcium and TOC behaviors in river particulates. Finally, the increase of TE enrichment is discussed here when considering the wide occurrence of mining districts, associated smelters and industrial activities and urban evolution at the basin scale. 4.1. Eutrophication of the Loire River and the calcium pattern The observed gradual Ca enrichment in the upper core levels, from the late 1940s to nowadays, can be assigned to eutrophication of the Loire River. Total organic carbon in studied sediments started to increase in the early 1950s. Chlorophyll a peaks, when first surveyed in the early 1980s, already exceeded 100 μg/L (Crouzet, 1983). In details, calcium contents and Ca/Al ratios were nearly constant from 400 to 130 cm suggesting a steady proportion of detrital sources (30% of volcanic inputs, 55% of granitic inputs including metamorphic rocks, and 25% of sedimentary inputs; Meybeck et al., in preparation). Above 130 cm, the gradual Ca enrichment in the sediment material, associated to high inorganic carbon contents, could be related to endogenic calcite formation as already described by Grosbois et al.

Fig. 5. Temporal evolution of Cd/Al vs. Zn/Al ratios in the b63 μm sediment core (a — core sediments on the period 1900–1945, r2 b0; b — core sediments on the period 1945–1980, r2 =0.72; c — core sediments on the period 1980–2009, r2 =0.98; d — bed sediments in coal- and metal-mining sub-basins (AELB database and this study); e — bed sediments in pristine monolithological sub-basins (Meybeck et al., in preparation); f — average background value at the Loire basin scale; and g — treated sewage sludges from 1980 to 2000 (Grosbois et al., 2006)).

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(2001, 2010), Manickam et al. (1985), Négrel (1997). Endogenic calcite is well identified by its specific habitus in suspended particulate matter sampled at low flow resulting from a polynuclear crystalline growth (Fig. 2a), very different from detrital rhombohedric calcite grains, observed during winter high flow (Grosbois et al., 2001). These endogenic calcite grains are 20–40 μm long and are formed around nuclei, often made of siliceous diatom skeletons (Fig. 2a). The precipitation of endogenic calcite is commonly described in carbonated eutrophic lakes (i.e. Anderson, 2011; Revel-Rolland et al., 2005; Stabel, 1986) but barely described in river waters except when travertine deposits take place. Surface waters of the Loire River can be very close to calcite saturation during the summertime (Grosbois et al., 2001). Calcite saturation and subsequent precipitation takes place during summer algal blooms, characterized by extreme diurnal cycles of dissolved oxygen and pH (pH ranging from 7.95 to 9.85; Moatar et al., 1999). From the late 1940s to the 1980s, some enrichment of phosphorus was noted in the Loire sediments (Fig. 2a), and a similar increase is expected in the dissolved fraction but only surveyed