Behavior of mercury in the Valdeazogues riverbank soil and transfer to Nerium oleander L

Journal of Geochemical Exploration 123 (2012) 136–142

Contents lists available at SciVerse ScienceDirect

Journal of Geochemical Exploration

j ourna l homepage: www.e lsev ie r .com/ locate / jgeoexp

Behavior of mercury in the Valdeazogues riverbank soil and transferto Nerium oleander L.

R. Millán ⁎, M.A. Lominchar, I. López-Tejedor, J. Rodríguez-Alonso, T. Schmid, M.J. SierraCIEMAT-Environmental Department, Avenida Complutense 40, E-28040 Madrid, Spain

⁎ Corresponding author. Tel.: +34 913466704; fax: +E-mail address: [email protected] (R. Millán).

0375-6742/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.gexplo.2012.07.002

a b s t r a c t

Article history:Received 21 November 2011Accepted 1 July 2012Available online 10 July 2012

Keywords:MercurySoilHeavy metalsRiverbanksSequential Extraction ProcedureNerium oleander

The Almadén mining district (Spain) is a singular case of natural mercury concentration in the World. The in-tensive mining activities during the last two thousand years have caused a dispersion and remobilization ofmercury within the area. This has generated interactions between soil and mercury as well as the incorpora-tion of this trace element into living organisms of the area. As mercury is a global pollutant, well known for itstoxicological and harmful effects, it is necessary to gather more knowledge about the environmental behaviorof this metal. In this framework, the study is focused on the mercury behavior in soil from the banks of theValdeazogues River, the main water course traversing the Almadén mining district, and to evaluate the mer-cury transfer to Nerium oleander shrubs that grow on the riverbanks.The results show that total Hg concentrations in soil range from 116.7±24.3 to 245.5±59.6 mg kg−1 of Hg,with peak concentrations reaching 350.9±68.6 mg kg−1. However, using a six-step Sequential ExtractionProcedure, it could be determined that the available Hg concentration is a smaller percentage than 0.16%of the total Hg measured in the samples, while the metal is associated with the more resistant soil fractions:crystalline Fe–Mn oxyhydroxides, organic matter absorbed and final residue. Regarding Hg absorption byN. oleander L., results show that the distribution of Hg in the plant is not homogeneous in the aerial part.Values are significantly higher in the leaves, followed by stems and fruits.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Mercury (Hg) is an element considered a global pollutant and oneof the most toxic metals. The highest natural concentration of mercu-ry in the world is located in Almadén, Spain (Hylander and Meili,2003; Saupé, 1990). Mining activities in this area have been reportedover the past two thousand years with different purposes and inten-sity and were continued until the beginning of the 21st century, whenthe mercury mine was closed down (Gray et al., 2004; Newman,2002). Almadén was the most important European mercury mine,followed by further mines located in Idrija (Slovenia) and MonteAmiata (Italy). As a consequence of these intensive mining activities,prolonged dissemination and redistribution of Hg within the sur-rounding areas occurred. Even areas further away from the centersof exploitation were affected by the atmospheric and hydrologicaltransport of Hg (Fitzgerald et al., 1998; Higueras et al., 2003; Hineset al., 2000). The mining district covers an area of approximately300 km2 and includes open pits, underground exploitation, metallur-gical complexes, mineral loading areas and transport systems. There-fore, it comes as no surprise that Hg has been found in all theenvironmental compartments of this mining area.

34 91346269.

rights reserved.

The Almadén area is influenced by a Mediterranean climate withsemiarid conditions. Average yearly temperatures of 14 to 16 °C andrainfall of 550–690 mm are registered mainly between Decemberand March. Summer is characterized as being very dry with tempera-tures reaching 30 °C. The landscape consists of narrow valleys andmountains that range between 200 and 1000 m above sea level. TheAlmadén mining district is geologically dominated by the HercynianCentral Iberian tectonics with a WNW structural trend (Higueraset al., 2003). The predominant lithology is composed of quartzites,quartz arenite rocks and black shales. In this case, cinnabar (HgS)was the main mineral used for primary mercury production in thearea. There are no important aquifers in the area and, regarding su-perficial water, there are two important reservoirs in this area. TheValdeazogues River is the main river that traverses the mining area,and is fed by tributaries that are influenced by contrasting seasonalchanges. This means that the tributaries contribute to the main riveronly during the rainfall season. In summer these tributaries normallydry up and during this process stagnant pools of water are formed.

