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Chemie der Erde

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Factors controlling the heterogeneous distribution of Cr(VI) in soil, plants andgroundwater: Evidence from the Assopos basin, Greece

Maria Economou-Eliopoulos ∗, Ifigenia Megremi, Charalampos VasilatosDepartment of Geology & Geoenvironment, Section of Economic Geology & Geochemistry, University of Athens, Panepistimiopolis, 15784 Athens, Greece

a r t i c l e i n f o

Article history:Received 16 October 2009Accepted 5 January 2011

Keywords:AssoposGreeceGroundwaterSoilPlantsContaminationHexavalent chromium

a b s t r a c t

The effective influence of industry or ultramafic rocks by natural processes to soil, plants and groundwatercontamination by chromium, which is often a subject of debate, was investigated for the case of theAssopos basin, Greece. The Neogene Assopos basin, is mainly composed by Tertiary and Quaternarysediments of more than 400 m thick and is characterized by brittle type deformation (fault zones, faults).Chromium in soil, ranging from 67 to 204 ppm, is mostly hosted in chromite, Fe-chromite, Cr-bearinggoethite and silicates.

Special attention was given to the plants, which are a major source of organic matter that serves asthe driving force for Cr(VI) reduction. The increase of the Fe, Mn and Ni contents, with the increasingCr content in the plant-roots, in particular at the external parts of roots and those of bulb-type plants,suggest reduction and immobilization of Cr(VI) and that redox reactions play a significant role to thetranslocation processes from root to shoot.

Groundwater samples from the Assopos aquifer showed a wide spatial variability, ranging from <2 to180 ppb Crtotal content [almost same to the Cr(VI)-values] despite their spatial association. The presenceof Cr(VI)-contaminated groundwater at depths >200 m is attributed to a direct injection of Cr(VI)-richindustrial wastes at depth rather than that Cr(VI) is derived from the Assopos river or by the interactionbetween water and Cr-bearing rocks. The heterogeneous distribution of Cr in groundwater may be relatedwith the intense neotectonic deformation, as is exemplified by several sharp tectonic contacts betweensediment types, while the Cr content in soil is mostly depend on the transported chromite grains.

© 2011 Elsevier GmbH. All rights reserved.

1. Introduction

The environmental impact of chromium, sources of chromiumpollution, oxicological/health effects and remediation are inter-est topics for many researchers. The behaviour of the chromiumin environmental systems depends on the valance or oxidationstate, essentially the electrostatic charge on the atoms. In nature,chromium generally carries a charge of +3 or +6, correspondingto the common names trivalent [Cr(III)] and hexavalent [Cr(VI)]chromium, respectively. However, Cr(VI) occurs as oxyanions,HCrO4

− (bichromate ion), CrO42− (chromate ion), and Cr2O7

2−

(dichromate ion). At low pH Cr(VI) exists predominantly as HCrO4−

and at high pH CrO42− remains in solution (Reddy et al., 1997).

Cr(III) is an essential element in human and animal physiologysince it plays a role in glucose and lipid metabolism. A commonform of Cr(VI) is a known mobile contaminant entering the cells,extremely toxic, carcinogen, in particular in large doses. It is highlytoxic to plants as well and is detrimental to their growth and

∗ Corresponding author.E-mail address: econom@geol.uoa.gr (M. Economou-Eliopoulos).

development (Anderson, 1989; Yassi and Nieboer, 1988). Thus,besides total element determination the speciation analysis (oxi-dation state) is of particular significance due to the diverse natureof biological interactions like toxicity, mobility, bio-availability,and bio-accumulation. Furthermore, Shanker et al. (2009) havefocussed on the recent developments in the field of Cr–plant inter-actions and proposed a model for the Cr–plant interaction, sincethey have important consequences on the bioavailability and haz-ard of Cr.

Hexavalent chromium in natural environment usually is largelyderived from Cr(VI)-bearing wastes derived from the extensiveindustrial usage of chromium. In addition to the Cr(VI)-related toanthropogenic activities an other source of aqueous Cr(VI) val-ues exceeding the World Health Organization’s limit for totalchromium (50 ppb or �g/l) in water may be derived by natu-ral processes from mafic–ultramafic complexes. Mafic–ultramaficcomplexes hosting Cr mainly in chromite and silicates (olivine,pyroxenes or serpentine) are widespread. Well known exampleshave been reported in New Caledonia, California, Italy, and Mexico(Golightly, 1981; Palmer and PuIs, 1994; Blowes et al., 1997;Blowes, 2002; Fantoni et al., 2002; Becquer et al., 2003; Oze et al.,2007).

0009-2819/$ – see front matter © 2011 Elsevier GmbH. All rights reserved.doi:10.1016/j.chemer.2011.01.001

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40 M. Economou-Eliopoulos et al. / Chemie der Erde 71 (2011) 39–52

Fig. 1. (a) Sketch map showing part of the geotectonic zones of Greece, the distri-bution of ophiolites and the location of the Assopos basin, (b). schematic sectionof the Assopos basin. The Neogene Assopos basin is mainly composed of Tertiaryand Quaternary sediments. The uppermost horizons, dominated in the studied area,are continental sediments consisting of conglomerates with small intercalations ofmarl, marly limestone, metaclastic schist, sandstone, clays and flysch. At the lowestparts there are alternations of lake yellow marls and marl limestones hosting smallblack lignite horizons. Coordinates and the working area (Thiva, Oinofita, Oropos,Avlona) are better explained in the Fig. 4.

Since 1969 the Assopos River rising from Viotia region and dis-charging into the South Euboic Gulf (Fig. 1) has been proclaimed a“processed industrial waste receiver” Although the control of thechromium content in the industrial wastes is a requirement theAssopos basin (Thiva, Oinofita, Oropos, Avlona) was known recentlyfrom the impact of groundwater by Cr(VI), due to intensive indus-trial activity at this area, such as plating metals, leather processing,stainless steel, Cr-bearing alloys resistant to corrosion and oxida-tion, which may be discharged into the environment.

The objective of the present study is the assessment of the pol-lution of the soil, plant and groundwater and the determination ofCr-hosts in the Assopos basin, Greece. Special attention was given tothe plants, which are a major source, along with micro-organisms,for organic matter that serves as the driving force for Cr(VI) reduc-tion, and to assess the bio-accumulation of Cr by different plantspecies grown on potentially contaminated soils of Assopos basin.The presented analytical data are compiled with previously pub-lished data in an attempt to define the source/es, processes ofchromium accumulation (the effective influence of industry vs.natural process) and interpret the heterogeneous distribution ofchromium in the Assopos basin.

1.1. Geotectonic outline of the Assopos valley

The Neogene Assopos basin, crossed by the Assopos river, ismainly composed of Tertiary and Quaternary sediments, of morethan 400 m thickness and an expansion of approximately 700 km2.The actual morphotectonic structure and evolution of this basinare the result of several normal faults striking E–W to WNW–ESE,covering a zone of approximatelly 500 m, during a long time period(Pliocene to present time). The deformation of the whole region canbe distinguished to the alpine one characterized by folds, reversefaults and thrust faults, and the post-alpine or neotectonic one,characterized by brittle type of deformation (fault zones, faults),which has affected both alpine and post-alpine formations. Amongpost alpine basins, due to neotectonic deformation is the large basin

of Assopos extenting to the Euboic Gulf (Fig. 1) and several smallerbasins (Papanikolaou et al., 1988; Chatoupis and Fountoulis, 2004).

The Assopos basin is composed by lake-shallow marine sed-iments and continental clastic formations. More specifically, atthe lowest parts there are alternations of lake yellow marls andmarl limestones hosting small black lignite horizons. In contrast,the uppermost horizons are continental sediments consisting ofconglomerates (pebbles derived from carbonates, basic and ultrabasic rocks) with small intercalations of marl, marly limestone,metaclastic schist, sandstone, clays and flysch, suggesting changesduring sedimentations. This sequence, overlying small peridotitemasses and Triassic metamorphic basement (Fig. 1), is dominatedby conglomerates in the region of Oropos–Sykamino–Chalkoutsi.In addition, due to the intense neotectonic deformation, whichhas been superimposed onto primary sedimentary features andthe complicate tectonic structure of the Assopos basin is a char-acteristic feature of the above region, as is exemplified by severalsharp tectonic contacts between sediment types (Chatoupis andFountoulis, 2004).

