Mineralogy and chemical forms of lead and zinc in abandoned mine wastes and soils: An example from Morocco

Journal of Geochemical Exploration 113 (2012) 56–67

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Journal of Geochemical Exploration 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 / j g e o ex p

Mineralogy and chemical forms of lead and zinc in abandoned mine wastes and soils: An example from Morocco Pietro Iavazzo a, c, Paola Adamo a,⁎, Maria Boni b, Stephen Hillier c, Mariavittoria Zampella a a b c

Dipartimento di Scienze del Suolo, della Pianta, dell'Ambiente e delle Produzioni Animali, Università Federico II di Napoli, 80055 Portici (Na), Italy Dipartimento di Scienze della Terra, Università Federico II di Napoli, 80134 Napoli, Italy The James Hutton Institute, AB15 8QH, Aberdeen, UK

a r t i c l e

i n f o

Article history: Received 19 November 2010 Accepted 9 June 2011 Available online 23 June 2011 Keywords: Abandoned mines Heavy metal mobility Sequential extractions XRPD Quantitative mineralogy

a b s t r a c t Chemical extractions coupled with quantitative X-ray powder diffraction (XRPD) were used to define the chemical and mineralogical forms of Pb and Zn in abandoned wastes and soils from the Upper Moulouya mining district (Morocco). The aim was to provide baseline data required to assess metal mobility and bioavailability. Wastes and soils were sampled inside the mine sites of Zeïda, Mibladen and Aouli, both in exploitation and processing areas. Additional potentially unaffected soil samples were taken outside the Mibladen site. pH of wastes and soils is alkaline as a consequence of carbonate abundance (on average 36%). Total Pb and Zn concentrations have a wide spread of values (Pb: 0.041–17.25 g kg−1; Zn: 0.051– 276.5 g kg−1), with tailings from all mines and soils from Mibladen processing area exhibiting the highest concentrations. Very low or no detectable contamination characterizes the soils from exploitation areas and those collected outside Mibladen. Zinc contamination is mainly restricted to Mibladen processing area, where Zn ores from other Moroccan mines were possibly processed. The sequential extraction procedure for metal fractionation indicates that in contaminated samples Pb and Zn are mainly present in the acetic acid extractable fraction, likely as carbonates, (Pb up to 80%; Zn up to 52%), while in less or not contaminated soils both metals are mostly associated with the reducible fraction, presumably as iron oxides (Pb up to 68%; Zn up to 80%). Eight minerals containing Pb and Zn were identified: cerussite, anglesite, galena, hydrozincite, smithsonite, sphalerite, willemite and hemimorphite. Cerussite is the most important Pb-host. Hemimorphite and smithsonite account for most Zn. According to the alkaline conditions and to the low solubility of Pb and Zn mineral phases, it can be suggested that within the studied environment mobilization into solution in aqueous systems and bioavailability of Pb and Zn have a low potential. Nevertheless, given aridity and strong winds, inhalation of airborne particulates may be a concern. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The use of metals in human history has been cause of great benefits, as well as of unexpected harmful consequences. In fact, mining activities often represent an important source of pollution of heavy metals, which may be introduced into the atmospheric, terrestrial and aquatic ecosystems (Passariello et al., 2002). Many cases of chemical contamination have been described in former mining areas, where significant amounts of various elements were mobilized by weathering and leaching from abandoned mining wastes (for example Abrahams and Thornton, 1987; Hamilton, 2000). In semi-arid areas, the dispersion of soluble and particulate metals is often enhanced because soils are typically scarcely vegetated (Navarro et al., 2008). Furthermore, where soil consists of fine mine

⁎ Corresponding author. Tel.: + 39 0812539172; fax: + 39 0812539186. E-mail address: [email protected] (P. Adamo). 0375-6742/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2011.06.001

detritus, severe erosion problems caused by wind and water runoff may occur (Chopin et al., 2003). The extent and degree of heavy metal contamination around mines vary depending upon the mineralogical and geochemical characteristics of both ore and host rocks, as well as on the degree of mineralization of the tailings (Johnson et al., 2000). The fate and transfer of metals are complex and depend on the transport process involved, on the size and mineralogy of eroded particles and on the soil and sediment properties (Razo et al., 2004). Evaluation of total concentration of metals and metalloids in soils is generally used as the first reference indicator for comparing pollution level with legislative limits; nevertheless the natural occurrence of toxic elements in soils, especially in disused mining areas, requires further analyses to detect the mobilization due to erosion and leaching to groundwater (Giuliano et al., 2007). The total amount of heavy metals in solid samples is an efficient approach to detect environmental contamination but it is not enough to evaluate the influence of each element in potential pollution of the ecosystem (Ure and

P. Iavazzo et al. / Journal of Geochemical Exploration 113 (2012) 56–67

Davidson, 2002). The biological availability, the potential toxicity, the interaction with organic-mineral constituents and the metal mobility, all depend on the form in which they are present in solid samples (Allen, 1997). An approach to this problem is to apply various chemical extractants, either singularly or sequentially in order to assess the forms, or at least the main pools, in which metal contaminants occur in solid samples (e.g. Adamo and Zampella, 2008; Ure and Davidson, 2002). However, it is now widely recognized that the forms determined by chemical extractions are inevitably operationally defined (Bacon and Davidson, 2008; Hillier et al., 2001). Mineralogical techniques such as electron microscopy and X-ray powder diffraction (XRPD), among many other instrumental methods, are often used to elaborate on the data obtained by chemical extractions (e.g. HudsonEdwards et al., 1996; Ryan et al., 2002; Schön, 1995). Many different procedures can be used to obtain quantitative information from XRPD data but they have traditionally required a considerable investment of time compared to the relatively straightforward use of XRPD for phase identification (Hillier et al., 2001). In recent years, however, there have been significant developments in the use of the full pattern fitting methods for quantitative mineralogical analysis (Omotoso et al., 2006). Direct methods are more sophisticated than chemical methods and need a high level of specialization to be routinely included in metal speciation studies. Nevertheless, a complementary use of the chemical and mineralogical approaches may provide a more realistic picture of the actual forms of heavy metals in solid matrices (Adamo et al., 2002; Hillier et al., 2003; Ryan et al., 2008; Venditti et al., 2000a,b). In this study we have applied heavy metal speciation by sequential chemical extractions and quantitative mineralogical determination by XRPD, to define Pb and Zn chemical and mineralogical forms and phases in mine wastes and soils from the former lead mining district of the High Moulouya valley (Morocco). The aim was to provide the base line data required to assess metal mobility/bioavailability based on the combined use of chemical and mineralogical methods to assess the forms in which the metals reside.

