Contamination of woody habitat soils around a former lead smelter in the North of France

Science of the Total Environment 407 (2009) 5564–5577

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Science of the Total Environment j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v

Contamination of woody habitat soils around a former lead smelter in the North of France F. Douay a,⁎, C. Pruvot a, C. Waterlot a, C. Fritsch b, H. Fourrier a, A. Loriette a, G. Bidar a, C. Grand c, A. de Vaufleury b, R. Scheifler b a b c

Laboratoire Sols et Environnement, Groupe ISA, 48 boulevard Vauban 59046 Lille cedex, France Laboratoire Chrono-Environnement, Université de Franche-Comté, UMR CNRS UsC INRA, Place Leclerc 25030 Besançon cedex, France ADEME, Direction Déchets et Sols, Département Sites et Sols Pollués, 20, Avenue du Grésillé, BP 90406 49004 Angers cedex, France

a r t i c l e

i n f o

Article history: Received 17 February 2009 Received in revised form 27 April 2009 Accepted 10 June 2009 Available online 7 August 2009 Keywords: Contamination Metal trace element Risk assessment Smelter Woody habitat soil

a b s t r a c t The contamination of the topsoil of 262 woody habitats around a former lead smelter in the North of France was assessed. In this urbanized and industrialized area, these kinds of habitats comprise of hedges, groves, small woods, anthropogenic creations and one large forest. Except for the latter, which is 3 km away, these woody habitat soils often present a high anthropization degree (a significant amount of pebbles and stones related to human activities) with a high metal contamination. In the studied woody habitat topsoils, Cd, Pb and Zn concentrations largely exceeded those of agricultural topsoils located in the same environmental context. Therefore, atmospheric emissions from the smelter are not the only cause of the high contamination of the woody habitat soils. This last one is related to the nature and the contamination level of deposit in relation with human activities (rubbles, slag, soils, etc). With regard to the results obtained with chemical extractions, the mobility of Cd, Pb and Zn in these soils is also greater than in agricultural soils. In the forest, pollutant solubility is increased by soil acidic pH. The variability of the physico-chemical parameters and the high metal contamination of the topsoils are the main characteristics of the woody habitats located around the former smelter. Although never taken into account during risk assessment, the disturbance of these environmental components could have important biogeochemical impacts (nutrients and metal cycles). Moreover, any modification of the soils' use could potentially cause mobilization and transfer of the pollutants to the biosphere. Six years after the closure of the smelter, and as social and economic pressures considerably increase in this area, the study of these peculiar ecosystems is necessary to understand and predict the bioavailability, transfer, bioaccumulation and effects of pollutants in food chains. © 2009 Elsevier B.V. All rights reserved.

1. Introduction All over the world, many studies on soil metallic pollution and on the mobility of pollutants, including their transfer to biota and the risks to the population, have been carried out in the proximity of metallurgical sites (Barcan, 2002; Kabala and Singh, 2001; Martley et al., 2004; van Straalen et al., 2001; von Lindern et al., 2003). In Northern France, a former lead smelter, named Metaleurop Nord and located at Noyelles-Godault, released large quantities of metalcontaminated dust into the atmosphere for more than a century until its closure in 2003. These atmospheric discharges highly contaminated the soil around the smelter. Previous works have dealt with the contamination of agricultural soils caused by the smelter fallout (Sterckeman et al., 2000; Sterckeman et al., 2002a).

⁎ Corresponding author. E-mail address: [email protected] (F. Douay). 0048-9697/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2009.06.015

These authors showed that the contamination of the cultivated field soils is limited to a depth of 25–30 cm and that a close relationship exists between the metal trace element (MTE) concentrations in the ploughed layer and the distance from the smelter. Near the smelter, the total cadmium (Cd), lead (Pb) and zinc (Zn) concentrations in agricultural soils can exceed 21, 1132 and 1378 mg kg− 1, respectively. Agricultural practices, particularly liming, result in a neutral or even slightly basic pH for ploughed soils which reduces metal mobility. On the scarce grasslands around the smelter, the Cd, Pb and Zn concentrations in the upper organic layers reached 67, 4890 and 2685 mg kg− 1, respectively, and a transfer of pollutants was detected to a depth of 60–80 cm. This leaching was explained by a slight acidity of the surface organic and organo-mineral horizons (pH = 5.8 to 6.7) coupled with an intense biological activity. Recently, Douay et al. (2008) characterized the physico-chemical parameters of urban soils used as kitchen gardens around Metaleurop Nord. These authors underlined the very large spatial variability of these soils and, above all, their high contamination. In topsoil, the Cd,

