Toxicological assessment of heavy metals accumulated in vegetables and fruits grown in Ginfel river near Sheba Tannery, Tigray, Northern Ethiopia

Ecotoxicology and Environmental Safety 95 (2013) 171–178

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Toxicological assessment of heavy metals accumulated in vegetables and fruits grown in Ginfel river near Sheba Tannery, Tigray, Northern Ethiopia Abraha Gebrekidan a,b,n,1, Yirgaalem Weldegebriel c, Amanual Hadera a, Bart Van der Bruggen b a b c

Department of Chemistry, College of Natural and Computational Sciences, Mekelle University, P.O. Box 231, Mekelle, Ethiopia Department of Chemical Engineering, Process Engineering for Sustainable Systems (ProcESS), KU Leuven, Belgium Ezana Analytical Laboratory in Ezana Mining Development PLC, P.O. Box 788, Mekelle, Tigray, Ethiopia

art ic l e i nf o

a b s t r a c t

Article history: Received 11 December 2012 Received in revised form 17 May 2013 Accepted 29 May 2013 Available online 20 June 2013

The accumulation of heavy metals in vegetables resulting from irrigation with contaminated water obtained from industrial effluents may create a potential public health risk. We quantified the concentration of heavy metals (Cu, Zn, Fe, Mn, Cr, Cd, Ni, Co and Pb) in soil, vegetables and the water used for irrigation at two sites (Laelay Wukro and Tahtay Wukro) around Wukro Town, Tigray, Northern Ethiopia. The concentrations of heavy metals in irrigation water measured during this study were lower than permissible limits of heavy metals allowed for irrigation water. The mean concentrations of heavy metals in irrigated soil samples obtained from Tahtay Wukro were higher for Mn, Zn, Cr, and Cu. The overall results of soil samples ranged 2.62–827, 1.4–51.6, 25.5–33.6, 23.5–28.2, 2.52–25.1, 15–17.8, 3–4, 2.5– 40.49 and 0.7–0.8 mg/kg for Mn, Zn, Cr, Ni, Cu, Co, Pb, Fe and Cd, respectively. Higher concentrations of heavy metals were also observed in vegetable samples from Tahtay Wukro. Pb was found to accumulate the most in all vegetable samples. It was observed that green pepper and lettuce accumulate high amounts of Cu and Zn; Swiss chard accumulates excessive amounts of Fe, Mn, Cr, Cd, Ni and Co; lettuce and tomato higher amounts of Cd; and green pepper, tomato and onion a higher concentration of Pb. Significant differences in the elemental concentrations between the vegetables analyzed from Laelay and Tahtay Wukro were observed. This was attributed in part to the geological nature of the study area and the discharges from the town and from a tannery. The results also indicate that Fe, Pb and Cd have high transfer factor values (mean values: 42.89, 0.84 and 0.37, respectively). The transfer pattern for heavy metals in different vegetables showed a trend in the order: Fe4Pb 4Cd 4Mn4 Cu 4Zn4Ni 4Zn4 Cr¼Co. The heavy metal contamination of vegetables grown in Tahtay Wukro, located downstream of the tannery, may pose increased health risks in the future to the local population through consumption of vegetables. & 2013 Elsevier Inc. All rights reserved.

Keywords: Heavy metals Vegetables Health risk Irrigation Tannery

1. Introduction Food safety is a major public concern worldwide. During the past decades, the increasing demands for food safety have stimulated research regarding the risk associated with consumption of food products contaminated by pesticides, heavy metals and/or toxins (D’Mello, 2003). Heavy metals are among the major

n Corresponding author. Current address: Department of Chemical Engineering, Process Engineering for Sustainable Systems (ProcESS), W. de Croylaan 46 bus 2423, 3001 Heverlee, KU Leuven, Belgium. Fax: +32 16 32 29 91. E-mail addresses: [email protected] (A. Gebrekidan), [email protected] (Y. Weldegebriel), [email protected] (A. Hadera), [email protected] (B.V.d. Bruggen). 1 Fax: +251 344 401090.

0147-6513/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ecoenv.2013.05.035

contaminants of vegetables (Zaidi et al., 2005). Heavy metals are not biodegradable, have long biological half-lives and have the potential for accumulation in the different body organs leading to unwanted effects (Nabulo et al., 2011; Singh et al., 2010; Jarup, 2003; Sathawara et al., 2004). Most heavy metals are extremely toxic, and because of their solubility in water, contamination may readily reach toxic levels (Arora et al., 2008). Food chain contamination is one of the most important pathways for the entry of these toxic pollutants into the human body (Wang et al., 2011; Ma et al., 2006; Ferner, 2001; Harmanescu et al., 2011). Mineral fertilizers, which are used for enhanced food production, constitute an important source of environmental pollution (Oyedele et al., 2006; Baig and Kazi, 2012; Jamali et al., 2007). The increasing demand for food and food safety has drawn the attention of researchers to the risks associated with consumption