In order to study the Hg soil-plant transfer in this area, Neriumoleander was selected. This species is an evergreen shrub, commonin the Mediterranean climate, and has been used in numerous inves-tigations to study the distribution of heavy metals in soils from differ-ent countries (Aksoy and Öztürk, 1997; Sawidis et al., 1995). Withinthe study area, this pluriannual species serves as an indicator of theHg accumulation in its aerial part for the past decades. This is the

137R. Millán et al. / Journal of Geochemical Exploration 123 (2012) 136–142

case of selected shrubs that were present when the mine was still inoperation (Millán et al., 2006), and are the most relevant specimentaking into account their abundance and size (up to 5 m high).

This work is focused on the study of the Hg behavior and distribu-tion within a riverbank area. To achieve this general objective, themain physico-chemical soil parameters were analyzed. Furthermore,the distribution of Hg in the different soil fractions, the quantificationof the total and available Hg, and the influence of the organic matter(OM) and the oxidizable fractions were determined. Finally, the ab-sorption and distribution of Hg in N. oleander shrubs were studied.

2. Material and methods

2.1. Study area and sampling procedure

The selected area is situated on the riverbanks of the ValdeazoguesRiver, which flows in an east–west direction through the Almadénmining district. The river meanders past some of the main mercuryextractive sites such as the Entredicho open pit. Furthermore, thesampling area is located a few meters after the confluence with theAzogado Stream that flows close to the Almadén mine and metallur-gical complex, and passes through one of the biggest mine tailingareas. The sampling area is after the confluence of the ValdeazoguesRiver and the Alcudia River, the latter traversing the Alcudia Valleyconsisting of agricultural and farming areas without the mercurymining influence. In addition, the area under investigation is closeto a main road and the abandoned railway station of Chillón thatwas used to load and transport cinnabar ore.

In this scenario, six sampling points were selected in the floodingarea along a 250 m stretch of the river (Fig. 1). At each point, soil fromthe riverbank and the corresponding N. oleander samples were col-lected. This species is common throughout the area and therefore,was used as an indicator of Hg transfer. The sampling area isinfluenced by seasonal variations in the river flow, causing changesin the redox potential and deposition of sediments from the upstreamarea.

Fig. 1. Location of sampling sites. Elaborated using aerial images (IGN, 2007) and a topo

The samples were collected in the months of November of 2007and 2008. Soil samples correspond to the first 15 cm of the soil toplayer under the oleander shrubs. Regarding N. oleander samples, thesame six specimens were sampled during the study period, andleaves, stems and fruits were collected from each specimen. Soil sam-ples were stored in plastic bags, whereas plant samples were kept inpaper envelopes. All samples were transported to the laboratory in acool box. The location of each oleander specimen was registered witha portable Global Positioning System (GPS) in order to perform thesampling in the same location and on the same shrubs.

2.2. Soil and plant analysis

Soil samples were air dried and sieved to obtain the finefractionb2 mm, to be used for standard physical and chemical analy-ses. The soil physical and chemical characterization was carried outmeasuring: pH (H2O, 1:2.5), electrical conductivity (EC), OM usingthe Walkley–Black method and soil texture using Bouyoucos methodaccording to standard procedures (Page et al., 1987). In this case, ECwas carried out in a solution with a ratio soil to water of 1:5. Thiswas then expressed as the conductivity of the saturated paste extract(ECe) by multiplying with a factor of 6.4 (Loveday et al., 1972).

Furthermore, a specific six-step Sequential Extraction Procedure(SEP) developed by Sánchez et al. (2005) was carried out to studythe distribution of Hg in soil samples. Besides Hg, further elementswere measured in the soil extracts obtained from the SEP. Thisincludes concentrations of Al, Ca, Cu, Fe, Mg, Mn, Pb, Zn, K and Nathat were measured using atomic absorption spectroscopy equip-ment (Perkin Elmer 2280).

Plant samples were separated into different fractions (leaves,stems and fruits) and then rinsed several times with distilled waterusing an ultrasonic bath (Ultrasons-H, Selecta) to remove externalcontamination (4 cycles of 10 min) at room temperature. Finally,the samples were placed into individual beakers and dried at roomtemperature till a constant weight was attained.

A fraction of the soil and the plant samples were specifically pre-pared for Hg analysis. Soil and plant samples were homogenized

graphic thematic map from the National Geographic Institute of Spain (IGN, 2011).

138 R. Millán et al. / Journal of Geochemical Exploration 123 (2012) 136–142

and ground with an agate mortar and electric mill, respectively, toobtain a fine particle size of 420 μm.

Hg content in all samples (soil, extracts of the SEP and plants) wasmeasured using an atomic absorption spectrometer designed for Hgdetermination (Advanced Mercury Analyser — AMA 254, LECO Com-pany). This equipment analyzes solid and liquid samples without aneed of a chemical pre-treatment with a detection limit of 0.01 ngof Hg.