F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577

Pb and Zn total concentrations reached 31, 3711 and 5830 mg kg− 1, respectively. This contamination was sometimes over a depth of 1.50 m. Furthermore, no simple relationship seemed to exist between pollutant spatial distribution in urban topsoils and the distance from the contamination source, unlike agricultural topsoils. This was explained by the soil uses, the past or present anthropogenic activities (removal and deposition of various contaminated materials, gardening practices, atmospheric fallout linked to circulating traffic, urban heating, and industrial activities), the corrosion of building materials, etc. Urban soils were often developed on composite materials and their spatial heterogeneity is a typical feature (de Kimpe and Morel, 2000). The situation is more complex in industrialized and urban areas such as the North of France where anthropogenic pressures have deeply influenced landscapes and soils. A few studies of other ecosystems, such as the soils of woody habitats, have been carried out around Metaleurop Nord although they are widely present in the landscape. In this highly urbanized and industrialized region, woody habitats correspond, for the most part, to small groves, woods, poplar groves, anthropogenic linear wooded creations (road embankments, hedges, old railways linked to coal mining, etc), and, secondarily, to a forest (the state-owned forest of Phalempin). Historically, anthropogenic pressures have always been very strong in this region and natural areas are quite rare. The main objective of this study is the characterization of the woody habitat soils located in the area which was highly affected by the past atmospheric emissions of the lead smelter Metaleurop Nord. This approach aims to complete the multidisciplinary investigations initiated over 15 years ago on the region. More precisely, it is part of a program named STARTT whose objectives are to evaluate the bioavailability, transfer and effects of metallic elements in terrestrial food webs at various spatial scales and biological levels (Scheifler et al., 2007). In this paper, the specific objectives were to (1) evaluate the degree of contamination of woody habitat topsoils in Cd, Pb and Zn around the former smelter Metaleurop Nord and to compare their contamination levels with those of agricultural and urban soils, (2) compare the contamination levels of these soils with local and regional values, (3) study the pollutants' behaviour by chemical extractions, and (4) contribute to an assessment of the environmental and health risks. 2. Materials and methods 2.1. Area description The area of our study was located in the former mining area of Northern France, where a lot of industries using coal as the energy source (coking plant, lead and zinc smelter, metallurgical industries) were established at the end of the 19th century. These industries' emissions have highly contaminated and damaged the environment. The largest producer of primary lead in France and one of the largest producers in Europe, the Noyelles-Godault smelter has released significant quantities of dust. In 2002, about 16.9 tons of Pb, 31.6 tons of Zn and 1 ton of Cd were released despite the implementation of significant technical improvements during the 1970s (DRIRE, 2003). Dust emitted by smokestacks, resuspended particles from industrial activities (truck traffic, ore and elaborated product transfer) and more particularly, from the storage site of slag resulting from the ore smelting should be added to these figures. In 2002, these diffuse emissions have been estimated at 29 tons of Pb (DRIRE, 2003). Franssens et al. (2004) have studied the spatial extension of the dry deposition plume by assessing the isotopic signature of the Pb deposition. They showed that the decrease of the Pb dry fallout reached about 80% within a distance of 1500 m from the smelter. In addition, these authors suggested that diffuse emissions from the top of the Metaleurop Nord slag heap have widely contributed to the pollutants' dispersal. The dust fallout affected a large area, highly industrialized and populated with

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various degrees. Moreover, about 3.5 km from Metaleurop Nord, an important Zn smelter (Umicore in Auby) also released large quantities of metallic pollutants into the atmosphere. Considering the existence of this other, important contamination source, the studied area was limited to a 40 km² (5 × 8 km) surface, which had an SW/NE aspect following the direction of prevailing winds. This area includes the former smelter Metaleurop Nord in its centre and is crossed by many transport links (roads, highways, railways, canal) (Fig. 1). It is important to note that urban areas represent 25% of the studied zone, and that areas with soils anthropized with moderate to high levels reached around 47%. The sector which seems to be the least affected by human activities is the state-owned forest of Phalempin, which stretches nearly 4 km2 in the northern part of the studied area. 2.2. Strategy of soil sampling The studied area was subdivided in 160 squares of 25 ha each. For each square, the dominant landscape type was determined by a quantitative analysis of data coming from the pan-European project Corine Land Cover database. The mapping of soil uses has been clarified in order to obtain a resolution of 4 m by scanning aerial photographs and field observations. From a hierarchical cluster analysis, 7 distinct groups have thus been identified according to the dominant landscape type of squares: ploughed fields, urban areas, forests, scrubs/shrubs, mixed-urban areas/ploughed fields, mixedforests/grasslands and the former smelter. Metaleurop Nord occupied an area of about 40 ha. The soil characterization of woody habitats was first carried out with a large scale (i.e. on the whole studied site). For 8 squares with the “ploughed fields” dominant landscape type located, for the most part, in the south of the studied area, and for the square corresponding to the former industrial site, any woody habitats were observed. Among the woody habitats present in each of 151 squares, only one was selected in each of the six dominant landscape types in order to simplify the approach. So, at the end of this first step, 151 woody habitats were chosen. Secondly, the previous step was completed by a more accurate characterization with a selection of 32 squares chosen among the 151 squares and according to the six dominant landscape types and the presumed soil contamination degree (low to very high). For each of these 32 squares, the main woody habitats were selected and their number varied from 1 to 10 per square. A total of 143 habitats, including 32 that had already been selected during the first stage, was selected, i.e. 111 additional habitats for this second step. For each of the 262 habitats selected by the two scales (151 and 111 selected in the first and second steps, respectively), soils were described using a manual auger to a depth of 1.20 m in the absence of obstacles in the soil (coarse elements). In the soil surface layer, a mixed sample was constituted at least from about 15 elementary samplings. Each elementary sampling represented the same weight part in the mixed sample. The OL horizon constituted by the accumulation of no- or few decomposed leaves and woody fragments on the soil surface was deleted from the sampling. However, the OF horizon made of fragmented residues was mixed and sampled together with the top mineral soil material, in accordance with the most frequently recommended protocol in Europe (Theocharopoulos et al., 2001). For each woody habitat, a topsoil sample was constituted. Considering the diversity of soil uses, the availability of financial resources, and in order to compare physico-chemical parameters, and particularly the degree of soil contamination of woody habitats, sampling was carried out on the first 25 cm. This depth was also chosen during previous investigations on agricultural and urban soils of the studied area (Sterckeman et al., 2002a; Douay et al., 2006; Pruvot et al., 2006; Douay et al., 2008). That allowed the comparison of soil parameters whatever their uses.

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F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577

Fig. 1. Soil use in the studied area.