since 1980 (Moatar and Meybeck, 2005). In fact, a gradual collection of urban waste waters took place at the end of the 1950s but without any phosphorus treatment until the 1990s. A similar history of waste water treatment has been observed and modeled in the Seine River (Even et al., 2007). It has, therefore, contributed to the eutrophication of Loire surface waters with algae blooms and subsequently to endogenic calcite formation during these blooms. As previously noted, this Ca and calcite increase in the Loire sediments corresponded to TOC increase, related to eutrophication, and led to a general dilution of all detrital particles and to a 20% decrease of Al, Fe, and P and all TE contents. In addition, the general trend of Ca enrichment presented four well-marked peaks, noted at 128–123 cm (EF=2.8), at 105–100 cm (EF=2.4), at 80–75 cm (EF=8.2) and at 55–50 cm (EF=6.0). These Ca peaks correspond both to high inorganic carbon contents (9.2 to 11.8%) and to enrichments of Ag, Bi, Cd, Cr, Cu, Hg, Ni, Sn, Pb and Zn (Fig. 2b). These TE evolutions were not only linked to eutrophication and related endogenic calcite as calcite was a dilutant for TE inputs, but also these 4 Ca peaks could be linked to smelting processes, using CaO and/or CaF2 to remove bedrock gangues of metallic ores. 4.2. Anthropogenic impacts of TE temporal trajectories 4.2.1. Coal mining and coal use impacts on As, Bi, Cd and Hg Coal can be enriched in various TE like As, Cd, Cr, Cu, Co, Hg, Se, Sb, Pb and Zn (Braggs et al., 1998; Kolkeer, 2012; Xu et al., 2003). Hence, coal mining and burning can be regarded as a major source for these elements, either by atmospheric pathways or/and direct waste inputs. Coal mining activities have been important in the headwaters of the Loire basin like in the Furan sub-basin (St. Etienne coal-basin, downstream the industrial city of Saint Etienne with an actual population of 375,000 people and where major 20th century industries were also the nation leaders for hunting guns, gun powder and ammunitions) and in the Bourbince–Arroux sub-basins (Blanzy–Montceau coal-basin). Both districts have been in operation since the 18th century up to the late 1990s and represented respectively the third and fifth most important coal productions in France (600 Mt and 192 Mt of total extracted coal respectively). The maximum production occurred between 1945 and 1960. The last shafts closed down in the 1990s and coal processing operations stopped in the early 2000s. River bank sediments from both coal districts were sampled to characterize a specific Loire coal signature. Although the Bourbince and Arroux river bank profiles did not show any suspicious enrichment (Table 4), the sampled Furan River bank profile provided an evidence of marked contamination in a dark-brown layer (Table 4). In this layer, TOC was the highest (19.4%) with abundant coal debris observed when compared to the clayey bottom of the bank profile