Certified reference materials weremeasured to determine the accu-racy and precision of the measurements. These reference materials areSRM1573a (tomato leaves, 0.034±0.004 mg kg−1), BCR151 (skimmedmilk powder, 0.101±0.010 mg kg−1), SRM2709 (San Joaquin agricul-tural soil, 1.40±0.080 mg kg−1) and BCR 150 (skimmedmilk powder,9.4±1.70 μg kg−1). Themean value determined for tenmeasurementsusing the AMA 254 equipment was 0.037±0.001 mg kg−1; 0.098±0.003 mg kg−1; 1.346±0.042 mg kg−1 and 9.5±0.1 μg kg−1, respec-tively. At a 95% confidence level, no significant differences weredetected between the certified value and the experimental one, sothis method was considered to be accurate for total Hg determination.

If the samples contained a high Hg concentration, out of the equip-ment range limit (b600 ng), they were pre-processed by an acidicdigestion following EPA 3052 Method (US EPA, 1996) and performedin a microwave MARS 500 (CEM Corporation). Recovery percentagefor certified reference material (CRM 051) was 99%.

Furthermore, easily available Hg in soils for plants was studied.This was considered as the sum of the soluble and exchangeableforms obtained in the first two steps of the SEP.

2.3. Statistical analysis

Standard descriptive analyses as well as the Kendall's Taub corre-lation tests were applied. Furthermore analysis of variance wasperformed. ANOVA was carried out with the Turkey test when theLevene test assumed homogeneity of variance, or the Welch's testwith the Games–Howell test when the Levene test rejected homoge-neity of variance. A probability of 0.05 or lower was considered as sig-nificant. These analyses were carried out with the statistical packageSPSS for Windows (version 11.5).

3. Results and discussion

3.1. Chemical and physical characteristics of soil

The following characteristics were determined for the soil samples(Table 1). According to Porta et al. (1999), the soils are slightly acidic.The EC has a range of 207 to 954 μS cm−1 and taking into account theECe indicates in certain cases very slightly and slightly saline condi-tions (Schoeneberger et al., 2002). Reasons for this condition are re-lated to wastewater discharge into the Valdeazogues River andagricultural activity that takes place along the tributary Alcudia River.

Table 1Results of soil characteristics from the Valdeazogues study area.

Samplingpoint

Date pH(1:2.5)

EC (1:5)(μS cm−1)

ECe(mS cm−1)

OM(%)

1 Nov-07 5.8 954 6.1 6.12 6.3 431 2.8 8.23 6.2 327 2.1 6.84 6.5 268 1.7 6.75 6.5 508 3.3 6.66 6.2 284 1.8 5.81 Nov-08 6.4 207 1.3 5.42 5.7 631 4.0 7.23 5.5 853 5.5 7.64 5.5 658 4.2 6.45 6.2 315 2.0 6.46 5.8 534 3.4 6.8

OM contents range between 5.4% and 8.2%. In semiarid areas,values of OM above 3% are considered elevated (Consejería deAgricultura and Comercio de Extremadura, 1992). The formation ofOM in this area is favored by wet soil conditions and the presenceof abundant vegetation.

These results were compared with results obtained from other lo-cations within the Almadén area (Millán et al., 2006; Schmid et al.,2005), which include a pastureland area with open Mediterraneanoak forest (Dehesa de Castilseras) as well as an abandoned metallur-gical plant near Almadenejos, east of Almadén. The studied soils fromthe surroundings of the Chillón railway station have a similar pH, buta higher EC and OM content than values obtained for pastureland areawith open forest. Soils from the abandoned metallurgical plant have aslightly higher acidity and EC and lower OM content than Chillón.

3.2. Mercury in soils

3.2.1. Total mercuryTotal Hg concentration (Table 2) values range between 117 and

250 mg kg−1. The exception is the sample from point 6 in 2008,which reaches values of 351 mg kg−1. According to the ANOVA test,no significant differences were found in the Hg concentration be-tween these two years. According to the Welch test for both years,there were no differences among sample points except for samplepoint number 4, which has the lowest Hg concentration.

According to other studies within the same area, Berzas Nevadoet al. (2003, 2009) and Millán et al. (2006) show Hg concentrationin sediments of 107.2±2.72 mg kg−1, 74±4 mg kg−1 and 146±13 mg kg−1 respectively. But, these three values are lower if theyare compared with those recorded by Hildebrand et al. (1980),which measured Hg concentration of up to 1085±681 mg kg−1 inthe same area during 1974–1977. The reasons for the decrease inHg concentration in the last four decades are most likely related tothe Hg treatment installation plant that is operational since 1977,the safe Hg waste storage site, the gradual decrease of mining activi-ties till they ceased in 2004 and the recovery of mine tailings and theEntredicho open pit mine.

