land degradation & development

Land Degrad. Develop. 16: 213–228 (2005)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ldr.659

THE EFFECT OF UNCONFINED MINE TAILINGS ON THEGEOCHEMISTRY OF SOILS, SEDIMENTS AND SURFACE WATERS OF

THE LOUSAL AREA (IBERIAN PYRITE BELT, SOUTHERN PORTUGAL)

E. FERREIRA DA SILVA,1* E. CARDOSO FONSECA,1 J. X. MATOS,2 C. PATINHA,1 P. REIS3

AND J. M. SANTOS OLIVEIRA4

1Departamento de Geociencias, Universidade de Aveiro, P-3810-193 Aveiro, Portugal2Instituto Geologico e Mineiro—INETInovacao, Beja Portugal

32 CVRM/Centro de Geo-Sistemas, IST, Technical University of Lisbon, Portugal4Instituto Geologico e Mineiro—INETInovacao, Porto Portugal

Received 10 January 2004; Revised 7 June 2004; Accepted 5 July 2004

ABSTRACT

The former Lousal mine, on the SW limb of the Lousal anticline, closed in 1988. Today Lousal is a small village, with muchevidence of environmental and landscape disturbance. This is largely the legacy of mining and mine tailings dumped around thesite, which, in turn, caused soil contamination and acid mine drainage. Particular attention is being given by the PortugueseGovernment to this sort of problem, bearing in mind the State’s responsibilities for the abandoned mine sites that occur in manyparts of the Country.

Despite the semiarid climatic conditions of the area, a visual inspection of the mine site indicates that the tailings are affectedby considerable water erosion, particularly during large rainfall events.

Significant amounts of Cu, Pb, Zn, As, Cd and Hg, occur within the soil collected near the tailing deposits (292–7013 mg kg�1 Cu, 871–12 930 mg kg�1 Pb, 126–7481 mg kg�1 Zn, 597–6377 mg kg�1 As, 0�2–16�4 mg kg�1 Cd and 1–130 mg kg�1 Hg) and in stream sediments downstream of the tailings site (1–1986 mg kg�1 Cu, 41-5981 mg kg�1 Pb,17–1756 mg kg�1 Zn, 6–1988 mg kg�1 As and 0�2–5�7 mg kg�1 Cd). All the soil samples collected in the tailings depositsexceed the permissible levels.

Near the mine site, significant acid mine drainage is associated with the pyritic material and such waters show values of pHranging from 1�9 to 2�9 and concentrations of 9249 to 20 700 mg L�1 SO4

2�, 959 to 4830 mg L�1 Fe and 136 to 624 mg L�1 A1.Meanwhile, the acid effluents and mixed stream waters also carry high contents of SO4

2�, Fe, Al, Cu, Pb, Zn, Cd, and As,generally exceeding the Fresh Water Aquatic Life Acute Criteria. Copyright # 2005 John Wiley & Sons, Ltd.

key words: mine pollution; tailings erosion; enrichment index; AMD; selective chemical extraction; southern Portugal

INTRODUCTION

Mine waste material containing sulphide waste is an important threat to the environment, being able to affect

extensively mined areas or localized concentrations in ‘hot spots’. Small portions of metals occurring in the mined

ores, in general, are not totally recovered by mill and processing operations, and thus are left in tailings deposits.

Such mining waste, containing significant concentrations of metals, is a source of present-day chemical pollution

that possibly will persist for a long time (Marcus, 1997). Due to the inherent chemical and physical (slope)

instability and other potential environmental problems, such contamination is a matter of long-term public concern

(OSHA, 1990). The mode of occurrence of this waste (in general, fragmented and finely-ground materials)

Copyright # 2005 John Wiley & Sons, Ltd.

�Correspondence to: E. Ferreira da Silva, Departamento de Geociencias, Universidade de Aveiro, P-3810-193 Aveiro, Portugal.E-mail: eafsilva@geo.ua.pt

Contract/grant sponsor: Instituto de Apoio as Pequenas e Medias Empresas e ao Investimento (IAPMEI), Ministerio da Economia, Portugal.

enhances and/or promotes the development of a number of chemical reactions. When iron sulphides are present,

oxidation produces acid mine drainage (AMD). Oxidation of metal sulphides in mines, mine dumps and tailing

impoundments produces acid metal-rich waters that can contaminate local surface waters, groundwaters and

stream sediments. AMD has been recognized as a major environmental pollution problem over the past three

decades (e.g. Letterman and Mitsch, 1978; Kleinman et al., 1981; Gray, 1997). One of the most important

questions associated with AMD is the degradation of water quality in response to stream acidification due to blow-

outs, surface water run-off over tailing deposits, or erosion of tailings into the watercourses during high rainfall

events.

In order to characterize the number of old mines without owner or property rights, the Portuguese Government

has taken the responsibility to carry out an inventory and assessment of abandoned mine sites. With this objective a

comprehensive program has been developed in order to: (a) characterize and identify the generated impacts; (b)

assess the symptoms of the risks inherent to former mining operations; and (c) promote measures that best fit the

rehabilitation of the environmentally affected sites (Santos Oliveira et al., 2002).

The results of this program led to the identification and hierachical organization of more than 100 mine sites

according to their hazardousness. In the assessment, emphasis was given to the geology and mineralogy of the ore

deposit, processes of ore extraction and processing, even size, composition and stability of the mine landfills

(tailings), type and magnitude of chemical anomalies in soils, stream sediments and waters as well as actual

mining safety, visual impact, degree of land use and archaeological (museum) relevance.

