Acid drainage at the inactive Santa Lucia mine, western Cuba: Natural attenuation of arsenic, barium and lead, and geochemical behavior of rare earth elements

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Applied Geochemistry 25 (2010) 716–727

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Applied Geochemistry

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Acid drainage at the inactive Santa Lucia mine, western Cuba: Natural attenuationof arsenic, barium and lead, and geochemical behavior of rare earth elements

Francisco Martín Romero a,*, Rosa María Prol-Ledesma b, Carles Canet b,Laura Núñez Alvares c, Ramón Pérez-Vázquez c

a Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., Mexicob Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., Mexicoc Facultad de Geología y Mecánica, Universidad de Pinar del Río, Cuba

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 June 2009Accepted 9 February 2010Available online 15 March 2010

Editorial handling by L. Munk

0883-2927/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.apgeochem.2010.02.004

* Corresponding author. Tel.: +52 55 5622 4284; faE-mail address: [email protected] (F.M. Ro

A detailed geochemical study was conducted at the inactive Zn–Pb mine of Santa Lucia, in western Cuba.The studied mine-wastes are characterized by high total concentrations of potentially toxic elements(PTE), with average values of 17.4% Fe, 5.47% Ba, 2.27% Pb, 0.83% Zn, 1724 mg/kg As and 811 mg/kg Cu.Oxidation of sulfide minerals in mine-waste dumps and in the open pit produces acid mine effluents(pH = 2.5–2.6) enriched in dissolved SO2�

4 (up to 6754 mg/L), Fe (up to 4620 mg/L) and Zn (up to2090 mg/L). Low pH values (2.5–2.8) and high dissolved concentrations of the same PTE were found insurface waters, up to 1500 m downstream from the mine. Nevertheless, concentrations of As, Ba andPb in acid mine effluents and impacted surface waters are relatively low: 0.01–0.3 mg/L As, 0.002–0.03 mg/L Ba and 0.3–4.3 mg/L Pb. Analysis by X-ray diffraction and electron microscopy revealed theoccurrence of lead–bearing barite and beudantite and the more common solid phases, reported else-where in similar environments including Fe-oxyhydroxides, jarosite, anglesite and plumbojarosite.Because the reported solubilities for barite and beudantite are very low under acidic conditions, theseminerals may serve as the most important control in the mobility of As, Ba and Pb. In contrast, Fe-oxy-hydroxides are relatively soluble under acidic conditions and, therefore, they may have a less significantrole in PTE on-site immobilization.

Mine-wastes and stream sediments show a light REE (LREE) and middle REE (MREE) enrichment rela-tive to heavy REE (HREE). In contrast, acid mine effluents and surface waters are enriched in HREE relativeto LREE. These results suggest that the LREE released during the oxidation of sulfides are captured by sec-ondary (weathering) minerals, while the MREE are removed from the altered rocks. The low concentra-tions of LREE in acid stream water suggest that these elements can be retained in the sediments morestrongly than HREE and MREE. One possible explanation for the sharp decrease in dissolved LREE mightbe their retention by low-solubility secondary minerals such as anglesite, barite and jarosite.

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1. Introduction

Historic mining activities have produced vast quantities of inac-tive sulfide-rich mine-wastes in several regions of the world. Thesesulfide-rich wastes constitute potential environmental pollutionsources due to oxidation of sulfide minerals that may result in gen-eration of acid mine drainage (AMD). The AMD is characterized bylow pH values (pH < 4) and high levels of dissolved toxic elements.Potentially toxic elements (PTE) of concern vary extensively withdifferent deposit types, but can include As, Pb, Cd, Fe, Cu, Zn and,in some cases, Tl or Se. In addition to PTE, acid mine drainagemay contain high concentrations of rare earth elements (REE)(Verplanck et al., 2004).

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x: +52 55 5622 4317.mero).

AMD has been recognized as the main environmental prob-lem derived from mining activities (Dold and Fontbote, 2001;Holmstrom et al., 2001). The released PTE may be transportedto the surrounding environment and contaminate soils, sedi-ments, ground and surface waters (Bain et al., 2000; Armientaet al., 2001; Jung, 2001).

Waste rock dumps and tailings impoundments have been cate-gorized as main sources of AMD, although open-pit highwalls,underground workings and ore stockpiles can contribute signifi-cant volumes of AMD (US-EPA, 2005). Considering that AMD isan important and costly environmental concern in the miningindustry, increasing attention has been given to the problemscaused and to the processes and pathways by which PTE are trans-ported away from abandoned mine sites. Understanding the geo-chemical processes which control release or retention of PTE inabandoned sulfide mines is crucial for the formulation of models

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F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727 717

that predict the environmental impact of such sites and allowestablishing remediation guidelines.