Comparing these concentrations with other areas in Almadén, sig-nificant differences can be found. The forestry and farming area ofDehesa de Castilseras shows soil Hg concentration between 14 and21 mg kg−1, whereas the abandoned old metallurgical plant presentssoil Hg concentrations of up to 550±58 mg kg−1 (Millán et al., 2006,2011; Sierra, 2009).

3.3. Soil chemical analysis by Sequential Extraction Procedure

3.3.1. Mercury distributionThe six-step Sequential Extraction Procedure (SEP) developed by

Sánchez et al. (2005) was performed in the soil samples taken in2007 to study the affinity of Hg to different soil components. The pro-cess gave recovery percentages higher than 65% in all samples exceptfor point 2. Therefore, this sample was not included in the results. Theaverage percentages of Hg in each soil fraction are shown in Table 3.

Table 2Total and available mercury concentrations in soils from the Chillón study area.

Samplingpoint

2007 2008

Total Hg(mg kg−1)

Available Hg(mg kg−1)

Total Hg(mg kg−1)

Available Hg(mg kg−1)

1 158.7±56.6 0.039±0.001 211.4±27.3 0.017±0.0002 245.5±59.6 0.105±0.000 167.5±9.9 0.112±0.0153 206.0±25.3 0.130±0.009 155.2±14.2 0.257±0.0154 116.7±24.3 0.020±0.002 119.1±11.9 0.089±0.0055 164.1±41.7 0.055±0.009 147.6±23.8 0.074±0.0096 186.8±35.0 0.045±0.001 351.0±68.6 0.199±0.003

Table 3Percentage concentration of mercury in different SEP fractions for Chillón railway sta-tion, Dehesa de Castilseras (Sánchez et al., 2005; Sierra, 2009) and Almadenejos(Millán et al., 2011).

Fraction Chillón trainstation Hg (%)

Dehesa de CastilserasHg (%)

AlmadenejosHg (%)

Water soluble b0.02 0 b0.2Exchangeable b0.14 b0.4 b0.2Carbonates b0.05 0.1–0.4 b0.2Easily reducible b0.06 b0.1 b0.26 M HCl soluble 4–21 82–90 22–59Oxidizable 15–43 1.0–3.5 5–20Final residue 34–82 9.0–12 31–70

139R. Millán et al. / Journal of Geochemical Exploration 123 (2012) 136–142

According to the obtained results, Hg is mainly associated with thethree last fractions (6 M HCl soluble, oxidizable and final residue).This element is mainly found in the fraction assigned in the final in-soluble residues that correspond to resistant Hg sulfides. This wouldbe related to the loading and transportation of the ore that tookplace at the Chillón railway station. Thereafter, the next highest Hgconcentration is found in the oxidizable fraction that can be associat-ed with OM and traces of elemental Hg. The next major amount of Hgis released with 6 MHCl that indicates the Hg associated to crystallineFe–Mn oxyhydroxides. The remaining fractions (water solublephases, exchangeable fraction, carbonates and easily reducible frac-tion) have a significant very low percentage of Hg compared withthe former three fractions according to the Welch's test. They areless than 0.2% of the total Hg concentration in the soils.

The easily available Hg has been considered the sum of the firsttwo fractions (water-soluble Hg and exchange Hg) in the six-stepSEP and it is the Hg concentration that could be absorbed by theplants and then transfer to the rest of the biosphere. In this case,the easily available Hg concentrations in soils close to theValdeazogues River banks are between 0.04% and 0.16%. This valueagrees with what was found by Millán et al. (2006) in the same loca-tions. Furthermore, this low ratio is supported by studies carried outby Lucena et al. (1992) and Millán et al. (2011) in other Almadénsoils that reported the Hg availability for plants was usually low. Ithas to be taken into account that in these samples, Hg is mainlyfound in the very stable mineral form cinnabar.

Regarding the easily available Hg, the soluble fraction can betransported by leaching and contaminate the groundwater. In thiscase, there is a low rate of soluble Hg and the risk of leaching is alsovery low.

Sánchez et al. (2005) and Sierra (2009) performed this six-stepSEP on agricultural and pastureland soils from Almadén and theyobtained different results except in the first four SEP fractions. Inthese soils, the major amount of Hg was associated with crystallineFe–Mn oxyhydroxides (Table 3). The final insoluble Hg residue variedin the range of 9 to 12% and the amount of Hg associated to the oxi-dizable fraction was the lowest of these three fractions.

