Use of crops for in situ phytoremediation of polluted

Use of crops for in situ phytoremediation of polluted soils following a toxic flood from a mine spill Author(s): María Auxiliadora Soriano and Elías Fereres Source: Plant and Soil, Vol. 256, No. 2 (October 2003), pp. 253-264 Published by: Springer Stable URL: http://www.jstor.org/stable/24124246 Accessed: 08-01-2017 10:14 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

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Plant and Soil 256: 253-264,2003. 253

© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

Use of crops for in situ phytoremediation of polluted soils following a toxic flood from a mine spill Maria Auxiliadora Soriano1 & Elias Fereres1,2,3 1 Departamento de Agronomia, Universidad de Cordoba, Apartado 3048, 14080 Cordoba, Spain. 2Instituto de Agricultura Sostenible, CSIC, Apartado 4084, 14080 Cordoba, Spain. 3Corresponding author* Received 21 October 2002. Accepted in revised form 13 May 2003

Key words: crops, heavy metals, metal uptake, phytoremediation

Abstract

Following a toxic flood from a mine spill that affected over 45 km2 in Southern Spain, experiments were conducted in 1999 to test the feasibility of using crops for phytoremediation of the area, after the mechanical removal of the

mud. Two cereals, barley and triticale, and two Brassica spp., rapeseed and ethiopian mustard, were planted in three contaminated plots, 50 x 100 m each, and in a control plot outside the affected area. Soil and plant contents of As, Cd, Cu, Pb, T1 and Zn were measured and bioaccumulation coefficients (BC) were calculated at maturity. The four crops tested accumulated Cd and Zn in the above-ground biomass only in the plot on acid soil. Both species of Brassica accumulated T1 (average BC of 3.6 and 1.4 for rapeseed and mustard, respectively, in contaminated plots). None of the four crop plants accumulated As, Cu and Pb under the experimental conditions. Maximum plant uptake values from soil were 5.4 mg m~2 of As, 0.54 mg m~2 of Cd, 9.7 mg m-2 of Cu, 7.0 mg m-2 of Pb, 3.4 mg m-2 of Tl, and 260 mg m~2 of Zn. Total crop uptake gave estimates for successful phytoremediation of at least five decades, casting doubts on the feasibility of using these crops for decontamination of the area. Nevertheless, cereal grains had mineral contents below toxicity levels for livestock, therefore it might be possible to use these crops for livestock feed while reducing deep percolation and gradually removing metals from polluted soils.

Introduction mud indicated that soils affected by the spill remained

polluted, with highly irregular spatial distribu

A toxic flood caused by the failure of a dam hold- the contaminants (Ayora et ing over 5 000000 m3 of acid water and toxic mud 2002). The principal pollutan originated from mine spoil, occurred on April 25, As, Cd, Tl, and Pb (CSIC, 1999 1998, over 45 km2 in Aznalcollar, southern Spain. Among the techniques propo Over 30 km2 of that area were farm lands. A task of contaminated soils, phyt force of many research groups was created to invest- tion of contaminants by p igate various aspects of the mine spills (Grimait and as a cost-effective and an e Macpherson, 1999). Simultaneously, the toxic mud ternative (Cunnigham et al., was removed mechanically during the period of May- Salt et al., 1995) to the con December, 1998. Initial results indicated severe heavy techniques (U.S. Army Tox metal pollution in the surface layers of most of the ials Agency, 1987). Among affected, sludge-covered, soils (Cabrera et al., 1999; in phytoextraction, some a Löpez-Pamo et al., 1999; Simon et al., 1999). Sub- lating very high concentratio sequent studies after the mechanical removal of the their tissues. Such plants are ors (Baker and Brooks, 1989; Brooks et al.,

FAX No: +34 957 499 252. E-mail: [email protected] However, hyperaccumulator p

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254

growth rates and annual biomass production (Robin- the flooded zone in an area between plots C2 and son et al., 1997, 1998). Slow growth rates limit their C3. Preliminary soil analyses indicated that the three extraction capacity per unit soil surface, making the locations selected varied widely in their soil contamin time required for an eventual site decontamination a ant levels, and represented the range of contaminants critical factor in phytoextraction technology. Many hy- found within the affected area. Before the accident,