AMD can be neutralized by pH-buffering reactions and the con-centration of dissolved PTE can be attenuated by a series of precip-itation and sorption reactions. Secondary minerals that precipitateduring the oxidation-neutralization reactions can permanently ortemporally sequester PTE (Levy et al., 1997; Holmstrom et al.,2001; Sánchez España et al., 2005). Secondary minerals precipi-tated in acid mine environments may be the dominant sink for re-leased PTE and, therefore, from a remediation standpointcontribute to natural attenuation of PTE. In this aspect secondaryminerals may be a more effective means of removing PTE inAMD in comparison with other engineered designs (Levy et al.,1997).

Since REE concentration patterns can record subtle geochemicalprocesses in natural systems (Taylor and McLennan, 1988), theycan help to better understand geochemical processes duringAMD formation and subsequent mobilization of PTE. In low tem-perature aqueous environments, subtle differences in electronicconfiguration across the lanthanide series lead to differences inthe behavior of REE with respect to aqueous complexation, ionadsorption, and mineral precipitation. Because of these differences,REE may be fractionated during weathering and solute transport,leading to changes in the REE profiles in both solid phases andaqueous media (Gammons et al., 2003). REE may preserve geo-chemical signatures that reflect source rocks along surface flowpaths. Thus, REE can be used as tracers to identify diffuse inflowof REE-rich acid mine drainage from the dumps into surface watersand sediments (Verplanck et al., 2004).

The REE have been increasingly applied as tracers of geochem-ical processes in acidic surface waters, both in systems in whichthe acidity is produced by natural processes and in systems af-fected by human activities, e.g., mining impacted environments.NASC-normalized patterns with middle REE enrichments havebeen commonly reported for acidic surface water (Johannessonand Zhou, 1999; Gimeno Serrano et al., 2000; Worrall and Pearson,2001). However, other researchers have shown that contrarytrends exist in acidic waters, including light REE enrichment(Bozau et al., 2004), heavy REE enrichment (Gammons et al.,2003), and middle REE depletion (Verplanck et al., 2004). There-fore, it is possible to conclude that there is no general REE patternsthat is universally characteristic of acidic water.

Detailed geochemical and mineralogical investigations wereconducted in the inactive Zn–Pb mine of Santa Lucía, western Cuba,in order to develop an effective management strategy to preventdamage to wildlife or their habitat and to other natural resources,as well as to protect public health and safety. The objectives of thisstudy are to: (i) characterize the mine-wastes of the inactive SantaLucia mine; (ii) study the processes generating AMD and to evalu-ate the contribution to environmental pollution due to PTE release;(iii) identify the solid-phase controls on the aqueous mobility ofsulfide-bound elements, in particular of As, Ba, Pb, Cu and Zn;and (iv) evaluate REE behavior during AMD generation and thesubsequent transport.

2. Site description

The Zn–Pb deposit of Santa Lucia is located in western Cuba, inthe vicinity of the homonymous town in the province of Pinar delRío; it together with the ore deposits of La Esperanza, Castellanosand Matahambre are grouped in the Santa Lucia-Matahambre min-ing district (Fig. 1). These deposits belong to the sedimentary-exhalative (SEDEX) type, and the age of mineralization is Jurassic.They are hosted by the silici-clastic sedimentary rocks of the SanCayetano Formation (Lower–Upper Jurassic), which is the oldest

formation in western Cuba and is composed of sandstones, grit-stones and black shales (Valdes-Nodarse, 1998).

In the Santa Lucia district, more than 20 ore bodies rich in Pb(average grade: 1.95%) and Zn (average grade: 6.14%) have beendocumented. The ore bodies are stratiform and concordant, andare hosted by carbonaceous black shales. The exposed ore bodieshave undergone extensive oxidation, developing a cap of Fe-oxyhy-droxides or gossan. The principal sulfide minerals are pyrite, sphal-erite and galena, with subordinate amounts of pyrrhotite andchalcopyrite. The supergene alteration assemblage includes goe-thite, hematite, magnetite, lepidocrocite, cerussite, anglesite, bar-ite, smithsonite, covellite and chalcocite. Among the main nonmetallic minerals are quartz, calcite, dolomite, chlorite and feld-spar (Pérez, 2000).

The Santa Lucia mine was under exploitation from 1980 to 1998and was mined for pyrite, for the production of H2SO4, using theopen pit mining method. These surface mining operations gener-ated great amounts of sulfide-rich waste rock, which were storedin a waste dump adjacent to mine operations. The rocks exposedin the open pit and waste dump show strong signs of weatheringdue to sulfide oxidation.

An intermittent stream flows through the open pit mine and thewaste dump, and drains into the Santa Lucia River, which drains to-wards the north, directly to the Caribbean Sea. Rainy season is fromMay to October, ranging from 1200 to 1400 mm�1 with dry periodsusually extending from November to April. The mean annual tem-perature varies between 25� and 28 �C, with high mean annualevapotranspiration (1600 mm a�1) and mean relative humidity is86% (Huenelaf and Huatatoca, 2006).

3. Sampling and analytical methods

3.1. Sample collection

A total of 21 samples of sulfide-rich waste rocks, surface waterand streambed sediments were collected in and around the SantaLucia mine site (Fig. 2). Additionally, four samples of unmineral-ized rock outcrops from a nearby control area were collected withthe aim of determining the natural and local background PTE con-centrations. The location of this control area is not shown in Fig. 2.