Millán et al. (2011) studied the Hg distribution in a soil from theabandoned metallurgic site at Almadenejos, where there was an in-complete mineral roasting process. Therefore, most Hg was associat-ed with resistant HgS as well as Hg associated with metacinnabardue to the roasting process (De la Cruz Gómez, 1995) and accordingto the results obtained with the final residue and 6 M HCl solublefraction, respectively. In the Chillón station, Hg was mainly associatedwith HgS. This last result does not agree with the case of our studyarea where there was no mineral roasting process. However, theamount of Hg associated to the first four SEP fractions is very lowand comparable to the present study area.

The fact that the percentage of Hg in oxidizable fraction of thestudy area was similar to the case of Almadenejos and significantlyhigher than the Dehesa de Castilseras could be related to the OMvalues in the different areas. A linear regression between Hg in the

oxidizable fraction and the amount of OMwas carried out. The resultsobtained, show that there is a significant positive correlation and con-firm the high affinity of Hg with the OM (Biester et al., 1999;Ravichandran, 2004; Warren et al., 1966).

3.3.2. Relation between mercury and other ionsApart from the Hg in each fraction of the soils, the concentration of

Al, Ca, Fe, P, Pb, Zn, K and Na was measured with their correspondingdistribution within the soils (Fig. 2).

Ca is the element with the highest proportion within the threefirst steps of the SEP. This distribution is directly related to the soil li-thology. Furthermore, high levels of Fe in the 6 M HCl soluble fractionare related to the presence of crystalline Fe–Mn oxyhydroxides insoils that play an important role in the adsorption of other elements.The high percentage of Al, K and Na in the final residue indicatesthe resistivity of aluminosilicates when applying the sequentialprocedure.

Although Hg extraction and processing were the main mining ac-tivities in Almadén, it coexisted with the mining of Pb and Zn mines.In this case, Pb is mainly found in carbonate and Fe–Mn oxyhydroxidefractions. This agrees with work carried out by Durn et al. (1999),whose study was developed in a Pb–Ag mining site. However, the ex-changeable and soluble Pb fractions are the lowest in the SEP. Pitchelet al. (2000) also determined low exchangeable Pb concentrations incontaminated soils affected by foundry, battery recycling and second-ary Pb smelting operations.

According to the study of soils affected by mining activities elabo-rated by Ramos et al. (1994), Zn is mainly associated with the crystal-line Fe-oxyhydroxide fraction, but the amount of Zn associated withthe carbonates and amorphous Fe-oxide fractions was also signifi-cant. These findings are in agreement with the study results obtainedin this work.

Comparing the distributions of Hg, Zn and Pb, there are differencesin the way that these metals are associated with each soil fraction. Asmentioned above, Hg is mainly associated with the last three soil frac-tions which are the most resistant, whereas the percentage of availableHg is very low. In contrast, Zn is found in all soil fractions, especiallythose fractions containing carbonates and Fe–Mn oxyhydroxides. Fur-thermore, it is the metal with the highest concentration in availablefractions. Finally Pb, unlike the other two metals, is not associatedwith either soluble fractions and is not present in the final residueeither. This could indicate that the Pb in the study area comes from alocation further upstream and is transported and deposited depen-ding on the river flow. Thus, Pb has preference to be adsorbed inthe intermediate soil fractions and is higher in the crystalline Fe–Mn oxyhydroxides, followed by amorphous oxyhydroxides, and ulti-mately, by carbonates.

Regarding the relationship between Hg and other ions in each SEPfraction, there is a positive correlation between Hg and Pb in the car-bonate fraction. However, there is a negative correlation with Zn andP in the carbonate fraction and easily reducible fraction, respectively.

3.4. Mercury in plant

Total Hg concentration was analyzed in different aerial plant frac-tions of N. oleander. Hg concentration in leaves varied from 0.28 to0.94 mg kg−1, in stems from 0.075 to 0.483 mg kg−1, and in fruitsfrom 0.030 to 0.081 mg kg−1. In addition, seeds were taken fromfruits of plants from sampling points 1, 3 and 6 to test their Hg con-centration. The results varied from 0.025 to 0.059 mg kg−1. The dis-tribution of Hg through the aerial part of the plant is shown inFig. 3. According to the Games Howell test carried out for bothyears, the Hg concentration in the leaves is significantly higher thanin the stems followed by the fruits.

The distribution tendency is in agreement with similar studiesimplemented by Huckabee et al. (1983), who observed in Almadén

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Al Ca Fe P Pb Zn K Na

Final residue

Oxidizable

6M HCl soluble

Easily reducible

Carbonates

Exchangeable

Water soluble

Fig. 2. Average distribution of major, minor, and trace constituents in the different extraction fractions.