peraccumulators (such as Thlaspi and Alyssum; Baker the locations selected were devoted to annual crops, et al., 2000; Brooks, 1998) are members of the Cru- primarily arainfed, sunflower-wheat rotation, ciferae family, which also includes crop plants, such In each location (CI, C2, C3 and CT), an experi as rapeseed. The use of crop plants for phytoremedi- mental plot of 50 m x 100 m was tilled, fenced and ation of contaminated soils has the advantages of their prepared for planting. Each plot was subdivided into high biomass production and adaptive capacity to vari- four strips 12 m wide and 100 m long, and each of the

able environments. However, to succeed they must four crops was assigned to a strip in each of the four be tolerant to the contaminants and be capable of plots. accumulating significant concentrations of phytotoxic Soon after crop emergence, the seedlings exhibited chemicals in their tissues. Additionally, crops could substantial variability in visible toxicity symptoms make the long time-periods for decontamination more within cropped areas, particularly in the CI and C3 acceptable, economically and environmentally. If the plots. Evidently, the variability in symptoms was re concentration of contaminants in biomass is below fleeting the spatial variability in the soil concentration critical levels for livestock consumption (Murillo et of toxic elements following the mechanical removal al., 1999), crops could have an economic value dur- of the mud and the surface soil. The experimental ing the phytoextraction process. Use of crops also design was adapted to this situation by selecting, in provides soil protection and reduces dust and potential side each cropped strip, homogeneous areas where leaching as positive environmental features. However, growth and symptoms were uniform within each ho there is the risk of the contaminants moving into hu- mogeneous area. The selected areas differed gradually

man and wildlife food-chains (Madejon et al., 2002; from severe symptoms and stunted growth up to nor

Mehargetal., 1999). mal growth without visible toxicity symptoms. Each

Field experiments were conducted to test the feas- individual selected area was considere

ibility of using the following four crops for phytore- plot where soil and plants were sampled mediation the contaminated soils from the mine spill at ant concentrations. Within each croppe

Aznalcollar: two cereals, barley (Hordeum distichum of five elemental plots were selected

L.), the cereal crop most tolerant to salinity (Ayers and experimental design was thus a split plo

Wescot, 1985) and known to accumulate Cd and Zn crop as a major factor and degree of p (Ebbs and Kochian, 1998), and triticale (Triticosecale as a minor factor. Such experimental

Wittmack), a crop well adapted to acid soils. The two peated in the three contaminated locations

other crops were species of the Brassica genus; rape- C3) while in the CT, only three sam seed (B. napus L.), which is tolerant to salinity and selected within each crop at regular in capable of accumulating Zn (Ebbs and Kochian, 1997) the cropped strip, and ethiopian mustard (B. carinata A. Braun) which

has been characterized as a hyperaccumulator of Pb Soil sampling and analysis

(Kumar et al., 1995).

To characterize the soil, each experimental plot was

subdivided before planting into four 50 m x 25 m Materials and methods areas, and four soil samples from the surface (0 0.30 m deep) and the subsoil (0.30-0.60 m) were

Experimental layout within each area with a soil sampling tube, 32 mm in

diameter, following a random path, and mixed. Corn

Three locations within the area flooded by the spill posite soil samples were homogenized and sieved (to were chosen to establish the experimental plots. They 2 mm) and pH and soil texture were determined. At were located to 850 m (CI), 1700 m (C2) and 2500 m around emergence, gravimetric soil water content, and (C3) away from the point where the dam was frac- soil depth, were measured with a soil tube, by taking tured. A control plot (CT) was also established outside 0.30 m samples, down to 1.50 m depth where possible,

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255 Table 1. Soil characteristics3 of affected (Cl, C2 and C3) and unaffected (CT) soils Experimental

Depth

Plot

(cm)

pH(H20) pH (H20)

Sand (%)

Silt (%)

Clay (%)

Mean Texture

Maximum

Soil Depth

(cm) CT

0-30

30-60

CI Cl

0-30

30-60

C2

0-30

30-60

C3

0-30

30-60

8.3

25.4

(8.1-8.4)

(24.6-26.1) (39.9-40.9) (33.0-35.5)

8.4

20.9

48.4

41.2

(8.4-8.4)

(18.2-23.6)

(36.6-39.2)

(39.9-42.6)

5.6

43.5

37.7

18.7

(5.1-6.2)

(30.4-65.0)

(25.5-43.9)

(9.5-25.8)

5.7

52.8

31.6

15.6

(5.3-5.9)

(39.7-71.8)

(19.5-38.2)

(8.7-22.1)

8.1

10.0

51.8

38.2

(8.0-8.1)