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of contaminated food products, particularly heavy metals in vegetables (Uwah et al., 2011; Khan et al., 2009; Prabu, 2009). Fresh vegetables and fruits are vital to the human diet as they contain essential components needed by the human body, i.e., carbohydrates, proteins, vitamins, minerals and trace elements (Itanna, 2002). There is a gradual increase in consumption of vegetables particularly among urban communities, due to increased awareness on food value of the vegetables, as a result of the exposure to proper education (Itanna, 2002). However, vegetables may also contain toxic elements over a wide range of concentrations. It is well known that plants take up metals by absorbing them from contaminated soils as well as from deposits on parts of the plants exposed to the air in polluted environments (Oves et al., 2012; Krishna and Govil, 2007; Khairiah et al., 2004; Chojnacha et al., 2005, Bigdeli and Seilsepour, 2008). Leafy vegetables have a propensity to accumulate heavy metals and the accumulation of these metals into vegetables is a health concern, because they are potential carcinogens or cause human organ dysfunction. For instance, lead and cadmium are among the most abundant heavy metals and are particularly toxic. The excessive content of these metals in food is associated with a number of diseases, especially with cardiovascular, kidney, nervous as well as bone diseases (WHO, 1992, 1995; Steenland and Boffetta, 2000; Jarup, 2003, Powers et al., 2003; Llobet et al., 2003; Ikem and Egiebor, 2005; Yargholi and Azimi, 2008; Mitra et al., 2009). In addition, they also cause carcinogenesis, mutagenesis and teratogenesis (IARC, 1993; Pitot and Dragan, 1996). Other metals such as copper and zinc are essential for important biochemical and physiological functions and necessary for maintaining human health. Zinc deficiency results in a variety of immunological defects, whereas copper deficiency is characterized by anemia, neutropenia and skeletal abnormalities (Prentice, 1993; ATSDR, 1994; Linder and Azam, 1996). However, an excess of copper is associated with liver damage; a high amount of zinc may produce adverse nutrient interactions with Cu and reduces immune function and the levels of high density lipoproteins (FDA, 2001). A high concentration of Fe and Mn also causes pathological events such as deposition of iron oxides in Parkinson’s disease (FDA, 2001). Recently, the European Union (EU) and other countries have recommended maximum levels of toxic heavy metal pollutants. In view of the potential toxicity, persistent nature and cumulative behavior as well as the consumption of vegetables and fruits, there is a need to test and analyze food items to ensure that the levels of these contaminants meet the agreed international requirements. Regular survey and monitoring programmes of the concentration of heavy metals in food products have been carried out for decades in most developed countries (Jorhem and Sundstroem, 1993; Pennington et al., 1995a, b; Milacic and Kralj, 2003; Saracoglu et al., 2004, Camazano et al., 1994; Mohamed et al., 2003; Devagi et al., 2007; Sobukola et al., 2010). However, in developing countries like Ethiopia, limited data are available on heavy metals in food products. Some data have been reported for leafy vegetables (Weldegebriel et al., 2012; Itanna, 2002). In northern Ethiopia, in an effort to address the problem of recurrent drought and related famine and food insecurity, it is attempted to harvest all water sources including ground waters, rivers and runoff water in micro-dams for use both in households and for small-scale irrigation schemes. It is recognized that an appropriate utilization of ground waters, rivers and constructed micro-dams for irrigation will result in sustained productivity. Food security is a major concern in the region and irrigation has numerous potential benefits. Most importantly, it may contribute substantially to food security and economic progress, which in turn provides rural households with improved access to health care delivery services and education (Keiser et al., 2005).

This study presents data on the level of heavy metals (copper, zinc, iron, manganese, chromium, cadmium, nickel, cobalt and lead) in selected leafy vegetables and their fruits grown from different agricultural activities that uses Ginfel River, which receives effluents from Wukro town and a tannery, for irrigation (Gebrekidan et al., 2009). This study also aims to identify the extent and severity of heavy metal pollution in soils and vegetables and analyze the transfer factor of individual heavy metal from different agricultural activities in the intensively farmed area in the study site, Wukro, Northern Ethiopia. These aspects have not previously been studied in the region (Tigray in Northern Ethiopia), and have an extreme importance in evaluating the environmental impacts on agriculture.