(1.8%), mercury content was one order of magnitude higher (3.6 mg/kg vs. 0.3 mg/kg in the bottom layers) and well correlated to TOC (r=0.92). High contents of As, Bi, and Cd, and of some chalcophilic metals like Au, Cu, Ni, Pb, Sb, Sn and Zn were associated. Hence, the very high mercury enrichment observed in sediments of the 1900–1945 period of Montjean core (24bEF (Hg)b36 in the 230–130 cm levels), the only trace element of this time period well-correlated with TOC, could therefore be attributed to coal mines and coal-burning power plants. Further chemical and mineralogical analyses with in-situ techniques (SEM and EPMA) on these samples are planned to determine associated TE in coal particles. 4.2.2. Mining and smelting impacts on Ag, As, Au, Cu, Pb, Sb, Sn, U, W and Zn The Loire basin is characterized by a dense distribution of metallicore deposits, few of them already identified during the Gallo–Roman era, (Négrel et al., 2004; Carroué, 2010). In its upstream part of the Loire reach, specific Cu, Ag–Pb, Sn and Zn districts were exploited (Fig. 1a). Some districts were closed very early after their discovery in the 19th century but major mining companies have been at their maximum production between 1910 and 1950 (mining sites and their respective history in a database available at sigminesfrance.brgm.fr) and most of the sites closed in the 1980s when metal prices decreased. In details, the Allier sub-basin (Fig. 1a) was the richest in terms of mining site number and ore production (sigmines.brgm.fr). Some districts like Pontgibaud Ag–Pb district (1853–1897) and the Brioude– Massiac Sb district (1850–1936) were very productive. The first one produced 50,000t of Pb up to 1900 and the second one was the most Sb-productive of the world between 1890 and 1909 (Périchaud, 1971). Tin has been exploited in the Charrier district (760t of Sn from 1872 to 1883 and 1896–1953) and in the Echassieres district (2000 t of Sn from 1915 to 1919, 1936–1939 and 1945–1962). This latter district was polymetallic with W oxide exploitation and it still contains an important supply for rare metals like Li, Be, Nb, and Ta. In sub-basins in the Middle Loire basin, some important mining districts were also exploited like the Montrebras Sn district (300t in 1868–1914) in the Creuse basin, the Lucette Au–As–Sb in the Mayenne River basin (1898–1934), and Au in the Sarthe River basin until 1993 (Fig. 1a). Uranium was exploited in the Vienne River basin between the early 1950s and 2005 (Mimausa database available at mimaubdd. irsn.fr). The limited impact of uranium mines in Montjean sediment record should be noticed (EFb1.3 for U and Th), despite important U mining districts (U production of 26,000 t at the Loire basin scale with 72% produced in the Vienne basin). The absence of U-enriched particles does not mean that U mines impacts did not occur since this element is much more soluble in oxic conditions than most other sensitive TE (Gaillardet et al., 2003), being complexed with organic, inorganic and/ or iron colloids (Borole et al., 1982; Crançon and Van der Lee, 2003; Graham et al., 2008; Murphy et al., 1999; Porcelli et al., 1997). A well marked contamination can be noted for tungsten in Montjean sediments in the early 1950s (EF maximum of 3.1; Fig. 4). It could be attributed to two important W mining districts, active during the 1940–1960 period, the Puy-les-Vignes district (4000t WO3 from 1938 to 1957; Vienne tributary) and the Echassieres district (3900 t WO3 from 1945 to 1962; Allier tributary). After 1950, mining activities gradually decreased but in many cases, the ore processing and smelting activities that had been developed were maintained in operations using imported ores and bullions as in Le Palais near Limoges (Vienne basin, Lestel, 2011). The maximum level of contamination for Ag, As, Au, Bi, Cu, Sb, Sn, Pb, and Zn, observed at Montjean between 1945 and 1980 remained stable (maximum EF between 5 and 6 for Cu, Pb and Zn), while antimony (EF max 4.5) and tin (EF max 5.6) gradually decreased. The chemical composition of these 1945–1980 sediments was clearly influenced by these various mining and ore processing activities as shown by the Cd–Zn relationship (Fig. 5). The Cd/Zn ratios in pristine monolithologic streams vary from 0.002 to 0.04 (0.0035 on average at the basin