Sulfide-rich waste rock samples exhibiting different degrees ofweathering were collected from surface mine workings and wastedumps. A total of 12 sulfide-rich rock samples, of 1.5 kg each, wereprepared as composites of five subsamples. Samples from sites y1to y5 are rocks collected from traverses across the open-pit, andsamples from sites t1 to t7 are rocks collected from waste dumps(Fig. 2). These solid samples were stored in hermetically sealedplastic bags to minimize contamination during transport to thelaboratory.

Water samples were collected from seepages below mine-wastedumps, surface mine workings and downstream from the minesite. From seepages emerging from the waste rock dump and sur-face mine workings, three samples of mine effluents (L1, L2 and L3in Fig. 2) were collected in order to assess the acidity and the con-centration of dissolved PTE that may be transported from the SantaLucia mine site to the surrounding environment.

Along the stream, at distances of 200, 1250, 1500 and 2500 mdownstream from the mine site, four surface water samples werecollected (A1–A4 in Fig. 2) in order to evaluate the contributionof mine effluents to environmental pollution. At the sampling site,pH and electric conductivity (EC) were measured in situ. The mineeffluents and surface water samples were collected in clean poly-ethylene bottles, filtered in the field using a plastic syringe witha 0.45 lm pore-size membrane. Filtered water samples were di-vided into two subsamples. Subsamples for metal and metalloid

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San Cayetano FormationLower - Upper Jurassic

Castellanos MemberUpper Jurassic

Artemisa FormationUpper Jurassic - Lower Cretaceous

Esperanza FormationUpper Jurassic - Lower CretaceousQuaternary

Manacas FormationPaleogen

Fig. 1. Geological map showing the location of the Santa Lucia-Matahambre district, Cuba.

718 F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727

analyses were acidified to pH 1.5 using concentrated reagent-gradeHNO3 and were stored on ice until analysis. Subsamples for SO2�

4

analyses only were stored on ice until analyzed.In parallel with surface waters samples, in order to estimate the

level of potentially toxic elements bound to the solid phase, twostreambed sediment samples (0–15 cm deep) were collected atthe same site where the surface water samples A2 and A3 were ta-ken (s1 and s2 in Fig. 2). Each streambed sediment sample com-prised a composite of five subsamples gathered from �2 to 4 m2

areas. These stream samples were sieved through a 2-mm stainlesssteel mesh and approximately 1.5 kg of the <2 mm fraction wasstored in hermetically sealed plastic bags to minimize contamina-tion during transport to the laboratory.

3.2. Analyses of solids

Solid samples were crushed, quartered, pulverized in an agatemortar to 200 mesh, and homogenized. The finely-milled and

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195000 195500 196000 196500 197000 197500 198000 198500 199000315000

315500

316000

316500

317000

317500

318000

L1

L2

L3

A1

A2

A3

A4

Legend

Sulfide-rich waste rock samples from open-pit

Sulfide-rich waste rock samples from waste dumps

Mine effluents samples

Surface water samples

Streambed sediment samples

Intermitent stream

Santa Lucia River

y4

t3

A1

s1

t7

t6

t5

t4t2

t1 t3

y5

y4

y3y2

y1

L1

s2

s1

Santa Lucía Mine

0 m 500 m 1000 m 1500 m

Fig. 2. Detailed map of the study area including sampling locations.

F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727 719

homogenized samples were analyzed using a Portable X-ray Fluo-rescence (PXRF ‘‘Niton XL3t”) for the elements As, Ba, Cu, Fe, Pb andZn, according to the EPA 6200 method (US-EPA, 2007). The qualityof these analyses has been verified by comparing the results withvalues obtained by conventional techniques. Thus, mine-wastesamples were analyzed first using PXRF and subsequently withICP–OES (after acid digestion). The correlation coefficient (r2) be-tween data from FPXRF and ICP–OES was 0.98 (Zamora Martínezet al., 2008).

A rigorous quality control was implemented including blanks,duplicate samples and certified reference materials. One blankwas analyzed for every five samples and the concentrations of ana-lyzed elements were below detection limits. The recoveries of PTEin the standard reference materials (sulfide ore mill tailing RTS-3Canadian standard) were in the range of 90–115%, and analysesof duplicates on five samples showed precision errors between5% and 8%.

Bulk mineral composition was obtained by X-ray diffractionanalysis (XRD) using a Philips 130/96 diffractometer (Cu Ka radia-tion, k = 1.5406 Å). In order to complete the mineralogical charac-terization, samples were also studied by scanning electronmicroscopy (SEM) JEOL JXA-8900R, equipped with energy disper-sive X-ray spectroscopy (EDS). The XRD and SEM analyses weredone in Laboratories at National Autonomous University of Mexico.