The Lousal mine is a typical ‘abandoned mine’ case study with all sorts of problems occurring as consequence

of the cessation of the industrial activity and resulting lack of maintenance of the industrial-mining infrastructure.

The objectives of the present study were: (1) to perform the overall chemical characterization of the site, with

the aim of assessing land degradation; (2) to evaluate the extent of pollution in soils and sediments impacted by

mining activities and by erosion of the tailings; (3) to identify the geochemical characteristics of the AMD formed

in the abandoned mine adits and on the tailing deposits of the Lousal mine; (4) to investigate the mechanisms of

dilution and chemical precipitation of selected metals in the Corona stream, which is impacted by AMD; (5) to

evaluate the extent of AMD pollution; and (6) to delineate guidelines regarding future clean-up operations in the

affected area.

GEOLOGICAL AND ENVIRONMENTAL SETTING OF THE STUDY AREA

The Lousal polymetallic massive sulphide mine is located in the NW region of the Iberian Pyrite Belt (IPB), in a

Volcano-Sedimentary Complex (the Lousal Complex), which also includes the old Caveira pyrite mine (Figure 1a).

The lithology is dominated by argillaceous and siliceous schists, lavas, basic and acid tuffs, and layers of jaspers

interbedded with sulphides and some manganese ores.

The massive sulphides of the Lousal mine are composed predominantly of pyrite (FeS2), together with minor

chalcopyrite (CuFeS2), galena (PbS), sphalerite (ZnS), pyrrhotite (FeS), marcasite (FeS2), bournonite (CuPbSbS2),

tetrahedrite (Cu12Sb4S13), arsenopyrite (FeAsS), cobaltite (CoAsS), magnetite (Fe2O4), saphlorite and native gold

(Au) (Strauss, 1970).

The geomorphology of the region is controlled by rocks of the South Portuguese Zone Palaeozoic Basement

on the NW, and by Tertiary sediments on the SE (Matzke, 1971; Schermerhorn et al., 1987; Matos & Oliveira,

2002a,b). The relief close to the principal streams is moderate (2 per cent–5 per cent), but the neighbouring hills are

dominated by long, steep slopes (10 per cent–35 per cent).

The Lousal mine was worked between 1900 and 1988, basically for pyrite. With this objective in mind, surface

and underground works were developed to a depth of about 500 m (Shaft No. 1) (Matos & Oliveira, 2002). In 1882

a local farmer discovered the gossans of Sul and Extremo Sul orebodies, located near the Corona stream. Later, in

1901, several shafts were worked from NW to SE, and these were assigned to different gossans: the Oeste, the

Central, the Sul and Extremo Sul orebodies). These main gossans and their supergene enrichment zones were

worked for copper ore (Orey, 1901). During the 1930s the mining increased significantly in the Lousal mine due to

the needs of the SAPEC phosphate plant (built in 1928 in the town of Barreiro) to produce high tonnages of pyrite

concentrates (Silva, 1996). Until the 1920s, superficial zones of the Sul, Extremo Sul, Central, Oeste, and Norte

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orebodies (gossans and/or supergene enrichment zones) were worked mainly for Cu (Strauss, 1970). Mining works

were later oriented for deeper lenses taking into account the chemical company’s goal of producing more pyrite

concentrates. In the late 1950s and early 1960s, the development of new mechanization technology for mineral

extraction and the improvement of the processes of ore treatment, in Lousal mine, allowed an annual production of

230 000–250 000 tons of concentrates, with 45 per cent of S and 0�7 per cent of Cu (Matzke, 1971). Thadeu (1989)

in Leistel et al. (1998) estimated a total tonnage of 50 Mt of ore containing 0�7 per cent of Cu, 0�8 per cent of Pb

Figure 1. (a) The NNW Sector of the Iberian Pyrite Belt (ad. Oliveira et al., 2001). Baixo Sado VSC lineaments: 1 Vale Agua/Chaparral, 2—Rio Moinhos/Fig. Cavaleiros W, 3—Porto Mel—Valverde, 4—Lagoa Salgada/Agua Derramada, 5—Outeirao/Pedrogao, 6—Salgueiral. AltoSado VSC lineaments: 7—Lagoas Paco/Ervidel, 8—Carregueira/Furadouro, 9—Nabos/Milhouros/Aljustrel, 10—Lousal/Alvalade. Mainstructures: grabens–BG—Batao, LSG—L. Salgada NE; horsts—MOH—Martim Afonso/Outeirao, PVH—Pedrogao/Valverde, PSH—Piugada/Mte. Sobral; faults—GF—Grandola, MF—Messejana, FF—Ferreira-Ficalho. Orebodies: ALJ—Aljustrel, GAV—Gaviao,LOU—Lousal, CAV—Caveira, LS—Lagoa Salgada. (b) Lousal topographic map (Matos et al., 2003, IGeoE—Carta Topografica 1/25 000No 518). 1—Lousal open pit, 2—Lousal acid water spring, 3—Lousal southern gossan, 4—Lousal Mining Museum; A—Clean waters dam;

B—Acid waters dam; T—Tailing deposits; R—Railway; * Acid Rock Drainage; inputs.

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and 1�4 per cent of Zn. Matzke (1971) presents the following average grades for the Lousal pyrite: 39 per cent of

Fe; 0�7 per cent of Cu; 0�8 per cent of Pb; 1�4 per cent of Zn; 0�08 per cent of Co; 0�01 per cent of Ni; 45 per cent

of S; 0�03 per cent of Sb; 0�05 per cent of Mn; 0�2 per cent of Sn; 0�01 per cent of Cr; 8�5 per cent of SiO2;

1�5 per cent of Al2O3; 0�1 per cent of CaO; 0�5 per cent of MgO; 0�02 per cent of BaO and 1 g t�1 of Au. One main

open-pit was developed in association with the underground works.