(9.7-10.4)

(45.4-58.1)

(32.2-44.2)

8.2

18.1

45.5

36.4

(8.0-8.4)

(15.5-20.7)

(39.0-51.9)

(32.6-40.3)

7.2

30.2

41.4

28.3

(6.7-7.6)

(17.1-52.7)

(30.9-45.2)

(16.4-38.3)

7.6

39.9

37.1

23.0

(21.8-43.4)

(7.8-32.4)

(7.5-7.8)

(24.2-70.4)

40.4

34.2

Clay loam

>150

Clay

Loam

60 to >150

Sandy loam

Silty Clay loam

>150

Silty Clay loam

Clay loam

90 to >150

Loam

aMean value (range).

at four points within each of the 50 m x 25 m area. All experimental plots were harvested at crop ma

Table 1 presents information on texture, pH and depth. turity, a total of four elemental plots per crop in CI and

Following identification of the elemental plots C2, five in C3, and three in CT. Harvested areas varied within each cropped strip, composite soil samples (0- between 2 and 5 m2 depending on the homogeneity of

0.30 m) were taken at each elemental plot for heavy the elemental plot. Plant material was cleaned, seeds metal analyses, at the crop vegetative stage. A total were separated from the rest of the above-ground bio

of five elemental plots per crop differing in plant mass, and all material was oven dried at 65-70 °C to symptoms were sampled in all three contaminated loc- constant weight to determine total biomass and crop

ations while only three plots per crop were sampled yield, in the control. At crop maturity, each harvested area

was sampled in identical fashion for soil heavy metal Soil and plant heavy metal and arsenic analysis analyses.

Seeds and the rest of the aerial biomass were ground

Crops and agronomy separately in a metal-free mill. The pulverized plant

samples were analyzed to determine As, Cd, Cu, Pb,

Barley (Hordeum distichum, cv. 'Trebon'), triticale T1 and Zn content. The average above-ground biomass

(Triticosecale, cv. 'Trujillo'), rapeseed (Brassica ion concentrations were calculated as the weighted

napus, cv. 'Comet') and ethiopian mustard (B. average of seed and rest of biomass concentrations. carinata) were planted between 14 and 30 January, Soil samples were air-dried and passed through 1999. The cereals were planted in rows 0.11 m apart at a 2 mm mesh screen. The screened fraction was a rate of 11.5 g m~2. The Brassica spp. were planted analyzed for total metal and arsenic contents (As, in 0.33 m rows with 1 g m-2 of seed. Prior to planting, Cd, Cu, Pb, T1 and Zn). Chemical analyses were 5 g N, 9.5 g P2O5 and 9.5 g K2O per m~2 were incor- carried out in the Svensk Grundämnesanalys AB porated into the soil, and on 29 March, 9.5 g N m"2 (SGAB) laboratories, Luleâ Tekniska Universitet,

were applied to all four crops as urea. in Sweden. The SGBA laboratory is accredited by

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256

the Swedish Board for Accreditation and Conform- (Table 3). The accumulation of Zn was pH dependent

ity Assessment(SWEDAC). Methods used to determ- (Table 1), thus the highest Zn accumulation was ob ine element concentrations were Inductively Coupled served in the plot on acid soil (CI), (Table 3), even Plasma-Atomic Emission Spectrometry (ICP-AES) though higher levels of Zn in the soil were measured and Inductively Coupled Plasma-Mass Spectrometry in C3 (Table 2). Element concentrations in the tis (ICP-MS). The analyses were performed according to sues of crops grown in CT were around normal values US EPA methods 200.7 and 200.8 (modified) for ICP- (Kabata-Pendias and Pendias, 1992).

AES and ICP-MS, respectively (USEPA, 1994). The Figure 1 presents the relationships observed soil and plant tissue samples were dried at 50 °C be- between plant and soil element concentrations, which fore digestion and analysis, and the soil samples were were linearly related in log-log scale, except for Cu homogenized by grinding prior to digestion. Sample (Figure 1). There was no detectable accumulation of digestion was carried out with high purity concen- Cu in aerial biomass for increasing Cu concentration trated nitric acid and hydrogen peroxide in closed in the soil, given that concentration in plants did not teflon vessels in a microwave digestion system (Jones exceed 20 mg kg_1Cu even though soil concentrations