140 R. Millán et al. / Journal of Geochemical Exploration 123 (2012) 136–142

oaks (Quercus pyrenaica Willd.) that the highest concentration of Hgwas in the leaves and the stems, while the lowest was in the fruits(acorn). Zornoza et al. (2010) found the same tendency in soil andhydroponic experiments with Lupinus albus. Sierra et al. (2008a)also showed in studies with common vetch (Vicia sativa), that thehighest concentration of Hg was in roots, followed by leaves, stemsand fruits. Analogous results were determined in plant speciesgrown on Almadén soils: Solanum melongena (Sierra et al., 2008b),and Hordeum vulgare (Sierra et al., 2011).

According to the Welch's and the Games–Howell tests, the con-centration of Hg in leaves and stems is significantly higher in 2007than in 2008. However, there are no significant differences betweenthe Hg concentrations in fruits between both years.

In order to determine Hg air pollution deposited on oleanderleaves, washed and unwashed leaves (untreated leaves) were ana-lyzed separately. In both years, Hg concentrations in unwashed leaveswere higher than in washed samples (Fig. 4). However, according tothe Welch test, there were only significant differences in 2008. AsFig. 4 shows, the median and the data distribution considering bothyears separately are similar, thus the influence of Hg deposition inthe oleander aerial part seems to be small.

To evaluate the Hg transfer from soils to N. oleander, bio-accumulation factor (BAF) was calculated taking into account the

Fig. 3. Distribution of Hg in different fractions of the N. oleander aerial part.

Hg concentration in tissues and easily available Hg in soils (Hg in tis-sue/Hg available in soil). The BAF in the leaves is the highest, followedby stems and fruits for both years (Fig. 5).

The average Hg concentration in the oleander aerial fraction, tak-ing into account all samples and fractions is 0.32±0.13 mg kg−1.The result is close to half the Hg concentration found by Millánet al. (2006) in the same study area and sampling the same shrubsin 2002–2003. Nevertheless, this former study shows that the avail-able Hg fraction in soil was close to four times higher than the valuespresented in this work.

4. Conclusions

Total Hg concentration for the riverbank soils presents values be-tween 117 and 350 mg kg−1, which are considered as normal withinthe mining area. However, the available Hg is less than 0.16% of thetotal due to Hg being associated with the crystalline Fe–Mn oxyhy-droxides, absorbed in the OM and the final residue fraction. Further-more, there is a relationship between OM and the concentration ofHg in the exchangeable and the oxidizable fraction.

Comparing the distributions of Hg, Zn and Pb, there are differencesin the manner that these metals are associated with each soil fraction.Hg is mainly associated with the last three soil SEP fractions whichare the most resistant, whereas the percentage of soluble and

Fig. 4. Hg concentration in N. oleander leaves considering previous treatment (washedand unwashed).

Hg

(m

g K

g-1

)Leaf BAFStem BAFFruit BAF

YEARS

Fig. 5. Nerium oleander BAF values (2007 and 2008) for leaves, stems and fruit.

141R. Millán et al. / Journal of Geochemical Exploration 123 (2012) 136–142

available Hg is very low. In contrast, Zn is found in greater abundancein all soil fractions, especially those fractions containing carbonatesand Fe–Mn oxyhydroxides and it is the metal with the highest con-centration in the available fractions. Pb was mainly found in carbon-ate and Fe–Mn oxyhydroxides fraction, and the exchangeable andsoluble Pb fractions give the lowest values.

Total Hg concentration in N. oleander leaves is significantly higherthan in the stems followed by the fruits, for both sampling periods.Furthermore, the concentration of Hg in leaves and stems is signifi-cantly higher in 2007 than in 2008. There are no significant differ-ences between the Hg concentrations in fruits between the twoyears. The average Hg concentration in the oleander aerial fraction,taking into account all samples and fractions is 0.32±0.13 mg kg−1.This value is not particularly high; however, the high biomass pro-duced would mean that a significant amount of Hg could be extractedfrom the soil. The N. oleander bioaccumulation factor (BAF) in theleaves is the highest, followed by stems and fruits for both years.

N. oleander could become a future candidate to be used inphytoremediation technologies. This is favored by a high biomassand as it is a toxic and inedible species, would prevent problems oftransfer through the trophic chain. Therefore, this species couldserve as a phyto-barrier on riverbanks.