This approach has been completed by the selection of two squares of “scrubs/shrubs” type and located over 10 km northeast of Metaleurop Nord in the same morpho-pedological context. Within these two squares, a total of 15 woody habitats, considered as a reference because they were far enough from Metaleurop Nord to be highly affected by atmospheric releases, were selected. The soils of these woody habitats were described according the protocol previously used. 2.3. Methods of sample preparation and analysis All soil samples were dried at a temperature below 40 °C, and then gently crushed to pass through a 2 mm sieve. Pebbles and stones were removed. On the 262 soil samples taken from the studied area, a granulometric analysis was carried out, according to the NF X 31-107 standard, by the dispersion of mineral particles after destruction of the organic matter by hydrogen peroxide and the separation of the particles into different classes by sedimentation (particles b 50 mm and sieving particles N 50 mm) (Gee and Bauder, 1986). Soil pH (water suspension) was measured according to the NF ISO 10390 standard. The organic carbon was determined by sulfochromic oxidation according to the NF X 31-109 standard. The total carbonate content (measurement of the volume of CO2 released by reaction with HCl) was measured according to the NF ISO 10693 standard. The Cation Exchange Capacity (CEC) was

measured after percolation of 1.0 M ammonium acetate solution at pH 7 (Lefevre, 1961) according to the NF X 31-130 standard. For each sample, a representative subsample was crushed and passed through a 0.250 mm sieve. As described by the NF X 31-147 standard (1996), ashing at 450° and a mixture of hydrofluoric (HF) and perchloric (HClO4) acids were used for the total dissolution of MTE. Concentrations of Cd and Pb were determined by inductivelycoupled argon plasma mass spectrometry (ICP-MS) and Zn was quantified by inductively-coupled argon plasma atomic emission spectrometry (ICP-AES). These analyses were performed by the Soil Analysis Laboratory of INRA (Arras, France) where all precautions were taken with respect to the protocol application and the calibration. Quality control was based on the use of certified samples (BCR 141 and 142; GBW 07401, 07402, 07404, 07405 and 07406), samples from inter-laboratory comparisons as well as internal control samples and duplicates of the analysis. Selective extractions with calcium chloride (CaCl2 0.01 M) were also performed in three replicates (Kabala and Singh, 2001; Novozamsky et al., 1993). For this, 3 g of soil sieved at 0.250 mm was shaken with a soil/extractant ratio of 1/10 (w/v). The contact time of the samples with the chemical extractant was 2 hours. The extract was separated from the solid residue by centrifugation (4530 rpm) for 20 minutes at room temperature and the solution was filtered on an acetate Millipore membrane characterized by a 0.45 μm porosity. The

F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577 Table 1 Sequential extraction procedure (from Rauret et al., 2000). Steps

Reagents

Nominal target phases

1

0.11 mol L− 1 acetic acid

2

0.50 mol L− 1 hydroxylammonium chloride at pH 2 8.8 mol L− 1 H2O2, followed by 1.0 mol L− 1 ammonium acetate at pH 2 Hydrofluoric (48%) and perchloric (70%) acids

Fraction A: Exchangeable, water- and acid-soluble Fraction B: Reducible

3 4

Fraction D: Oxidizable Fraction R: Residual

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element distribution in the landscape and to evaluate the environmental risks. The pollutant partitioning in the soils of woody habitats, the regrouping of habitats by type and the research of decisive factors for the behaviour of pollutants were evaluated by principal component analysis and then by a hierarchical cluster analysis. Statistical analyses have been performed using XLSTAT or STATISTICA. 3. Results 3.1. Location and characteristics of woody habitats

solution was put into a polypropylene container, acidified with 92 μL of nitric acid solution (65%) and stored at 4 °C for analysis. The extracted metal concentrations were determined by using AtomicAbsorption Spectrometry (AAS, AA-6800, Shimadzu). Moreover the fractionation of Cd, Pb and Zn was performed for the 143 samples which correspond to the soil characterization of 32 squares. A three-stage sequential extraction procedure was used according to Rauret et al. (2000). Each of these steps was noted as fractions A, B or D. The extracted metal concentrations were determined by using AAS. A fourth step (fraction R) was added to the procedure of Rauret et al. with the total metal dissolution in the residual fraction (Table 1). Total dissolution and the MTE measurement were carried out by the accredited Soil Analysis Laboratory of INRA. The BCR CRM 701 was used as standard reference material. The variability of obtained results was b10%. The total digestion results were in a good agreement with the sum of recoveries of individual extraction steps (R2 = 0.935). On the 15 samples corresponding to reference habitats, total Cd, Pb and Zn concentrations were also determined. Among them, the analytical procedure was completed for 10 samples by the study of the mobility of MTE using CaCl2 extractions as previously described.

Because of the sampling strategy used, the habitats that were characterized are evenly distributed over the studied area. Sixty-six percent of the total habitats were located between 1000 and 3500 m from the smelter. It is noteworthy that about 25% of the habitats are located under the prevailing winds of the former lead smelter and that 10% are located under the intermediate winds, with a North–North-East direction. Fig. 2 shows the distribution of habitats according to their nature and the soil anthropization degree. This degree was mainly estimated with regard to the amount of coarse elements in the soils. In the area affected by atmospheric releases from Metaleurop Nord, the percentage of coarse elements in the surface layer varies from a few percent (low to medium anthropization) to more than 90% (very high anthropization). For about 55% of the studied habitats, the soil anthropization was described as medium to high. The soil depth is less than 80 cm for 31% of cases. This information attested to the important anthropogenic reworking of the soils. The habitat soils corresponding to hedges show a medium to high anthropization in 80% of cases. In groves and woods, the soil anthropization is less frequent (37%). Conversely, for the forest it is low, or even absent (Fig. 2a). The majority of reference habitats correspond to woods with a low anthropization degree of soils (Fig. 2b).

2.4. Diagnosis of topsoil contamination 3.2. Physico-chemical parameters of woody habitat topsoils All the analytical data was collected in a database. For each habitat, dominant landscape type, habitat feature (hedge, wood, grove, forest, ploughed field, urban area, etc), distance from the smelter, angle between geographic North and the site, as well as pedological parameters (depth and hydromorphy degree of soils, percentage of coarse elements, anthropization degree of soils) were also recorded. The anthropization assessment of soil includes the presence of signs at various depths that show a deposit or embankments related to human activities. It mostly concerns coarse elements (brick, concrete, waste rock from coal mining). In the North of France, soils are mainly developed in Quaternary loess formations, and to a lesser extent in Tertiary clays, alluvium and colluvium deposits (Sterckeman et al., 2006). Therefore the presence of coarse elements in the soils of the studied area is exclusively linked to human activities. The total Cd, Pb and Zn concentrations measured in the soils of woody habitats located around Metaleurop Nord were compared, on the one hand, with those measured in reference soils that had been slightly affected by dust fallout coming from the smelter. On the other hand, the studied element concentrations were compared with various values: agricultural or urban local values, and regional agricultural data. Local values were only determined in the area affected by the dust emissions of Metaleurop Nord smelter. These data were obtained in different studies by applying the same procedures that those described in this present paper (sampling, analysis). Regional agricultural values are the most frequently observed concentrations in the ploughed layer of agricultural soils located in the North of France, without any major pollution source or industrial and mining activities. The contaminated areas were excluded from the regional agricultural sampling (Sterckeman et al., 2006; Baize, 2001). Maps of Cd, Pb and Zn concentrations measured in extraction solutions (total and CaCl2) have been drawn in order to represent the

Results concerning physico-chemical characteristics of woody habitat topsoils are presented in Table 2, and the soil textural data

Fig. 2. Distribution of woody habitats according to their type and degree of soil anthropization (a: around Metaleurop Nord, b: reference area).