Peridotites overthrusted on the Triassic–Jurassic carbonateswith a maximum visible thickness 200 m, are dismembered partsof ophiolite complexes dominated towards central and northernparts of Greece. They are highly melt-depleted mantle harzbur-gite, and to a lesser extent lherzolite, dunite and crustal magmaticrocks, which are, parent rocks of Ni-laterite deposis (Valeton et al.,1987; Eliopoulos and Economou-Eliopoulos, 2000). Although ophi-olitic rocks are rare throughout the Assopos basin the presence ofchromite, Cr-bearing goethite, and silicates in soils derived fromultramafic rocks cannot be precluded.

2. Samples and methods of investigation

The soil samples covering some cultivated sites of the basin werecollected from the rhizosphere of plants using a plastic spatula wasand they stored in plastic sample bags. They were air dried, crum-bled mechanically and then passed through a sieve with a 2 mmmesh. Samples containing large stones or clods were first sieved ona 10 mm mesh and then a 5 mm mesh. Then they passage through a2 mm mesh, and grains finer than 2 mm were pulverized and usedfor analysis. The elements Al, B, Ba, Cu, Fe, K, Li, Mn, Na, Ni, P, S, Se,Si, V and Zn, were analyzed by Inductively Coupled Plasma MassSpectroscopy (ICP/MS) after Aqua Regia Digestion. The detectionlimit of the method was 0.1 ppm for the elements Cu, Pb, Ni, Co, Cd,Sb and Bi, 0.5 ppm for As, 1 ppm for Cr, Mn, Zn, Ba, La and Sr, 2 ppmfor V, 0.01 wt% for Fe, Ca, Mg, Al and K, 0.05 wt% for S, 0.001 wt% forP, Ti and Na. The results of standard (STD) and black (BLK) analysisare given at the end of the Table 1.

Also, plant samples (n = 20) were analyzed after cleaning anddrying at 70 ◦C. They powdered in an agate mortar and ana-lyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP/MS),after Aqua Regia Digestion, at the ACME Analytical Laboratoriesin Canada. The detection limit of the method was 1 ppm for Mn,0.1 ppm for the elements Cr, Zn, As and Ba, 0.01 ppm for Cu, Pb, Coand Cd, 0.01 wt% for Ca, Al, K and S, 0.001 wt% for P, Na, Mg and Fe.Reference material and Black (BLK) results are given at the end ofTable 1. Microsoft Office XP professional was used for statistics.

The mineralogical composition of soil was investigated by opti-cal microscopy, X-ray diffraction (Siemens D5005 power diffractmeter) and phase mineral analysis. XRD data were obtained using aSiemens Model 5005 X-ray diffractometer, Cu Ka radiation at 40 kV,40 nA, 0.020◦ step size and 1.0 s step time. The XRD patterns wereevaluated using the EVA 2.2 program of the Siemens DIFFRAC andthe D5005 software package.

Polished sections prepared from soil, after carbon coating wereexamined by reflected light microscopy and scanning electron

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M. Economou-Eliopoulos et al. / Chemie der Erde 71 (2011) 39–52 41

Table 1Concentration of major and trace elements in soils from the Assopos basin and other areas.

Location/sample Concentration (ppm)

Cr Mn Cu Pb Zn Ni Co As Ba Sr Cd Sb

Assopos basinGM1s 200 910 28 15 60 470 39 10 81 69 0.3 0.2GM2s 190 860 28 13 62 480 36 8 70 73 0.3 0.1GM3s 200 920 47 13 60 450 39 9 78 73 0.4 0.2KT1s 150 920 50 19 71 320 31 16 89 104 0.3 0.3KT2s 130 860 36 18 70 300 28 15 73 108 0.2 0.3LGMs 190 860 26 14 57 470 36 9 77 71 0.4 0.1PKTs 130 810 37 19 76 260 29 16 84 125 0.4 0.4PEP1s 67 690 28 16 66 130 19 17 54 98 0.2 0.5PGMs 180 880 40 28 115 470 37 9 79 79 0.4 0.2KA1s 140 680 41 22 58 340 30 7 61 80 0.3 0.3KA2s 140 670 30 12 47 330 29 7 51 63 0.2 0.2KTs 130 800 41 20 93 260 30 16 81 118 0.3 0.4MRSs 89 660 24 18 49 190 21 14 57 31 0.2 0.4PRK 130 830 31 18 65 290 28 16 76 103 0.3 0.3Outside of the Assopos basinHollargosX1-s 110 1180 48 62 100 130 28 25 140 79 0.5 0.9X2.1-s 89 950 30 76 130 137 23 21 94 18 0.8 1.4MesseniaMVAT2.s 30 116 10 9.4 16 17 7 6 18 9 <0.1 0.1MVAT3.s 40 100 12 10 14 20 8 7 25 13 <0.1 <0.1VourinosV.Wh.s 250 850 22 17 54 633 51 6 100 17 0.2 <0.1V.V1.s 180 760 9 2.5 27 1890 88 7 15 15 <0.1 <0.1V.V2.s 370 1300 13 3.6 38 2380 150 3 35 19 <0.1 <0.1HalkisHA.1.s 330 800 22 15 48 530 36 7 101 26 0.3 <0.1HA.2.s 805 1020 39 16 60 1850 96 4 31 34 0.3 <0.1Euboeaa

(average, n = 16) 1300 1300 28 35 50 2800 150 4 50 50 0.3 0.4Reference materialSTD 192 624 107 71 390 55 10 52 395 65 6.5 5.5BLK <1 <1 <0.1 <0.1 <1 <0.1 <0.1 <0.5 <1 <1 <0.1 <0.1

ppm wt%

Sample V La Fe Ca Mg P Ti Al Na K S Corganic

AssoposGM1s 44 11 2.96 7.74 2 0.08 0.02 1.67 0.01 0.29 0.06 0.86GM2s 42 11 2.81 6.85 1.89 0.08 0.01 1.39 0.01 0.27 <0.05 0.82GM3s 44 10 2.82 7.68 1.89 0.12 0.02 1.59 0.01 0.24 0.06 0.97KT1s 38 10 3.31 7.21 1.74 0.1 0.01 1.8 0.04 0.32 0.06 1.04KT2s 37 9 2.96 7.06 1.7 0.09 0.01 1.57 0.01 0.25 <0.05 1.22LGMs 45 11 2.9 7.28 2.05 0.06 0.02 1.62 0.01 0.26 <0.05 0.8PKTs 34 10 3.02 7.38 1.62 0.21 0.01 1.84 0.01 0.3 <0.05 1.37PEP1s 23 8 2.69 5.6 1.14 0.08 0.01 1.25 0.01 0.17 <0.05 0.7PGMs 41 10 2.84 8.71 1.68 0.14 0.01 1.51 0.01 0.33 <0.05 1.16KA1s 31 7 2.19 7.19 1.7 0.22 0.01 1.04 0.01 0.4 <0.05 1.4KA2s 29 7 2.1 6.84 1.61 0.08 0.01 0.99 0.01 0.25 0.05 1.9KTs 34 9 3.05 7.2 1.64 0.08 0.01 1.79 0.03 0.27 <0.05 1.2MRSs 31 9 2.39 6.14 0.83 0.05 0.01 1.2 0.02 0.28 <0.05 1.15PRK 33 9 2.92 7.01 1.61 0.09 0.01 1.45 0.03 0.26 <0.05 1.2Outside of the Assopos basinHolargosX1-s 53 16 3.21 11.03 1.06 0.1 0.02 2.3 0.06 0.67 0.07X2.1-s 48 15 3.21 2.2 0.54 0.06 0.02 1.62 0.01 0.28 0.08MesseniaMVAT2.s. 27 9 1.66 0.2 0.08 0.03 0.01 0.92 0.01 0.11 <0.05MVAT3.s. 30 8 1.72 0.3 0.1 0.03 0.02 1.04 0.01 0.12 0.05VourinosV.Wh.s 49 18 4.08 1.16 0.77 0.06 0.02 2.39 0.01 0.27 <0.05V.V1.s 12 2 4.14 5.39 11.08 0.01 0.01 0.52 0.01 0.08 <0.05V.V2s 21 3 6.04 1.54 11.67 0.02 0.01 0.55 0.01 0.17 <0.05HalkisHA.1.s 58 15 3.56 3.08 1.87 0.04 0.03 2.51 0.03 0.33 <0.05HA.2.s 37 6 5.78 2.1 8.13 0.07 0.02 1.25 0.02 0.37 <0.05Euboeaa