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Table 4 Concentration ranges (min–max; mg/kg except Al and TOC in wt.% and Au in μg/kg) of representative bed and bank sediment of the Loire sub-basins showing an anthropogenic influence. n = number of analyzed samples; n.d. = not determined; bd.l. = under detection limit. Sampled sub-basin

Main anthropogenic activity

Al

TOC

Ag

As

Au

Bi

Cd

Cr

Cu

Hg

Mo

Ni

Pb

Sb

Sn

U

W

Zn

Aix (n=1) bed sediment Arroux (n=8) bank profile Bourbince (n=5) bank profile Cher (n=6) bank profile Creuse (n=14) bank profile Furan (n=15) bank profile Lignon (n=1) bed sediment Ondaine (n=1) bed sediment Sioule (n=4) bed sediment Vienne (n=7) bank profile

(Pb–Ag) mining and related activities

7.6

4.7

0.3

44

11

1.4

2.1

75

32

0.06

0.5

25

216

0.6

13

10.6

9

205

Coal mining and related activities 8.0– 8.9

1.3– 4.1

0.2– 0.3

23– 44

4– 13

1.2– 2.4

0.4– 0.7

109– 479

22– 35

bd.l.– 0.20

0.9– 32– 2.6 103

70– 136

0.4– 0.6

18– 47

13.1– 17.9

10– 147– 14 237

Coal mining and related activities 7.2– 8.7

0.7– 9.2

bd.l.– 14– 0.7 24

1– 12

0.4– 1.5

0.2– 0.6

75– 89

12– 32

0.02– 0.22

0.6– 23– 1.4 33

30– 52

0.2– 0.4

7–10

8.5– 18.0

6–8 81– 180

(Fe, coal) mining and related activities

6.4– 7.0

4.8– 6.2

0.4– 1.2

21– 40

0.7– 0.8

0.4– 0.7

75– 89

23– 32

0.30– 0.40

0.4– 29– 0.6 33

43– 63

0.2– 0.4

8–9

3.3– 5.1

3–4 125– 158

U-mining and related activities

8.0– 10.8

0.2– 7.3

bd.l.– 11– 0.5 42

1– 52

0.5– 2.0

bd.l.– 75– 0.9 130

12– 42

bd.l.– 0.45

0.3– 29– 4.1 50

21– 247

0.2– 1.5

10– 23

6.5– 13.8

4–9 47– 180

Urban tributary with industries and coal mining activities

8.0– 11.0

1.1– 19.5

bd.l.– 25– 2.3 217

1– 38

0.8– 10.0

bd.l.– 103– 5.1 253

29– 200

0.10– 3.65

0.9– 41– 4.6 95

32– 291

0.3– 6.4

18– 1129

7.0– 19.4

4– 24

106– 402

(Pb–Zn) mining and related activities

9.0

4.6

0.8

24

170

1.5

1.0

96

43

0.15

0.7

32

70

0.4

13

13.0

6

222

Urban tributary with industries

7.5

6.0

0.6

114

28

2.2

2.4

253

222

0.40

8.9

96

522

4.7

48

9.8

40

522

(Pb–Ag, Sb, Sn, and W) mining and related activities

0.8– 3.0

n.d.

n.d.

9– 176

n.d.

n.d.

0.2– 9.2

bd.l.– 11– 39 29

0.03– 0.21

n.d.

8– 3– 1432 26

1.0– 10.0

bd.l.– n.d. 25

n.d.

77– 798

(U and W) mining and related activities

8.4– 9.2

2.7– 4.4

0.5– 1.7

40– 76

24– 135

1.4– 4.3

0.7– 2.3

96– 137

0.50– 4.80

0.7– 27– 1.4 47

0.5– 0.8

16– 25

6– 13

188– 296

8– 21

scale) for the whole basin and well-marked the low Cd/Zn end-member (Fig. 5). The highest Cd–Zn geochemical signature can be firstly considered as a mining end-member determined with bed sediments sampled downstream various mining districts (Table 4). The maximum contamination during the 1945–1980 period corresponds to Cd/Zn ratios around 0.015, somewhat higher than Cd/Zn median ratio of stream samples impacted by coal and metal mining sub-basins (0.007; Fig. 5). This suggests an additional Cd–Zn source like urban sources including multiple plating industries. A typical urban Cd/Zn ratio can be represented by Paris treated sewage sludges which vary from 0.025 in the early 1980s down to 0.004 at the end of the 1990s related to law regulation and application by the Seine basin water authority (Grosbois et al., 2006). The highest values of Cd/Zn urban ratios in the early 1980s can fully account in a first approach the highest Cd/Zn ratios observed in the 1945–1980 Loire sediments (Fig. 5) even if the use of Paris sludge as a substitute for city inputs in the Loire basin remains questionable. The Cd/Zn ratios in the Loire sediments then converge from 1980 to 2009 towards the background Cd/Zn ratio (0.0035). Hence, the different geochemical end-members of pollutant sources can be dynamic over time, either by ore and bullions various origins and treatments or as a result of joint environmental efforts. The resulting contamination trajectory in sediments at a basin scale is therefore very complex. 4.2.3. Industrial sources: example of Cr–Ni It is difficult to separate what was due to ore extraction only and to ore and metal processing and to differentiate industrial sources of contamination from urban sources without further investigation on