2. Materials and methods 2.1. Description of study area The study area, Wukro town, is located within Ethiopia, in the Eastern zone of the Tigray region, North of Mekelle city at an elevation between 2140 and 2250 m above sea level. A tannery, Sheba Tannery, capable of processing 6000 hides a day, is active in Wukro. The major economic activities of the town are: services (hotels and restaurants, traditional beverages, pensions, and groceries), urban agriculture, trades and retails, wood works, and metal works. The town has a poor solid and liquid waste management system, which causes severe environmental and health problems. Two farm lands used for growing different vegetables, named Laelay Wukro and Tahtay Wukro, were selected to study the level of heavy metal contamination. Laelay Wukro farm land is located on the upper side of Wukro town. This farm is irrigated with water from Ginfel River upstream of Sheba Tannery and most other sources of pollution. Tahtay Wukro farm is located in the southern part of the city, downstream of Sheba Tannery. Ginfel River at this point has received the liquid waste discharged from households, institutions, and Sheba Tannery. This polluted river water is used for bathing, washing, and urban agriculture, mainly for growing vegetables and fruits. 2.2. Instrument and chemicals A drying oven with forced air and timer (FED 53, USA) was used for drying samples and A11 Basic Analytical MILL (IKA-Werke GmbH & Co.KG, 79219 Staufen, Germany) for grinding vegetables. A Varian AA240 FS Fast Sequential Atomic Absorption Photometer (Varian, Australia), fully automated and PC-controlled using Apectra AA Base and PRO software versions and equipped with fast sequential operation for multi-element flame determinations, using four lamp positions and automatic lamp selection, was used for the determination of metals (Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb and Zn). The chemicals used were: 70 percent HNO3 (BDH, England), 70 percent HClO4 (Aldrich, Germany), 37 percent HCl (Riedel-de Hä en, Germany), and calibration standards SPECTROSCAN, (Industrial Analytical (pty) Ltd, South Africa) for the metals Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn. The accuracy of analytical procedures was checked by analyzing a certified reference material (IPE682 for plants and GBM3995 for soil) wheat (straw)/ Triticum aestivum (Wageningen Evaluating Programs for Analytical Laboratories, The Netherlands) obtained from Ezana Mining Development P.L.C. Analytical Laboratory (Mekelle, Ethiopia). All samples and standards were diluted with distilled and deionized water. 2.3. Collection, preparation and preservation of samples Samples of vegetables, soil and water were collected twice from the selected farms, prepared and preserved in the laboratory until analysis was done. 2.3.1. Vegetable samples Composite samples of vegetables (Leafy and fruits) including cabbage, Swiss chard, tomato, lettuce, pepper, potato, and onion were collected from the study agricultural plots depending on their availability. The samples were brought to the laboratory in paper bags, cleaned with deionized water to remove dust and extraneous matter and dried in an oven at 105 1C for 12 h. The dried samples were then ground and homogenized and stored in tightly closed clean sample bottles until analysis. 2.3.2. Soil samples Composite soil samples were collected randomly, from eleven agricultural plots with a stainless steel auger at 0–30 cm depths and stored in plastic bags. Each composite soil sample, of about 1 kg, was taken from five thoroughly mixed

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subsamples taken at random sampling sites within the study area. All samples were well mixed, riffled and one-fourth of each sample was dried in an oven at 105 1C for 12 h. The dried samples were then ground and sieved with a 75 μm mesh size sieve and analyzed.

The means are compared with the corresponding certified values using a student t test at 95 percent confidence interval to determine the acceptability of the test results. The critical t test value for n¼15 at 95 percent CI is t0.95 ¼ 1.761, for n¼ 9 at 95 percent CI is t0.95 ¼1.859 and for n¼6 at 95 percent CI is t0.95 ¼2.015. The actual value qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of t was calculated using: t calc ¼ jx−μj=ð s2 =n þ δ2 Þ, where n refers to the number of

2.3.3. Water samples River water samples were collected in 250 mL plastic bottles from the canals and Ginfel River. Two samples from each study site were brought to the laboratory, centrifuged to remove the suspended particles, filtered through 0.45 mm micro pore filter sand preserved with 1.0 mL of 70 percent HNO3. The filtered samples were stored in a refrigerator to minimize volatilization and biodegradation between sampling and analysis.

determinations, x is the analysis mean, μ is the certified mean, s is the standard deviation, and δ is the certified standard deviation. When the tcalc for all tests is lower than tcrit, the tests conducted for the vegetables are accepted (Mullins, 2003).

2.7. Transfer factor The transfer factor expresses the bioavailability of a metal at a particular position on a species of plant (Khan et al., 2009). The transfer factor calculated in this study was based on the total metal content of the whole plant (edible part) without taking into consideration the various parts of the plant. The formula used was stated by Cui et al. (2004)

2.4. Digestion procedures 2.4.1. Digestion of vegetable samples A sample of 0.20 g of dried powdered plant was weighed in a test tube. 2.0 mL 30 percent H2O2 was added into the test tube and digested at 150 1C on a hot plate for 30 min. Then 2.0 mL of HNO3 was added to the sample and further digested on the hot plate for another 30 min. Digestion was continued with 2.0 mL of HClO4 for 30 min, cooled, carefully transferred into 50 mL volumetric flask, rinsed and diluted with distilled water and shaken. Finally, sample was taken by 16  150 mm test tubes and analyzed for trace metals using FAAS.

TF ¼

Concentration of metal in edible part Concentration of metal in soil

2.8. Statistical methods used The elemental concentrations of water, soil and vegetable samples were determined using the statistical software SPSS 15.0 for Windows.