2. Materials and methods 2.1. The study area Morocco with its large number of metalliferous sites is considered a traditional mining region since antiquity (Chronicle of Mineral Research and Exploration, 1998). One of these sites is the Upper Moulouya lead district which contained one of the largest concentrations of lead in Morocco, with a total output of more than 1 million metric tons (Rajlich, 1983). Currently the area is characterized by a low population density (less than 20 inhabitants per km 2) and poor economic conditions and only few studies have been undertaken to acquire information about the environment contamination by heavy metals from past mining activities (Bouabdli et al., 2005; El Hachimi et al., 2006; El Khalil et al., 2008). Our interest for the area was also due to the fact that it is crossed by the largest Moroccan river, the Moulouya, which, with its 520 km of length and tributaries, drains approximately 53,500 km 2 in the eastern part of the country, spreading contamination far away from the source up into the Mediterranean Sea where it finally flows. The site of interest, the Upper Moulouya lead district, corresponds to the south-western region of the Oranaise Meseta bounded by High Atlas on Southeast and by Middle Atlas on Northwest (Piqué and Michard, 1989). The region is composed of two separate Paleozoic massifs (Bou-Mia and Aouli). The Paleozoic basement that crops out in these massifs consists of pelitic and quartzitic lithotypes intruded by Hercynian granites and unconformably overlain by a Mesozoic cover consisting of Triassic evaporites/siliciclastics and Jurassic and Cretaceous carbonates/shales (Bouabdli et al., 2005). In the Upper

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Moulouya district three main lead–barite deposits occur: Zeïda, Mibladen and Aouli (Jébrak et al., 1998) (Fig. 1). Zeïda (Z) mining area (1490 m a.s.l.; period of active exploitation: 1972–1985) is located 30 km NW of the small town of Midelt, along the Moulouya river's course. At Zeïda the mineralization occurs as stratabound levels in sub-horizontal Permo-Triassic arkosic sandstones, unconformably covering the granite basement (El Jaouani, 2001; Emberger, 1965). In this mine the exploited ore body consisted mainly of cerussite (70% of Pb recovered) and galena (30%) associated with minor chalcopyrite, pyrite and barite. Minor anglesite, wulfenite, vanadinite, pyromorphite and rare sphalerite were also found at Zeïda (Amade, 1965; Direction des Mines, 1990). Mibladen (M) ore deposit (1400 m a.s.l.; period of active exploitation: 1936–1985) extends over an area of about 60 km 2. The mining area is located 15 km ENE of Midelt in a plateau consisting mainly of Mesozoic carbonates covering the basement (Felenc and Lenoble, 1965). Mainly galena and less frequently barite have been extracted from this deposit, but rare chalcopyrite and pyrite have been encountered as well (Petris, 1963). The most common oxidation products at Mibladen are cerussite, anglesite and vanadinite. Galena, often in association with barite, occurs as impregnations, disseminations or layers of variable thickness in shaly–dolomitic and calcareous– dolomitic sediments (Emberger, 1961; 1965). The Zeïda deposit may correspond to a Triassic metallogenic event, focused along fault systems or within permeable sandstone. Deep fluids were mobilized during the early extensional movements associated with the opening of the Atlas rift basin. The Mibladen mineralization is related to a distinct metallogenic event superimposed on the first one, and may represent a remobilization of earlier concentrations or a more recent event with metals originating from the same sources, but with a more pronounced contribution of local organic matter (Jébrak et al., 1998). It could be broadly considered as genetically belonging to the Mississippi Valley type class of deposits. Aouli (A) mine (1130 m a.s.l.; period of active exploitation: 1926– 1985) is located 26 km NE of Midelt and at 12 km from the village of Mibladen, in a narrow gorge cut by the Moulouya river. The Aouli ore zone extends over an area of 300 km 2 and consists of a network of veins hosted by metamorphic schists and granites; minor veins occur also in a cover of Permo-Triassic sedimentary rocks (Emberger, 1965). The main ore mineral at Aouli is galena, associated with barite and fluorite in a quartz gangue. Minor amounts of sphalerite, pyrite, chalcopyrite and rare malachite, azurite and cerussite have been also recorded (Nasloubi, 1993; Saunier, 1963). Mining activity has seriously modified the natural landscape of the High Moulouya valley (Fig. 2). In the three mining areas, different sites are devoted to exploitation and processing activities. Deep excavations filled with ground- and run-off water (used by locals for irrigation and water holes for cattle), mine adits and abandoned pits occur in exploitation sites, where also mine wastes are accumulated in elongated banks up to 20 m high. Processing plants, usually built in close proximity to living areas, are characterized by the presence of several abandoned (and ruined) facilities and waste tailings. Most tailings are accumulated in dumps, preferentially located along fluvial banks without visible safety control. The almost general lack of vegetation cover, coupled with the typical high temperatures and strong winds of this part of Morocco, enhances erosion and transport of waste materials. The nearby Moulouya and Mibladen rivers, along with their tributaries, run through the area and are periodically subjected to flooding, further enhancing the dispersion of contaminants. Climate of the study area corresponds to that of the Upper Moulouya region, with annual precipitation of 100–400 mm and mean annual temperatures of 12–14 °C, this depending on different locations (Combe and Simonot, 1971; Derrar, 1996). In the region, climate is semi arid and the average annual minimum temperature is reached in January, around 0 °C. In summer the average annual maximum temperature is reached in July (32–33 °C) (Raynal, 1961). The annual rainfall is almost

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P. Iavazzo et al. / Journal of Geochemical Exploration 113 (2012) 56–67

Fig. 1. Geological setting of the study area and sampling locations (arrows indicate the Moulouya river flow direction).

250 mm. Rainy seasons are normally spring and autumn. Rainfall events have torrential character. They are frequent, brief and violent showers. Water does not have time to infiltrate into the soil and this favors violent streams of water full of silt, resulting in severe flooding (Ngadi, 1995). The high Moulouya rarely knows days without wind. In winter, winds blow from W or NW, in summer from E or SSW. These winds, often violent, reinforce the drought in the basins areas and affect the distribution of plant associations. In summer they cause persistent clouds of dust that can last several days (Raynal, 1961). The most common land use is rough grazing for cattle and sheep. Recently planted orchards, containing mainly apple trees, can be found in specific areas around the town of Midelt, where availability of water is less limited due to the occurrence of privately-owned wells.