In addition, REE concentrations in solid samples were deter-mined by Activation Laboratories in Canada using inductively cou-

pled plasma-mass spectrometry (ICP-MS) after partial digestionwith hot (90 �C) aqua regia (HCl–HNO3–H2O) according to theirstandard methods and QA/QC procedures.

3.3. Analyses of waters

Water samples were analyzed for PTE, aluminosilicate-boundelements and REE by Activation Laboratories in Canada using acombination of inductively coupled plasma (ICP) optical emissionspectroscopy (OES) and mass spectroscopy (MS) according to theirstandard methods and QA/QC procedures. The SO2�

4 concentrationswere determined by turbidimetry through the formation of BaSO4

by adding BaCl2, in laboratories at the Universidad Nacional Auto-noma de México.

4. Results and discussion

4.1. Mine-waste rock

Mine-waste rock from open-pit highwalls and waste dumpsfrom Santa Lucia mine are characterized by high total concentra-tions of PTE, with average values of 17.42% Fe, 5.47% Ba, 2.27%Pb, 0.83% Zn, 1724 mg kg�1 As, and 811 mg kg�1 Cu (Table 1).

The PTE total concentrations in mine-waste were compared toaverage local background concentrations, in order to determine

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Table 1Total concentration of selected elements and mineralogy of analyzed solid samples.

Sample Fe(%)

Ba (%) Pb (%) Zn (%) As(mg/kg)

Cu(mg/kg)

Primary mineral Secondary mineral

Mine-waste rocky1 13.78 6.17 10.38 2.19 5737 172 Quartz (SiO2), illite

(KAl2Si3AlO10(OH)2) phlogopite(KMg3(Si3Al)O10(OH)2) Pyrite (FeS2),Sphalerite (ZnS), Galena (PbS)

Jarosite (KFe3(SO4)2(OH)6) Barite (BaSO4), Anglesite(PbSO4) Gypsum (CaSO4 � 2H2O)Beudantite(PbFe3[(AsS)O4]2(OH)6) PlumbojarositePbFe6(SO4)4(OH)12 Hematite (Fe2O3), Goethite(FeO(OH))

y2 28.95 7.17 1.98 1.62 1846 86y3 15.59 0.55 0.06 1.51 235 3747y4 24.99 0.54 0.37 0.33 964 141y5 43.06 0.08 0.06 0.12 506 97t1 10.64 2.07 1.46 0.04 1448 88t2 20.88 7.49 2.26 0.08 3578 160t3 18.64 6.97 2.1 0.21 2163 172t4 9.88 7.81 4.82 0.66 2082 10t5 5.14 0.94 0.13 0.32 83 85t6 10.65 1.14 0.1 0.05 107 4962t7 6.82 24.73 3.49 2.86 1940 10Average 17.42 5.47 2.27 0.83 1724 811

Stream sedimentss1 10.96 0.58 0.21 0.08 330 75 Quartz (SiO2), illite

(KAl2Si3AlO10(OH)2)Hematite (Fe2O3), Goethite (FeO(OH))

s2 13.45 0.99 0.66 0.07 327 87 Magnetite (Fe3O4), Gypsum (CaSO4* 2H2O)

Average 12.2 0.79 0.44 0.08 328 81 Beudantite (PbFe3[(AsS)O4]2(OH)6)

Sample Fe(%)

Ba (mg/kg)

Pb (mg/kg)

Zn (mg/kg)

As (mg/kg)

Cu (mg/kg)

Background levelsA.1 0.35 26 2 25 <7.0 10A.2 3.18 257 7 124 <7.0 22.4A.3 1.37 46 130 47 <7.0 16A.4 1.41 285 9 22 9.7 20.5Average 1.58 154 37 55 5.6 17

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enrichment ratios of waste rocks relative to unmineralized rocksfrom the nearby control area. The average enrichment ratios forPb, Ba, As, Zn, Cu and Fe were 613, 355, 308, 152, 47 and 11,respectively.

Minerals in the mine-waste rock from the study site may beclassified into two groups: primary and secondary (Table 1). Pri-mary minerals are those that constituted the ore and gangueassemblages, and secondary minerals are those that form throughoxidation and dissolution/precipitation reactions. The presence ofboth primary and secondary minerals indicates that the studiedmine-waste samples are not completely oxidized. The ore bodiesexposed at the surface were extensively oxidized prior to mining,forming secondary minerals. Subsequently, post-mining oxidationprocesses have also resulted in the formation of secondary miner-als. The distinction of pre- from post-mining secondary minerals isdifficult because some minerals might have formed in both stages.

The identified primary ore minerals include the sulfides pyrite,sphalerite and minor galena (Table 1, Fig. 3A), whereas primarynon-sulfide phases are largely dominated by quartz, muscoviteand phlogopite. It is important to emphasize that carbonate miner-als were not observed. Oxidation and acid-neutralization reactionshave partially dissociated the sulfide minerals and promoted thedevelopment of secondary oxyhydroxides, arsenates and sulfates.XRD analyses show that secondary minerals are dominated byjarosite (KFe3(SO4)2(OH)6), barite (BaSO4), anglesite (PbSO4), gyp-sum (CaSO4�2(H2O)), beudantite (PbFe3[(AsS)O4]2(OH)6), plumboj-arosite (PbFe6(SO4)4(OH)12), hematite (Fe2O3), and goethite(FeO(OH)) (Table 1, Fig. 3B).