Acknowledgments

The authors are grateful to Minas de Almadén y Arrayanes(MAYASA) for their collaboration and support during the work. Fur-thermore, this work is supported by the R&DProgramme of the SpanishMinistry of Science and Innovation for the REUSA (CTM2005-04809-CO2/TECNO) and MERCURIO (CGL2009-13171-C03-02) projects. Authorsappreciated the collaboration of the CIEMAT Research Unit on MassSpectrometry and Geochemical Applications.

References

Aksoy, A., Öztürk, M.A., 1997. Nerium oleander L. as a biomonitor of lead and othermetal pollution in Mediterranean environments. The Science of the Total Environ-ment 205, 145–150.

Berzas Nevado, J.J., García Bermejo, L.F., Rodríguez Martín Doimeadios, R.C., 2003. Dis-tribution of mercury in the aquatic environment at Almadén, Spain. EnvironmentalPollution 122, 261–271.

Berzas Nevado, J.J., Rodríguez Martín-Doimeadios, R.C., Jiménez Moreno, M., 2009.Mercury speciation in the Valdeazogues River-La Serena Reservoir system:influence of Almadén (Spain) historic mining activities. The Science of the TotalEnvironment 407, 2372–2382.

Biester, H., Gosar, M., Müller, G., 1999. Mercury speciation in tailings of the Idrijamercury mine. Journal of Geochemical Exploration 65, 195–204.

Consejería de Agricultura y Comercio de Extremadura. 1992. Interpretación de Análisisde suelo, foliar y agua de riego: Consejo de abonado (normas básicas). Ed. Mundi-Prensa (Madrid). Spain.

De la Cruz Gómez, C., 1995. Doctoral Thesis: Estudio de los fenómenos de transporte enla reacción de tostación del cinabrio en un reactor de lecho fluidizado. EditorialUniversidad de Castilla-La Mancha. 446 pp.

Durn, G., Miko, S., Čović, M., Barudžija, U., Tadej, N., Namjesnik-Dejanović, N., Palinkaš,L., 1999. Distribution and behaviour of selected elements in soil developed over ahistorical Pb–Ag mining site at Sv. Jakob, Croatia. Journal of Geochemical Explora-tion 67, 361–376.

Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., Nater, E.A., 1998. The case for atmosphericmercury contamination in remote areas. Environmental Science & Technology 32, 1–7.

Gray, J.E., Hines, M.E., Higueras, P.L., Adatto, I., Lasorsa, B.I., 2004. Mercury speciationand microbial transformations in mine wastes, stream sediments, and surface wa-ters at the Almadén Mining District, Spain. Environmental Science & Technology38, 4285–4292.

Higueras, P., Oryazum, R., Biester, H., Lillo, J., Lorenzo, S., 2003. A first insight intomercury distribution and speciation in soils from the Almadén Mining District,Spain. Journal of Geochemical Exploration 80, 95–104.

Hildebrand, S.G., Huckabee, J.W., Sanz DÍaz, F., Janzen, S.A., Solomon, J.A., Kumar, K.D.,1980. Distribution of mercury in the environment at Almadén, Spain. Oak RidgeNational Laboratory, Oak Ridge, Tennessee. ORNL/TM-7446.

Hines, M.E., Horvat, M., Faganeli, J., Bonzongo, J.C., Barkay, T., Major, E.B., Scott, K.J.,Bailey, E.A., Warwick, J.J., Lyons, W.B., 2000. Mercury biogeochemistry in the IdrijaRiver, Slovenia, from above the mine into the Gulf of Trieste. EnvironmentalResearch 83, 129–139.

Huckabee, J.W., Sanz, F., Janze, S.A., Salomon, J., 1983. Distribution of mercury in vege-tation at Almadén, Spain. Environmental Pollution 30, 221–224.

Hylander, L.D., Meili, M., 2003. 500 years of mercury production: global annualinventory by region until 2000 and associated emissions. The Science of the TotalEnvironment 304, 13–27.

IGN, 2007. National Geographic Institute of Spain. Plan Nacional de OrtofotográfiaAérea de España. Cartografía del Instituto Geográfico Nacional.

IGN, 2011. National Geographic Institute of SpainAccessed October 31, 2011 at http://www2.ign.es/iberpix/visoriberpix/visorign.html, 2011.

Loveday, J., Beatty, H.J., Norris, J.M., 1972. Comparison of currents methods for evaluat-ing irrigations soils. CSIRO Division of Soils Tech. Bulletin 14, Canberra.

Lucena, J.J., Hernández, L.E., Olmos, S., Carpena, R., 1992. Valoración de métodos deextracción de mercurio en suelos contaminados. Suelo y Planta 2, 747–755.