Large volumes of waste were generated by the mining activities, ranging from barren overburden and barren

rock, to various types of tailings (the amount of waste stored on the site is estimated to be greater than 1 Mt).

Rainwater circulates and percolates easily over and through these weakly cemented materials causing significant

erosion and the transport of tailings debris to areas nearby and downstream.

To control the hazards presented by this closed metal mine, preventive measures should have been adopted, but

this was not the case at the Lousal mine. Fieldwork carried out at the Lousal mine revealed that some measures

were undertaken, like levees surrounding the tailing deposits, however they were not effective because some of

them have been breached due to erosion, allowing the uncontrolled movement of waste material and contaminated

water from the mine area. Likewise, the dam constructed to avoid the AMD input into the Corona stream, has

infiltration occurring at its base and contaminates the water. Due to the uselessness of these remedial measures,

sediments located downstream and close to the tailings show a black tinge caused by the input of eroded tailings,

while the stream water has acquired a reddish hue due to acid mine drainage. Three main sources of material input

were identified in the Corona stream (Figure 1b).

MATERIALS AND METHODS

To investigate the impact caused by the dismantling and erosion of the tailings around the mine site, soil, stream

sediment and water samples were collected in the area.

Soil, Stream Sediment and Water Sampling and Analysis

A total of 409 soil samples (0–15 cm of depth from surface) were taken for the centre of the contaminated area at

nodes of 50 m� 50 m grids and for the N and S borders of the studied area nodes of 100� 100 m were used

(Figure 2). The sampled area covered 1�9 km2 (the approximate area of the mining field), corresponding to a

sampling density of 215 samples km�2. This included the area affected by the old mining activities and also the

surrounding areas.

Soil and stream sediment samples were oven dried before dry sieving at a temperature of 40�C, until a constant

weight was attained. Samples were disaggregated and passed through a 177 mm aperture plastic sieve. The fine-

grained (< 177 mm) fraction of soil and stream sediment samples was submitted to multi-elemental analysis in an

accredited Canadian laboratory (ACME Anal. ISO 9002 Accredited Lab-Canada). A 0�5 g split was leached in hot

(95�C) aqua regia (HCl-HNO3-H2O) for 1 hour. After dilution to 10 ml with water, the solutions were analysed by

Inductively Coupled Plasma-Emission Spectrometry (ICP-ES) for 35 chemical elements. Included in this

analytical package were Ag, Al, As, Au, Ba, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, S,

Sb, Sc, Sr, Th, Ti, U, Vand Zn. The accuracy and analytical precision were determined using analyses of reference

materials (standards C3 and G-2) and duplicate samples in each analytical set.

Minerals of the tailings were determined by powder XRD using a Phillips powder diffractometer, model

PW3040/601, equipped with an automatic slit. A Cu-X-ray tube was operated at 40 kV and 30 mA. Data were

collected from 2 to 70� 2� with a step size of 1� and a counting interval of 0�6 seconds.

Water samples were collected at 15 selected sites in September 2000 during normal flow conditions, after rainy

weather. Samples no. 2, 18 and 19 were taken on the discharging points; samples no. 3, 4, 6, 7, 8, 9, 11, 12 and 13

on the Corona stream; sample no. 10 on the Lousal stream; and samples no. 14 and 15 on the Sado River (Figure 2).

At each selected site, samples were collected by using a clean 1L polyethylene bottle and stored by cooling to

4�C until further analysis. In order to analyse the dissolved phase, 250 ml of water from each sample was taken and

filtered on-site through 0�45 mm Millipore membrane filters using an all-plastic pressurized filtering system

(ASTM, 1984). Samples for metal analysis were immediately preserved after collection in the field, with the

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Fig

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ENVIRONMENTAL GEOCHEMISTRY AT LOUSAL MINE, PORTUGAL 217

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reduction of pH to 2�0 using HNO3 (acidified waters). All the water samples were stored at 4�C pending further

analysis.

Water samples were analysed for a number of dissolved trace metals, major cations (Ca, K, Na and Mg) and

anions (Cl�, NO3� and SO4

2�). Major cations and trace elements (Fe, Cu, Pb, Zn, Mn, Cd, Bi and Sb) were

determined using inductively coupled plasma-atomic emission spectrometry (ICP-MS). Sulphate (SO42�) and Cl�

were analysed by ion chromatography (IC) and HCO3� by titration on filtered unacidified samples.

Temperature, pH, and conductivity were measured in situ and HCO3� was determined by titration in the

laboratory. For the pH determinations, a portable HI 8424 microcomputer pH meter was employed, which was

calibrated using two Titrisol pH 4 and 7 buffers.

Rigorous water data quality control was undertaken by inserting reagent blanks and duplicate samples into each

batch (Ramsey et al., 1987).

Method for Enrichment Index Estimation

Enrichment Index (EI) was used in this study to evaluate the degree of trace metal contamination (Nishida et al.,

1982; Chon et al., 1995; Kim et al., 1998; Lee et al., 1988). The permissible level is the element concentration in

the soil, from which crops produced are considered as unsafe for human health (As 20 mg Kg�1; Cd 3 mg Kg�1; Cu

100 mg Kg�1; Mo 5 mg Kg�1; Pb 100 mg Kg�1; Zn 300 mg Kg�1; Hg 1 mg Kg�1; Sb 5 mg Kg�1; Kloke, 1979;

Reimann and Caritat, 1998).