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Table 2 Minimum, mean, median, maximum values and standard deviation (SD) of physicochemical parameters of the woody habitat topsoils around Metaleurop Nord smelter (n = 262).

Min Mean Med Max SD

Clay

Silt

Sand

Organic carbon

CaCO3

g kg− 1

g kg− 1

g kg− 1

g kg− 1

g kg− 1

29 204 193 685 77

90 456 466 745 135

52 339 328 881 168

5.3 48.9 35.8 260.5 36.8

b1 36 16 208 44

pH

CEC cmol+ kg− 1

3.8 6.8 7.2 8.3 1.3

7.04 15.41 14.45 33.30 4.95

are graphically shown in Fig. 3. Similar to the coarse element percentage and the depth of soils, the physico-chemical parameters vary highly according to the sites. In most cases, the topsoil texture classes are silty loam to loam according to the USDA classification. They are sometimes sandy or clayed. Similarly, topsoil pH can vary highly (from 3.8 to 8.3). Even if they are often neutral, they might be locally acid or slightly basic (Table 2). Organic carbon contents are also variable (from 5.3 to 260.5 g kg− 1). In addition, the description of soil profiles has often shown some signs of winter water logging. For 33% of profiles, oxido-reduction mottles appear before a depth of 40 cm. For 22% of soils, they are located between 40 and 80 cm deep. With regard to the high variability of these physico-chemical parameters, the soil functioning could be greatly different according to the woody habitats. 3.3. Contamination degree of woody habitat soils Table 3 shows the distribution of total Cd, Pb and Zn concentrations in woody habitat topsoils in the studied area, those of the references

as well as those relating to local and regional agricultural soils and urban soils of the area affected by Metaleurop Nord atmospheric releases. For woody habitats in the studied area, the great variability in the measured values is noteworthy. For example, the spatial distribution of total Cd, Pb and Zn concentrations in woody habitat soils is depicted in Fig. 4. The map shows that the Pb concentrations in soils often exceed 500 mg kg− 1 within a radius of 2 km around the former smelter and how the affected area stretches to the North-East due to the prevailing wind direction. The highest MTE concentrations are observed in a woody habitat located on dredging deposits which run alongside the former smelter (2402 mg of Cd, 41,959 mg of Pb, and 38,763 mg of Zn per kg of soil). Particularly elevated concentrations have been also measured in the immediate vicinity of the smelter next to a motorway where the habitat soils are developed on ballast (236 mg of Cd, 6809 mg of Pb, and 6035 mg of Zn per kg). Conversely, the lowest Cd and Pb concentrations corresponded to a hedge bordering an unsealed road from about 4.5 km North-East of the smelter (0.1 mg of Cd, 16 mg of Pb, and 51 mg of Zn per kg of soil). After the removal of data relating to soils of the two habitats which show high concentrations, linear relations between total Cd, Pb and Zn concentrations were observed ([Cd] = 0.0123 [Pb] + 2.2503 − R2 = 0.62; [Zn] = 0.8543 [Pb] + 236.08 − R2 = 0.79; [Zn] = 34.968 [Cd] + 387.59 − R2 = 0.57). These relations explain the similarities noted between the distribution of the three elements in woody habitat soils although it was significantly lower than agricultural soils, for which the determination coefficient was 0.85 (Sterckeman et al. 2002a). For all the soils of the woody habitats, the average ratio Zn/Pb was 1.5. Nevertheless for the Phalempin forest soils, this one was only 0.7.

Fig. 3. Texture of 262 woody topsoils around Metaleurop Nord smelter (USDA classification).

F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577 Table 3 Distribution of total Cd, Pb and Zn concentrations in woody habitat topsoils around Metaleurop Nord smelter, in reference woody habitat topsoils, in agricultural and local topsoils, and in local urban topsoils (minimum, mean, first quartile, median, third quartile, maximum values and standard deviation). Cd

Pb

Zn

16 748 193 303 614 41,959 2718 43 112 80 110 136 200 47 58 324 153 240 397 3005 293 13 32 23 29 37 198 17 45 673 266 434 872 3711 637

44 885 290 460 838 38,763 2512 89 178 140 177 199 278 59 115 450 264 350 527 2515 306 24 68 54 67 80 206 25 98 935 459 709 1030 5830 859

mg kg− 1 Woody habitat soils

Studied area (n = 262)

Reference (n = 15)

Agricultural soils

Local values (n = 337) (unpublished data)

Regional valuesa (n = 232)

Urban soils

a

Local values (n = 165) (unpublished data)

Min Mean Q1 Med Q3 Max SD Min Mean Q1 Med Q3 Max SD Min Mean Q1 Med Q3 Max SD Min Mean Q1 Med Q3 Max SD Min Mean Q1 Med Q3 Max SD

0.1 19.2 2.9 5.0 10.3 2402.1 149.0 0.9 1.5 1.2 1.5 1.7 2.4 0.5 1.4 6.2 3.4 4.8 7.5 44.2 4.5 0.03 0.44 0.29 0.41 0.52 1.36 0.21 0.8 9.5 4.1 6.9 12.6 31.4 7.5

Without distinction of the parental material nature (from Sterckeman et al., 2002b).