Millán, R., Gamarra, R., Schmid, T., Sierra, M.J., Quejido, A.J., Sánchez, D.M., Cardona, A.I.,Fernández, M., Vera, R., 2006. Mercury content in vegetation and soils of theAlmadén mining area (Spain). The Science of the Total Environment 368, 79–87.

Millán, R., Schmid, T., Sierra, M.J., Carrasco-Gil, S., Villadóniga, M., Rico, C., SánchezLedesma, D.M., Díaz Puente, F.J., 2011. Spatial variation of biological and pedolog-ical properties in an area affected by a metallurgical mercury plant: Almadenejos(Spain). Applied Geochemistry 26, 174–181.

Newman, H.R., 2002. The mineral industry of Spain in 2002. U.S. Geological SurveyMinerals Yearbook. : Area Reports: International, vol. III.

Page, A.L., Miller, R.H., Heeney, D.R., 1987. Methods of soil analysis, Part 2. Chemicaland Microbiological Properties2nd edition. American Society of Agronomy, IncSoil Science Society of America, Inc, Madison, Wisconsin, USA.

Pitchel, J., Kuroiwa, K., Sawyerr, H.T., 2000. Distribution of Pb, Cd, and Ba in soils andplants of two contaminated sites. Environmental Pollution 110, 171–178.

Porta, J., López-Acevedo, M., Roquero, C., 1999. In: Mundi-Prensa (Ed.), Edafología parala agricultura y el medio ambiente. Bilbao (Spain). 849 pp.

Ramos, L., Hernandez, L.M., Gonzalez, M.J., 1994. Sequential fractionation of copper,lead, cadmium and zinc in soils from or near Doñana National Park. Journal ofEnvironmental Quality 23, 50–57.

Ravichandran, M., 2004. Interactions between mercury and dissolved organic matter —a review. Chemosphere 55, 319–331.

Sánchez, D.M., Quejido, A.J., Fernández, M., Hernández, C., Schmid, T., Millán, R.,González, M., Aldea, M., Martín, R., Morante, R., 2005. Mercury and trace elementfractionation in Almadén soils by application of different sequential extractionprocedures. Analytical and Bioanalytical Chemistry 381, 1507–1513.

Saupé, F., 1990. The geology of the Almadén mercury deposit. Economic Geology 85,482–510.

Sawidis, T., Marnasidis, A., Zachariadis, G., Stratis, J.A., 1995. A study of fair pollutionwith heavy-metals in Thessaloniki city (Greece) using as biological indicators.Archives of Environmental Contaminated Toxicology 28, 118–124.

Schmid, T., Millán, R., Sánchez, D., Quejido, A., Fernández, M., Sierra, M.J., Pirotta, I.,Vera, R., 2005. Mining influences on soils in the district of Almadén (Spain).Abstract in Proceedings CD of the 9th International Conference on ContaminatedSoil — ConSoil 2005. Bordeaux (France). 3–5 October 2005.

Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., Broderson, W.D., 2002. Field book fordescribing and sampling soils, version 2.0. Natural Resources Conservation Service.National Soil Survey Center, Lincoln, NE.

Sierra, M.J., 2009. In: CIEMAT (Ed.), Alternativas de uso agrícola para suelos con altoscontenidos de mercurio: aplicación a la comarca minera de Almadén. Madrid(Spain). 300 pp.

Sierra, M.J., Millán, R., Esteban, E., Cardona, A.I., Schmid, T., 2008a. Evaluation ofmercury uptake and distribution in Vicia sativa L. applying two different studyscales: greenhouse conditions and lysimeters experiments. Journal of GeochemicalExploration 96, 203–209.

Sierra, M.J., Millán, R., Esteban, E., 2008b. Potential use of Solanum melongena inagricultural areas with high mercury background concentrations. Food andChemical Toxicology 46 (6), 2143–2149.

142 R. Millán et al. / Journal of Geochemical Exploration 123 (2012) 136–142

Sierra, M.J., Millán, R., Cardona, A.I., Schmid, T., 2011. Potential cultivation of Hordeumvulgare L. in soils with high mercury background concentrations. InternationalJournal of Phytoremediation 13, 765–778.

US EPA, 1996. Method 3052. Microwave Assisted Acid Digestion of Siliceous andOrganically Based Matrices. US EPA Office of Solid Waste, Washington D.C.

Warren, H.V., Delavault, R.E., Barakso, J., 1966. Some observations on the geochemistryof mercury as applied to prospecting. Economic Geology 61, 1010–1028.

Zornoza, P., Millán, R., Sierra, M.J., Seco, A., Esteban, E., 2010. Efficiency of white lupin inthe removal of mercury from contaminated soils: soil and hydroponic experi-ments. Journal of Environmental Science 22 (3), 421–427.