According to the Global Soil Regions map, based on a reclassification of the FAO–UNESCO World Soil Map, soils in the region of the Upper Moulouya valley are mainly entisols and inceptisols (USDA, 2005). 2.2. Sample collection Soil sampling was based on a study area survey aimed to differentiate exploitation from processing mining centers and to design different areas for land use. After preliminary minipit excavation, several representative soil profiles (code SP) were opened, described and sampled inside the exploitation and processing sites of Zeïda and Mibladen mining areas; several potentially unaffected surface (0–10 cm) soil samples were taken randomly outside Mibladen mining

P. Iavazzo et al. / Journal of Geochemical Exploration 113 (2012) 56–67

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Fig. 2. Mining exploitation and processing areas in the Upper Moulouya district; A, B: groundwater filling the deep open pit in Zeïda; C, D: waste dumps and processing plant facilities in Mibladen; E, F: ruined processing plant and abandoned mining village in Aouli.

area and mixed to form a single composite sample (code SS). Tailings (code T) of all mines and coarse waste rocks (concassé) (code C) from the Mibladen mine were sampled at random from the surface (0 to 10 cm depth) of waste dumps. Sampling was done in February and November 2007; all sampling points were georeferenced by GPS (Fig. 1 and Table 1). In field we determined the soil color with Munsell Color Charts, the texture by hand, and we estimated the carbonate content by observing sample effervescence after adding 10% v/v HCl (Siebe et al., 1996).

2.3. Analytical procedures In laboratory, all samples were air dried at room temperature and sieved at 2 mm, obtaining the soil skeleton (N2 mm fraction) and the fine earth (b2 mm fraction). All analyses were carried out soon after sampling.

Table 1 Location of studied mine wastes and soils. No.

Sample code

Site

Latitude N

Longitude W

Mine Z (Zeïda) 1 ZP1 2 ZT

Expl. Proc.

32° 48′ 26″ 32° 50′ 24″

4° 58′ 17″ 4° 57′ 05″

Mine M (Mibladen) 3 MP1 4 MP2 5 MP3 6 MT 7 MC SS

Expl. Proc. Proc. Proc. Proc. Out

32° 32° 32° 32° 32° 32°

4° 4° 4° 4° 4° 4°

Mine A (Aouli) 8 AT

Proc.

32° 48′ 40″

45′ 16″ 45′ 42″ 45′ 53″ 45′ 44″ 45′ 15″ 40′ 57″

39′ 02″ 38′ 41″ 38′ 41″ 38′ 40″ 39′ 10″ 37′ 27″

4° 35′ 34″

P = soil profile; T = tailings; C = coarse waste rock; SS = composite surface soil; Expl. = exploitation area; Proc. = processing area; out = outside mining area.

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Fine earth was analyzed for the determination of moisture content at 105 °C, pH-CaCl2 (1:2.5 soil:0.01 M solution ratio), electrical conductivity (EC) (on 1:5 soil:water extracts at 25 °C), organic carbon (Walkley and Black, 1934), carbonates (pressure Dietrich–Fruehling calcimeter method, Loeppert and Suarez, 1996), particle-size analysis (after ultrasonic treatment at 20 kHz and 75 W for 15 min) and Pb and Zn total content (by XRF). Separation of the particle size fractions (sand: 2–0.02 mm, silt: 0.02–0.002 mm, clay: b0.002 mm) was carried out by wet sieving and centrifugation. Determination of total metal content was carried out by X-ray fluorescence spectroscopy (XRF) under contract at GAU-Radioanalytical Laboratories, Southampton University, UK. Samples, ground at 1 part sample:9 parts pure silica glass for dilution, were prepared as pressed powder pellets and analyzed using a Philips Magix-PRO WD-XRFS, endwindow 4 kW rhodium target tube and SuperQ4 software. During analysis the reference sample USGS GXR-1 was analyzed as a check on accuracy. Chemical speciation in the fine earth was assessed by BCR sequential extraction procedure, modified according to Renella et al. (2004) in order to differentiate the carbonate bound fraction (Table 2). Concentration of Pb and Zn in the various extracts was determined by ICP-MS. Lake sediment reference material BCR CRM 601 was extracted by BCR sequential extraction procedure to assess the quality of the Pb and Zn data obtained. Results indicated that, with the exception of zinc in 1 M H2O2/NH4OAc (+27% of recovery), agreement with literature values (Rauret et al., 2000) was acceptable and in the range of ±17%. For X-ray powder diffraction (XRPD) samples (3 g) were wet ground for 12 min in a McCrone micronizing mill to reduce the particle size to nominally less than 10 μm and the resulting slurry sprayed dried directly from the mill (Hillier, 1999). The resulting powder samples were top loaded into 2.5 cm diameter circular cavity holders and XRPD patterns recorded on a Siemens D5000 with a θ/θ goniometer and Co-Kα radiation, chosen with a diffracted beam monochromator. Diffraction patterns were obtained by step scanning from 2 to 75° 2θ, with a step size of 0.02°. Mineral identification was made by reference to the International Center for Diffraction Data (ICDD) and the International Crystal Structure Database (ICSD) patterns. Minerals identified were quantified by a full-pattern fitting, reference intensity ratio method, similar to that described for ‘participant 18’ in Omotoso et al. (2006). Briefly, measured XRPD patterns were simulated as weighted sums of experimental pattern of standard pure minerals using a least squares optimization procedure to minimize the difference between experimental and simulated patterns. Reference minerals were obtained from the Macaulay Institute mineral collection and from private mineral collectors. Metal fractionation and mineralogical analysis were carried out on all tailings, on concassé from Mibladen and on selected soil samples.