et al., 1991). exceeded 300 mg kg-1 Cu (Figure 1). No differences

were detected in the apparent accumulation capacity

of the four crops for As, Pb and Zn (Figure 1 and

Results Table 3). However, for the same soil concentrations,

there were differences in the Cu and T1 concentratio

Heavy metal and arsenic contents in soils in the cereals vs- the Brassica spp.,

centration in barley was lower than in both Bras

Table 2 presents total element concentrations in the species (Figure 1 and Table 3). I first 0.30 m of soil for As, Cd, Cu, Pb, T1 and Zn for cereals uptake was about twice t the three affected areas (Cl, C2 and C3) and for the spp. (average values of 11.2 mg control (CT). In all cases, soil element concentrations and 6.6 mg kg-1 Cu in the Bra were significantly higher in the contaminated areas cumulation of T1 differed among the

than in the CT soil (Table 2) because, despite of the there was essentially no T1 in the a

rapid removal of the sludge, significant contamination than 0.4 mg kg""1 Tl), while the Br

remained in the affected area (Madejon et al., 2002; mulated Tl in above-ground biomas Moreno et al., 2001). In fact, the average content of up to 9.9 and 2.6 mg kg""1 in rap

toxic elements in the soils of the contaminated areas, mustard, respectively, (Figure 1, and

relative to CT, were from 5 to 40 times higher in As, Bioaccumulations coefficients ( 3-25 times in Cd, 1-10 times in Cu, 2-15 times in ment concentration ratio) are presen Pb, 4-25 times in Tl, and 3-20 times in Zn (Table 2). the most contaminated plots (CI Among the contaminated areas, the highest concen- Zn, average BC's were much highe

trations for all elements were in C3 and the lowest in seed and 1.1 in mustard, for Cd, and 1.7 for Zn in

C2. The maximum concentrations observed in some Brassica spp.) than in C3 (0.5 for Cd and 0.4 for Zn, locations within the C3 plot were about two orders of in both Brassica species), for corresponding total soil magnitude higher than the maximum values measured concentrations below 500 mg kg""1 Zn and 2 mg kg""1

in the CT soil (Table 2). Cd. As indicated in Table 1, CI had light soil texture

and a pH below 6.2, and it has been shown that Cd a

Heavy metal and arsenic contents in plant tissues Zn availability and uptake are enhance soils (Kabata-Pendias and Pendias, 1992; Mahler Table 3 presents the concentration of As, Cd, Cu, al., 1982). The BC for As and Pb wer Pb, Tl, and Zn in the above-ground biomass of each 0.1, respectively, (Table 4), indicating

crop sampled at maturity in the four locations. In gen- lation of As and Pb in the aerial bioma

eral, the highest plant concentrations were measured crops at physiological maturity. Hua in the C3 and CI plots, which also had the highest ham (1996) in short-term pot experim soil concentrations (Table 2). The Cu concentration in Pb of 2500 mg kg-1, observed BC v

aerial biomass of the four crops growing in the affected corn and 0.05 for wheat. Plants did n soils was not significantly different from the control (BC below 0.55 for all crops in the cont

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257

Table 2. Total concentration (mean and minimum and maximum values, mg kg-') of heavy metal and arsenic in soils (0-0.30 m) in affected (Cl, C2 and C3) and unaffected (CT) soils. Means followed by the same letter within each column are not significantly from each other using LSD test (P = 0.05)

Element (mg kg 1)

Experimental Plot

As

Cd

CT

6.25 a

(4.9-7.8) CI

57.1 c

(17.4-152)

(0.14-1.46)

(32.3-108)

C2

33.4 b

0.333 b

22.4 a

(7.4-127)

(0.12-1.21)

(9.8-63.8)

248 d

2.42 d

(53.8-709)

(0.48-9.19)

C3

Cu

Pb

T1

Zn

0.094 a

13.0 a

27.0 a

0.094 a

40.0 a

(0.07-0.12)

(9.4-15.3)

(20.4-32.9)

(0.05-0.13)

(36.1-43.2)

0.577 c

74.1 b

148 c

0.526 b

216 c

(74.5-235)

(0.08-1.28)

(118—462)

73.8 b

0.444 b

118b

(19.6-264)

(0.10-1.61)

(47.5^130)

148 c

456 d

2.67 c

787 d

(44.7-564)

(110-1690)

(0.59-12.9)

(160-3260)

Table 3. (a) Total concentration (minimum and maximum values, mg kg~1 ) of heavy metal and arsenic in aerial biomass of barley, triticale, rapeseed and ethiopian mustard at harvest, growing in affected (Cl, C2 and C3) and unaffected (CT) soils, (b) Average for the four crops in the different pfots. Means followed by the same letter within each column are not significantly from each other using LSD test (P = 0.05) Plot