52– 103

63– 109

10.2– 12.6

each tributary. Preliminary results on industrial sources of Cr and Ni are presented here as a working hypothesis. Chromium and nickel were not mined in the Loire basin. In France, they are also not associated to urban impacts as noted for Paris megacity (Meybeck et al., 2007; Thévenot et al., 2007). Since Cr and Ni were enriched in the Montjean core between 1950 and 1980 (maximum EF (Cr)=3.4 and EF (Ni)=2.8), industrial sources are therefore likely. In the Arroux sub-basin draining granites, the median content of Cr was around 40 mg/kg and Ni~13 mg/kg in pristine bed stream sediments (Meybeck et al., in preparation). A carboniferous coal deposit was also exploited in this area (Montceau–Blanzy district) and induced a multi-phase ferrous steel and stainless metallurgy using Co–Ni and Co– Cr superalloys since the mid 20th century (Shedd, 2011). Hence, the very high Cr and Ni contents in the Arroux sediments (Cr~480mg/kg and Ni~100 mg/kg, Table 4) could be attributed to this type of industry. Industrial sources of metals for many other elements as Ag, Au and Hg due to plating activities, Bi, Cu, Pb and Zn linked to fuses industry and W to lamp filament industry will be now looked closely in the basin. We are currently developing a large scale project on river bank and reservoirs sediments in all the Loire tributaries to differentiate these multiple sources of metals and their chronologies. 4.2.4. Urban sources: example of silver Urban sources of metals like Cd, Cr, Cu, Hg, Pb, and Zn are diffuse and multiple, runoff on streets, commercial areas and roofs, leaching of contaminated atmosphere (Eckley and Branfireun, 2008; Hall and Ellis, 1985; Garnaud et al., 1999; Sörme and Lagerkvist, 2002; Wong et al., 2006) and inputs from domestic and industrial wastewaters connected

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to urban sewage network (de Miguel et al., 2005; Gasperi et al., 2010; Garcia-Delgado et al., 2007; Seidl et al., 1998). Among the metals with an urban origin, silver is usually considered as a tracer of wastewater effluents and generally attributed to its use in photography (Ayrault et al., 2010; Feng et al., 1998; Lanceleur et al., 2011; Ravizza and Bothner, 1996; Sanudo-Wilhelmy and Flegal, 1992). However, detailed source quantification remains uneasy (Cui et al., 2010). The circulation of metal-containing materials in urban areas is barely documented and/or only established from national statistics which have to be deconvoluted for specific urban sources (Lestel et al., 2007). Metal use in France has gradually been increased by a factor 3 to 5 from 1950 to 2000, except for Hg which has been regulated since the 1970s. After 1980, new environmental measures to limit metallic pollutant dispersion have been gradually developed in France like industrial recycling of metal wastes and reduction of atmospheric emissions. These environmental regulations, in addition to a gradual decrease of mining and industrial related activities, contributed to a TE decrease in treated urban sewage sludges as measured in Paris sewage treatment plant (Grosbois et al., 2006). The Loire basin has never encompassed a large urban surface or a large population at a national scale as the Seine and the Rhône basins did. The Loire basin population represented from 19% of the national population in the early 1900s down to 13% in 2001. The collection of urban waste waters has taken place after 1950 in most cities of the basin but their treatment (primary and secondary levels) has only been gradually developed from the 1970s to the 1990s. Urban impact on river sediments was probably at maximum from 1960 to late 1970s. This period corresponds to the highest EF for Ag (EF (Ag)= 17.6 in 1966 and 16.0 in 1980). However, in the Loire basin, silver cannot be totally ruled out as mining Pb–Ag districts has been exploited in the upper part. The Ag–Pb relationship in the Montjean sediments (Fig. 6) allows to lift this uncertainty: the Ag–Pb contamination was at its maximum when Ag/Pb ratios, originally between 0.013 and 0.016 (1945–1980), went up to 0.019 in the early 1980s. These Ag/Pb ratios were very similar to the ratio of Loire bed sediments collected just a few meters downstream a sewage water plant in the Loire upstream reach (Ag/Pb=0.020; Fig. 6), like in other basins (de Miguel et al., 2005). In comparison, bed sediments, sampled in various Pb–Zn–Ag mining districts, presented lower Ag/Pb ratios (0.002bAg/Pbb0.007). The urban source of silver in the Loire is therefore more likely. From 1980 to 2009, there is a marked Ag and Pb decontamination with a constant Ag/Pb ratio. According to studies, carried out in the USA on silver use, photographic wastes, spent catalysts and electronic material could be a major source of urban silver, at least until 2000 (Hilliard, 2011). These first results on the metal contamination of the Loire are in general agreement with the previous analyses of metal contamination trends and material flow analysis for metals at the Seine basin