3. Results 3.1. Morphological features The morphological features of studied wastes and soils are summarized in Table 3. Table 2 Sequential extraction procedure for metal fractionation. Step

Extractant

Presumed metal forms

1 2 3

1 M NH4NO3 0.11 M HOAc 0.5 M NH2OH HCl

4 5

1 M H2O2/NH4OAc HCl/HNO3/HF 3:1:1

Soluble and exchangeable (labile pool) Carbonate-bound (HOAc-extractable) Occluded in ‘easily reducible’ Mn and Fe oxides (reducible) Organic matter associated and sulfides (oxidizable) Residual mainly in primary minerals lattice structure

In field all tailings have a fine texture, are pink in color and, except those from Zeïda (ZT), are rich in carbonates. Soil profiles from the processing area of Mibladen (MP2, MP3) are extremely complex, as a consequence of the heavy impact of human activities on the site. They are characterized by several horizons consisting of different materials produced during the processing activities, variously mixed with earth components. The layers exhibit high variability in color, structure, compactness, grain-size and mottling, this pointing to a variable extent of sediment heterogeneity. None of them shows any significant evidence of pedogenesis. By contrast, soil profiles from the exploitation areas of Zeïda (ZP1) and Mibladen (MP1) are less complex. Their appearance is more natural than “man-made”, containing a higher content of earthy granular material and organic matter, and, only in surface horizons, some rests of mine spoils and wastes. The color of all wastes and soils, ranging from red (2.5 YR 4/6) to yellowish brown (10 YR 5/6), suggests occurrence of variable amounts of iron (hydr) oxides (Viscarra Rossel et al., 2010). 3.2. Textural and chemical properties Textural and chemical characteristics of mine wastes and soils are presented in Table 4 and Fig. 3. No particles larger than 2 mm exist in tailings consisting of relatively barren materials, which were crushed and separated from the rich ore during processing, and in the majority of soil samples. Particles with diameter N2 mm were found in ZP1 (with an average relative abundance of 36%) and MP2 (47%) soil profiles, and in MC waste sample (57%). According to the Standard US Department of Agriculture Soil Textural Classification Triangle (Brady and Weil, 2001), studied soils can be classified as loamy sand (ZP1), sandy loam (MP3 and SS) and silt loam (MP2 and MP1). All tailings have a silt–silt loam texture, presumably as a result of the desliming processes of ore slurries, intended to eliminate the finest argillaceous fraction. pH in CaCl2 suspensions of both wastes and soils is slightly alkaline in all samples and ranges between 7.6 and 8.5, presumably as a consequence of carbonate abundance (on average 357 g kg −1) due to the limestone environment. Waste and soil samples with the highest carbonate contents are concentrated in Mibladen processing area (MC, MT, MP2, MP3). In Zeïda tailings (ZT), carbonate was present only in very small amounts (as cerussite, see below); because of the granite and sandstone composition of the host rock common carbonate minerals such as calcite and dolomite are absent at ZT. The majority of studied samples had no or very low content of organic matter, which in only a few cases was higher than 5.0 g kg −1 and reached a maximum value of 12.0 g kg −1 in the 15–45 cm depth horizon of MP1 profile. According to literature (Bouabdli et al., 2005; El Hachimi et al., 2006; El Khalil et al., 2008), the main metallic contaminants in the studied area are Pb and Zn. Their total content in mine wastes and soils is reported in Table 4. The MC waste sample from Mibladen and all tailings are highly enriched in Pb (2.59–7.65 g kg −1) and Zn (0.54–1.03 g kg −1) in the following order: MC ≅ MT N ZT N AT. Indeed, soil profiles from Mibladen processing area (MP2 and MP3) contain both metals in much higher amounts compared with the soil profile sampled at Mibladen exploitation site (MP1), which is essentially barren. In MP2 and MP3 profiles, only Zn shows a decrease with depth, while Pb content has an uneven behavior, with deeper horizons still being richer in Pb than surface horizons. The soil profile from Zeïda exploitation area (ZP1), as well as the composite soil sample from Mibladen surroundings (SS) also contains both metals in lower amounts compared with MP2 and MP3 profiles. However, in ZP1 and SS the Pb and Zn values are higher than those detected in MP1. Neither Pb nor Zn total contents were significantly correlated with pH, clay, organic matter and carbonate values, this suggesting that the above parameters do not control the distributions of the metals in soils and wastes, but rather that the control is anthropic, as expected.

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Table 3 Morphological features of the studied mine wastes and soils. Waste/soil

Horizon

Depth (cm)

Limits

Main soil forming factor

Color [dry]

Ck 2Ck 3Ck –

0–6 6–36 36–52 0–20

Abrupt smooth Clear smooth – –

Anthropic Natural Natural –

2.5 7.5 7.5 7.5

MT MC SS

Ak 2Bk 3Ckm Ck 2Ck 3Ckm Ck 2Abk 3Ck 4Ck 5Ck – – –

0–15 15–45 45–75 0–23 23–50 50–90 0–35 35–45 45–72 72–92 92–150 0–20 0–20 0–10

Abrupt smooth Abrupt smooth – Abrupt smooth Abrupt smooth – Abrupt smooth Clear wavy Abrupt smooth Abrupt smooth – – – –

Anthropic Anthropic Natural Anthropic Anthropic Anthropic Anthropic Anthropic Anthropic Anthropic Anthropic – – –

Aouli AT

–

0–20

–

–

Zeïda ZP1

ZT Mibladen MP1

MP2

MP3

Structure

Skeleton

Organic matter, roots

Mottling

HCl effervescence

Incoherent Incoherent Massive –

40% 40% 30% Absent

Few Few Absent Absent

None None None –

Strong Strong Strong Very slight

5 YR 5/6 (yellowish red) 5 YR 4/6 (yellowish red) 7.5 YR 5/6 (strong brown) 7.5 YR 6/4 (light brown) 7.5 YR 7/4 (pink) 2.5 YR 4/6 (red) 7.5 YR 6/4 (light brown) 5 YR 7/4 (pink) 7.5 YR 8/3 (pink) 7.5 YR 7/3 (pink) 7.5 YR 8/3 (pink) 7.5 YR 8/3 (pink) 5 YR 4/6 (yellowish red) 10 YR 5/6 (yellowish brown)

Granular Granular Massive Granular Granular Massive Platy Granular Platy Platy Platy – – –

Absent Absent Absent Absent 70% 40% Absent Absent Absent Absent Absent Absent 70% Absent

Many Many Few Few Absent Absent Few Common Absent Common Absent Few Few Many

Few Few None Clear None Clear Few None Few None Clear – – –

Slight Slight Slight Strong Strong Strong Slight Slight Slight Slight Slight Strong Strong Strong

7.5 YR 8/3 (pink)

–

Absent

Absent

–

Strong

YR YR YR YR

4/6 4/6 4/6 8/3

(red) (strong brown) (strong brown) (pink)

Considering that Zn minerals were not generally exploited in the studied area, the high levels of this element detected in soil profiles from Mibladen processing area and in all tailings suggest that Zncontaining ores from diverse Moroccan mines were also stockpiled and processed in the flotation plants of the High Moulouya district. This is a common practice in a mining area with an efficient processing plant, where different kinds of ores can be treated successfully. At Mibladen itself no zinc ore has ever been recorded. 3.3. Chemical speciation The Pb and Zn fractions operationally defined by sequential extraction are here in referred to as ‘labile’, ‘HOAc-extractable’, ‘reducible’, ‘oxidizable’ and ‘residual’, respectively. The distribution of Table 4 Chemical properties and total content of Pb and Zn of studied mine wastes and soils. Sample Depth code (cm)