This index is useful to evaluate the degree of multiple element enrichment. An enrichment index over 1�0indicates that, on average, metal concentrations are above the permissible level and any element enrichment may

be from anthropogenic inputs or natural geological sources (Nimick and Moore, 1991).

The denominated Enrichment Index (EI) was used in this study to evaluate the degree of trace metal

contamination (Nishida et al., 1982; Chon et al., 1995; Kim et al., 1998; Lee et al., 1998), which was computed

by averaging the ratios of the element concentration (mg Kg�1) to the hazard criteria, the permissible level for each

element.

In this study, eight elements (As, Cd, Cu, Mo, Pb, Zn, Hg and Sb) were selected to calculate the enrichment

factor in each sample using the following equation:

Enrichment Index ðEIÞ ¼As20þ Cd

3þ Cu

100þ Mo

5þ Pb

100þ Zn

300þ Hg

1þ Sb

5

8

Selective Chemical Extraction Procedure

The knowledge about metal partitioning among different geochemical phases is particularly important to assess

the potentially bioavailable fractions and any risks of ecotoxicity. Trace metals can be transported in the secondary

geological environment by different chemical mechanisms (Gibbs, 1973): (a) in solution or adsorbed on solids,

being readily available in these conditions; (b) in organic compounds or metallic hydroxides, for which chemical

changes are required before they are released; or (c) in the crystal structure of some minerals, where they are

generally unavailable for the ecosystems.

Many schemes of sequential extraction to assess chemical speciation have been developed. In these, samples are

sequentially treated with different reagents in order that metals with different affinities for the mineral matrix can

be liberated (Tessier et al., 1979; Meguellati et al., 1983; Rapin and Forstner, 1983; Quevauviller et al., 1994;

Gomez-Ariza et al., 2000).

In this study, selected elements (Cu, Pb, Zn, As and Fe) were analytically partitioned into labile and residual

fractions using a six-step sequential extraction procedure, with the purpose of: (a) determining the mode of

occurrence of the metals in the tailing materials; and (b) establishing the geochemical patterns of trace metals,

which are useful for predicting their release ability into the aquatic environment and into the ecosystem (Cardoso

Fonseca and Ferreira da Silva, 1998).

A sequential chemical extraction procedure, previously discussed by Cardoso Fonseca and Ferreira da Silva

(1998), was adopted in this study. According to Cardoso Fonseca (1982), the following extractants and

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operationally-defined chemical fractions were taken: (a) ammonium acetate (1M NH4Ac, pH 4�5) for fraction 1—

water soluble and dissolved exchangeable ions, specifically adsorbed and carbonate-bound; (b) hydroxylamine

hydrochloride (0�1 M NH4OH�HCl, pH 2) for fraction 2—ions bound in Mn oxyhydroxides; (c) ammonium

oxalate (dark) (0�175 M (NH4)2C2O4–0�1 M H2C2O4, pH 3�3 in darkness) for fraction 3—ions linked to

amorphous Fe oxides; (d) H2O2 35 per cent for fraction 4—ions associated with organic matter (on this step

sulphide-bound as primary sulphide minerals could not be totally leached out according to Rapin and Forstner

(1983) and Khebonian and Bauer (1987)); (e) ammonium oxalate (UV) (0�175 M (NH4)2C2O4–0�1 M H2C2O4, pH

3�3 under UV radiation for fraction 5—ions associated with crystalline Fe oxides; (f) mixed acid

(HClþHNO3þHF) attack for fraction 6—ions associated to matrix elements in lattice positions, resistant

oxides and sulphides. After each reaction timing, the solutions were centrifuged and filtered. Metal analyses were

carried out by AAS (for Cu, Pb, Zn, Fe) and by hydride generation combined with AAS (for As). The accuracy of

the sequential treatment, considered as a whole, may be estimated by the comparison of the total sum of the

amounts obtained after each sequential extraction step with the total amount obtained after hot mixed-acid attack

of the same sample.

RESULTS AND DISCUSSION

Enrichment Index

Heavy metal pollution of the surface environment due to mining carried out in past is commonly associated with a

number of contaminants, rather than with a single element.

One approach for comparing metal concentrations is to compute an index which averages the accumulation of

each metal in a sample.

As expected, the highest EI values occur in the tailings and also in the eroded soils, indicating that those residual

deposits are the main source of chemical contaminants. According to Figure 3, it was possible to identify two main

areas (the A and B areas) associated with the tailing dumps. The EI calculated for representative samples of these

two areas varies from 16�26 to 70�65 (21�22–70�65 for the A area and 16�26–54�92 for the B area), showing that the

tailings are metal enriched at a level likely to be toxic to the ecosystem as defined by Nimick and Moore (1991).

Chemical analysis of minesoil samples collected at the A and B areas (tailing deposits) revealed high

concentrations of Pb, As, Hg, Cu, Mo and Zn (Table I). Values ranging between 272–12 930 mg kg�1 Pb, 597–

6377 mg kg�1 As, 1–130 mg kg�1 Hg, 292–7013 mg kg�1 Cu, 1–130 mg kg�1 Mo, and 126–7481 mg kg�1 Zn were

recorded for the tailings. All the values exceed the Pb, As, Hg, Cu, Mo and Zn permissible levels (Kloke, 1979;

Reimann and Caritat, 1998).