scale for the period 1935–2000 (Meybeck et al., 2007) and at the Paris scale for the period 1815–2009 (Lestel, 2011). The critical analyses of metal flow data and metal supplies (Lestel et al., 2007) have shown the following: - A declining river contamination, which began in the 1960s, due to the de-industrialization and industry closure (Meybeck et al., 2007). At the same time, industries have greatly improved waste water treatment and atmospheric emissions in several steps from 1970 to the early 1990s. Industrial solid wastes also have been gradually recycled and/or stored in secured wastes depositories. For instance, the average content of most metals in the sludges, treated by Paris mega wastewater treatment plant in Achères (8million people) has decreased by an order of magnitude since 1979 (Grosbois et al., 2006). - The population density on French river basins and the metal consumption are both increasing, except for Cd and Hg, although, the per capita metal leak into river system has been decreased by an order of magnitude between 1960 and 2000. In addition to the general industrial transformation, specific consumption habits for Cd and Hg have been drastically modified in the last 30 years by several European directives, promoting the collection and recycling of metal-containing products, like batteries, and establishing then the total ban of metal use like for mercury. From 1815 to 2009, each metal has its own material flow trajectory with marked regional differences, as shown for Pb, Cu and Zn by Lestel (2011). Specific history of the Loire River basin, with regards to metal use and contamination, should now be detailed on each main tributary and for each potential pollution site. Numerous industrial sites could be targeted like Saint Etienne, the nation leaders for hunting guns, gun powder and ammunitions and cycles, active during the whole 20th century, Le Palais (near Limoges, Vienne basin) and Couëron (near Nantes in the estuary, downstream of the studied station), where much of the French copper and lead transformations were once realized (Lestel, 2011), Bourges with numerous army industries, Monluçon and Clermont-Ferrand with rubber industries. 4.3. How are the Loire basin sediments contaminated in comparison to other West-European basins? The Loire River basin is often presented as the last “wild” river in France for its hydrological, morphological and biological characteristics (Rode, 2010). We are demonstrating here that this global ecological status is not in contradiction with a metal contamination. In order to compare its degree of contamination with other river basins, the geoaccumulation index Igeo (Müller, 1979), based on the log-ratio between the measured concentration of an element X in the core level [X] and the natural geochemical background of X, were calculated for Cd, Cr, Cu, Hg, and Pb as follows:   Igeo ðX Þ ¼ log2 ½X sample =1:5½X background :

Fig. 6. Temporal evolution of the Ag/Al vs. Pb/Al ratios in the b63 μm sediment core (a — core sediments on the period 1900–1945, r2 =0.47; b — core sediments on the period 1945–1980, r2 =0.80; c — core sediments on the period 1980–2009, r2 =0.53; d — bed sediments in Pb–Zn–Ag-mining sub-basins (this study); e — bed sediments downstream a sewage water plant in the Loire upstream basin).

The Muller index was associated to a qualitative scale of a general contamination level: absence of pollution for Igeo (X)b1, moderate pollution for 1bIgeo (X)b3, strong pollution for Igeo 3b(X)b5 and extreme pollution for Igeo >5. It must be noted that Muller considered an effective contamination when sediment concentrations exceeded the background reference by 50%, which seems high. This scale reflects the general appreciation on metal contamination in the 1970s as an Igeo ~5 corresponds to an EF about 25. Metal contamination data are available in the Seine basin (Le Cloarec et al., 2011; Meybeck et al., 2004), the Rhine basin (Gocht et al., 2001; Middelkoop, 2000; Berner et al., 2012), the Garonne basin (Grousset et al., 1999) and the Rhône basin (only Pb available; courtesy of M. Desmet). Many of these basins have been more