Fine pH Carbonates Pb Zn Organic earth % (CaCl2) matter g kg− 1 g kg− 1 g kg− 1 g kg− 1

Mine Z (Zeïda) ZP1 0–6 6–36 36–52 ZT 0–20

63 62 66 100

8.3 8.1 8.1 7.9

Absent Absent Absent 3.0

204 259 266 2

Mine M (Mibladen) MP1 0–15 100 15–45 100 45–75 100 MP2 0–23 92 23–50 34 50–90 32 MP3 0–35 100 35–45 98 45–72 100 72–92 100 92–150 100 MT 0–20 100 MC 0–20 43 SS 0–10 100

7.7 7.9 7.9 7.6 7.8 7.9 8.5 8.4 8.1 8.4 8.5 8.1 8.2 7.9

9.0 12.0 7.0 6.0 4.0 Absent 4.0 6.0 4.0 8.0 5.0 3.0 2.0 5.0

231 209 154 452 500 350 470 514 540 456 604 500 520 211

Mine A (Aouli) AT 0–20

100

0.17 0.17 0.16 3.48

Pb and Zn in concassé, tailings and selected soil samples within the operationally defined fractions is reported in Tables 5a and 5b. The relative amounts of Pb and Zn expressed as a percent of the cumulative total extracted are given in Fig. 4. For Pb a satisfactory agreement (r = 0.99; p b 0.001) is found between gross total obtained by a single determination (in Table 4) and cumulative total as extracted by the sequential extraction scheme (Table 5a). In the majority of the samples Pb cumulative total falls within − 12% and +9% of gross total, sometimes deviating by 26–55% and in the SS sample by 361%. For Zn the deviation between cumulative and gross total amounts is larger than for Pb. Zn cumulative total falls within −35% and +21% of gross total in the majority of the samples, deviating up to 578% in the others. Samples inhomogeneity and differences in analytical methodologies can be taken into account to explain the deviations which are found. For example, soil samples contaminated by mining/industrial

Clay 100% soil

0.12 0.12 0.10 0.66

waste

clay

8.1

5.0

337

0.062 0.051 0.060 0.064 0.041 0.056 12.13 276.5 11.45 159.7 0.32 1.18 6.51 3.48 13.04 0.29 3.13 0.28 17.25 0.17 5.66 0.15 6.41 1.03 7.65 0.56 0.057 0.19

2.59

0.54

silty clay sandy clay

silty clay loam

clay loam sandy clay loam loamy sand

ZT

silt loam

loam

sandy loam

silt

sand

Sand 100%

ZP1

SS

MC MP3

MP2

AT

MP1 MT

Fig. 3. Textural classification of the studied mine wastes and soils.

Silt 100%

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Table 5a Content (mg kg− 1) of lead in the sequentially extracted fractions and cumulative totals in studied mine wastes and soils. Sample Depth code (cm)

Labile HOAcReducible Oxidizable Residual Cumulative pool extractable total

Mine Z (Zeïda) ZP1 0–6 0 ZT 0–20 230

28 2149

128 1110

77 20

37 85

270 3594

Mine M (Mibladen) MP1 0–15 0 MP2 0–23 68 MP3 0–35 52 35–45 44 72–92 173 MT 0–20 61 MC 0–20 218 SS 0–10 0

16 3936 5826 11,740 13,503 8240 2618 2

22 5558 1185 1300 1721 1007 715 172

2 1743 452 1025 1521 808 506 49

21 2324 669 155 287 329 71 39

61 13,629 8184 14,264 17,205 10,445 4128 262

1772

1228

178

161

3350

Mine A (Aouli) AT 0–20

11

activities are known to be characterized by metal contaminants that are generally not uniformly distributed throughout the soil, but strongly associated with or segregated into specific solid phases (Davidson et al., 1999). Thus inhomogeneity can produce large deviations in the measured total values as a result of sampling. Moreover, in this study total analysis is a single determination carried out by XRF, while chemical fractionation and therefore cumulative total determination is performed by acid digestion followed by ICPMS. In the majority of the studied soils and mine wastes, Pb is mostly concentrated in the HOAc-extractable and reducible fractions, although it is also present in other fractions (Table 5a). In samples most highly enriched in Pb (tailings: ZT, MT, AT; soil: MP3; mine waste: MC) from 53 to 82% of total Pb is associated with the HOAcextractable fraction. In contrast, in less contaminated samples, including ZP1, MP1 and MP2 soils, the reducible followed by the HOAc-extractable fraction contains most of the Pb (from 58 to 70%). In SS soil, the greatest percentage of Pb (66%) is present in the reducible fraction. In MP1 soil, the residual fraction (36%) also contains a significant portion of total Pb. The proportion of Pb associated with oxidizable phases (likely sulfides and in less extent organic matter) is always low and decreases with the decrease of Pb cumulative content. The percentage contribution of labile fraction to total Pb is relatively low in all studied materials (from 0% in ZP1, MP1 and SS to 6% in ZT). Nevertheless, in wastes and soils containing very high Pb levels, the

Table 5b Content (mg kg− 1) of zinc in the sequentially extracted fractions and cumulative totals in studied mine wastes and soils. Sample Depth code (cm) Mine Z (Zeïda) ZP1 0–6 ZT 0–20

Labile HOAcReducible Oxidizable Residual Cumulative pool extractable total 1 3

3 94

243 327

53 20

0 39

300 483

Mine M (Mibladen) MP1 0–15 0 MP2 0–23 489 MP3 0–35 63 35–45 4 72–92 3 MT 0–20 30 MC 0–20 34 SS 0–10 0

4 44,477 1152 673 409 310 1772 2

11 80,972 518 69 56 90 108 120

0 48,474 520 145 88 70 264 93

27 14,932 223 88 0 52 0 18

42 189,344 2476 979 556 552 2178 233

157

126

31

58

375

Mine A (Aouli) AT 0–20

3

Fig. 4. Percentage distribution of lead (A) and zinc (B) in the chemical fractions sequentially extracted from studied mine wastes and soils.