3.3.1. Comparison with values of reference woody topsoils Median values of total MTE concentrations of woody habitat soils affected by releases are three times higher than those of the reference habitat soils which are 1.5 mg of Cd, 110 mg of Pb and 177 mg of Zn per kg of topsoil. The variability of these element concentrations in the reference soil is noteworthy. For Cd and Zn, the ratio (minimum– maximum values) was 3 while for Pb it was 5. However, the reference soil concentrations are significantly lower than those measured in the studied woody habitats (p b 0.005, p b 0.011 and p b 0.001, respectively for Cd, Pb and Zn, with an error of 5%).

3.3.2. Comparison with local and regional agricultural topsoil values In the studied area, even if the median values are quite similar, the contamination degree of woody habitat soils is significantly higher than that of agricultural soils (p b 0.0001 with an error of 5% for the 3 elements). However the difference observed between reference woody and regional agricultural topsoils is not significant.

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3.4. Influence of the distance from the former smelter After the removal of data relating to the habitat soil developed on the dredging sludge of the canal, a no linear relation (power model) between total MTE concentrations in woody habitat soils and the distance from the lead smelter is highlighted (Fig. 5). Although the highest concentrations correspond to habitats located less than 500 m from the source, there does not seem to be a strong relation (R2 = 0.49). In another study, Douay et al. (2008) showed less significant relationship whereas Sterckeman et al. (2002a) and Douay et al. (2006) have observed a close relationship with agricultural topsoils (y = 5 × 106x− 1,23, with y as the Pb concentrations and x, the distance to the smelter; R2 = 0.85). 3.5. Fractionation of Cd, Pb and Zn in soils Fig. 6 shows the distribution of MTE in fractions A, B, D and R, evaluated by using sequential extractions. Values are expressed for each element as the percentage of the sum of all fractions. With regards to this data, the results indicate that Cd is mainly in the exchangeable, water- and acid-soluble fraction (fraction A, 55% in average), and to a lesser extent to reducible fraction (fraction B, 26%). The oxidizable fraction of Cd (fraction D) and the residual fraction (R) are relatively weak with 13 and 5%, respectively. Lead is highly present in fraction B with about 45%. In fraction A, 30% of Pb is extracted with a higher variability than that of the other fractions. Fractions D and R represent 9.5% and 14.5%, respectively. Zinc is mainly in fraction A (34%) but also in fractions B (26%) and R (26%). The part of Zn in the fraction D is relatively low (13%). Altogether, the results confirm that Cd is the most mobile among the studied elements. The percentage of Pb extracted by acetic acid solution (fraction A) is on average about 1.8-fold less than the percentage of Cd. For Zn, the distribution between A (the most mobile fraction), B and R (the least mobile fraction) is similar. Lead appears to have an equal, or even higher, mobility to that of Zn. 3.6. Extractability of Cd, Pb and Zn with CaCl2 Table 4 presents the distribution of Cd, Pb and Zn concentrations extracted with CaCl2 (0.01 M) in woody habitat soils (around the former smelter and in reference area). With regard to the total concentrations of woody habitat soils, CaCl2 Cd-extractable was ranged from 0.2 to 114.7%, while for Pb, it is 0.0 to 8.4%, and for Zn the percentages varied from 0.0 to 38.9%. Results show that Cd is the most easily removed element using CaCl2. They underline the high variability of the studied elements according to soils and their physico-chemical parameters. For Cd and Pb, the extractions with CaCl2 are clearly higher than those obtained on the soils of reference habitats (0.3± 0.2% and 3.6 ± 0.8%, respectively). However, Zn concentrations are quite similar (6.1 ± 6.0%). The very high variability for this element is also noteworthy. Fig. 7 depicts the spatial distribution of Cd, Pb and Zn concentrations extracted with CaCl2 (0.01 M) in the woody habitat soils around the former smelter. In addition to the high variability of the measured concentrations, it appears that the Pb concentrations extracted with CaCl2, and also those of Cd and Zn are among the highest in the Phalempin forest soils whereas the total concentrations are among the lowest. 3.7. Typology of the woody habitat soils

3.3.3. Comparison with local urban topsoil values For Cd and Pb the results show that there is not a significant difference between the soils of woody habitats and the urban soils. On the other hand, the Zn contamination degree of woody habitat soils is significantly higher than that of local urban soils (p b 0.001 with an error of 5%).

In order to bring out the structural variables, a principal component analysis (PCA) was carried out, including different variables such as pedological data, total Cd, Pb and Zn concentrations of studied woody habitat soils as well as the geographic position (distance and angle) in relation to the smelter. The first statistical

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Fig. 4. Maps of total Cd, Pb and Zn concentrations in woody habitat topsoils in the studied area.

F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577

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Fig. 4 (continued).

treatment, collecting all the 262 habitats, shows that two points particularly influence the scatterplot distribution, considering the very high total concentrations of Cd, Pb and Zn in their soils. The following statistical treatment was performed after the removal of these two values. Fig. 8 presents the correlation circle and the first factorial plane defined by the first two axes of the PCA. The analysis of the results shows that a structuring of data exists according to the soil texture, the distance from the smelter and the anthropization degree. The first two factorial axes of PCA reveal a contrast between the soils with a high anthropization and high MTE concentrations and between deeper and low contaminated soils. In addition, PCA tends to confirm the absence of a simple structuring of data according to the Cd, Pb and Zn concentrations in soils. The most differential factors seem to be the soil anthropization degree (soil depth and coarse element percentage) and the organic carbon concentration. The gradient of soil contamination secondarily influ-

ences the data structuring. According to the statistical analysis (PCA and hierarchical cluster analysis), the 262 woody habitats are classified in 8 clusters (Fig. 9), pooled on 4 main groups noted A to D. Table 5 shows the main characteristics of each cluster. Group A joins Clusters 1 and 4, which comprise of 92 habitats and which are located near Metaleurop Nord (1.7 km) and under the prevailing winds. They mainly assemble agricultural hedges and some wooded creations. Their soils are moderately anthropized (or even slightly), have a silty-loam texture, a neutral pH, a high CEC. Cluster 1 joins together 28 sites which are under the prevailing winds and where soils are the most hydromorphic and contaminated whereas Cluster 4 joins 63 sites located under intermediary winds with soils among the least contaminated. With regards to environmental and physico-chemical characteristics of soils, one of two habitats removed from the hierarchical cluster analysis is linked to Cluster 1. This habitat

Fig. 5. Relationship between Pb total concentrations in woody habitat topsoils and distance to Metaleurop Nord.