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contaminated than the Loire basin for Cd, Hg, Pb and Zn (Fig. 7). For Pb and Zn, the Loire basin sediments were among the least contaminated with the Garonne basin during the 20th century. The Seine basin was the second‐most contaminated after the Rhine basin. For Cd, the Loire basin was the second most contaminated basin among the cited basins (Fig. 7), even more contaminated than the Garonne basin where a historic heavy metal pollution, resulting from mining and smelting activities in the Lot tributary (Audry et al., 2004a). The relatively lower Garonne contamination may be related to the importance of the dilution effect of suspended particulate matter fluxes from the Lot basin by the Garonne sediment fluxes (Audry et al., 2004b) and by the Cd desorption and Cd plankton uptake in the Garonne estuary where the core was sampled (Dabrin et al., 2009; Schäfer et al., 2002; Strady et al., 2011). For Hg, only the Loire and the Seine studies could be compared and these basins are both highly polluted in Hg, the Seine basin more than the Loire basin. The Hg decontamination started earlier in the Seine basin (early 1960s) than in the Loire basin (mid 1970s) and seems very slow in comparison with other TE indexes due to the geochemical persistency of this element. Contamination scales used here are the original ones from Müller; they can be different from today contamination criteria. This preliminary comparison of West-European rivers reveal common features of metal contamination history: i) a high level of contamination for some elements before 1939, ii) a maximum contamination for most elements between 1950 and 1970–1980 and iii) a general decontamination after 1980 that started 10 to 20 years before in some basins and/or for some elements. 5. Conclusions and perspectives The environmental history of the Loire basin for the last 100 years can be browsed for the first time. The analysis of 55 element contents in a floodplain core of the Lower Loire basin reveals a wide range of contamination patterns, resulting from mining, smelting and their

303

associated industries and from urban sources. Few elements did not show any enrichment with regards to aluminum for the 1900–2009 period. They can be considered as “non-sensitive to Human impacts”: K, Mg, Na, Si and Ti for major elements and Ba, Be, Co, Cs, REE, Rb, Sc, Sr, Th, V, Y and Zr. In contrast, a sensitivity scale to combined Human impacts can be drawn from the most sensitive TE to the last ones: Ag, Au, Bi, Cd, Hg>Sb, Sn≥Cu, Zn≥Pb≥As, Cr, Mo, W>Ni while Ca, Mn and P can also be influenced by human activities. Single contamination peaks were attributed to mining activities (Sb, Sn, and W) or to industrial sources (Bi, Cr, and Ni), and multiple synchronous peaks of Cu, Cr, Pb, and Zn after 1945 were more linked to smelting and specific industries. This unveils the contamination history of the Loire River sediments in three main periods: (i) the early contamination (1900s–1945) for Hg, Sn, Sb, and Au, already highly enriched in sediments, probably due to intensive coal mining, to coal use and to Au, Sb–Sn mining; (ii) the severe contamination (1945–1980) for Ag, Au, Bi, Cd, Cu, Hg, Pb, Sb, Sn, W, and Zn, attributed to industries and ore-processing activities (including imported ores and/or bullions) while mines were gradually closing and to the collection of urban waste waters without an appropriate treatment; and (iii) the generalized and gradual decontamination (1980 to 2009) for all previously listed TE except gold. An important river eutrophication is also registered in the core with the precipitation of endogenic calcite and a dilution effect of TE contents by 20%. These results will have to be confirmed and made sure to be precise through the development of other sedimentary archives (cores and/or bank sediment) in order to characterize specific and local metal sources in targeted tributaries where potential metal sources have been identified. The material flow analysis of metals (Sibley, 2011) can provide clues on the contamination sources which can also be tracked by their isotopic signature, e.g. for Cd, Hg, Pb and Zn (Arnaud et al., 2006; Cloquet et al., 2006, 2008; Graney et al., 1995; Sonke et al., 2008). The comparison of the Loire basin with other basins will be completed for

Fig. 7. Temporal variations of Muller indexes of Pb and Cd at the studied site and comparison with other sediment cores of various West-European basins. (Database for the Seine basin from Le Cloarec et al. (2011), for the Garonne basin from Grousset et al. (1999), for the Rhine basin from Gocht et al. (2001) and Berner et al. (2012), and for the Rhöne basin courtesy of M. Desmet).

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