2.4.2. Digestion of soil samples A sample of 20+0.05 g of pulverized (75 μm) soil was weighed in a 400 mL tall form beaker. An acid mix of 50 mL HCl and 20 mL HNO3 was slowly added to the sample while swirling, to ensure that the sample is properly wetted and simmered on the hot plate for a minimum of 45 min at 160 1C, stirring with a glass rod. It was removed from the hot plate before becoming dry, cooled and diluted in a 200 mL volumetric flask with distilled water, shaken and poured back into the beaker and settled for 30 min. Finally, sample was taken by 16  150 mm test tubes and analyzed for trace metals using FAAS.

3. Results and discussion 3.1. Concentration of heavy metals in water and soil The concentration of heavy metals found in soils and water used for irrigation is summarized in Table 3. The concentration of heavy metals (μg/L) in water used for irrigation was the highest for Fe, followed by Zn, Cr, Pb, Ni, Cu, Co, Cd and Mn. The concentrations of Cu, Zn, Mn, Cr, Cd, Ni and Co in irrigation water measured during the study were higher in Tahtay Wukro than in irrigation waters used in Laelay Wukro (Table 3). The higher detectable amounts of heavy metals recorded in Laelay Wukro could be due to that of the non-point sources, agricultural and surface run-off, and the discharges from the town and Sheba Tannery. The concentrations of heavy metals in irrigation water measured during this study were below the permissible limits of heavy metals allowed in the irrigation water (Ayers and Westcott, 1985). The mean concentrations of heavy metals (mg/kg) in the irrigated farmland soil samples obtained from Tahtay Wukro show a somewhat elevated level of concentrations in Mn, Zn, Cr, and Cu (Table 3). This indicates a contamination of soils of this irrigated farmland by heavy metals. The overall results ranged 2.62–827, 1.4–51.6, 25.5–33.6, 23.5–28.2, 2.52–25.1, 15–17.8, 3–4, 2.5–40.49 and 0.7–0.8 mg/kg for Mn, Zn, Cr, Ni, Cu, Co, Pb, Fe and Cd, respectively. Although the concentrations of heavy metals under study are below the permissible limits of concentrations for soil (Ewers, 1991; Pendias and Pendias, 1992), there is an indication that the concentrations of the heavy metals in the soils of Tahtay

2.5. Analytical procedures Stock standard solutions containing 1000 mg/L of Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn (SPECTROSCAN, Industrial Analytical (pty) Ltd, South Africa) were used for preparing working standards (0, 0.50, 1.00 and 2.0 mg/L). Using a micro pipette, exactly 1.00 mL of the 1000 mg/L stock standard was poured into a labeled 100 mL volumetric flask; 20 mL of concentrated hydrochloric acid (Analytical Reagent) was added to the flask and diluted to the mark with distilled water. Then 0, 5.0, 10.0 and 20.0 mL of 10 mg/L working standard was added in to clean 100 mL volumetric flasks for preparing 0, 0.50, 1.00 and 2.0 mg/L standards, respectively. Analyses of the vegetables, soil and water samples were performed after determining the detection limits and validating the procedures with recovery tests. 2.5.1. Method detection limits The detection limits for analytical methods for vegetables, soil and water were obtained from three times the pooled standard deviation, i.e., 7 3 s of fifteen determinations of the reagent blanks and analyzed in three groups of five blanks each and for soil samples was obtained by taking low level samples and analyzing in the same way as for vegetables and water (Fifield and Kealey, 1995). Instrument working conditions and detection limits are presented in Table 1. 2.6. Validation of the analytical data using t test For validation of the analytical procedure, a recovery study was carried out by using standard reference materials obtained from Ezana Mining PLC, Mekelle, Tigray, Ethiopia. The mean of each of the elements analyzed for the certified reference material (IPE682 for plants and GBM399-5 for soil) is given in Table 2. Table 1 Instrument working conditions and detection limits. Element Lamp current (mA) Fuel Support Wave length (nm) Slit width (nm) 1 IDL Instrument Water (mg/L) Soil (mg/L) Vegetables (mg/L)

Cd

Co

Cr

Cu

Fe

Mn

Ni

Pb

Zn

4 C2H2 Air 228.8 0.5

7 C2H2 Air 240.7 0.2

7 C2H2 N2O 357.9 0.2

4 C2H2 Air 324.7 0.5

5 C2H2 Air 248.3 0.2

5 C2H2 Air 275.9 0.2

4 C2H2 Air 232.0 0.2

5 C2H2 Air 217.0 1.0

5 C2H2 Air 213.9 1.0

0.002 0.01 1.00

0.005 0.01 1.00

0.006 0.02 1.00

0.003 0.01 1.00

0.003 0.02 1.00

0.002 0.02 1.00

0.010 0.03 1.00

0.010 0.01 1.00

0.001 0.05 1.00

0.01

0.01

0.02

0.01

0.02

0.02

0.03

0.02

0.05

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Table 2 Validation of the analytical method using plant and soil certified reference materials (mg/kg dry wt). Metals