concentration of labile (soluble and exchangeable) forms is not negligible (ZT: 230 mg kg −1; MP3 72–92 cm: 173 mg kg −1; MC: 218 mg kg −1). Like Pb, most of Zn in studied soils and mine wastes is present in the HOAc-extractable (1–81%) and reducible (5–81%) fractions. The HOAc-extractable fraction tends to prevail in the most Zn-enriched materials (from 552 to 189,344 mg kg −1), which are all from the Mibladen processing area. In the case of MP1 soil, containing the lowest total content of Zn (42 mg kg −1), the greatest percentage of Zn (64%) occurs in the residual fraction. In SS and ZP1, containing 233 and 300 mg kg -1 of cumulative Zn total, the reducible is the most important fraction (respectively, 51% and 81%), followed in SS by the oxidizable one (40%). Only in MP2, the soil with the highest Zn total content, the concentration of labile forms of Zn is considerably high (489 mg kg −1). The major association of Pb and Zn with the HOAc-extractable fraction in soils and wastes containing the metals in the highest amounts is in agreement with the occurrence in these materials of Pb and Zn carbonates (cerussite and smithsonite, see below). As the total content of Pb and Zn in waste and soil samples decreases, the proportions of both metals associated with these mineral phases decrease, and those associated with reducible Fe oxides increase.

P. Iavazzo et al. / Journal of Geochemical Exploration 113 (2012) 56–67

3.4. Mineralogical analysis In total, 24 crystalline phases were identified and quantified in waste and soil samples from the three investigated mining sites of the High Moulouya district (Table 6 and Fig. 5). The main mineralogical non-metallic phases consist of dolomite and calcite, along with quartz; the samples from the processing area of Mibladen are also rich in barite. The soil profile from Zeïda exploitation site (ZP1) is especially rich in calcite (on average 24%), quartz (37%) and clay minerals (24%). In ZT tailings, the quartz content is still very high (50%), but carbonates and clay minerals are almost absent and replaced by K-feldspars (29%). In soil from the Mibladen exploitation site (MP1), quartz is the dominant mineral (on average 55%) followed by dolomite (15%) and calcite (11%). The main mineralogical groups in soil samples from the Mibladen processing area (MP2 and MP3) are quite different, where the main mineral phases consist of dolomite (on average 39%), quartz (16%) and barite (15%). Also samples from the Mibladen mine wastes, MC and MT, contain mainly dolomite (44%) and barite (22%). The composite surface soil sample taken outside the Mibladen mining area consists mainly of K-feldspars (22%), calcite (22%) and various clay minerals (21%). The main mineralogical phases detected in tailings from Aouli mine site (AT) are quartz (24%), dolomite (24%), clay minerals (15%) and calcite (15%). Iron oxides are present in substantial quantities in SS soil (5.7% of goethite, 3.3% of hematite, 0.5% of magnetite), while in other samples their content was less than 3% (in ZP1 from 1.7 to 2.4% of hematite with increasing depth; in MP2 3.0% of hematite at 50–90 cm depth; in MP3 from 1.1 to 3.0% of goethite with increasing depth; in AT 1.4% of hematite). Gypsum (up to 5%) is only found in the surface horizon of MP1 soil, where water is likely to accumulate temporarily; lower amounts (up to 1.4%) are present in MP3, SS and AT. Minor fluorite was detected only in tailings from Zeïda (ZT, 2.4%).

63

Eight out of all the minerals identified and quantified contain in their structures Pb [galena, cerussite and anglesite] and Zn [sphalerite, hydrozincite, smithsonite, willemite and hemimorphite]. Cerussite has been detected in waste materials from all mines (ZT: 0.5%, MC: 0.4%, MT: 1.2%, AT: 0.3%), as well as in soil profiles from Mibladen processing area (on average 0.9% in MP2 and 1.0% in MP3). Cerussite, along with many other lead minerals is a strong diffractor and its identification and quantification are routinely possible at subpercent level by XRPD. In Mibladen processing area minor amounts of anglesite (in MP2 at 50–90 cm: 0.4%) and galena (in MP3 at 35– 45 cm: 0.4%; at 72–92 cm: 0.3%; in MT: 0.1%) have also been found. Zinc minerals only occur in soil and tailings from Mibladen processing area; the following minerals have been detected in the first two horizons of MP2 (at 0–23 cm: willemite 8.2%, smithsonite 8.9%, hemimorphite 17.2%, hydrozincite 5.9%; at 23–50 cm: willemite 5.0%, smithsonite 9.1%, hemimorphite 9.3%, hydrozincite 5.4%), and in MT (sphalerite 0.4%). 4. Discussion This paper provides a general view of the heterogeneity and complexity of a large abandoned mining area such as the UpperMoulouya mining district, and of the difficulties of obtaining a clear picture of contamination in such a complicated environment. The whole investigation illustrates how appropriate it is to provide a better understanding of contamination to improve assessment of environmental risks and remediation feasibility. The non-metallic minerals identified in soils and wastes from the study area are all derived from local lithologies such as Triassic sandstones, Liassic carbonates and Hercynian granites. The absence of galena, the main Pb-bearing ore mineral in the Upper-Moulouya mining district, in the Mibladen tailings could be due to its optimal recovery during ore processing as well as to transformation to cerussite or oxyhydroxides inside the deposit during supergene oxidation. Moreover, as Ramos Arroyo and Siebe (2007) indicate, PbS may decrease with aging

Table 6 Mineralogy (wt.%) for wastes and soils. Mineralogy Carbonates Dolomite Calcite Cerussite Hydrozincite Smithsonite Oxides Goethite Hematite Magnetite Sulfates Barite Anglesite Gypsum Sulfides Galena Sphalerite Silicates Quartz K-Feldspars Plagioclases Pyroxenes Willemite Hemimorphite Clay mineralsb Other Fluorite a b

Mineral formula (approximate)

ZP1a

CaMg(CO3)2 CaCO3 PbCO3 Zn5(CO3)2(OH)6 Zn(CO3)

2.9 23.9

FeO(OH) Fe2O3 Fe2+Fe3+2O4

ZT

MP1a

MP2a

MP3a

SS

15.3 11.2

27.6 12.4 0.9 3.8 6.0

45.4 3.2 1.0

21.6

0.5

1.7 2.0

BaSO4 Pb(SO4) Ca(SO4)2H2O

1.0

7.5

1.1 0.1 3.4

PbS (Zn,Fe)S SiO2 (K,Al,Si,O) (Na,Ca,Al,Si,O) (Ca,Na)(Mg,Fe,Al)(Si,Al)O Zn2SiO4 Zn4Si2O7(OH)2H2O

23.5 CaF2

Average value along horizons. Kaolinite, illite, montmorillonite, chlorite.