SEM-EDS analysis showed small (<5 lm), dense particles scat-tered throughout the mine-waste that contain sulfide-bound ele-ments. Four distinct types of particles containing these elementswere identified: (a) Pb–S, (b) Pb–Fe–S, (c) Pb–Ba–S and (d) Pb–Fe–S–As. The presence of Si and Al detected in these particles aredue to the presence of bulk quartz and aluminosilicate in the ma-trix samples. These phases correspond to anglesite, with traces ofBa and Ce, plumbojarosite, Pb-bearing barite and beudantite,respectively (Fig. 4).

Anglesite and plumbojarosite have been widely identified asweathering products of primary sulfides in mine-waste elsewhere(Davis et al., 1993; Lin, 1997). However, Pb-bearing barite andbeundantite are much less common. Some authors have reportedthat Pb can be incorporated into barite by Ba/Pb substitution inmine-waste rock piles (Courtin-Nomade et al., 2008). Accordingto Roussel et al. (2000) and Romero et al. (2007), the formationof beudantite results in efficient trapping of As and Pb in minetailings.

4.2. Mine effluent

Mine effluents from the inactive mine of Santa Lucia are charac-terized by low pH values (2.5–2.6) and high electric conductivity(4.4–13.6 mS/cm). Furthermore, high dissolved concentrations ofsulfide-bound metals (Fe, Zn and Cu) and aluminosilicate- andcarbonate-bound elements (Na, Mg, Al, Si, K, Ca and Mn) were de-tected. However, these effluents have relatively low concentrationsof dissolved the PTE As, Ba, and Pb.

Extensive oxidation of pyrite and sphalerite in mine-waste rockhas resulted in low pH mine effluents and therefore, they are a po-tential source of AMD. The oxidation of these sulfide mineralsmight form secondary minerals containing S, Fe and Zn. The disso-lution of these secondary minerals was favored under acidic pHconditions and it may be the main source of high dissolved concen-trations of SO2�

4 (up to 6754 mg/L), Fe (up to 4620 mg/L) and Zn (upto 2090 mg/L) registered in acidic mine effluents (Fig. 5). Mineral-ogical analyses did not allow identification of any source of Cu,which is in agreement with the relatively low concentrations ofCu in the studied mine-waste (Table 1). The oxidation of traceamounts of unobserved chalcopyrite (CuFeS2) is a probable sourceof Cu. A maximum dissolved Cu concentration of 1.8 mg/L was de-tected within the acidic mine effluents.

The acidic conditions, generated during pyrite and sphaleriteoxidation, favored the dissolution of aluminosilicate minerals inmine-wastes (quartz, muscovite, illite and phlogopite) and, conse-quently, the release of Na (up to 44 mg/L), Mg (up to 780 mg/L), Al

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2 Theta

2 Theta

B

2 Theta

2 Theta

2 Theta

2 Theta

2 Theta

2 Theta

A

Py

0

Fig. 3. (A): Polished section showing sulfide minerals in mine-waste rock. Abbreviations: Py: pyrite, Sp: sphalerite, Gn: galena. (3B): XRD patterns of (B.1) mine-wastesamples and (B.2) stream sediment samples. Abbreviations: M: muscovite (KAl2[Si3Al]O10(OH)2); J: jarosite (KFe3(SO4)2(OH)6); G: goethite (FeO(OH)); Q: quartz (SiO2); A:anglesite (PbSO4); H: hematite (Fe2O3); JPb: plumbojarosite (PbFe6(SO4)4(OH)12); B: beudantite (PbFe3[(AsS)O4]2(OH)6); Ba: barite (BaSO4).

F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727 721

(up to 174 mg/L), Si (up to 44 mg/L), K (up to 3.0 mg/L), Ca (up to420 mg/L) and Mn (up to 91 mg/L) to acidic mine effluents. Cal-cium and Mg can be released also by calcite and dolomite dissolu-tion. The strongly acidic conditions and the lack of carbonateminerals in the mine-waste materials indicate their total dissolu-tion, with the subsequent release of Ca and Mg to the mineeffluents.

4.3. Solid-phase control on the mobility of As, Ba and Pb

Arsenic, Ba and Pb, which are of major environmental and tox-icological concern, are among the PTE with the highest enrichmentfactors in the mine-wastes of Santa Lucia, potentially representinga major source of pollution to the environment. Nevertheless, As,Ba and Pb concentrations in acid mine effluents are relativelylow (As = 0.01–0.29 mg/L, Ba = 0.002–0.35 mg/L, and Pb = 0.33–0.66 mg/L), which implies that dissolved concentrations of theseelements have been naturally attenuated.