(average, n = 16) 50 7 7.95 7.18 4.1 0.06 0.04 1.33 0.01 0.18 <0.05 0.92Reference materialSTD 85 10 2.33 0.88 0.98 0.08 0.11 0.97 0.101 0.45 0.19BLK <2 <1 <0.01 <0.01 <0.01 <0.001 <0.001 <0.01 <0.001 <0.01 <0.05

Symbols: STD and BLK are standard and black, respectively used by the analytical Laboratory of ACME.a Afrer Megremi (2010).

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42 M. Economou-Eliopoulos et al. / Chemie der Erde 71 (2011) 39–52

Fig. 2. Backscatter SEM images from soil of the Assopos basin containing rounded and angular fragments of goethite (goeth) with alumino-silicate and oxide intergrowths (aand b) hosting chromite (chr), REE-minerals (REEm) and quartz (Qtz), isolated chromite grains or fragments (a–d and f) and serpentine (e). Mineral compositions are givenin the Tables 3 and 4).

microscope (SEM) and Energy Dispersive Spectroscopy (EDS) anal-ysis. Microprobe analyses and SEM images were carried out atthe University of Athens, Department of Geology and Geoenvi-ronmemt, using a JEOL JSM 5600 scanning electron microscope,equipped with automated energy dispersive analysis system ISIS300 OXFORD, with the following operating conditions: accelerat-ing voltage 20 kV, beam current 0.5 nA, time of measurement 50sec and beam diameter 1–2 �m. The spectra were processed usingthe ZAF program (3 iteration).

Groundwater samples were collected from domestic and irri-gation wells and municipality of the Assopos basin. Hexavalentchromium determined colorimetrically within 24 h following the1,5-diphenylcarbohydrazide method, using a HACH DR/4000 spec-trophotometer (American Public Health Association, 1989). Theconcentration of Cr(VI) in a few samples was checked after a fewdays and no difference was observed. The estimated detection limitof the method was determined at 4 (g/L. The physical and chemi-cal parameters of the water samples [pH, Eh, CND (conductivity),

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TDS (total dissolved solids)] were measured using a portable Con-sort 561 Multiparameter Analyzer. The analyses of total chromiumwere performed by GFAAS (Perkin Elmer 1100B) and the estimateddetection limit of the method was determined at 1 (g/L. Other ele-ments (Al, As, B, Ba, Cu, Fe, K, Li, Mn, Na, Ni, P, S, Se, Si, V and Zn),were analyzed, in the acidified portion of the samples, by Induc-tively Coupled Plasma Mass Spectroscopy (ICP/MS) at the ACMEAnalytical Laboratories in Canada.

The organic matter in the soil samples was determined by mea-suring organic carbon, following a wet oxidation method (Walkleyand Black, 1934), using 1 N K2Cr2O7 solution. The heat gener-ated when two volumes of H2SO4 are mixed with one volumeof the dichromate assisting the reaction. The remaining dichro-mate was titrated with ferrous sulphate (at the University ofAthens).

3. Results

3.1. Chromium content and Cr-host minerals in soils

The soil samples, of alluvial type, were collected from the rhizo-sphere of plants, from a maximum depth of approximately 20 cm(horizon B) and grains finer than 2 mm were used for analysis, inorder to investigate probable relationship between metal contentsin plants and soil chemistry. They are mostly composed of roundedfragments of quartz, calcite, serpentinite [Mg3 Si2O5(OH)4], chlorite[(Mg,Al,Fe)12(Si,Al)8O20(OH)16] as well as chromite [(Mg, Fe2+)(Cr,Al, Fe3+)2O4], Fe-chromite, magnetite [Fe2+)(Fe3+)2O4], ilmenite(FeTiO3), rutile (TiO2), zircon (ZrSiO4) and epigenetic rare earthelement carbonates. Micro-organisms, with various morpholog-ical forms, mostly as filament-like and occasionally spherical tolenticular bacteria are common. A salient feature is the presence ofrounded fragments of goethite containing fragments of chromite,with a wide compositional variation, and rare earth element min-erals (phosphates), reflecting a contribution from mafic-ultramaficrocks and multistage weathering and transportation (Fig. 2a andb). The concentration of chromium and other trace elements in soilof the Assopos basin exhibits a relatively small variation (Table 1)compared to soils located in the vicinity of ultramafic rocks, Ni–Fe-laterite deposits, mining excavations or ore beneficiation units, likethose in the Euboea island (Megremi, 2010).

A portion of the chromium in soils from the Assopos basin ishosted in chromite grains or fragments, Cr-bearing goethite andsilicates transported as residual component inherited from theophiolitic parent rocks and Ni-laterite deposits, as well as sec-ondary minerals formed by epigenetic processes. It is in a goodagreement with the good positive correlations of Cr to Ni, Co, Mn,V and Ti (Table 2). There is a wide variation in the compositionof chromite, the Cr# [Cr/(Cr + Al)] ratio ranging from 0.51 to 0.8(Table 3; Fig. 2) throughout the Assopos basin, which is comparableto that of the chromite in both ophiolite complexes and Ni-lateritedeposits of Lokris (Valeton et al., 1987; Eliopoulos and Economou-Eliopoulos, 2000). Also, due to alteration processes, by Al and Mgloss to and Fe gained from silicates, an increase of the Cr# anddecrease of the Mg# ratios results in a wide compositional variation(Fig. 3). Also, Cr-magnetite grains containing up to 6.64 wt% TiO2,reflecting the participation of mafic members of ophiolite com-plexes are occasionally present. Fragments of ultramafic rocks aremostly composed by serpentine appearing a difference in the colourand composition from core outward. (Fig. 2e; Table 3). Serpentinewith the deeper colour (srp2) is iron depleted compared to the core(srp1) (Table 4). Sulphur content in soil samples ranges from 600to less than 500 ppm, while organic carbon ranges between 0.7 and1.9 wt% (average 1.1). Ta

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1

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Table 3Representative microprobe analyses (wt%) of oxides from soils of the Assopos basin.