49.5 29.4 6.7

4.1 2.4

55.4 3.3 0.2

13.5 0.5 0.5

11.2

4.4 8.8 19.4

MC

AT

43.8

43.9 7.7 0.4

23.5 14.7 0.3

1.0 0.9

1.4

6.7

9.0

1.2

5.7 3.3 0.5

22.8

0.5

0.4

0.9

0.1

37.3 8.3 2.2

MT

37.4

1.4 0.1 0.4

18.1 0.7 0.3 6.4

8.4 22.0 9.7

10.3 2.2

19.1 1.0 1.0

24.1 3.8 6.4

6.2

21.0

4.8

18.6

15.4

0.1

64

P. Iavazzo et al. / Journal of Geochemical Exploration 113 (2012) 56–67

Fig. 5. Mineralogical composition of studied mine wastes and soils.

of tailings, as is the case in the semi-arid climate of Mexico, where it becomes sequestered in oxides or hydroxides after galena decomposes. The ore recovery appears to have been less selective for the oxidation minerals as cerussite, which is commonly found in all wastes and in the Mibladen soils. The presence of willemite in MP2 soil is rather peculiar, because this mineral can be either derived from hypogene Znnonsulfides ore deposits (certainly not those considered in the High Moulouya district) (Boni, 2003) or as a by-product of Zn-ore smelting processes. Thus it seems likely that Zn ores from Moroccan localities other than the Upper-Moulouya district were treated at Mibladen processing plant. This may also be the reason that Zn minerals are present in soils of the processing area, where they have been stockpiled (oral communication of former mine employee) before undergoing flotation in the plant. The sequential extraction used in this study is useful to indirectly assess the potential mobility and bioavailability of metal contaminants in soils and mine wastes. Given that bioavailability is related to extractant strength and phase solubility, then metal bioavailability decreases in the order: labile N HOAc-extractable N reducible N oxidizable N residual fraction. Based on our results, the amount of Pb and Zn present in the residual fraction is low in all studied materials, with the exception of the SS soil. On the contrary, a large percentage of total Pb and Zn was detected in the non-residual fractions, mainly associated with HOAcextractable (carbonates) and reducible (Fe oxides) phases, with low contributions by oxidizable (sulfides and organically bound) and labile (soluble and exchangeable) forms. Therefore, our results suggest that in studied soils and wastes from the Moulouya mining district, the risk associated to metal contamination is appreciable from the standpoint of potential mobility and bioavailability of Pb and Zn in solid materials. The low readily mobile and bioavailability of contaminants in the studied area are in agreement with the low content of both metal contaminants observed by Iavazzo (2010) in surface (Pb: 2–30 μg L −1; Zn: 8–80 μg L −1) and subsurface (Pb: 2–28 μg L −1; Zn: 5–34 μg L −1) waters. Also in regard to the waters, the samples from Mibladen mining site exhibited the highest concentrations, never exceeding the limit of 50 μg L −1 for Pb in the waters defined by the Moroccan legislation (Norme Marocaine Qualité Eaux, 2002). 4.1. Comparison between chemical and mineralogical data The utility of obtaining quantitative mineralogical data for the samples is mainly due the insight it can offer into the relative

importance of different forms of Pb and Zn by reconciling it with the chemical data. Using typical compositions for Pb and Zn mineral phases identified and the concentrations of each phase in the investigated samples, it is a simple matter to calculate the concentrations of Pb and Zn present in soil and waste samples, to assess their phase speciation by comparison with directly determined geochemical data and also to validate the quantitative mineralogical analysis. In Fig. 6 the results of these calculations are compared with XRF results. In general, the two approaches to quantifying total metal content produce consistent results, supported by positive and very strong correlation between total contents (in mg kg −1) of Pb and Zn measured by XRF and calculated through the mineralogical data (for Pb, r = 0.95, p = 0.01; for Zn, r = 0.99, p = 0.01). In the fractionation scheme (Table 2), the 0.11 M HOAc solution is thought to remove from the tailings and soils predominantly carbonate-bound metals. XRPD analysis indicates that both Pb and Zn are present as carbonates, namely cerussite, smithsonite and hydrozincite, and that these forms contribute almost solely to the total metal contents. The second stage of extraction involves the use of hydroxylamine hydrochloride, a reducing agent used to dissolve Mn and Fe oxide/hydroxide minerals that in soil and sediment are unstable to anoxic conditions and strong scavengers for ore metals and other trace elements (Post, 1999). XRPD analysis detects Fe oxide/hydroxides almost exclusively in soils, where the proportion of Pb and Zn associated with these minerals increases as total metal content decreases. The third stage of fractionation procedure is considered to extract mainly organic and oxidizable sulfides leading to the release of associated metals. XRPD analysis indicates that only a small number of studied samples (MT and MP3) contained galena and sphalerite and in very low amounts. Moreover, the organic matter content of all studied materials is very low. The contribution of these forms, although not negligible in the most contaminated waste and soil samples, was always smaller than that of carbonates and Fe oxides. The residual soil should contain mainly primary and secondary silicate minerals, which may hold heavy metals within their structure. XRPD analysis indicates that willemite and hemimorphite occur in soils and tailings from Mibladen processing area, likely contributing to their Zn enrichment. Anglesite is expected to contribute to the total metals extracted during this step. According to Ramos Arroyo and Siebe (2007), anglesite remains insoluble even at acid pH values, protecting galena (armouring) very efficiently from further weathering. Considering that Fe oxides and sulfides dissolve incompletely

P. Iavazzo et al. / Journal of Geochemical Exploration 113 (2012) 56–67

65

Fig. 6. Comparison between total contents of Pb and Zn calculated by XRPD mineralogical data and measured by XRF analysis in the mine wastes and soils.