IPE682

GBM399-5

Measured value

Cu Zn Pb Ni Co Mn Fe Cr Cd

Certified value

Measured value

t test (tcalc)

n

Mean

Std dev

Mean

Std dev

9 9 9 9 9 9 6 6 6

2.27 11.4 6.479 0.346 0.061 21.8 117 0.568 0.309

0.211 1.517 0.695 0.118 0.016 0.947 18.242 0.078 0.036

2.2 10.5 6.77 0.315 0.059 21.5 114 0.615 0.319

0.706 2.06 0.874 0.123 0.013 2.27 17 0.153 0.028

0.221 0.54 0.386 0.248 0.121 0.226 0.154 0.47 0.265

Certified value

n

Mean

Std dev

Mean

Std dev

15 15 15 15 15 15 15 15 15

28,633 9430 21,611 24,205 42.7 261 4.2 532.9 9.3

954 371 1141 899 3.2 14.8 0.44 33.4 0.7

29,424 9493 21,173 24,412 46 na 4.47 557 na

1446 504 1402 1248 5 – – – –

t test (tcalc)

0.539 0.123 0.306 0.163 0.658

Table 3 Mean concentration of heavy metals in water and soil used for irrigation in Laelay and Tahtay Wukro. Sampling site

Soil from farm land of

Cu

Zn

Fe

Mn

Cr

Cd

Ni

Co

Pb

Laelay Wukro (LW)

Cabbage Green pepper Lettuce Swiss Chard Tomato Mean values for LW soil samples Guidelines for soil (mg/kg)b Water (μg/L) used for irrigation Cabbage Green pepper Onion Potato Swiss Chard Tomato Mean values for TW soil samples Guidelines for soil (mg/kg)b Water(μg/L) used for irrigation Guidelines for irrigation water (μg/L)a

30.50 30.50 8.00 8.00 8.00 17.00 100.00 10.00 25.10 21.70 27.80 25.10 25.10 26.7 25.25 100.00 12.00 17.00

61.20 61.20 22.20 22.20 22.20 37.80 300.00 104.00 51.60 56.80 53.40 51.60 51.60 46 51.83 300.00 118.00 200.00

2.80 2.80 1.10 1.10 1.10 1.78 5000.00c 189.00 2.80 2.50 2.90 2.80 2.80 2.4 2.70 5000.00 143.00 500.00

817.20 817.20 200.30 200.30 200.30 447.06 2000.00c 2.00 827.00 656.30 784.80 827.00 827.00 789.3 785.23 2000.00 3.00 20.00

19.70 19.70 24.00 19.70 24.70 21.56 100.00 18.00 30.30 30.30 32.80 33.60 33.60 25.50 31.02 100.00 28.00 550.00

0.80 0.80 0.50 0.80 0.30 0.64 3.00 4.00 0.70 0.70 0.90 0.70 0.70 0.80 0.75 3.00 7.00 50.00

31.20 31.20 21.30 31.20 10.70 25.12 50.00 10.00 23.50 23.50 28.20 27.00 27.00 26.80 26.00 50.00 13.00 1400.00

18.30 18.30 13.00 18.30 7.00 14.98 50.00 6.00 15.00 15.00 17.80 16.90 16.90 16.50 16.35 50.00 21.00 50.00

5.00 5.00 4.00 5.00 22.20 8.24 100.00 16.00 3.00 3.00 4.00 3.30 3.30 3.00 3.27 100.00 18.00 65.00

Tahtay Wukro (TW)

a b c

Source: Ayers. Source: Ewers. Source: Pendias and Pendias.

Wukro were high especially for Cr. This may be due to the discharges of waste from the effluents of the Sheba Tannery. 3.2. Levels of heavy metals in vegetables Toxic heavy metals entering the ecosystem may lead to bioaccumulation, particularly by eating fruits and vegetables (Kashif et al., 2009). This may cause an excessive build-up of heavy metals in the body. Some heavy metals that are most often found to be responsible for harmful damage to humans are lead, cadmium, chromium, cobalt and nickel. Some heavy metals such as copper, chromium, iron, zinc and manganese, are necessary for the body but in case of overexposure, they can lead to heavy metal toxicity symptoms. Heavy metal concentrations vary among different vegetables, which may be attributed to a differential absorption capacity of vegetables for different heavy metals (Singh et al., 2010). All heavy metals concentrations in the vegetables from Tahtay Wukro were higher than those obtained from Laelay Wukro (which is used as a reference site). Concentrations of the studied heavy metals (Cu, Zn, Fe, Mn, Cr, Cd, Ni, Co and Pb) in vegetables (dry weight) are presented in Table 4. The concentrations of heavy metals were generally higher in samples from Tahtay Wukro, which is located downstream of the Shaba Tannery and receives effluents from the tannery and effluents of Wukro town discharged into the river; Laelay Wukro is located upstream of the Sheba Tannery. The results in Table 4 are