Sample MRs

Mineral chr chr chr chr Fe-chr Fe-chr Cr-mt Cr-mt Fe-chr Cr-mtAssociated spt spt chl spt spt spt spt chl spt cal

Cr2O3 44.52 40.67 44.57 59.27 36.37 24.27 1.27 41.19 51.97 n.d.Al2O3 24.37 24.57 25.6 9.98 1.82 3.21 0.38 16.41 1.81 1.81Fe2O3 1.29 2.48 0.01 1.36 34.65 45.52 69.8 9.91 15.75 64.9FeO 16.87 24.58 16.51 21.31 17.54 11.36 21.86 28.23 23.99 31.05MgO 12.29 7.34 12.64 8.06 9.25 12.83 5.97 4.07 4.92 n.d.TiO2 n.d. 0.61 n.d. n.d. n.d. n.d. n.d. n.d. 1.21 1.28MnO n.d. n.d. n.d. n.d. n.d. 2.57 n.d. n.d. n.d. n.d.NiO n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Total 99.34 100.3 99.33 99.74 99.63 99.76 99.28 99.81 99.65 99.04

Cr/(Cr + Al) 0.55 0.53 0.54 0.8 0.93 0.83 0.68 0.63 0.95Mg/(Mg + Fe2+) 0.56 0.35 0.58 0.4 0.48 0.67 0.33 0.2 0.29

Sample GM3s KA1s PEPs

Mineral chr chr chr chr Cr-mt Cr-mt Ti-mt chr chr mt

Associated cal spt spt chl cal spt cal cal cal cal

Cr2O3 41.88 43.61 46.58 43.59 19.16 1.78 22.85 41.61 29.41 n.d.Al2O3 25.03 26.37 23.08 25.62 0.36 n.d. 11.03 25.16 38.18 0.73Fe2O3 0.49 0.11 0.64 0.94 53.36 68.11 22.14 0.11 2.59 68.85FeO 24.84 17.55 16.89 17.16 18.05 27.95 28.14 25.84 12.89 26.81MgO 7.19 12.17 10.72 12.31 n.d. 2.02 7.81 6.64 16.32 1.62TiO2 0.42 n.d. n.d. 0.24 n.d. n.d. 6.64 0.51 n.d. n.d.MnO n.d n.d. 1.11 n.d. 1.35 n.d. n.d. n.d. n.d. n.d.NiO n.d n.d. n.d. n.d. 6.74 n.d. n.d. n.d. n.d. 2.08

Total 99.85 99.81 99.02 99.86 99.02 99.86 98.61 99.87 99.39 100.09

Cr/(Cr + Al) 0.51 0.53 0.57 0.53 0.52 0.35Mg/(Mg + Fe2+) 0.34 0.55 0.53 0.56 0.32 0.69

Abbreviations: spt = serpentine; chl = chlorite; cal = calcite; chr = chromite; mt = magnetite; n.d. = below detection limit.

Fig. 3. Compositional fields showing the variation of Cr# (Cr/(Cr + Al) vs. Mg#(Mg/(Mg + Fe2+). (a) In the studied soils from the Assopos basin and (b) chromitefrom ophiolite complexes and Ni-laterite deposits of Lokris (after Eliopoulos andEconomou-Eliopoulos, 2000).

3.2. Variation of the chromium content in groundwater

More than 100 groundwater samples coming from deep(180–200 m) domestic and irrigation wells throughout the Asso-pos basin (Fig. 4) exhibit a wide variation in the Cr andother element concentrations (Fig. 4), although they are locatedin a similar aquifer horizon composed by continental sedi-ments consisting of conglomerates with small intercalations

of marl, marly limestone, metaclastic schist, sandstone, claysand flysch. The total chromium concentration in aquifer ofthe Assopos basin varies widely, ranging from <2 to 180 ppbor �g/l. Certain groundwater samples were analyzed severaltimes during the 2007–2010 period in order to define thespatial and temporal variability of chromium contamination(Fig. 6).

The groundwater samples on the basis of their compositioncan be classified into one group with Crtotal concentration lowerthan 2 ppb, a second group with Crtotal concentration lower than50 ppb, and a third one with Crtotal concentrations higher than50 ppb, although any spatial association is not obvious (Fig. 4).The range of pH (7.2–7.8) and Eh (−0.011 to −0.061 V) measure-ments of the groundwater indicate slightly alkaline and oxidizingconditions. Also, a discrimination between water samples basedon the differences of their chemical composition has been pre-sented by the triangular plots of Na–Ca–Cr(VI) and Na–Mg–Cr(VI)(Vasilatos et al., 2008). Besides the water subsaline due to sea-water supply to the aquifer near the coastline the natural watersfrom the Assopos basin exhibit relative low Na, K and B valuesand show a significant variation in the Ca, Mg and Si (Vasilatoset al., 2008). There is a clear difference concerning the Ca/Mgratio in the Mavrosouvala karst type groundwater (∼5), while thatof groundwater throughout the Assopos basin this ratio is lowerthan 1, the lowest value (0.4) being at the area of Thiva (Fig. 4;Vasilatos et al., 2008; Gannoulopoulos, 2009). Since Cr(VI) val-ues are commonly much closer to the total (r = 0.98) the Cr totalwas used. The Mavrosouvala karst-type aquifer, belonging to thegroup of groundwater with <2 ppb Cr, show the lowest Si concen-tration (average 5 ppm). However, there are several groundwaterwell with Si ranging between 10 and 25 ppm and <2 ppb Cr. Also,the variation of the Cr concentration in more than 100 ground-

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M. Economou-Eliopoulos et al. / Chemie der Erde 71 (2011) 39–52 45

Fig. 4. Location of the groundwater boreholes and distribution of the Crtotal concentration (in ppb) throughout the Assopos basin. Data: present study, Vasilatos et al. (2008),and Giannoulopoulos (2009). The groundwater samples were classified into tree groups: (a) lower than 2 ppb Crtotal, (b) lower than 50 ppb Cr total, and (c) higher than 50 ppbCrtotal.

water samples, in an increasing order of the Cr concentration(ppb) is not accompanied by increasing Ca, Mg or Si concentrations(Fig. 5).

The temporal variability of the Cr(VI) concentration, sinceSeptember 2007 in groundwater wells from Municipality urbanwater supply (Sarandari, ChryssopygiI and ChyssopygiII) and irri-gation wells is given in the Fig. 6. It is obvious that thereis a difference in the Cr concentration between wet and dryperiod, during a year time. The maximum Cr concentrations ingroundwater wells throughout the Oropos area was observed dur-ing September–October (dry period) while during February (wetperiod) the lowest values in the Cr concentration were recorded(Fig. 6). Any significant variation in the Crmax concentration ofgroundwater wells at the area of Thiva, during same period(2007–2010) was not recorded.

3.3. Chromium content in plants

Special attention was given to the plants, which are a majorsource of organic matter that serves as the driving force for Cr(VI)reduction, is an essential micronutrient in human metabolic pro-cesses, but also because of its important consequences on thehuman health. The majority of the selected plants are coming fromcultivated areas in order to assess the bioaccumulation of Cr bydifferent vegetable plant species grown on potentially contami-nated soils of the Assopos basin. In addition, numerous of plant

samples (along with the corresponding soil samples from their rhi-zosphere) were collected from sites outside the polluted area of theAssopos basin, like Holargos (surrounding of Athens), the Messenia(Pelloponessos) area, which is characterized by the absence of ophi-olites (Cr-bearing rocks), as well as from Halkis (Euboea, centralGreece) and Vourinos (northern Greece) in the vicinity with ophi-olites where Cr related with natural processes. Sometimes morethan one plant species growing on a single site correspond to thesame soil (Tables 1 and 5).

The Cr content in plants ranges from 0.6 to 3.7 ppm (aver-age 1.25 ± 0.92) in shoots, from 0.7 to 8.0 ppm in roots (average4.7 ± 3.7) and 0.7 ppm in bulb plants. The Cr contents in plantsfrom unpolluted areas (Hollargos and Messenia) range from 0.6 to1.4 ppm in shoots, 1–2 ppm in bulbs and 5.5 ppm in roots. The Crcontents in plants from areas in vicinity with ophiolites (Euboeaand Vourinos) range from 1.8 to 2.3 ppm in shoots, 1.3 to 100 ppmin roots and 1.5 ppm in a bulb plant (Table 5). In general, the Crvalues in shoots are higher compared to those in roots, and the Crcontents in plants from the Assopos basin are relatively lower com-pared to the average value (30 ppm) of plants grown in the vicinityof ultramafic rocks, Ni–Fe-laterite deposits, mining excavations orore beneficiation units located in the Euboea island and Vourinoscomplex (Table 5; Megremi, 2010). Such Cr contents are higherthan normal or sufficient values (0.1–0.5 ppm Cr) in plants (Kabata-Pendias and Pendias, 2000), although crucial questions such as thereduction or increase of Cr uptake in crops to regulate minimal

Table 4Representative microprobe analyses of silicates and goethite associated with chromite in soils from the Assopos basin.