Mineralogical analysis indicates the presence of anglesite,plumbojarosite, barite and Pb-bearing barite in mine-waste rock.Reported values of solubility constants for anglesite (Ksp = 10�7.79),barite (Ksp = 10�9.98) and plumbojarosite (Ksp = 10�28.43) indicatethat they are relatively insoluble minerals (Eary, 1998; Gaboreauand Vieillard, 2004). Therefore, these minerals could be the mostimportant solid-phase controls on the aqueous mobility of Pband Ba in Santa Lucia mine.

In addition, the presence of beudantite in the mine-waste hasbeen confirmed. The occurrence of beudantite in Santa Lucia isremarkable because it is an insoluble mineral that plays an impor-tant role in the natural attenuation of As and Pb in sulfide oxidationprocesses. Roussel et al. (2000) estimated a value of 10�15 for thesolubility constant of beudantite, at 298 K, using the standard Gibbsenergy of formation of plumbojarosite of �727.5 kcalmol�1, whichis very similar to the predicted Gibbs energy of formation ofbeudantite (Gaboreau and Vieillard, 2004). This value indicatesthe low solubility of beudantite. Considering that beudantite is

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Fig. 4. SEM-EDS analysis in the BSE mode in mine-waste rock: (A) Pb–S particles (anglesite); (B) Pb–Fe–S particles (plumbojarosite); (C) Pb–Ba–S particles (Pb-bearingbarite); (D) Pb–Fe–S–As particles (beudantite). Silicon and Al peaks correspond to the interference from fine-grained quartz and aluminosilicates.

722 F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727

characterized by very low solubility over a wide range of pH and Eh(Kolitsch and Pring, 2001), the precipitation of this mineral may bethe most important control on the mobility of As under acidic con-ditions within mine-waste materials from Santa Lucia.

4.4. Stream sediments

High total concentrations of PTE were determined in streamsediments collected at approximately 1300 and 1500 m down-

stream of the mine site. These stream sediments were enrichedin Fe (10.96–13.45%), Ba (0.58–0.99%), Pb (0.21–0.66%), Zn (0.07–0.08%), As (327–330 mg/kg), and Cu (75–87 mg/kg). The PTE totalconcentrations in stream sediments are lower than the concentra-tions determined on mine-waste samples, but they were higherthan local background concentrations (Table 1). These results re-flect waste material transport from the mine site, and indicate thatthe inactive mine is a potential source of PTE contamination to thesurrounding environment.

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Fig. 5. SEM-EDS analysis in the BSE mode in stream sediments: (A) Pb–S particles (anglesite); (B) Pb–Ba–S particles (lead-bearing barite); (C) Fe–Pb (Fe-oxyhydroxides withtraces of Pb). Silicon and Al peaks correspond to the interference from fine-grained quartz and aluminosilicates.

F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727 723

XRD analyses show that stream sediment mineralogy is domi-nated by quartz, illite, gypsum, hematite, goethite and beudantite(Table 1). SEM-EDS analysis revealed the occurrence of scatteredparticles containing Pb, Fe, Ba and S, which could correspond toanglesite, Pb-bearing barite and Fe-oxyhydroxides with traces ofPb (Fig. 5).

4.5. Surface stream water

Surface waters, collected approximately at 200, 1250 and1500 m downstream of the mine site, reach pH values of 2.6, 2.8and 2.5, respectively. These pH values are similar to those of themine effluents. Nevertheless, the dissolved concentrations of sul-fide-bound elements and aluminosilicate-bound elements in thestream water were lower than those determined in mine effluents(Fig. 6). These stream water samples contained up to 2589 mg/LSO2�

4 , 557 mg/L Ca, 416 mg/L Mg, 253 mg/L Fe, 165 mg/L Zn,44 mg/L Al, 22 mg/L Si, 21 mg/L Mn, 0.22 mg/L Cu, 0.03 mg/L Baand 0.02 mg/L As (see Table 2).

Additionally, relatively high concentrations of Na, K and Pbwere found in the acidic stream water, with maximum contents

(mg/L) of 75, 13 and 4.3, respectively. High concentrations of Naand K may be explained by cation exchange with the solid phasesof the stream sediments. On the other hand, SEM-EDS analysis ofstream sediments collected at the same site where the streamwater samples A2 and A3 were collected (Fig. 2), showed the pres-ence of Fe-oxyhydroxides with traces of Pb (Fig. 5C), which can bereleased under acidic conditions and, therefore, contribute to thedissolved Pb values (up to 4.3 mg/L) found in the stream waters(Fig. 6).

The results indicate that secondary minerals may not be consid-ered as relevant candidates for on-site immobilization of sulfide-bound metals (Fe and Zn) or aluminosilicate-bound elements. Onthe contrary, under acidic conditions, the dissolution of thesephases may contribute to the dissolved PTE. However, the rela-tively low concentrations of As and Ba in stream water would beexplained by the presence of barite and beudantite, which arerelatively insoluble under acidic conditions.