in agreement with previous studies on the uptake of heavy metals in edible parts of vegetables grown with continuous wastewater irrigation (Khan et al., 2007; Liu et al., 2005; Muchuweti et al., 2006; Sharma et al., 2007). The concentration of carcinogenic heavy metals in water samples from Ginfel river used for irrigation was in the order of Cr4Pb 4Ni 4Co 4Cd for Laelay Wukro and Cr4Co 4 Pb 4 Ni4Cd for Tahtay Wukro. In both cases a high concentration of Cr and low concentration of Cd was observed. The higher concentration of Cr in the Tahtay Wukro is probably due to the discharges of the Sheba Tannery. The order of the concentration of these carcinogenic heavy metals in soils from the selected vegetable irrigated farm lands was Ni4 Cr4Co 4Pb 4Cd for both Laelay and Tahtay Wukro. The high concentration of chromium in soil samples observed in Tahtay Wukro could also be attributed to high level of the effluents of the Sheba Tannery. The order of concentration of essential heavy metals in water samples was also in order Fe 4Zn4 Cu4 Mn for Laelay Wukro and Fe4Zn 4 Cu4 Mn for Tahtay Wukro. In both cases a high concentration of Fe and low concentration of Cd was observed. The higher concentration of Fe and low concentration of Cd in both sample sites could be attributed to the physicochemical nature of the soil samples in whole catchment of Wukro town. The order of the accumulation of essential heavy metals in soils from the selected vegetable irrigated farm lands was in the order of Mn4Zn4Cu4Fe for both Laelay and Tahtay Wukro sample sites.

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Table 4 Concentration of heavy metals (mean 7 SD) (mg/kg) in vegetables grown at Laelay and Tahtay Wukro using Ginfel River. Sampling site

Name of sample

Part of sample

Less concern Cu

Laelay Wukro

Cabbage Green Pepper

Lettuce Swiss chard Tomato

Tahtay Wukro

Cabbage Green Pepper

Onion

Potato

Swiss chard Tomato

Edible Edible Non edible, leaves Edible Edible Edible Non edible, leaves Edible Edible Non edible, leaves Edible Non edible, leaves Edible Non edible, leaves Edible Edible Non edible, leaves

High concern Zn

Fe

Mn

2.66 70.12 1.08 70.05 43.81 7 0.82 5.62 70.09 1.47 70.23 45.99 7 3.19 6.78 70.16 2.58 70.06 104.627 2.83 5.30 70.34 4.72 70.18 4.01 70.40 4.48 70.11

72.45 7 0.65 8.36 7 1.15 99.03 7 2.97

4.82 70.45 112.95 7 5.92 118.65 7 4.52 3.27 70.22 114.32 7 2.05 460.127 25.72 2.96 70.18 70.777 1.03 12.247 2.10 5.98 70.43 117.647 2.93 152.417 1.24

4.10 70.05 0.75 70.17 59.29 7 0.98 1.93 70.12 4.7070.23 76.187 1.85 10.10 70.16 2.17 70.14 122.26 7 1.19

Cr

Cd

Ni

Co

Pb

0.117 0.02 0.20 70.01 0.38 70.07 0.217 0.01 2.80 7 0.23 0.217 0.09 0.32 70.03 0.65 70.03 0.177 0.00 2.077 0.67 0.42 7 0.03 0.40 70.03 0.68 70.08 0.23 7 0.01 2.167 0.34 0.32 7 0.02 0.42 7 0.04 0.39 7 0.02 0.447 0.01

0.30 70.02 0.23 70.02 0.22 70.01 0.3370.04

0.7070.06 0.63 70.08 0.68 70.08 0.57 70.10

0.217 0.01 0.217 0.02 0.187 0.02 0.247 0.01

1.55 7 0.48 2.53 7 0.22 3.25 7 0.45 2.26 7 0.33

29.43 7 0.32 8.63 7 0.39 94.95 7 1.52

0.43 7 0.02 0.18 70.01 0.75 70.09 0.147 0.01 3.82 7 0.15 0.43 7 0.04 0.18 70.01 0.46 70.03 0.127 0.01 5.85 7 0.71 0.32 7 0.05 0.41 70.03 0.76 70.06 0.247 0.01 2.58 7 0.31

2.48 70.10 2.96 70.55 4.66 70.10 0.44 70.09

27.80 7 1.17 87.30 7 0.67

5.43 7 0.11 35.447 1.78

0.377 0.02 0.20 70.01 0.53 70.05 0.107 0.01 4.90 7 0.79 0.45 7 0.04 0.16 70.01 0.53 70.04 0.137 0.01 3.687 0.38