Sample MRs PEPs GM3s

Mineral spt1a spt2 spt1b chl chl chl chl goeth goeth spt

wt%SiO2 38.86 44.43 40.46 35.78 36.63 33.29 29.73 0.44 2.11 38.99Cr2O3 0.23 n.d. n.d. 0.54 0.4 n.d. n.d. 0.31 1.68 n.d.Al2O3 1.22 0.88 0.85 14.52 11.29 13.01 20.83 0.78 3.45 n.d.MgO 37.04 35.98 35.89 33.44 33.87 36.51 17.64 n.d. n.d. 38.87FeO 8.62 4.07 7.47 4.98 2.63 1.97 17.91 78.32 74.38 4.29MnO n.d. n.d. n.d. 0.34 n.d. n.d. 0.78 0.21 0.26 n.d.NiO 0.24 0.37 0.24 n.d. 0.32 n.d. n.d. n.d. 0.38 n.d.

Total 86.21 85.73 84.91 89.6 85.14 84.78 86.89 80.06 82.26 82.15

Abbreviations: spt = serpentine; chl = chlorite; goeth = goethite; n.d. = below detection limit.

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Fig. 5. Variation of the Crtotal (in ppb), Si, Ca and Mg concentrations (in ppm) in the groundwater wells from the Assopos basin. The presented increasing trend of theCrtotal concentration is not accompanied by any systematic variation between Crtotal and Si, Ca and Mg concentrations. Data: present study, Vasilatos et al. (2008), andGiannoulopoulos (2009).

uptake for growth and production, and to increase uptake in hyperaccumulators and keep toxicity to a minimum for the completionof its lifecycle still remain (Shanker et al., 2009).

A salient feature is the extremely high Cr content in roots ofcertain plants, such as wheat, onion and leek, all having a lot ofsmall and fine fibrous roots towards the surface of the soil, in con-trast to other specious, like beet and Verbascum, having a thickmain root extending far down into the soil, which have relativelylow Cr content. In addition, the percentage of soil chromium inplants is relatively small, ranging between 0.35 and 3.3% (aver-age = 0.93 ± 0.9) is shoots, 0.4 and 61.5% (average 14.5 ± 20.6) inroots (Table 6).

4. Discussion

Most contamination problems with chromium are related togroundwater contamination as a result of Cr(VI)-rich water. Thesource of hexavelent chromium in the groundwater of the Asso-pos basin, Greece, e.g. by the influence of industry or ultramafic

rocks is a subject of great interest. Since ultramafic rocks, con-taining chromite as an accessory mineral and Cr-bearing silicateminerals, are widespread throughout central Greece (Fig. 1) theinvestigation of both Cr(VI)-sources, anthropogenic activity andnatural processes within the Assopos basin was carried out. Themobility of chromium through soils is dependent upon its oxidationstage. Chromium(VI) reduction in soils occurs through a complexnetwork of synergistic and competing processes. Major factors con-trolling the oxidation stage are the redox soil conditions, soil pH, thepresence of certain other metals and organic compounds (Bartlettand Kimble, 1976; Richard and Bourg, 1991; Banerjee and Nesbitt,1999; Lin, 2000).

4.1. The behaviour of chromium in soils and the role of organiccarbon

Chromium(VI) usually exists in the environment in the formof chromate and dichromate. Since Cr(VI) occurs as an anion, it

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Fig. 6. Temporal variability of the Cr(VI) concentration, all in ppb (Y axis), in groundwater wells, three of the Municipality urban water supply (Sarandari, ChryssopygiI andChyissopygiII) and three irrigation wells, during the period 2007 to 2010. The abbreviations on the X axis (Date) correspond to the months/Year. Data: Present study andVasilatos et al. (2008).

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Table 5Trace element concentrations in plants from the Assopos basin and other areas.

Samples Species of plant ppm

Cr Ni Mn Fe Zn Cu Pb Co As

Assopos basinGM1-shoot Allium cepa (onion) 1.8 2 60 12 24 6 0.2 3.8 <0.1GM1-bulb Allium cepa (onion) 0.7 2.9 22 2 39 6 0.1 2.3 <0.1GM2-shoot Allium sativum (garlic) 1.4 1.8 34 14 28 4 0.2 2.0 <0.1GM2-bulb Allium sativum (garlic) 0.7 3.1 15 4 46 8 0.1 6.1 <0.1GM3-shoot Cucurbita Pero (cocozelle) 1.3 14 52 15 34 10 0.1 3.8 0.2KT1-shoot Beta vulgaris (beet) 0.8 1.3 115 8 11 7 0.4 2.4 0.2KT1-root Beta vulgaris (beet) 0.6 0.5 23 3 12 8 0.0 2.7 0.1KT1.E-root Beta vulgaris (beet) 0.9 2 29 9 21 12 0.1 5.3 0.2KT2-shoot Lactuca sativa (lettuce) 1 3.2 49 15 48 10 0.4 1.0 0.1LGM-shoot Brassica oleracea (cabbage) 0.8 7.7 22 5 15 2 0.0 1.9 <0.1PKT-shoot Allium porrum (leek) 0.7 1.6 16 5 17 5 0.1 2.0 <0.1P.KT-root Allium porrum (leek) 8.0 11 23 70 19 8 0.3 1.9 9.1PEP1-shoot Allium porrum (leek) 0.6 1.3 17 6 20 4 0.1 2.4 <0.1PEP1-root Allium porrum (leek) 5.4 8 40 120 25 7 0.7 1.4 7PGM-shoot Beta vulgaris (beet) 3.7 10 138 20 38 10 0.7 1.6 0.3KA1-shoot Lactuca sativa (lettuce) 1.3 2.7 31 17 17 10 0.3 2.9 <0.1KT-shoot Spinacea oleracea (spinach) 2.6 5 45 54 68 10 0.5 2.2 0.4MR1-shoot Cucurbita Pero (cocozelle) 0.9 13 56 17 37 10 0.1 4.0 <0.1MR2-shoot Cucurbita Pero (cocozelle) 0.6 11 34 15 23 7 0.1 3.7 0.2MR3-shoot Cucurbita Pero (cocozelle) 0.5 8 21 18 18 13 0.1 0.5 0.3Outside of the Assopos basinHollargosX1-shoot Sinapis arvensis (b. mustard) 0.6 0.8 73 90 33 5 0.3 0.9 0.1X2.1-shoot Sinapis arvensis (b. mustard) 1 2.5 28 100 41 4 0.2 0.8 <0.1X2.1-shoot Cichorium intybus (chicory) 1.4 2.4 57 300 66 10 0.7 1.5 2.1MesseniaMVAT2-bulb Allium cepa (onion) 1 2.4 25 110 17 4.2 0.2 0.4 <0.1MVAT2-root Allium cepa (onion) 5.9 6.6 460 3000 33 14 5.2 12 2.8MVAT3-bulb-ext Allium cepa (onion) 2.0 4.3 32 180 7.6 2.7 0.4 0.6 0.1MVAT3-bulb-int Allium cepa (onion) 1.0 0.2 8 30 11 3.8 0.05 0.1 <0.1M.Po. Solanum tuberosum (potatoe) 1.3 3.3 10 120 18 7.9 0.1 2.0 <0.1VourinosV.Wh-seeds Triticum spp (wheat) 1.3 3 30 70 26 4.6 0.4 6.5 <0.1V.Wh-shoot Triticum spp (wheat) 1.7 1.2 25 100 12 2.5 0.7 16 <0.1V.Wh-root Triticum spp (wheat) 100 150 140 3900 12 14 2.0 20 0.5V.V1.-shoot Verbascum spp (verbascum) 2.3 19 29 430 18 3.3 1.0 2.5 <0.1V.V1.-root Verbascum spp (verbascum) 1.3 14 4 120 22 4.4 0.6 52 <0.1V.V2.-shoot Verbascum spp (verbascum) 1.8 9 26 170 12 3.8 0.02 16 <0.1V.V2.-root-ext Verbascum spp (verbascum) 2.1 20 16 230 20 6.0 0.5 10 <0.1V.V2.-root-int Verbascum spp (verbascum) 1.0 6 5 50 39 6.4 0.3 12 <0.1HalkisXA1.-bulb Allium cepa (onion) 1.5 8.5 13 70 16 4.8 0.8 0.3 0.2XA1.-root Allium cepa (onion) 31 79 76 4200 22 7.1 1.8 5.0 0.7XA2.-bulb Allium cepa (onion) 1.5 2.6 13 70 10 2.8 0.2 0.2 <0.1XA2.-root Allium cepa (onion) 44 69 146 430 13 8.4 1.9 14 1.5XA2-po Solanum tuberosum (potatoe) 5.4 5.4 52 3600 13 9.5 3.1 3.5 2.2Central Euboeaa average (n = 31) 36 64 56 880 65Reference materialFLOUR 0.8 0.3 32 4 25 3.2 0.22 0.02 <0.1STD V14 0.8 1.5 2130 17 14 4.3 0.79 0.77 10.8STD V16 375 10 756 535 37 7.3 2.89 1.24 1.6