Fig. 6. Fractionation of Cd, Pb and Zn in woody topsoils around Metaleurop Nord smelter. Mean values and standard deviation are given (n = 143).

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Table 4 Distribution of Cd, Pb and Zn concentrations extracted with CaCl2 (0.01 M) in woody habitat topsoils around Metaleurop Nord smelter and in reference woody habitat topsoils (minimum, mean, first quartile, median, third quartile, maximum values and standard deviation). Cd

Pb

Zn

0.27 1.51 0.29 0.67 1.75 13.68 2.14 0.77 3.40 2.97 3.67 4.16 5.07 1.30

0.01 12.84 0.80 4.22 17.05 142.97 20.49 0.76 6.43 3.42 6.06 7.16 18.38 5.41

mg kg− 1 Studied area (n = 262)

Reference (n = 10)

Min Mean Q1 Med Q3 Max SD Min Mean Q1 Med Q3 Max SD

0.02 0.66 0.07 0.20 0.53 59.46 3.70 0.15 0.30 0.21 0.25 0.34 0.62 0.15

is closed to the smelter, its soil is highly anthropized, clayed sandy with a load of coarse elements (10%), and highly contaminated (236 mg of Cd, 6809 mg of Pb and 6035 mg of Zn per kg). Group B joins Clusters 2 and 5, which correspond to highly anthropized soils, and Clusters 6 and 7, which correspond to the highest anthropized shallow soils. They are silty loam to sandy loam, with a high quantity of coarse elements, a neutral to slightly basic pH and high organic matter content. These soils are highly contaminated and located from approximately 2 km from the former smelter. Cluster 2, with 47 habitats, mainly joins together road embankments which are moderately contaminated. Cluster 5, with 17 habitats, joins developed and urban habitats which are very anthropized, contaminated and showing a high organic carbon content. The sites attached to Cluster 5 are located between 2 and 3 km from the smelter. Clusters 6 (14 habitats) and 7 (5 habitats) are characterized by human created spaces, with shallow and carbonated soils showing either a very weak (Cluster 6) or a very high (Cluster 7) organic carbon content and the highest MTE concentrations. These sites are very close to the smelter (fewer than 600 m) but not located under the prevailing winds. Group C is composed by Cluster 3 which includes 86 habitats. It was dominated by forests and agricultural groves. The soils are loam, slightly acid, with lowest organic carbon content, slightly anthropized, moderately contaminated and a distance of about 4 km from the contamination source, but located under the prevailing winds. Group D is made up by only one habitat showing a very high contamination degree. It is located on dredging deposits from the canal, with a silty loam texture and a neutral pH. For statistical analysis, the data related to this habitat was previously suppressed. The potential environmental risks can be analyzed with regard to the CaCl2 extracts and the fractionation obtained by sequential extraction procedure. This assessment highly varied according to the groups and clusters previously described but also according to the used procedure. Whatever the last one, the behaviour of Cd and Zn was similar. On the other hand the classification of clusters was different according to the extraction. With CaCl2, the percentage of Cd and Zn extractable appeared to be the most important for the clusters 3, 1, 7 and 4. For Pb, the most important percentage was obtained for Cluster 3. With regard to first step of the sequential extraction, the percentage of Cd and Zn extractable appeared to be the most important for Clusters 8, 7, 6, 2. Except for Cluster 8 for which the percentage was the lowest, the classification was similar for Pb. 4. Discussion The aim of this work was to characterize the woody habitat soils in an area that has been highly affected by the past emissions of the lead

smelter, Metaleurop Nord. The behaviour of the main topsoil pollutants (Cd, Pb and Zn) was evaluated using chemical sequential and CaCl2 extractions. Except for the soils of Phalempin forest, which showed a pedogenetic development in keeping with the regional morphoclimatic context, the main characteristic of the studied woody soils is their high degree of anthropization, which occurs to such an extent that they can often be linked to Technosols, whose properties and functions are dominated by human activities (Rossiter, 2007; Lehmann and Stahr, 2007). The coarse element percentage might vary from a few percent to sometimes more than 80%. The latter figure applies particularly to the edges of roads and highways or to the location of former railway tracks. For most of the studied habitats, the means through which soil MTE contamination occurred include dust fallout due to past smelter activities as well as modifications and deposits of various kinds of materials. To our knowledge, soils from these kinds of habitats have never been the subject of a broad investigation and thus it was difficult to refer to other industrialized or unindustrialized sites. This work showed that woody habitat soils, located more than 10 km from the former smelter, and considered in this study as reference woody habitats, can also present anthropization signs and MTE concentrations noticeably higher than the regional agricultural values. These observations suggest that this situation is not the only one considering the nature of developments (edges, former brownfields, etc) or woody habitat specificities (age, history, use of space). In the studied area, the proximity of the smelter worsened the contamination of the woody habitat soils with the dispersal of various contaminated wastes in the surrounding area. The resulting contamination of soil might sometimes be increased with regard to that only generated by smelter atmospheric releases. Van Oort et al. (2001) made the same observation on agricultural soils affected by zinc smelter activities. Over the course of time and with soil use changes, the deposition of industrial wastes on lanes became pollution sources. The present research has also highlighted that the degree of Cd, Pb and Zn contamination in Phalempin forest topsoils was two or three times more significant than that in the surrounding agricultural soils. This could be explained by the foliar uptake and the canopy filtration of dust, aerosols and gases (Nieminen et al., 1999). Different works have shown, particularly in urban environments, that tree leaf concentrations depended on the sites' exposure to pollutants, species, seasons and years (Piczak et al., 2003; Monaci et al., 2000; Alriksson and Eriksson, 2001; Rademacher, 2001). Anyway, the decomposition of contaminated leaves as well as of twigs and barks progressively causes an increase in soil pollutant concentrations (Scheid et al., 2005). The methodology, set up in the present study, does not allow specifying the enrichment of pollutants at the soil surface. In fact, in order to compare the soil contamination degree of woody habitats, a soil sample was constituted from the 0–25 cm layer. However it is common knowledge that a gradient of MTE concentrations exists in forest topsoils not largely exposed to pollutions (Sterckeman et al., 2006; Alriksson and Eriksson, 2001). The surface organic layers are the most contaminated. In addition, the present work assessed the contamination degree of each woody habitat studied through a soil composite sample. Because of the importance of soil anthropization, a high spatial variability of topsoil contamination was presumed. Conversely, it could be thought that this variability is low in forest soils. But many factors influence the spatial distribution of physico-chemical parameters (Kirwan et al., 2005) and MTE in forest soils. These factors are (1) the absence of agricultural uses which, conversely to agricultural soils, lead to the differentiation of organic and/or organo-mineral layers with variable thickness, more or less enriched with MTE, (2) the ability of trees to accumulate and to trap MTE which is dependent on species (Alriksson and Eriksson, 2001), (3) the canopy structure which influences the throughfall water composition and the atmospheric depositions at the forest soil surface (Nieminen et al., 1999), (4) the stem flow which