in hydroxylamine and peroxide, it is likely that the metals associated with them can contribute to the residual fraction (Dold, 1999). The very small solubility products of cerussite and anglesite (logK = − 13.1 and logK = − 7.7; Magalhaes and Silva, 2003), and of hydrozincite, hemimorphite and smithsonite (logK = − 14.9, logK = − 24.0 and logK = − 9.9; Alwan and Williams, 1979) coupled to the general alkaline conditions, enhancing metal precipitation and cation retention processes, could explain the small contribution of water soluble and exchangeable forms to the total metals extracted sequentially. 5. Conclusions The studied wastes and soils of the mining areas of the UpperMoulouya district of Morocco can be classified as sandy loam to silty loam, with tailings verging to silt. pH is alkaline by virtue of carbonate abundance. Indeed, the main mineral assemblage consists of dolomite, calcite, barite and quartz, with feldspars, pyroxenes and phyllosilicates as minor minerals. The data indicate that abandoned tailings and “man-made” soils resulting from processing activities are all highly enriched in Pb and Zn. In Morocco no environmental guidelines exist to regulate maximum permissible heavy metal

concentrations in mine wastes and soils or to give reference values above which action should be taken to mitigate environmental risk. According to existing guidelines in Italy (Gazzetta Ufficiale della Repubblica Italiana, 2006), if the assessment is based on their total Pb and Zn contents (maximum regulatory limits established for soils of public, residential and private areas: Pb = 100 mg kg −1 and Zn = 150 mg kg −1; for soils of commercial and industrial areas: Pb = 1000 mg kg −1 and Zn = 1500 mg kg −1), most of the studied wastes and soils represent a potential source of contamination for the surrounding environmental compartments. Quantitative mineralogical analysis indicates that cerussite and anglesite are the most important Pb-host minerals at all sites. The main sources of Zn, abundant in all tailings and in soils from Mibladen processing site, are hydrozincite, hemimorphite and smithsonite. The sequential extraction data show that soluble and exchangeable Pb and Zn content (i.e. the most mobile and bioavailable fraction present in wastes and soils) is low in almost all samples, which is attributed to the low solubility of the major metallic mineral phases and to the alkaline pH conditions. Only in the most contaminated wastes (MC and ZT) and soils (MP2 and MP3) the soluble and exchangeable Pb and Zn concentrations are indicative of a potential risk of transfer of contaminants to the nearby environment via solution. Indeed, transfer in the studied area by wind

Table 7 Average ± SD, minimum and maximum concentrations (mg kg− 1) of Pb and Zn in tailings and soils from the Upper Moulouya mining district compared with ranges or average concentrations (± SD) measured in tailings and soils from others Moroccan and European Pb/Zn mining areas. Mining area

Upper Moulouya (Morocco) Almagrera (Spain) Cabezo Rajao (Spain) Gyöngyösoroszi (Hungary) Marrakech (Morocco) Bytom (Poland)

Pb

Zn

Reference

Tailings

Soils

Tailings

Soils

4163 ± 1997 2593–6411 9200–41900 1000–19000 25–1540

5010 ± 6068 41–17253

743 ± 253 539–1027 17600–61900 3400–53100 63–4547

31590 ± 82276 51–276510

255–5756 69–5260

1623–8361 42–12100

Álvarez-Valero et al., 2009 Navarro et al., 2008 Kovács et al., 2006 Boularbah et al., 2006 Ullrich et al., 1999

66

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and water erosion of fine metal enriched particles is likely to be a much more important vector for metal dispersion. Although comparisons with literature data on tailings and mining soils pollution are confounded by different geological features, sampling and analytical procedures, some comparisons help to put the levels of contamination into context. Thus the concentrations of Pb and Zn in tailings from the Upper Moulouya district are below those recently reported in tailings from the Spanish mining sites of Almagrera and Cabezo Rajao (Álvarez-Valero et al., 2009; Navarro et al., 2008), and, in regard to Pb, higher than the values measured by Kovács et al. (2006) in the Gyöngyösoroszi mine (Hungary) (Table 7). Whereas the average content of both elements in soils corresponds to maximum values measured by Boularbah et al. (2006) in a polymetallic mines near Marrakech (Morocco) and by Ullrich et al. (1999) in Pb/Zn mining and smelting contaminated soils around the town of Bytom (Poland). The combination of chemical fractionation by modified BCR sequential extractions and XRPD quantitative analysis has produced valuable and congruent data defining Pb and Zn speciation in the solid phase of studied mine wastes and soils. Application of this approach to the study of contaminated sites is highly advisable in order to obtain information of particular value in risk assessment and remediation feasibility. In the abandoned Upper Moulouya mining district of Morocco, while bulk metal concentrations are high, the combined XRPD and sequential chemical analysis indicate that the host minerals are largely insoluble in the semi-arid to arid environment and that contamination of surface water or groundwater is unlikely. This is a conclusion that would not be reached by merely examining bulk metal concentrations, and thus underlines the importance of the kind of combined chemical and mineralogical analysis presented in this study. Population exposure by inhalation to metal-enriched dust is rather a main health hazard. Providing information on mineralogy and chemical properties of the dust, and therefore, on its potential solubility in lung tissue, the results presented herein might also be useful in assessing a potential for inhalation risks. Acknowledgments The authors are grateful to Dr. Peter Ryan of the Geology Department, Middlebury College (USA) for the helpful review of the manuscript and for his suggestions which improved the presentation of the results. This study was supported by the Italian Ministry of University and Research (MIUR Funds for International University Cooperation), in the framework of the international agreement between the University Federico II of Napoli (Italy) and the University of Kénitra (Morocco). The authors are grateful to Professors Allal Douira and Abdelhak Bouabdli (Université de Kénitra), for information about the mine sites, and to Mr. M. Ait-Zaid (Agence du Bassin Hydraulique de la Moulouya — Midelt) and Mr. M. Rahdou (Auberge Jaafar), for admittance to the mine zones and facilities offered. Thanks are also due to Mr. Ian Phillips for his valuable assistance in the XRPD analysis. References Abrahams, P.W., Thornton, I., 1987. Distribution and extent of land contaminated by arsenic and associated metals in mining regions of southwest England. Transactions of the Institution of Mining and Metallurgy 96, B1–B8. Adamo, P., Zampella, M., 2008. Chemical speciation to assess potentially toxic metals (PTMs) bioavailability and geochemical forms in polluted soils. In: De Vivo, B., Belkin, H.E., Lima, A. (Eds.), Environmental Geochemistry: Site Characterization, Data Analysis and Case Histories. Elsevier, pp. 175–212. Adamo, P., Arienzo, M., Bianco, M.R., Violante, P., 2002. Heavy metal contamination of the soils devoted to stocking raw materials in the former ILVA iron–steel industrial plant of Bagnoli (South Italy). The Science of the Total Environment 295, 17–34. Allen, H.E., 1997. Importance of speciation of metals in natural waters and soils to risk assessment. Report of International Workshop on Risk Assessment of Metals and their Inorganic Compounds. International Council on Metals and the Environment, Ottawa, pp. 141–157.

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