Samples Plant ppm wt%

Cd Ba Se K Na Mg Ca S P

Assopos basinGM1-shoot Allium cepa (onion) 0.1 11 0.3 4.1 0.1 0.6 2.0 0.33 0.68GM1-bulb Allium cepa (onion) 0.1 4 0.4 2.3 0.0 0.2 0.5 0.40 0.53GM2-shoot Allium sativum (garlic) 0.1 40 0.5 3.2 0.0 0.7 1.7 0.39 0.53GM2-bulb Allium sativum (garlic) 0.1 9 0.6 1.7 0.0 0.1 0.4 0.60 0.52GM3-shoot Cucurbita Pero (cocozelle) 0.1 9 0.3 4.5 0.0 0.9 2.5 0.37 0.74KT1-shoot Beta vulgaris (beet) 0.2 50 0.9 3.9 8.2 1.5 1.1 0.48 0.17KT1.K-root Beta vulgaris (beet) 0.1 14 0.2 2.7 1.5 0.2 0.1 0.19 0.18KT1.E-root Beta vulgaris (beet) 0.1 9 0.3 3.1 0.9 0.3 0.2 0.18 0.38KT2-shoot Lactuca sativa (lettuce) 0.3 6 0.6 6.9 1.1 0.7 0.8 0.34 0.54LGM-shoot Brassica oleracea (cabbage) 0.0 3 0.3 3.0 0.1 0.2 0.5 0.50 0.27PKT-shoot Allium porrum (leek) 0.1 6 0.2 4.0 0.2 0.2 0.3 0.50 0.31P.KT-root Allium porrum (leek) 0.3 15 0.5 3.5 1.6 0.8 0.5 0.81 0.26PEP1-shoot Allium porrum (leek) 0.0 3 0.2 2.9 0.0 0.1 0.1 0.45 0.44PEP1-root Allium porrum (leek) 0.2 10 0.3 3.3 1.0 0.8 0.4 0.51 0.35PGM-shoot Beta vulgaris (beet) 0.2 16 0.6 6.1 4.4 1.2 0.8 0.39 0.43KA1-shoot Lactuca sativa (lettuce) 0.2 7 0.2 7.6 0.2 0.3 1.0 0.22 0.59KT-shoot Spinacea oleracea (spinach) 0.2 6 0.4 5.9 1.5 1.3 0.6 0.38 0.60

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Table 5 (Continued )

Samples Plant ppm wt%

Cd Ba Se K Na Mg Ca S P

MR1-shoot Cucurbita Pero (cocozelle) 0.04 8 0.3 4.5 0.0 0.9 2.5 0.37 0.74MR2-shoot Cucurbita Pero (cocozelle) 0.06 10 0.4 6.9 0.0 0.9 1.9 0.27 0.55MR3-shoot Cucurbita Pero (cocozelle) 0.07 17 0.3 8.0 0.0 0.5 1.1 0.26 0.62Outside of the Assopos basinHollargosX1-shoot Sinapis arvensis (b. mustard) 0.06 18 0.5 4.3 0.2 0.22 3.3 0.5 0.4X2.1-shoot Sinapis arvensis (b. mustard) 0.02 0.6 0.1 2.0 0.04 0.11 1.5 0.5 0.3X2.1-shoot Cichorium intybus (chicory) 0.4 4.7 <0.1 3.4 1.2 0.22 1.6 0.3 0.3MesseniaMVAT2-bulb Allium cepa (onion) 0.11 2.4 0.1 1.0 0.10 0.10 0.3 0.23 0.3MVAT2-root Allium cepa (onion) 0.75 152 0.1 1.3 1.27 0.22 1.12 0.56 0.23MVAT3-bulb-ex Allium cepa (onion) 0.06 18 <0.1 0.8 0.06 0.16 1.42 0.13 0.08MVAT3-bulb-int Allium cepa (onion) 0.02 0.6 <0.1 1.0 0.06 0.07 0.17 0.32 0.20M.Po. Solanum tuberosum (potatoe) 0.12 0.5 0.1 2.2 0.024 0.11 0.04 0.11 0.34V.Wh-seeds Triticum spp (wheat) 0.01 4.3 <0.1 0.6 0.00 0.13 0.04 0.09 0.38V.Wh-shoot Triticum spp (wheat) 0.01 16 <0.1 0.9 0.00 0.06 0.17 0.06 0.17V.Wh-root Triticum spp (wheat) 0.2 34 <0.1 1 0.76 1.06 0.92 0.34 0.12V.V1.-shoot Verbascum spp (verbascum) 0.21 3.2 <0.1 1.5 0.00 0.56 1.26 0.16 0.29V.V1.-root Verbascum spp (verbascum) 0.03 3.9 <0.1 0.8 0.01 0.10 0.09 0.12 0.24V.V2.-shoot Verbascum spp (verbascum) 0.04 136 <0.1 3.7 0.00 0.20 1.37 0.07 0.24V.V2.-root-ext Verbascum spp (verbascum) 0.06 128 0.1 4.5 0.00 0.18 0.70 0.10 0.19V.V2.-root-int Verbascum spp (verbascum) 0.03 55 0.2 2.3 0.00 0.08 0.25 0.05 0.20HalkisXA1.-bulb Allium cepa (onion) 0.06 3.1 0.3 1.82 0.08 0.22 0.17 0.6 0.40XA1.-root Allium cepa (onion) 0.07 15 <0.1 0.46 0.02 0.10 0.37 <0.01 0.14XA2.-bulb Allium cepa (onion) 0.19 5.6 0.2 0.91 0.04 0.09 0.4 0.17 0.20XA2.-root Allium cepa (onion) 0.07 34 0.3 1.07 0.96 0.57 1.45 0.26 0.11XA2-po Solanum tuberosum (potatoe) 0.31 15 0.2 0.63 1.13 0.18 1.07 0.27 0.09Reference materialFLOUR 0.03 2.7 <1 0.32 0.00 0.13 0.03 0.18 0.38STD V14 0.22 1.4 50 0.52 0.00 0.08 0.62 0.07 0.09STD V16 0.09 2 38 0.24 0.00 0.056 0.28 0.02 0.05

Symbols: FLOUR, STD V14, STD V16 are standards used by the analytical Laboratory of ACME; ext = external part; int = internal part.a Afrer Megremi (2010).

is readily adsorbed to positively charged surfaces such as oxidesand hydroxides of manganese, iron and aluminum (Kabeta-Pendias,and Pendias, 1984; Deng and Stone, 1996). Chromium reactionrates or kinetics from Cr(III) to Cr(VI) have been extensively stud-ied (Lin, 2000). The effect of various electron donors and acceptorson chromate reduction by Cr(VI)-reducing bacteria isolated fromCr(VI) contaminated sites and the mechanism of Cr(VI) reductionby enriched bacterial consortium have been investigate by severalauthors (Arnold et al., 1988; Daulton et al., 2001; Stepek et al.,2002, and references therein). More specifically, it has been sug-gested that the reduction of Cr(VI) can be explained by bacteria,either (a) direct enzymatic reduction, e.g. by soluble enzyme sys-tem or the membrane-bound system. Direct contact between cellsand the metal oxide is required for the energy conservation pro-cess. (b) Indirect reduction: the lower redox and pH required forthe reduction of Cr(VI), are the result of various biochemical reac-tions and the metabolites formed by bacteria. Also, it is well knownthat soil absorbs easily organic matter and reduces Cr(VI) to Cr(III).Normally, in the presence of any organic matter in the soil, Cr(VI) isreduced to Cr(III), however, if the rate of seepage of Cr(VI) exceedsthe reduction potential of the soil, Cr(VI) will migrate and may bereleased later when the ionic strength changes or another anionbecomes dominant (Bartlett and Kimble, 1976; Richard and Bourg,1991; Banerjee and Nesbitt, 1999). Experimental data have shownthe rate of the oxidation of Cr(III) to Cr(VI) is relatively slow, thet1/2 ranging from 0.6 to 37 years. In contrast the reaction rates forthe reduction of Cr(III) to Cr(VI) is very rapid, the t1/2 ranging from15 min to 21.5 d (Motzer, 2005).