At approximately 2500 m downstream of the mine, streamwaters reach a pH of 6.4, and show relatively low concentrationsof PTE (with values of 11 mg/L Fe, 0.27 mg/L Zn, 0.9 mg/L Ba,0.019 mg/L Cu, 0.017 mg/L Pb, and 0.004 mg/L As), most likely

Page 9

Fig. 6. pH, EC and dissolved concentrations of potentially toxic elements. Abbreviations: L1, L2, L3: mine effluents. A1, A2, A3: surface stream water collected approximately200, 1250 and 1500 m downstream of the mine site, respectively. A4: surface stream water collected approximately 2500 m downstream of the mine site.

Table 2Concentration of dissolved constituents in mine effluents and surface stream water.

Sample pH EC SO42� Na Mg Al Si K Ca Mn Fe Cu Zn As Cd Ba Pb

(mS/cm) (mg/L)

Mine effluentsL1 2.6 13.6 6754 43.6 780 174 43.5 3.00 420 90.7 4620 0.15 2090 0.293 1.2300 0.035 0.637L2 2.7 12.9 6399 30.9 708 151 29.4 1.33 374 78.9 4330 0.095 1890 0.191 0.9970 0.002 0.328L3 2.5 4.4 2149 27.5 92.2 140 41.9 0.59 113 48.1 264 1.79 585 0.005 1.5300 0.015 0.662

Surface stream waterA1 2.6 5.3 2589 31.5 416 4.82 16 9.82 557 21.3 253 0.028 165 0.006 0.0656 0.011 0.911A2 2.8 1.7 784 74.6 22.4 15.4 14.1 12.9 69 3.97 9 0.223 30 0.024 0.1100 0.033 1.18A3 2.5 2.4 1144 27 20.6 43.6 21.6 12.2 23 4.54 37 0.202 30.9 0.005 0.1300 0.01 4.27A4 6.4 0.2 34 13.6 4.57 0.03 6.1 4.09 7 3.59 11 0.019 0.3 0.004 0.0003 0.196 0.017

Guidelines for Drinking-water Quality (WHO, 2008)6.5 - 8.5 ND 500 200 ND 0.2 ND ND ND 0.4 0.3 2 3 0.01 0.003 0.7 0.01

ND = No guideline value drinking-water is proposed.

724 F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727

due to mixing with more alkaline natural surface waters andchemical attenuation (Fig. 6).

Although these surface waters are not used for drinking pur-poses, it is considered important to warn that the dissolved con-centrations of some contaminants associated with acid minedrainage, from the inactive Santa Lucia mine site, are above the

World Health Organization Guidelines for drinking-water quality(WHO, 2008). Of particular environmental concern are high dis-solved concentrations of SO2�

4 , Fe and elevated levels of other met-als such as Al, Cd, Mn, Pb and Zn.

Using the average values for dissolved concentrations in acidicsurface water it was found that the WHO guidelines were exceeded

Page 10

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F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727 725

by factors of 3 for SO2�4 , 33 for Fe, 106 for Al, 34 for Cd, 25 for Mn,

212 for Pb and 25 for Zn. In near neutral surface water impacted byacid mine drainage only Fe, Mn and Pb exceed the WHO guidelinesby factor of 37, 9 and 2, respectively. However, none of the surfacewater samples exceeded the WHO guidelines for Ba and Cu; andonly one sample of acidic surface water had a dissolved As concen-tration that exceeded the WHO guidelines.

4.6. Rare earth elements (REE) concentrations

REE concentrations of analyzed samples in this study variedover five orders of magnitude. The total

PREE concentration in

the stream sediments samples varied between 16 and 43 ppm,and between 16 and 20 ppm in the mine-waste. The highest dis-solved

PREE concentrations (3.7E�01 to 8.6E�01 mg/L) were reg-

istered in the acid mine effluents, while for stream water impactedby acid mine drainage the dissolved

PREE concentrations varied

between 3.5E�02 and 8.5E�02 mg/L. The lowest dissolvedP

REEconcentration (1.7E�05 mg/L) was obtained in the near neutralsurface water (pH = 6.4) collected at approximately 2500 m down-stream of the mine (Table 3). In this near neutral water only LREEand MREE La, Ce, Nd, Eu and Gd, were detected whereas all HREEwere under the detection limits.

REE concentrations were normalized to the North AmericanShale Composite ‘‘NASC” (Gromet et al., 1984). Plots of NASC-nor-malized REE patterns of solid samples (mine-waste material andstream sediment) show light LREE and MREE enrichments relativeto HREE. In contrast, NASC-normalized REE patterns observed in li-quid samples (acid mine effluents and stream water) show HREEand MREE enrichment relative to LREE (Fig. 7A–C). A similar MREEenrichment has been observed in low pH water environments, andhas been ascribed to solid–liquid exchange reactions and to disso-lution of surface coatings (Worrall and Pearson, 2001; GimenoSerrano et al., 2000; Johannesson and Zhou, 1999).