Crop

Element (mg kg 1) As

Cd

Cu

Pb

T1

Zn

Barley

0.39-1.79

0.051-0.088

9.4-17.1

1.67-6.23

0.023-0.005

21.9-30.6

Triticale

0.64-1.02

0.120-0.140

5.6-15.8

1.54-4.65

0.014-0.009

14.1-24.4

Rapeseed

0.24-0.43

0.230-0.255

4.2-5.3

0.89-1.13

0.106-0.152

19.4-22.4

Ethiopian mustard 0.30-0.39

0.322-0.364

4.3-5.0

0.95-1.09

0.033-0.057

21.8-24.0

Barley

2.21-6.74

0.195-0.436

6.8-13.5

3.62-6.93

0.025-0.070

174-334

Triticale

1.10-3.31

0.323-0.592

7.0-16.8

4.49-6.04

0.045-0.149

168-412

Rapeseed

2.26-5.76

0.253-0.567

6.3-12.7

3.30-9.29

0.59-3.35

148-273

Ethiopian mustard 1.83-5.26

0.269-0.450

6.2-11.1

6.02-10.5

0.324-0.659

203-309

(a) CT

CI

C2

C3

Barley

0.63-1.48

0.045-0.080

9.5-12.6

1.84-4.21

0.010-0.022

64.0-88.5

Triticale

0.84-1.87

0.059-0.177

6.4-11.5

1.85-3.38

0.020-0.068

57.1-96.1

Rapeseed

1.01-2.81

0.282-0.413

4.8-7.9

2.30-5.54

1.16-5.60

49.8-80.2

Ethiopian mustard 1.11-1.86

0.258-0.379

3.0—4.2

2.29-3.37

0.204-0.656

35.5-60.3

Barley

1.46-19.5

0.142-0.434

6.3-15.5

3.5-27.0

0.024-0.163

107-257

Triticale

2.82-21.2

0.234-1.93

7.5-27.5

4.7-37.3

0.050-0.362

93-588

Rapeseed

1.75-4.45

0.276-0.921

4.9-10.6

3.19-7.86

0.16-9.94

64-293

Ethiopian mustard 1.31-12.3

0.232-0.747

3.7-9.8

2.54-18.6

0.24-2.55

52-228

CT

All four

0.589 a

0.198 a

7.99 a

2.05 a

0.049 a

22.2 a

CI

All four

3.24 c

0.383 b

10.0 a

5.76 b

0.517 b

238 d

(b)

C2

All four

1.51 b

0.216 a

6.98 a

3.33 a

0.755 b

64.1b

C3

All four

6.13 d

0.473 b

10.0 a

10.3 c

1.31c

168 c

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a Barley a v Triticale Barley O Rapeseed o Ethiopian mustard v 10

10

-t

Triticale

10 10

Cd

y = 0.38 x037

AsAs

y = 0.23 x062

= 64 r2 = 0.31, n = 64 -| r2 = 0.62,r2 =n0.62,=n 64

r2 = 0.31, n = 64

V v

10

10™

-J

Dfi □ ij|s o®

10

io

A

n

'V

10 io" o

oA

io

10 -

\ 10

10°°

10'°

10"

10:

10

Pb

10 -

10 -

y = 0.37 x055

r2 = 0.54, n = 64 10

-1."

10'°

10"

10"

10'

10

0

y = 1.44 x0-89 10 -

r2 = 0.51, n = 32

o

®° rx^

10

eg®

o

¥□ v 2

10 -

^ aa A

10

y = 0.07 x066

r2 = 0.52, n = 32 10

10

10

10

10 10

SoilSoil concentration (mg kg"1) concentratio Soil concentration (mg kg"1) Figure 1. Trace elements concentration (As, Cd, Cu, Pb, T1 and Zn; mg kg-1) in plant tissues (above-ground biomass) of four crop species (barley, triticale, rapeseed and ethiopian mustard) at physiological maturity plotted against soil concentration (both on a logarithmic scale). Linear Regression equations are included in the graph where appropriate. The crossed symbols correspond to data obtained in the CI.