Sulphides are still abundant at the surface around the mine and are likely to undergo further sulphur oxidation

for a long time, unless some remediation measure is undertaken. Oxidation of the dominant sulphide minerals

(pyrite, pyrrhotite, chalcopyrite, and arsenopyrite) gives rise to a great variety of secondary minerals on these two

sites, many of them being identified by X-ray diffraction (jarosite, melanterite, and scorodite among others). The

solubility of these minerals is variable indicating that the release of trace metals (Cu, Pb, Zn, As, etc.) may not

coincide with the amount of sulphate generated, as noted by Kwong (1991).

The values achieved for EI in alluvium samples range from 0�9 to 8�2, mainly due to the presence of significant

contents of As and Pb. These high values are probably due to clastic (mechanical) movements of tailing materials

as a result of erosion.

During runoff events, rain water dissolves secondary minerals producing strong acid waters, which are collected

in small pools and wetlands. During dry seasons these pools evaporate progressively to originate efflorescent

sulphate salt crusts (jarosite and melanterite) indicating the presence of an extreme acidic environment. The

melanterite salt crust easily dissolves during rainfall events.

Selective Chemical Extraction

Using Selective Chemical Extraction it was possible to estimate the distribution of trace metals among the

geochemical fractions on eroded soils, which accounts for the relative proportions of each trace metal transported

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by the mechanical and chemical agents. Table II shows the geochemical partitioning of some selected heavy

metals and As in those fractions of each sample. The overall recovery rates (the sum of 6 fractions/total

concentration) ranged from 85 to 110 per cent. These results have the same order as those obtained by Cardoso

Fonseca and Ferreira da Silva (1998).

Figure 3. Mapping of EI (Enrichment Index) for soils in the Lousal study area (1, 2 and 3—AMD source). The bold line defines thesurrounding area.

Table I. Range and median concentrations of As, Cd, Cu, Hg, Fe, Mo, Pb, S, Sb and Zn in soil samples from the twocontaminated areas (A and B) and surrounding area. The concentrations of As, Cd, Cu, Hg, Mo, Pb, Sb and Zn values areexpressed in mg kg�1 and the concentration of Fe and S values in %

Element Area A Area B Surrounding area(n¼ 13 samples) (n¼ 19 samples) (n¼ 57 samples)

Median Minimum Maximum Median Minimum Maximum Median Minimum Maximum

As 1243 769 6377 1347 597 4864 562 164 2068Cd 4�5 0�5 10�9 0�9 0�2 16�4 0�4 0�2 3�6Cu 1579 403 7013 934 292 6398 471 103 3324Hg 48 21 130 26 1 92 10 1 40Mo 4�8 1�7 22 4�3 2 13�5 2�1 0�2 13�3Pb 6050 3394 12 930 5079 871 18 503 1070 74 2831Sb 123�1 70�8 431�8 92�7 26�5 278�6 28�3 5�3 84�5Zn 2760 335 7310 647 126 7481 339 31 1859Fe 35�50 13�58 43�19 16�97 6�51 41�01 7�71 2�69 18�39S 36�02 7�85 47�24 5�82 0�12 44�46 0�73 0�04 15�53

Bold values exceeds the permissible levels defined by Kloke (1979) and Reimann and Cavitat (1998).

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In areas exposed to AMD, soils and stream sediments usually consist of a complex mixture of geochemical

bearing phases containing potentially toxic metals. It is well known that the availability of such hazardous metals

strongly depends on their specific chemical forms and binding capabilities, rather than on the total element

concentrations.

Iron is mostly extracted by H2O2 in the tailing samples (62�7 per cent Fe extraction for the A area and

67�1 per cent Fe extraction for the B area), suggesting the presence of sulphides. However, a considerable

proportion of this metal (up to 30 per cent) appears to be accumulated in the residual fraction of a representative

eroded soil sample collected near the A tailing, or in the reducible fraction (up to 37 per cent). It must be pointed

out that the easily extractable Fe, in the tailings, is less than 10 per cent (jarosite and melanterite).

Lead, Zn and Cu show high (and similar) partitioning patterns in the A tailing (19�2 per cent, 43�8 per cent and

52�3 per cent of H2O2 extraction, respectively), suggesting that these elements are held in sulphides. This trend

probably reflects the presence of galena, sphalerite and chalcopyrite, which are common sulphides in the mineral

paragenesis (Cardoso Fonseca and Ferreira da Silva, 1998). It was, however, shown that Pb was also greatly

extracted with ammonium acetate (44�9 per cent of extraction—2730 mg kg�1 for the A area and 30�8 per cent of

extraction—2159 mg kg�1 for the B area) indicating that the exchangeable/carbonate fraction is probably a

preferential sink for Pb.

Moreover, part of this element was extracted with Tamm reagent in the dark in both samples (13�5 per cent of

extraction—820 mg kg�1 for the A area and 17�5 per cent of extraction—1195 mg kg�1 for the B area) and with

Tamm UV (B: 23�1 per cent of extraction—1619 mg kg�1) suggesting that Pb may also be linked, to some extent,

to amorphous and crystalline Fe oxides.

Zinc was also extracted by ammonium acetate (27�7 per cent of extraction—1072 mg kg�1 for the A area and

25�1 per cent of extraction—1759 mg kg�1 for the B area) indicating the presence of secondary minerals in the

tailing samples (probably smithsonite).

Different results for As distribution patterns were obtained for the two eroded tailings (A and B—Table II). The

highest As concentration in sample B indicates that this element is extracted by Tamm reagent and H2O2

suggesting that this element occurs as scorodite. In sample A, however, As appears to be linked to sulphides since a

considerable proportion is extracted by H2O2 (44 per cent of extraction—573 mg kg�1). This high percentage

associated to sulphides reflects the presence of arsenopyrite in the samples.