F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577

Fig. 7. Map of Cd, Pb and Zn concentrations extracted with CaCl2 (0.01 M) in woody habitat topsoils around Metaleurop Nord smelter.

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Fig. 7 (continued).

includes water running down tree trunks and branches (Draaijers et al., 1996; Beiera et al., 1993), (5) the high organic matter contents of soils which are responsible for the formation of stable complexes with Pb, and to a lesser extent with Cd, as well as the solubilization and the transfer of these elements during the organic matter solubilization and the soluble organic acid production such as fulvic acids (Bergkvist, 2001), and (6) the soil acidity which is always higher in the rhizosphere than in the bulk soil (Courchesne et al., 2006). Moreover, forest

edges are known to have an effect on throughfall water, stem flow, and ion deposition in the forest soil. Atmospheric deposition and dry deposition in particular are more elevated at upwind forest edges due to local advection and enhanced turbulent exchange (Devlaeminck et al., 2005; Erisman and Draaijers, 2003; Draaijers et al., 1988). This spatial variability of deposition in forest ecosystems could have important implications for hydrological, biogeochemical, and ecological processes on the forest floor (Staelens et al., 2005).

Fig. 8. Correlation circle and projection of main physico-chemical parameters of woody habitat soils. Cd, Pb and Zn contamination degree and their situation from the smelter along axes 1 and 2 issued from PCA.

F. Douay et al. / Science of the Total Environment 407 (2009) 5564–5577

Fig. 9. Distribution of woody habitats according to the distance from the smelter, the degree of soil anthropization, texture, hydromorphy degree and total Cd, Pb and Zn concentrations.

The very high degree of soil contamination in some woody habitats can be hazardous for the environment and the biosphere. The soil sampling strategy based on the constitution of composite samples from the 0–25 cm layer might underestimate the hazard evaluation. The soil surface layer of woody habitats might show a MTE contamination noticeably higher than measurements taken from the first 25 cm, and that the hazards were consequently increased. Compared with agricultural soils in the studied area (unpublished data), Cd and Zn contained in woody habitat soils are more present in the A and D fractions and less in reducible fraction (B). These elements are respectively 18% and 38% more present in the A fraction and 52% and 16% in the D fraction. The share of Pb is significantly more important in fractions A (800%) and R (70%). On the other hand, Pb is less present in fractions B (−32%) and D (− 55%) compared to the agricultural soils. Results obtained for Pb are in agreement with those of Ettler et al. (2005), who compared the lead speciation in forest and tilled soils heavily polluted by metallurgic emissions. These authors

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explained the Pb fractionation in forest soils by two binding mechanisms of this element to soil organic matter. A part of Pb is weakly bound to labile organic matter by electrostatic interactions. Another part is strongly bound to organic matter by inner-sphere complexes. The remaining Pb is adsorbed on Fe–Mn oxides and hydroxides (reducible fraction), and to a lesser extent Pb is present in the crystal lattice. Considering that the A fractions are easily available, it appears that Cd, Pb and Zn are, on the whole, more mobile in woody habitat soils than in agricultural soils. This is particularly true for soils of the Phalempin forest in which the higher mobility of these elements is explained by the acidic soil pH. It is well known that metal solubility depends on organic matter content and pH (McBride et al., 1997). This phenomenon could favour a pollutant transfer to food chains but also to deeper layers. For Pb hazards should be moderate with regard to the weak degree of contamination in Phalempin forest topsoils (only 2 to 3 times higher than the regional agricultural values) and the low mobility of this element. On the other hand metal elements more mobile than Pb, such as Cd and Zn, could migrate to depth. In comparison with agricultural topsoils the low Zn/Pb ratio calculated in the case of forest soils with acidic pH should indicated movement of Zn towards depth (Fernandez et al, 2007). In addition, the temporary soil waterlogging of habitats located at the north of the canal might also be a factor which influences the mobility of the studied elements. Indeed, groundwater frequently reaches the highly contaminated surface layers of soils in winter. This could contribute to the transfer of pollutants from the surface to the deeper layers. It is well known that reducing conditions induce indirect effects on the speciation and solubility of studied MTE, through decreasing pH and reducing Mn–Fe oxides (Chuan et al., 1996; Charlatchka and Cambier, 2000). The Pb and Zn mobility has been particularly highlighted in forest soils affected by the atmospheric releases of a former zinc smelter in the North of France. Thus, in a similar geomorphic context, Van Oort et al. (2001) showed an important leaching of Zn, an

Table 5 Main characteristics of the 8 clusters identified by the hierarchical cluster analysis. Group