2.52 70.13 1.40 70.10 5.45 70.10 4.73 70.18

40.497 1.15 61.93 7 0.44

2.62 7 0.11 59.707 0.94

0.39 7 0.06 0.18 70.04 0.25 70.06 0.107 0.01 2.58 7 0.36 0.497 0.06 0.17 70.03 0.47 70.07 0.177 0.01 3.30 7 0.41

2.58 70.24 0.46 70.06 150.09 7 2.66 174.95 7 1.83 2.43 70.15 1.89 70.39 94.687 2.06 13.717 0.27 5.45 70.23 0.72 70.10 177.40 7 1.66 224.307 2.90

0.60 7 0.06 0.38 70.02 0.73 70.06 0.217 0.01 2.50 7 0.24 0.377 0.04 0.18 70.02 0.34 70.07 0.107 0.01 3.78 7 0.19 0.62 7 0.03 0.34 70.03 0.51 70.07 0.247 0.01 2.317 0.30

The concentration of metals in vegetables mainly depends on the texture of the soil or media on which they grow but this also depends on the type and nature of plant (Kabata-Pendias and Pendias, 1984). The metal concentrations in leaves and fruit showed significant variations in the uptake by plants. Leaves (non-edible) parts of the vegetables accumulated higher concentrations of heavy metals as compared to the edible parts. This study also shows that there is a correlation between metals in soils and vegetables, which transfers heavy metals into the food chain. The application of contaminated water led to change the physicochemical characteristics of soil and consequently the uptake of heavy metals by vegetables (Arora et al., 2008). The heavy metal concentration in the edible parts of vegetables collected from Laelay and Tahtay Wukro sample sites is shown in Fig. 1. From this figure, it can be clearly observed that the concentration of all the heavy metals were higher in Tahtay Wukro compared to Laelay Wukro which should be attributed to the discharge of wastewater from Wukro town and Sheba Tannery. Carcinogenic heavy metal concentrations for all vegetable samples, except cabbage, from both sample sites (Laelay Wukro and Tahtay Wukro) followed the order Pb 4 Ni4Cr 4Cd4 Co. High bioaccumulation of concentration of lead and low bioaccumulation of cobalt was observed in all vegetable samples. Concentration of essential heavy metals from vegetable samples of Laelay Wukro were in the order of Mn 4 Fe4 Cu4 Zn for cabbage (C), lettuce (L), Swiss Chard (SC) and Fe 4Mn 4Cu 4Zn for green pepper (GP) and tomato (T). High concentration of iron than manganese was observed in vegetable samples of cabbage from Tahtay Wukro. For vegetable samples of green pepper, tomato and potato the trend was Fe 4Mn 4Cu 4Zn, and that of onion Fe4Mn 4Zn 4Cu. The level of heavy metals in onion (Cr, Co, Cu, Zn and Mn) collected from around Meki town and Lake Zeway, Ethiopia was found to be high and Pb concentration low (Kitata and Chandravanshi, 2012). The results of our findings indicate that green pepper and lettuce bioaccumulate high amounts of Cu and Zn; Swiss chard bioaccumulated excessive amounts of heavy metals of Fe, Mn, Cr, Cd, Ni and Co and green pepper bioaccumulates a high amount of Pb. Some studies (Awode et al., 2008) also showed that green

pepper accumulates high concentrations of heavy metals such as Cu, Zn and Pb. This could be attributed due to the different nature of the vegetable species that accumulate different metals depending on their environmental conditions, metal species, plant available and forms of heavy metals. Studies (Jung, 2008; Mido and Satake, 2003; Bingham et al., 1975) also showed that the uptake and accumulation of metals by different plant species depend on several factors such as soil pH, cation exchange capacity, soil organic content, soil texture and the interaction of soil–plant root–microbes which play important roles in regulating heavy metal movement from soil to the edible parts of vegetables. The ability of a metal species in its different forms to migrate from the soil through the plant parts and makes itself available for consumption can be represented by the transfer factor. The transfer factor is a function of different factors such as the soil pH, soil organic matter, metals availability and soil particle size. The transfer factors of different heavy metals from soil to vegetation are one of the key components of human exposure to metals through the food chain. The highest values (mean values: 42.89, 0.84 and 0.37, respectively) were found for Fe, Pb and Cd because these metals are more mobile in nature. Fe has the highest transfer factor (103.93), this could be attributed to the low retention rate of the metal in soil and therefore it is more mobile in the soil. The lowest was for Cr and Co (mean values: 0.01) probably because they can bind more to the soil and become part of the soil composition. This means that a larger percentage of the metal from the soil was adsorbed to the plant tissue for the former (Fe, Pb and Cd) and a very insignificant percentage for the latter (Cr and Co). The transfer pattern for metals is Fe 4 Pb 4Cd 4Mn 4 Cu4 Zn4 Ni4Zn 4Cr¼ Co. The TF values for Cu, Zn, Fe, Mn, Cd, and Pb for various vegetables varied significantly between plant species and sampling sites. Variations in transfer factor among different vegetables may be attributed to differences in the concentration of metals in the soil and differences in element uptake by different vegetables (Cui et al., 2004; Zheng et al., 2007). The transfer factor for Cu, Zn, Fe, Mn, Cd, and Pb for various vegetables varied significantly between plant species and sampling sites (Table 5). In this study the transfer factors values clearly indicate that by selecting particular vegetables, it is possible to