The percentage of easily mobilized phases for the four initial steps of the sequential extraction reached values of

76�9 per cent to 91�4 per cent for Cu, 75�6 per cent to 77�6 per cent for Pb, 84�5 per cent to 89�5 per cent for Zn and

78 per cent to 79�1 per cent for As.

The mineralogical study of tailing samples by XRD showed that they are composed mainly of quartz, feldspar

and primary and secondary micas (biotite, chlorite, muscovite, illite, and kaolinite). However, significant

proportions of iron oxides (haematite, ilmenite) and Fe, Cu, Pb and Zn sulphides (pyrite, chalcopyrite, galena,

Table II. Cu, Pb, Zn, As and Fe total concentrations and percentage of extraction by each reagent of the sequence in twoselected tailing samples

Total Sample from tailing A Sample from tailing B

Cu Pb Zn As Fe Cu Pb Zn As Fe3154 6082 3873 1433 32�59 1258 7009 2212 416 13�12

R1 15�3 44�9 27�7 11�8 3�7 10�9 30�8 25�1 7�2 10�9R2 0�5 1�9 1�0 0�6 0�2 0�0 0�3 1�4 0�0 0�0R3 9�3 13�5 13�0 23�3 0�7 6�0 17�5 1�2 37�4 2�2R4 52�3 19�2 43�8 44�0 62�7 74�5 27�3 64�4 28�0 67�1R5 0 6�7 0�9 5�1 0�5 3�5 23�1 3�1 17�1 14�3R6 22�6 13�7 25�4 15�2 32�1 5�1 0�9 3�3 10�4 4�4R1—ammonium acetate; R2—hydroxylamine; R3—Tamm dark; R4—hydrogen peroxide; R5—Tamm UV; R6—acid decomposition.Bold values exceeds the permissible levels defined by Kloke (1979) and Reimann and Cavitat (1998).

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sphalerite) are present. Iron sulphates (jarosite and melanterite) and Cu and Pb sulphates and carbonates (anglesite,

malachite, cerussite) are also present in the tailings.

Assessment of the Extension of Pollution on Surface Waters and Stream Sediments

In the study area, As and trace metal concentrations on surface waters and stream sediments vary greatly

depending on both the location of the sampling stations and the influence of the tailing deposits. According to the

results, three important AMD sources are responsible for the contamination of surface waters and stream

sediments (Figure 3 and Table III).

According to the results, the AMD sources shows high concentrations of Fe and SO42�, resulting from the

oxidation of sulphide minerals (mainly pyrite, but also pyrrothite, marcasite, arsenopyrite and chalcopyrite). The

general characteristics of these waters are: pH ranging from 1�9 to 2�8 and chemical concentrations from 9240 to

20 070 mg L�1 SO42�, 959 to 4830 mg L�1 Fe, and 136 to 624 mg L�1 Al.

In order to better visualize the impact of the AMD sources and the dispersion of heavy metals along the Corona

stream, related to erosion and to chemical reactions of element precipitation, concentrations of Fe, Al, Cu, Pb, Zn,

As and Cd were plotted versus the distance from the open pit (Figures 4 and 5).

Sample no. 10, collected on an uncontaminated upstream site, was taken as a reference water sample

(background values). Concentrations of 4 mg L�1 Cu, 2 mg L�1 Pb, 30 mg L�1 Zn, 180 mg L�1 Fe, 1 mg L�1 As,

0�05 mg L�1 Cd, 15 mg L�1 SO42� and 20 mg L�1 Al determined on this sample were adopted as local background

values. Meanwhile, the analysis of ‘contaminated’ surface waters revealed strong metal increases in relation to the

background concentrations. For example, Cu exceeds its background level by 11–1179 times, Pb- about 58–248

times, Zn- 70–424 times and As- 5–611 times. Moreover, the measured high concentrations of Fe (7422–

61 900 mg L�1), Al (3460–27 000 mg L�1), and SO42� (195–807 mg L�1) are consistent with those expected in

mining areas impacted with waste deposits and tailings (Reimann and Caritat, 1998).

With respect to the significant inorganic parameters, such as Cu, Pb, Zn and Cd, the samples 6, 7, 8, 9, 11, and 12

exceed the Fresh Water Aquatic Life Acute Criteria (WQC) proposed by USEPA (2002). Bacteria and algae were

the only forms of life observed in the Corona stream. Fish and other macroscopic organisms were absent in the

zone affected by AMD. According to Brake et al. (2001), the relative absence of aquatic life in this environment

suggests that contamination has probably disrupted food chain relationships, discouraged animal habitation and

interfered with normal biogeochemical cycling of the stream system.

Aluminium, in particular, has been shown to be lethal to fish at concentrations as low as 0�2 mg L�1 in a pH

range of 4�4 to 5�9 (Cronan and Schofield, 1979). More recently, studies by Witters et al. (1996), show that acute

respiratory dysfunction and mortality can occur in areas where aluminium-rich acidic waters mix with neutral

waters.

From Figure 4 it is clear that the stream sediments are seriously contaminated with As and sulphide-related

heavy metals (Cu, Pb, Zn, and Cd) despite some decrease noticed after the confluence with the Sado river (sample

no. 14). The water collected in the Sado river upstream from the confluence (sample no. 15) show that the

concentrations of Cu, Pb, Zn, Fe, As, Cd, SO42� and Al are low and near the background values (8 mg L�1 Cu,

2 mg L�1 Pb, 55 mg L�1 Zn, 192 mg L�1 Fe, 1 mg L�1 As, 0�05 mg L�1 Cd, 78 mg L�1 SO42� and 97 mg L�1 Al).