REE patterns of the acid mine effluents and acid stream waternormalized to the mine-waste rock and stream sediment, respec-tively, are shown in Fig. 7C. While liquid samples are always de-pleted in all REE, compared to the solid samples, acid mineeffluents and acid stream water are enriched in MREE and HREEcompared to LREE. This behavior suggests that the REE mightbe fractionated during sulfide oxidation and acid generation andsubsequent transport, so that MREE and HREE were preferentiallyenriched over LREE. According to Johannesson and Zhou (1999)the behavior of the REE during weathering of a parent rock couldbe induced by: (a) the abundance and distribution in mineralphases containing REE in the unweathered parent rock, (b) thestability of these REE bearing mineral phases with respect tothe aqueous fluids involved in the weathering reactions, (c) thechemistry of the aqueous fluids, e.g., pH, pE, concentrations ofinorganic and organic complexing ligands, and (d) the capacityof secondary minerals formed as a product of weathering to ac-cept REE removed from the primary minerals of the unweatheredparent rock.

The data are not sufficient to explain the behavior of the REE inthe study site. Nevertheless, a possible explanation could be theretention of the LREE by the low-solubility secondary mineralsanglesite, barite and jarosite, which were identified at the studysite, while HREE and MREE are removed from the altered rocks. An-other possible explanation could be the fact that REE are not re-leased uniformly from the parent rock, but individual mineralscould weather at different rates, and these mineral phases couldhave different REE patterns.

In the stream water impacted by acid mine drainage, dissolvedLREE concentrations are lower than MREE (Fig. 7C), which is differ-ent behavior to that reported by other authors in low pH surfacewaters. Borrego et al. (2005) have reported that in low pH

Page 11

Mine-waste rockStream sediment

Mine effluents Surface water impacted by mine effluents

Surface water non impacted by mine effluents

Fig. 7. (A) NASC-normalized patterns for all analyzed samples, (B) NASC-normalized REE patterns for mine-waste rock and mine effluents, (C) NASC-normalized REE patternsfor stream sediments and surface water impacted by mine effluents, (D) REE patterns normalized to mine-waste rock and stream sediment for mine effluents and surfacewater impacted by mine effluents, respectively.

726 F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727

water environments, LREE occur as dissolved species, while HREEand, to a lesser extent, MREE are preferentially adsorbed ontoparticle surfaces, which is explained by the preferential scavengingof Fe-hydrous oxides for HREE. Gammons et al. (2005) reported REEattenuation in acid water environments, due to their adsorptiononto Al-hydrous oxides. Other investigators have documented REEadsorption onto clay minerals (Coppin et al., 2002; Maza-Rodriguezet al., 1992) and reported that HREE were more strongly sorbed ontoclay minerals than LREE.

The low concentrations of dissolved LREE in acid stream waterfrom San Lucia may suggest that LREE can be retained in the solidphases of the sediments more strongly than HREE and MREE. Thereis no evidence in this study for retention of LREE in hydrous oxidesof Fe or hydrous oxides of Al and clay surfaces, which suggests thatLREE sorption on these surfaces is not an important control onaqueous REE mobility at the study site. Thus, one possible explana-tion for the sharp decrease in dissolved LREE might be their reten-tion by the low-solubility secondary minerals anglesite, barite andjarosite which are stable under the dominant acidic conditions. Forexample, SEM-EDS analysis showed small white particles (<5 lm)

dispersed throughout the mine-waste rock which contained Pb–Swith traces of Ce (Fig. 4A).

5. Conclusions

Oxidation of sulfide minerals in the inactive Santa Lucia mine hasgenerated acid mine effluents enriched in dissolved SO2�

4 , Fe and Zn.Transport of fine particles of mine-waste material and acid mineeffluents have contaminated surface waters and stream sedimentsup to 2500 m downstream from the mine site. However, the acideffluents and surface impacted water are characterized by relativelylow dissolved concentrations of toxic elements As, Ba and Pb, whichimplies that these toxic elements have been naturally attenuated.The behavior of PTE and the mineralogical analyses indicate thatthe main mechanism controlling the mobility of As, Ba and Pb isthe precipitation of secondary minerals anglesite, Pb-bearing bariteand beudantite. Although the results are not sufficient to completelyexplain the behavior of the REE, their possible retention by anglesite,barite and jarosite, would allow confirmation that precipitation oflow-solubility secondary minerals is the main control on the mobil-

Page 12

F.M. Romero et al. / Applied Geochemistry 25 (2010) 716–727 727

ity of contaminants. The results suggest that PTE and REE retentionon the surface of Fe-oxyhydroxides is not important in the inactiveZn–Pb mine of Santa Lucía, in western Cuba because of persistentlow pH.

Acknowledgements

We would like to thank to I. Puente Lee (Facultad de Química,UNAM), C. Linares (Instituto de Geofísica, UNAM), T. Pi and O.Zamora (Intituto de Geología, UNAM) for their laboratory assis-tance. This study was supported by Projects IN118709 and CONA-CYT-SEMARNAT-2004-01-350. The authors are indebted to P.L.Verplanck and L.A. Munk for their invaluable suggestions and com-ments that greatly improved an earlier version of this paper.

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