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259

Table 4), and the observed Cu concentration values For all crops, the maximum observed values for were within normal values in plant tissues (Kabata- total extraction of contaminants from the polluted Pendias and Pendias, 1992). Both species of Brassica soils were 5.4 mg m-2 of As, 0.54 mg m~2 of Cd, were bioaccumulators of Tl, but rapeseed had a higher 9.7 mg m~2 of Cu, 7.0 mg m of Pb, 3.4 mg m-2 de BC than the mustard (average BC of 3.6 and 1.4, re- Tl, and 260 mg m~2 of Zn. spectively, in the most contaminated plots, CI and C3). Discussion

Toxicity levels in plants

The average concentrations found in the soils affected

Table 5 presents the average concentrations of heavy by the spill, when compared to the limit values

metals and As in the grain and stover of the four crops agricultural lands proposed by the Commission of t

tested, in the affected soils. Seeds of all four crops European Communities (CEC, 1986, 1987) indica had values below the thresholds indicated as toxic for that soils in the C3 plot were contaminated with As,

livestocks with the exception of Tl accumulation in Cu, Pb, Zn and Tl, while in the CI and C2 soils, on Brassica spp. (Table 5). Accumulation in stover above As and Pb were above the limit values. If the m

thresholds occurred in the two Brassica species for Tl imum values reported in Table 2 are considered, then

and Cd, and in triticale for Cd (Table 5). in addition to the five elements mentioned above, C3 is contaminated with Cd, while CI and C2 have As,

Total uptake of heavy metals and arsenic by crops Pb, Zn, and Tl levels above the CEC limits.

The data in Figure 2 shows that, for a given level of

Table 6 presents the total above-ground biomass pro- tissue element concentration, there was great variabil duced by the four crops in CI and C3 plots (the most ity in total element uptake, particularly at concentra contaminated locations). No significant differences in tions below those that lead to maximum uptake. That the average biomass were found among the four crops, meant that other factors were affecting biomass pro but there were differences up to one order of mag- duction in our experiment. Rainfall during the growing nitude among the selected homogeneous areas within season in 1999 was only 150 mm and that caused water each crop (Table 6). The lack of differences in bio- deficits in all four crops. Soil water storage at planting mass, combined with the data of Figure 1, meant that was variable among the harvested areas within each all of them extracted about the same amounts of As, plot, because of differences in soil depth and texture

Cd, Pb, and Zn, per unit soil surface. For Cu and (Table 1). Estimated water-holding capacity of the Tl, there were clear differences in uptake between the potential root zone varied from 40 mm of water in

cereals and the Brassica species (Table 6). shallow, light textured soils, up to 190 mm in deep The highest extraction levels per unit surface were soils of medium and fine texture. Such differences in observed for the intermediate concentrations in plant available water meant that biomass production, and tissues (Figure 2). The highest concentrations in plant toxic elements uptake, varied substantially depending tissues were measured in the areas where soils had on the seasonal water supply of the site, regardless of the maximum concentration of pollutants (Figure 1), soil contaminant concentration up to a point, which impacted plant growth significantly, thus pro- For the practical application of phytoremediation, ducing very little biomass (Table 6) and consequently, it is necessary to estimate the number of years or crop low total extraction per unit ground surface (Figure 2). pings required to reduce the content of toxic elements In the heavily polluted areas, plant growth was so af- in polluted soils below limit values for agricultural fected that visual estimates of intercepted radiation at land. To perform such calculations, plant uptake and peak biomass were less than 20% of incident radiation, soil concentration of pollutants are required. Given the thus the very low biomass production (Table 6). The large variability in contaminant concentrations within plant tissue concentrations above which plant growth the affected area, the lowering of contaminant con was apparently limited by contaminants were 5-6 mg centration via crop extraction would depend on the kg-1 of As, 8-9 mg kg-1 of Pb, and 225-250 mg specific location and toxic element, but our estimates kg-1 of Zn; such values may be considered as crit- indicate that remediation times would be to the order ical toxicity concentrations under the experimental of several decades. For instance, if we consider an av

conditions. erage level for Zn of 450 mg kg-1 soil and calculate

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Plot C1: PlotCI:

Ethiopian mustard © Rapeseed a aBarley œ ffl Ethiopian mustard BarleywwTriticale Triticale© Rapeseed

Plot C3:C3: Plot

O Rapeseed □ Ethiopian mustard a Barley Barley v v Triticale Triticale O Rapeseed □ Ethiopian mustard 7.50 7.50

0.75

Cd 6.25

0.62

©

® 0.50 0.50 -

5.00 -

□ _

VD V □ □