Apart from the reduction of Cr(VI) to Cr(III), the application oforganic matter has been proposed to remove Cr(VI). On the basis ofsurface interactions between activated carbon and chromium theremoval of the later from mingled has been interpreted (Kay et al.,

1985; Mohan and Pittman, 2006). Key et al. (1985) in their attemptto establish the adsorption mechanisms of Cr(III) and Cr(VI) on var-ious adsorbents concluded that relatively low activation energy of60 kJ/mol suggests a diffusion-controlled adsorption on the acti-vated carbons from co-mingled waste, whereas relatively highvalues for the oxidized carbon of 92 kJ/mol indicates the processcontrolled by chemical reactions. The free energy of the adsorptionat all temperatures was negative indicating a spontaneous process.The positive entropy values indicate the existence of ion-exchangeand the substitution reactions. The overall processes were foundto be endothermic and the enthalpy changes of 3–11 kJ/mol havebeen considered to indicate the complex character of the Cr(III)adsorption mechanism (Key et al., 1985). In addition, the acti-vated carbon prepared from groundnut shell has been utilized bySuleman et al. (2009) for bio-sorption of Cr(VI) from wastewater,and the study of equilibrium, kinetics and thermodynamics of bio-sorption process. They concluded that the bio-sorption of Cr(VI)was endothermic and that ion exchange was the major removalmechanism.

4.2. Factors controlling the bio-accumulation of chromium toplants

The uptake of heavy metals by plants is considered to dependon the mobility and availability of metals in the soil. Factors thataffect the mobility of heavy metals include: (a) chemical parame-ters such as pH, Eh, organic matter content, salinity, concentrationsof completing ions, ionic or anionic complexes, moisture, (b) phys-ical parameters, penetrability, presence of one or an assemblageof clay minerals, their surface area and cation exchange capacities,presence of oxyhydroxides of Fe, Mn and Al and carbonate minerals,climate temperature) and (c) biological parameters, such as micro-

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Table 6Percentage of soil chromium in plants. Data from Tables 1 and 5.

Samples Species of plant ppm ppmCr-plant Cr-soil % Cr

Assopos basinGM1-shoot Onion 1.8 200 0.9GM1-bulb Onion 0.7 200 0.35GM2-shoot Garlic 1.4 190 0.7GM2-bulb Garlic 0.7 190 0.36GM3-shoot Cocozelle 1.3 200 0.6KT1-shoot Beet 0.8 150 0.53KT1-root Beet 0.6 150 0.4KT1.E-root Beet 0.9 150 0.6KT2-shoot Lettuce 1 130 0.76LGM-shoot Cabbage 0.8 190 0.42PKT-shoot Leek 0.7 130 0.53PKT-root Leek 8.0 130 61.5PEP1-shoot Leek 0.6 67 0.9PEP1-root Leek 5.4 67 8.1PGM-shoot Beet 3.7 180 2.1KA1-shoot Lettuce 1.3 140 0.93KT-shoot Spinach 2.6 130 2MR1-shoot Cocozelle 0.9 89 1MR2-shoot Cocozelle 0.6 89 0.67MR3-shoot Cocozelle 0.5 89 0.56Outside of the Assopos basinHollargosX1-shoot Black mustard 0.6 110 0.54X2.1-shoot Black mustard 1 89 1.12X2.1-shoot Chicory 1.4 89 1.57MesseniaMVAT2-bulb Onion 1 30 3.3MVAT2-root Onion 5.9 30 18.7MVAT3-bulb-ext Onion 2.0 40 5MVAT3-bulb-int Onion 1.0 40 2.5M.Po. Potatoe 1.3 50 2.03VourinosV.Wh-seeds Wheat 1.3 250 0.52V.Wh-shoot Wheat 1.7 250 0.56V.Wh-root Wheat 100 250 40V.V1.-shoot Verbascum 2.3 180 1.28V.V1.-root Verbascum 1.3 180 0.17V.V2.-shoot Verbascum 1.8 370 0.48V.V2.-root-ext Verbascum 2.1 370 0.57V.V2.-root-int Verbascum 1 370 0.27HalkisHA1.-bulb Onion 1.5 330 0.45HA1.-root Onion 31 330 9.4HA2.-bulb Onion 1.5 805 0.18HA2.-root Onion 44 805 5.5HA2-po Potatoe 5.4 805 0.67Central Euboea Average (n = 31) 36 1300 3

bial activity, vegetative root extension, and burrowing organisms(Liesack et al., 2000; Shanker et al., 2005, 2009).

Lin and Huang (2008) investigated the abiotic reduction of Cr(VI)by pyrite under anaerobic conditions. They indicated that the rateof Cr(VI) reduction was strongly dependent on pH and concludedthat relatively high values of the activation energy (73 kJ/mol) ofthe Cr(VI) reduction in the temperature range of 10–50 ◦C is a majorcontrolling factor of the reduction rate over this temperature range,during surface reactions. Zero-valent iron, aqueous Fe(II), Fe(II)-hydroxides, adsorbed Fe(II), have been shown to reduce Cr(VI) veryrapidly (Bartlett and James, 1988; Fendorf and Zasoski, 1992; Dengand Stone, 1996; Sedlak and Chan, 1997; Buerge and Hug, 1997,1998; Pettine et al., 1994; Li, 2000; Ludwig et al., 2007; Agrawalet al., 2009). They indicated a strong linear free-energy relation-ship between calculated Fe(III)-organic ligand reduction potentialsand second order Fe(II)-organic ligand Cr(VI) reduction rate con-stants and concluded that the relative reaction kinetics are stronglydependent upon the reduction potential of the Fe(III)/Fe(II)-ligandcomplex. Faster Cr(VI) reduction kinetics were observed for theorganic ligand-complexed Fe(II). The fast Cr(VI) reduction and

Fig. 7. Plots of Crtotal vs. Fetotal, and Crtotal vs. S contents in plants (both shoot androots) from the Assopos basin. Data from Table 5.

dependence on the formation of metal-ligand organic complexesis suggested by the presented data concerning the bioaccumula-tion of Cr to plants in the Assopos basin, as it is exemplified by thepercentage of soil Cr in plants.

Also, the accumulation of Cr in plants depends on its bio-availability. The calculated thermodynamic data on the stability ofchromite by Redfern et al. (1999) and goethite by Navrotsky et al.(2008) suggest a higher resistance of Cr in the chromite lattice thanthat of the absorbed Cr on goethite. The relatively small percent-age bio-accumulation of Cr in the Assopos basin, ranging between0.35 and 3.3% (average = 0.93 ± 0.9) is shoots, and 0.4 and 61.5%(average 14.5 ± 20.6) in roots (Table 6) is comparable to that (2.7%)recorded in Euboea (Megremi, 2010). In addition, the much lower(1.1%) Cr(VI) of the total Cr content in plants (Megremi, 2010), cou-pled with the increasing Cr content, along with the Fe, Mn and Nicontents, (a) in the plant-roots and (b) the external parts of the rootsas well as the external parts of the bulb-type plant, like the onion(Table 5) may point to the reduction and immobilization of Cr(VI)and that redox reactions play a significant role the root to shoottranslocation processes. High carbon contents in root-near envi-ronment cause efficient sulphate reduction, increasing enrichmentof S2− and S◦ in pore water (Wiessner et al., 2007). The presence ofsulphur mostly in the plant-roots from the Assopos basin (Table 5)and its increase with increasing Cr content (Fig. 7b) may suggestthat chromium is taken up by carriers of essential ions such assulphate or iron (Shanker et al., 2005) and a re-oxidation and re-immobilization of S as well.