A

C

B

Cluster

1

4

3

2

5

6

7

D 8

Number of habitats Distance from smelter (m) Angle/North (°) Hydromorphy Depth of soil (cm) Anthropization degree Amount of coarse elements (%) Clay (g kg− 1) Silt (g kg− 1) Sand (g kg− 1) Organic carbon (g kg− 1) Total CaCO3 (g kg− 1) pH CEC (cmol+ kg− 1) Cd total (mg kg− 1) Pb total (mg kg− 1) Zn total (mg kg− 1) Cd CaCl2 (%) Pb CaCl2 (%) Zn CaCl2 (%) Cd (%) A fraction B fraction D fraction R fraction Pb (%) A fraction B fraction D fraction R fraction Zn (%) A fraction B fraction D fraction R fraction

28 1942 62 3.5 118 2.0 0.4 321 458 221 55.1 10 6.8 24.08 12.03 763 812 5.3 0.2 2.5 45.9 41.8 7.1 5.2 24.3 53.0 13.2 9.5 29.3 33.8 12.8 24.2

63 1714 215 2.8 112 3.0 2.3 213 560 227 35.7 32 7.4 15.72 8.8 470 669 3.6 0.2 1.1 57.2 29.1 9.4 4.3 29.3 48.1 9.6 13.0 35.2 31.1 11.5 22.2

86 3868 50 3.2 118 1.8 1.1 186 395 419 35.2 2 5.6 12.82 3.1 222 282 28.2 1.9 11.4 48.6 21.4 24.3 5.7 21.3 51.8 9.8 17.0 23.6 22.2 18.2 36.1

47 2178 165 0.4 47 4.6 11.4 197 504 299 47.3 52 7.7 14.42 5.0 332 539 2.2 0.4 0.3 63.5 21.7 10.0 4.7 41.0 35.1 7.3 16.7 40.6 21.0 11.8 26.6

17 2744 121 0.1 38 5.0 41.8 118 270 612 129.2 37 7.5 17.36 8.7 700 1208 1.2 0.3 0.9 42.6 37.9 12.6 6.9 17.8 40.5 13.0 28.7 35.3 31.5 11.6 21.6

14 623 139 2.1 56 4.5 7.6 223 495 282 34.8 61 8.0 14.66 42.7 1693 2192 1.8 0.03 0.2 68.6 20.2 7.1 4.1 50.6 37.8 5.8 5.8 53.3 19.8 11.9 15.0

5 499 113 0.4 52 5.0 46.0 96 256 648 175.0 25 7.5 12.90 52.1 5179 4289 4.3 0.02 1.6 76.4 9.7 6.2 7.7 71.5 18.6 4.0 5.9 59.5 16.3 9.1 15.0

1 1622 112 4.0 120 5.0 0.0 116 507 377 100.9 115.0 7.5 14.95 2402.1 41,959 38,763 2.5 0.004 0.3 87.5 0.6 6.9 5.0 1.0 60.7 22.9 15.4 48.6 40.0 4.8 6.6

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immobilization of Pb in relation with organic matter, and an accumulation of the Pb-organic component fraction in Bh horizon. With regard to the Cd and Zn mobility, the habitats which represent the highest risk corresponded to cluster 8 (dredging deposits from the canal) then 7 (wooded creations and road hedges). Soils of these habitats presented a high degree of anthropization, a high contamination associated with an important content of organic carbon. For Pb, the risk is more important for the forest habitats and agricultural hedges which were characterized by an acidic pH and a low CEC. In this case, the soil pH appears as a major parameter explaining the solubility of Pb (McBride et al., 1997). Woody habitats are never taken into account in the management of areas contaminated by MTE in such a comprehensive way. In the studied area, the very high contamination of the woody habitat soils might noticeably increase exposure of the population, even if the tree cover, often associated with herbaceous vegetation, limits the resuspension of contaminated soil particles. Indeed these generally contribute to the exposure of the population living on contaminated soils. The incidental ingestion of contaminated dust or soil particles via hand-to-mouth transfer is the main means of exposure among young children (Vostal et al., 1974; Calabrese et al., 1999). The destruction of the vegetation cover, related to developments or soil use changes might increase the pollutant dispersion and the population exposition. Changes in land use could also result in changes of pH, redox potential and organic matter content, which subsequently lead to pollutant mobilization. 5. Conclusion Characterizing woody habitat soils across a large area, which has been greatly affected for more than a century due to lead smelter activities, is an original approach. It completes the data previously obtained for agricultural and urban soils. The study shows that the degree of Cd, Pb and Zn contamination in these topsoils is significantly higher than that found in agricultural topsoils, to the extent that it could not always be explained by atmospheric releases alone. In such a populated and industrialized area, the embankments and deposits give specific characteristics to soils. This work also highlights an impact of the tree canopy on the MTE enrichment of soils. The acidity of forest soils might favour pollutant mobility and their transfer to the deeper layers and the biosphere. This study is the first part of a large program focused on woody habitats, which particularly aims to understand and predict the bioavailability, transfer, bioaccumulation and effects of pollutants in trophic web through spatial variations. By improving the assessment of environmental and health risks, this work should guide the choices made by administrators of highly degraded spaces. Acknowledgements The authors wish to thank the “Agence de l'Environnement et de la Maîtrise de l'Energie”, the “Agence Nationale de la Recherche”, the Nord-Pas de Calais Council, the French Ministry of Research, and the European Regional Development Fund (FEDER) for the financial support of this research. References Alriksson A, Eriksson HM. Distribution of Cd, Cu, Pb and Zn in soil and vegetation compartments in stands of five boreal tree species in N.E. Sweden. Water, Air, and Soil Pollution: focus 2001;1:461–75. Baize D. Evaluer les contaminations diffuses en éléments traces dans les sols. 5èmes Journées GEMAS / COMIFER, Blois, novembre 2001, 281–295. Barcan V. Nature and origin of multicomponent aerial emissions of the copper–nickel smelter complex. Environ. Int. 2002;28(6):451–6. Beiera C, Hansen K, Gundersen P. Spatial variability of throughfall fluxes in a spruce forest. Environ. Pollut. 1993;81(3):257–67.

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