Table III. Physical-chemical characteristics and SO42�, Fe, Al, Cu, Pb, Zn, Cd and As total concentrations in the water

samples from the AMD sources

pH Cond. SO42� Fe Al Cu Pb Zn Cd As

mS cm�1 mg L�1 mg L�1 mg L�1 mg L�1 mg L�1 mg L�1 mg L�1 mg L�1

AMD1 2�8 9300 11 610 959 136 000 8864 306 170 000 267 2AMD2 1�9 10 620 20 070 4830 624 000 66 155 177 225 699 560 36 455AMD3 2�0 6259 9240 1629 327 000 49 610 2 130 236 300 4574

AMD1—spring water, located in the open-pit (sample 2); AMD2—stream draining the tailing deposit A (sample 18); AMD3—streamdraining the tailing deposit B (sample 19).

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Figure 4. Plots of Cu, Pb, Zn, As and Cd concentrations versus distance at Corona stream (&—stream sediment samples; *—Water samples).

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The highest concentrations of these potentially toxic elements were found close to the open pit (where a spring

is located) and to the tailing deposits, where values ranging between 1–1986 mg kg�1 Cu, 41–5981 mg kg�1 Pb,

17–1756 mg kg�1 Zn, 6–1988 mg kg�1 As, 0�2–5�7 mg kg�1 Cd, 1�74–27�7 per cent Fe, 0�65–8�22 per cent Al were

found. The highest concentrations were recorded in sediments of the Corona stream. On the other hand, sediments

taken in the immediate areas (near the open pit and tailing deposits), show significant amounts of oxy-hydroxide-

sulphate phases, and are strongly enriched in Fe (10�1–27�66 per cent), S (2�2 per cent–25�29 per cent) and Al

(0�76 per cent–8�22 per cent), whereas the farthermost area (downstream sediments) of the Corona stream show

lower concentrations of these elements (4�94 per cent of Fe, 0�76 per cent of S and 1�87 per cent of Al) in the AMD

sources results.

As illustrated in Table III, conclusions drawn on water sample data shows that sulphuric acid produced during

these chemical reactions is responsible for the acid properties of these waters as well as inducing oxidation

reactions, promoting the release of Al, As, Cd, Cu, Pb and Zn. According to the Ficklin diagram (Plumlee et al.,

1994) these waters are plotted in the ‘high acid/extreme metal’ portion and the metal concentrations are similar to

other AMD case studies (Boult et al., 1994; Brake et al., 2001; Sainz et al., 2003; Sracek et al., 2004).

The high concentrations of Al (Table III) found in these mine waters suggest that this element could also have

been released from the dissolution of aluminosilicates, associated with conditions of low pH, which occur widely

in the study area. However, a NNW intense kaolinitization corridor observed in the open pit, which is related to the

supergene alteration of the Central and Miguel outcropping orebodies (Matos and Oliveira, 2002; Matos et al.,

2003) is also suggested as another factor for the Al enrichment.

The water data derived from the Corona stream suggests that the Corona stream undergoes the influence of

drainage waters coming from surface and underground mining works and also from sulphide-bearing waste piles

and tailing deposits. The results indicate the existence of acid Fe-rich stream waters severely polluted with metals,

which according to Ficklin classification are ‘moderate acid and high metal’. The formation of neoformed

colloidal ferrous hydroxides appears to be responsible for the yellowish orange colour of the stream water.

Figure 5. Plots of Fe and Al concentrations versus distance at Corona stream (&—stream sediment samples; *—Water samples).

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Local fluctuations (and decreases) of the metal concentrations in the surface waters may be explained on the

basis of ‘dilution and precipitation of heavy metals’. The plot results (Figure 5) show that an important percentage

of Fe and Al precipitate on the stream bed as the pH increases. These high levels of iron and aluminium suggest

that both remain mostly in solution until the Lousal stream reaches the Corona stream. At this point mixing of

these waters takes place causing neutralization and fast chemical precipitation. Further downstream, pH of the

Corona stream water increases gradually reaching values greater than 6�0, which is accompanied by decreases of

Cu, Pb, Zn, As and Cd concentrations. These agree with Chapman et al. (1983) who found that mechanisms of

sorption, precipitation and dilution accounted for the attenuation of metals in streams of New South Wales,

Australia, affected by AMD.

The enrichment of the stream sediments in As, Cu, Pb, Zn and Cd (Figure 4) appears to be mainly due to the

erosion and dismantling of the tailing deposits, which originate sulphide input in the main stream. However, it

must be stressed that other mechanisms contribute to the secondary dispersion of the metals, such as: (a)

precipitation of hydroxide, oxyhydroxide, or hydroxysulphate phases from aqueous species due to the

increase of pH; and (b) adsorption of metals onto surfaces of these neoformed minerals, (for example,

carbonates or adsorbed on iron and manganese coatings), according to Nordstrom (1982) and Chapman et al.

(1983).

Proposal for Remediation of the Lousal Contaminated Area

In the Lousal case study, the former owner and the local council jointly devised an integrated development

programme with a cultural component that should reclaim Lousal economically and socially. The former mine

owner and the local council are brought together in the Frederic Velge Foundation, the Lousal Integrated

Development and Revitalization Programme (RELOUSAL Programme). Based on this programme the majority of

the abandoned mining buildings and industrial equipment will be reclaimed This Programme includes the building

of tourist facilities (small hotels, restaurants, leisure facilities, a campsite and opportunities for rural tourism) as

well as vocational training (Reis, 1998).