Anionic complexes have been identified in plant tissues extractsand xylem fluids, while trioxalatochromate have been reportedin plant leaves (Tiffin, 1971; Zayed et al., 1998). Speciation of

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M. Economou-Eliopoulos et al. / Chemie der Erde 71 (2011) 39–52 51

chromium can be determined by X-ray absorption near-edgestructure (XANES), X-ray absorption spectroscopy (XAS), anion-exchange HPLC–ICP-MS and ion-pairing HPLC with diode-array andICP-MS detection. Chromium(VI) is actively taken up in a metaboli-cally driven process, whist Cr(III) is passively taken up and retainedby cation exchange sites of the cell wall (Parsons et al., 2007; Yuet al., 2008; Strub et al., 2008). The investigation by X-ray absorp-tion near edge structure (XANES) on Cr speciation interconversionshowed that Cr(VI) is converted to Cr(III) by plants in the roots.The root to shoot translocation of Cr was extremely limited dueto the trend of Cr(III) to bind to cell walls (Zayed et al., 1998).Bluskov et al. (2005) reported that in plant tissues, Cr(III) wasdetected, primarily as acetate in the roots and oxalate in the leaves.X-ray microprobe showed the sites of Cr localization, and probablysequestration, in epidermal and cortical cells in the roots and epi-dermal and spongy mesophyll cells in the leaves. A combination ofX-ray synchrotron radiation microbeams and nuclear microprobefor quantitative chromium and chromium oxidation state mappingin cells has been used as well (Galushko and Schink, 2000). Theclear reduce of sulphur with decreasing chromium content in plant(Fig. 7c) may point to the role of S compounds in the Cr–plant inter-actions. It confirms the aspect that the transport of chromate mayinvolve sulphate carriers. It seems likely that the absorbed chro-mate is involved in sulphate assimilation and the accumulation ofcysteine and glutathione (Gardea-Torresdey et al., 2005).

4.3. Behaviour of chromium in groundwater systems

Groundwater systems seem to be fundamentally different com-pared to soil system, particularly in terms of potential reductant,since organic material in soil decreases with increasing depth, atthe water table and deeper. In general the species of Cr(VI) that aresoluble over a wide pH range are weakly adsorbed to soil solids,in contrast to Cr(III) species that insoluble over a wide pH rangeand are known to adsorb highly to soil solids (Griffin et al., 1977). Ifthere is limited organic material in groundwater and hence limitedopportunities to be reduced. Eh-pH diagrams are sometimes used inassessing redox conditions in soil environments for chromium. Onthe basis of the Eh-pH diagram for the Cr–O–H system the stabilityfield for Cr(VI) is restricted under oxidizing (Eh > 0), approximately−0.1 V to +0.9 V and alkaline conditions (pH > 6.0), while the possi-bility for converting Cr(III) to Cr(VI) under basic conditions is clear(Brookins and Jones, 1987; Ball and Nordstrom, 1998).

The composition of water may be affected by the solubility ofminerals which are major components of the aquifer and exchangereactions. The negligible Crtotal content and the relatively high(5) after “Ca/Mg ratio” in the Mavrosouvala wells of karstic type(Chatoupis and Fountoulis, 2004) are consistent with the obviousinteraction between water-carbonate rocks, consisting that aquiferhorizon. Although ophiolitic rocks are very limited at the Asso-pos basin, they may have been corroded and transported fromthe surrounding region. With respect to the order of solubility ofthe minerals which are major components of ultramafic rocks, thedecrease from olivine (the most soluble mineral) is followed bythat of the pyroxene, serpentine, chlorite talc and goethite (Golit-ghlt, 1981). However, the existence of a large group of groundwaterwells with Crtotal concentration less than 2 ppb (below detectionlimit), in a spatial association with highly Cr-contaminated wells(Fig. 4) and the lack of any relationship between the Cr concentra-tion (ppb) and those of Ca, Mg or Si concentrations (Fig. 5) wouldbe inconsistent with an interpretation on the basis only the inter-action between water and Cr-bearing rocks. Although much moreresearch is required the observed temporal variability of the Cr(VI)concentration (Fig. 6) is consistent with the expected differencebetween wet and dry period during a year time. The maximumCr(VI) concentrations in .groundwater wells was observed during

September–October (dry period). The lowest recorded values in theCr(VI) concentration in February (wet period) each year (Fig. 6),although commonly it occurs during April–May, may be related tothe permeability of the aquifer in uppermost horizons, composedby conglomerates (Fig. 1).

Therefore, the heterogeneous distribution of Cr(VI) in ground-water wells in a spatial association (Fig. 4) at any given time withinthe Assopos basin may indicate that (a) most groundwater contam-ination problems are a result of the direct injection of Cr(VI)-richindustrial wastes rather than Cr(VI) is derived from the interactionbetween water and Cr-bearing rocks, and (b) the heterogeneousdistribution of Cr(VI) in groundwater may be related with theintense neotectonic deformation, which has been superimposedonto primary sedimentary features, the complicate tectonic struc-ture of the Assopos basin (Chatoupis and Fountoulis, 2004), whichmay facilitate the migration of contaminated water along severalsharp tectonic contacts between sediment types.

5. Conclusions

The composition of soil, plant and groundwater samples cou-pled with geological, geochemical and phase mineral (Cr-hosts)data from the Assopos basin and other areas of Greece, lead to thefollowing conclusions:

1. Total chromium in soil, ranging from 67 to 204 ppm Cr, is mostlyhosted in chromite, Fe-chromite, and to a lesser extend to Cr-bearing goethite and silicates.

2. The percentage of soil Cr in plants is relatively small, rangingfrom 0.4 to 3.3% in shoots, and 0.4 to 62% in roots, suggests a lowdegree of bio-accumulation.

3. The increase of the Fe, Mn and Ni contents, with the increasingCr content (a) in the plant-roots and (b) at the external partsof roots and bulb-type plants, like the onion, suggest reductionand immobilization of Cr(VI) and that redox reactions play asignificant role to the root to shoot translocation processes.

4. The existence of relatively high Cr contents in roots of certainplants, having a lot of small and fine fibrous roots towards thesurface of the soil, may provide evidence for the bio-remediationof contaminated soils using specific specious.

5. There is a wide spatial variability of the Cr-content in groundwa-ter, ranging from <2 to 180 ppb, despite their spatial association.

6. The heterogeneous distribution of Cr(VI) in groundwater may berelated to the intense neotectonic deformation, which may facil-itate the migration of contaminated water along several sharptectonic contacts between sediment types.

7. The observed temporal variability of the Cr(VI) content ingroundwater wells is consistent with the expecting differencebetween wet and dry period. The maximum Cr(VI) contents wereobserved during September–October (dry period).

8. The oxidation of Cr(III) to Cr(VI) downward to depths >200 mwould be inconsistent with the existing physico/chemical con-ditions throughout the Assopos aquifer, suggesting a directinjection of Cr(VI)-rich industrial wastes at depth.

Acknowledgments

The Mayor and the Municipality of Oropos is acknowledged forthe financial support of this work (A.K. 70/3/9997). Mr. E. Michae-lidis, University of Athens, is thanked for his assistance with theSEM/electron probe analyses. The constructive criticism and sug-gestions of two anonymous reviewers to an earlier draft of themanuscript are greatly appreciated.

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