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HM Concentration (mg/Kg)

3.5

7

Laelay Wukro

Cr Cd Ni Co Pb

3.0 2.5 2.0

HM Concentration (mg/Kg)

176

1.5 1.0 0.5

Cr Cd Ni Co Pb

Tahtay Wukro

5 4 3 2 1 0

0.0 C

GP

L

SC

T

C

500 Laelay Wukro

Cu Zn Fe Mn

HM Concentration (mg/Kg)

HM Concentration (mg/Kg)

6

100

0 C

GP

L Sample

SC

200 180 160 140 120 100 80 60 40 20 0

GP

O

SC

T

Tahtay Wukro

T

C

GP

P Cu Zn Fe Mn

O SC Sample

T

P

Fig. 1. Carcinogenic and essential heavy metals of different vegetables grown in Laelay and Tahtay Wukro using Ginfel River: cabbage (C), green pepper (GP), lettuce (L), onion (O), Swiss Chard (SC), tomato (T) and potato (P) . Table 5 Transfer factor of heavy metals for different vegetables grown at Laelay and Tahtay Wukro using Ginfel River. Sampling site

Laelay Wukro

Tahtay Wukro

Name of sample

Cabbage Green pepper Lettuce Swiss chard Tomato Cabbage Green pepper Onion Potato Swiss chard Tomato

TF Cu

Zn

Fe

Mn

Cr

Cd

Ni

Co

Pb

0.09 0.18 0.66 0.59 0.5 0.16 0.09 0.09 0.1 0.1 0.09

0.02 0.02 0.22 0.15 0.13 0.01 0.08 0.06 0.03 0.01 0.04

15.65 16.43 102.68 103.93 64.34 21.18 30.47 9.59 14.46 53.6 39.45

0.09 0.01 0.59 2.3 0.06 0.04 0.01 0.01 0 0.21 0.02

0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01

0.25 0.4 0.6 0.29 0.73 0.26 0.26 0.22 0.26 0.54 0.23

0.01 0.02 0.03 0.02 0.06 0.03 0.02 0.02 0.01 0.03 0.01

0.01 0.01 0.02 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01

0.56 0.41 0.39 0.51 0.15 1.27 1.95 1.23 0.78 0.76 1.26

reduce the risk of human exposure to soil metal contaminations. Based on the results, lettuce and Swiss chard are high Fe, Mn and Cu accumulators; lettuce, Swiss chard and tomato are high Zn accumulators; tomato and lettuce are high Cd accumulators; green pepper, cabbage, tomato and onion are high Pb accumulators. Based on the toxicity of heavy metals more attention for Pb and Cd is needed. Therefore, we strongly suggest that in Pb/Cd contaminated areas one should avoid eating the vegetables listed above in order to reduce health risks. On the other hand, lower Pb and Cd accumulator vegetables are relatively safer to consume.

4. Conclusion A high concentration of heavy metals in soil and irrigated water led to the accumulation of heavy metals in vegetables. This study indicates that leafy vegetables (non-edible parts) accumulate relatively high concentrations of heavy metals compared to the edible parts. Significant differences were found for the elemental concentrations among the vegetables analyzed from Laelay and

Tahtay Wukro. This might be in part to the geological nature of the study area, the ability of plants and their specific parts to accumulate metals as well and the discharges from the town and Sheba tannery. Based on the results, lettuce and tomato were high accumulators of Cd and green pepper, tomato and onion were high accumulators of Pb. Although there is a general tolerable level of metals in most of vegetables under study from Wukro and its surroundings at the moment, there are exceptional cases of metal build up such as cadmium and lead in lettuce, tomato, cabbage and onion vegetables. At present the daily intake of these metals is not done due to lack of information on the consumption of vegetables per person and household in the community but the situation could raise health risk concerns in the future depending on the dietary pattern of the community and the amount of contaminants added to the Ginfel River and irrigable farmlands. The consumption of Cd and Pb-contaminated vegetables, which is higher than the permissible limits, by the local communities may pose health hazards. This has important implications for policy makers aimed at regular monitoring and controlling heavy metal concentrations in irrigation water sources not to exceed the

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permissible limits. Therefore, policies and programs need to be adapted so that agricultural practices are taken into account, and an appropriate local measure developed for mitigating heavy metal uptake by vegetables. Thus regular monitoring of heavy metal contamination in the vegetables grown in Wukro area is necessary and consumption of contaminated vegetables should be avoided in order to reduce the health risk caused by taking the contaminated vegetables. To avoid entrance of metals into the food chain, it could be suggested that municipal or industrial waste should not be drained into rivers and farmlands without prior treatment.

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