For a successful reclamation of this abandoned mine, a proper rehabilitation plan should be defined. The

assessment of contamination at this area showed three main contaminated sources. The deep slopes of the tailing

deposits promote the down slope movement of eroded material that easily reaches the Corona stream.

Based on the results of chemical analysis it is clear that the erosion and transportation of tailing materials is

responsible for the mechanical dispersion of the contamination.

Due to the spatial extensiveness of the contaminated area only the tailing deposits (A and B areas—Figure 3)

and the contaminated sediments/alluvium from the Corona stream must be excavated and shipped off-site to a

landfill. This landfill could be installed inside the open-pit. However this option may be costly as the price is by the

ton. Another approach is the consolidation of waste piles in order to reduce water quality impacts. More than 1 Mt

of contaminated soils, sediments and mine processing wastes must be consolidated on-site. The regrading of the

area will prevent the erosion by reducing water runoff and provide a more stable surface.

Once consolidated, a variety of measures must be taken including diversion trenches and culverts, evaporation

ponds and capping (impermeable layer and soil) in order to minimize contaminated runoff leaving the site. The

capping idea is that the remaining solid material, high in metals and/or acid-producing materials, will not be

exposed to superficial weathering. According to Costello (2003) this solution is a reasonable option for reducing

hazard to potential receptors (risks associated with dermal contact and/or incidental ingestion of surface soils) but

they generally do not reduce the toxicity or volume of the metals present in soil. Revegetation with seeds is

needed to reinforce the topsoil, to reduce soil erosion and runoff velocity and to remove water from the soil by

evaporation.

Related to the water resources, the flooded adits are an important source of pollution. The major concern is

the water table rebound, responsible for the formation of the pond of acidic waters and the groundwater spring

draining directly to the Corona stream. The underground connection between the abandoned adits and wells, the

pond and the groundwater spring, make this a complex system of ‘diffuse’ sources of AMD. To solve this

complex problem of AMD we propose a wetland system, located between the groundwater spring and the

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Corona stream. This system should be composed of two different sections, one with an aerobic environment

used basically for iron precipitation and another with an anaerobic environment for precipitation of heavy

metals.

The purpose for AMD abatement should be to restore the uses of the streams in the watershed rather than merely

improve the water quality. Results show that the high heavy metal concentrations in surface waters are hazardous

for fresh-water aquatic life. It is our believe that, depending on the decision upon the water use desired for this

area, further investigation may be needed to understand the processes occurring at the sediment–water interface,

essential to the maintenance of the Corona stream ecosystem.

With this approach the input of contaminated loads on the Corona stream are restricted, that way water quality is

maintained at an acceptable level and the accumulation of contaminated sediments is avoided.

CONCLUSIONS

In the Lousal area past remediation effort has been ineffective in eliminating or significantly reducing the acidity

of the drainage or the associated elevated concentrations of dissolved metals. With the present study it was

intended to investigate and quantify these disturbances in the surrounding environment, with a basis on

geochemical analysis of stream sediments, soils and surface waters.

The geochemical results obtained indicate that the Lousal mining area is strongly contaminated in Cu, Pb, Zn,

Cd, As, Fe and Al. The dispersion patterns of these elements in the stream sediments and surface waters located

downstream and downslope of the tailings site appear to be controlled by eroded and transported tailing materials

and by the action of strong acid waters formed from the sulphide oxidation. Many of the dissolved constituents

coprecipitate with, or adsorb to, the iron- and aluminium-rich precipitates.

An approach based on chemical sequential extraction demonstrates that the distribution/accumulation of trace

metals in the tailings is to a great extent, related to sulphides, but also to amorphous or poorly crystalline iron-

oxide geochemical phases. However, it was also noticed that a significant proportion of the trace metals is linked to

exchangeable and acid-soluble species, upon which desorption and ion-exchange reactions may originate the

release of these readily mobile phases. High concentrations of soluble metals at tailing deposit surfaces have been

explained by precipitation of hydrated metal sulphates resulting from evaporation during warm, dry periods. The

dissolution of these salts probably is the source of the most of the dissolved metals and acidity in leachates of the

tailings materials from the site. These salts may degrade water quality during storm events. Acid mine drainage

(AMD) formed around the Lousal mine is the main agent of contamination acting in the area. Decreases observed

in the contents of some metals downstream from the discharge points are explained on the basis of reactions of

precipitation and dilution.

The results achieved for the Lousal area, not only indicate the existence of environmental hazard, but also point

to the consequences of soil acidification together with leaching of As and heavy metals to deeper soil layers.

Beyond the measures mentioned above, a specific soil sampling campaign based on vertical profiles and soil

horizons is advisable to fully understand the distribution of metals with depth. This step is important to calculate

the total amount of the existing contaminated material to be removed, which is essential for estimating the duration

and costs of future rehabilitation operations.

acknowledgments

This study was made possible with the financial support provided by IAPMEI (Instituto de Apoio as Pequenas e

Medias Empresas e ao Investimento—Ministerio da Economia). Particular acknowledgments are addressed to

Instituto Geologico e Mineiro (INETI Inovacao) for authorizing the use and publishing of the geochemical

and hydrochemical data from the Project ‘Estudo do Controle Ambiental nas areas Mineiras Abandonadas de

Lousal e Caveira’. Appreciation is given to the anonymous reviewers for helpful comments and suggestions

who helped to improve the manuscript. Finally, the authors also express their special thanks to Professors

Gudrun Gisladottir and Michael Stocking for their support within the Editorial Board during the submission of

this paper.

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