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Environmental Monitoring and Assessment (2006) 120: 79–91 DOI: 10.1007/s10661-005-9050-3

c Springer 2006 

HEAVY METAL LOAD OF SOIL, WATER AND VEGETABLES IN PERI-URBAN DELHI S. SINGH∗ and M. KUMAR Division of Environmental Sciences, Indian Agricultural Research Institute, New Delhi 110012, India (∗ author for correspondence, e-mail: [email protected])

(Received 18 February 2005; accepted 22 September 2005)

Abstract. Peri-urban lands are often used for production of vegetables for better market accessibility and higher prices. But most of these lands are contaminated with heavy metals through industrial effluents, sewage and sludge, and vehicular emission. Vegetables grown in such lands, therefore, are likely to be contaminated with heavy metals and unsafe for consumption. Samples of vegetables i.e., spinach (Spinacia oleracea L.) and okra (Abelmoschus esculentus L.); soil and irrigation water were collected from 5 peri-urban sites of New Delhi to monitor their heavy metal loads. While heavy metal load of the soils were below the maximum allowable limit prescribed by the World Health Organization (WHO), it was higher in irrigation water and vegetable samples. The spinach and okra samples showed Zn, Pb and Cd levels higher than the WHO limits. The levels of Cu, however, were at their safe limits. Metal contamination was higher in spinach than in okra. Spatial variability of metal contamination was also observed in the study. Bio-availability of metals present in soil showed a positive relationship with their total content and organic matter content of soil but no relationship was observed with soil pH. Washing of vegetables with clean water was a very effective and easy way of decontaminating the metal pollution as it reduced the contamination by 75 to 100%. Keywords: bio-availability, contamination, heavy metals, okra, safe limits, spinach

1. Introduction Throughout the world, there is a long tradition of farming intensively within and at the edge of cities (Smit et al., 1996). However, most of these peri-urban lands (lands in the periphery of city) are contaminated with pollutants including heavy metals such as Cu, Zn, Pb, Cd, Ni, and Hg. These metals are contributed mainly through industrial effluents, sewage and sludge, vehicular emission, diesel generators and application of pesticides in agriculture. This loading of heavy metals often leads to degradation of soil health and contamination of food chain mainly through the vegetables grown on such soils (Jackson and Alloway, 1992; Rattan et al., 2002). When grown on contaminated soils toxic metals are accumulated in vegetables resulting in reduction of yields due to the inhibition of metabolic processes (Sanders et al., 1987). Heavy metals are cytotoxic, mutagenic and carcinogenic in nature and are posing serious threat to the urban population, which rely on vegetable

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and foliage crops grown in peri-urban lands (Kumar et al., 1991; Fargasova, 1994). Peri-urban lands of New Delhi, the capital city of India, are intensively cultivated with vegetable crops. The objectives of the present study were to (1) estimate the heavy metal loads of soil, irrigation water and vegetables of peri-urban lands of New Delhi, (2) understand the mode of heavy metal contamination in these areas and (3) formulate strategies for safe and sustainable cropping in such areas. 2. Materials and Methods 2.1. S ELECTION

OF SAMPLING SITES AND VEGETABLES

Based on the intensity and types of vegetable grown in the peri-urban Delhi, five major vegetable growing sites viz., Yamunapusta, Okhla, Najafgarh, Alipur and Ballabhgarh (Figure 1) were selected for assessing the heavy metal contamination load of soil, irrigation water and vegetables in these sites. These five sites are located in four different directions of Delhi and are subjected to different sources of heavy metal contamination. Yamunapusta site is prone to contamination through aerial deposition from a coal-based thermal power plant operating in this area. At Okhla site thermal power plant and sewage water are the major sources of contamination. At Najafgarh the main source of contamination is sewage water while at Ballabhgarh the main source is industrial emission. At Alipur sources of heavy metals are mainly agro-chemicals (micro-nutrients and fungicides) as there are almost no industries in this area. Two important vegetables namely okra (Abesmoschus esculentus L.) and spinach (Spinacia oleracea L.), grown extensively in these sites were selected for this study. Four heavy metals i.e., copper (Cu), zinc (Zn), lead (Pb) and cadmium (Cd) were identified for assessing their load in soil, water and vegetables. These elements were selected on the basis of their occurrence and severity in the city. 2.2. C OLLECTION

AND ANALYSES OF SOIL , WATER AND VENGETABLE

SAMPLES

Surface soil (0–15 cm), irrigation water and vegetable (okra fruits and spinach leaves) samples were collected from the farmers’ fields of selected sites during rainy season (August to September) of 2003. In order to represent the whole vegetable growing areas of the selected sites, three sub-sites within each site were marked for soil, water and vegetable sampling. Further, three samples were collected from each sub-site. The samples of soil, water and vegetables were processed immediately after collection for chemical analyses. A part of the vegetable samples were washed

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Figure 1. Delhi map showing the location of sampling sites of soil, irrigation water and vegetables.

thoroughly with tap water to examine the extent of heavy metal removed by the process. The soil samples were air-dried while the vegetable samples were dried in hot air oven at 70 ◦ C for 48 hours. After drying the samples were ground to fine powder for further analyses. The soil and irrigated water samples were analyzed for organic carbon content and pH as per the procedure described by Walkley and Black (1949) and Singh et al. (1999), respectively. For total content of metals (Cu, Zn, Pb, Cd) in the soil, vegetables and water, samples were digested by the wet digestion method and analyzed by atomic absorption spectrophotometer (Singh et al., 1999). Available heavy metal contents in soil were estimated by DTPA-CaCl2 -TEA method (Singh et al., 1999).

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3. Results and Discussion 3.1. PHYSICO-CHEMICAL

PROPERTIES AND HEAVY METAL CONTENTS OF SOIL AND WATER OF THE PERI - URBAN DELHI

The soils of the five sites were slightly alkaline with pH 7.5 to 8.2 (Table II). Organic carbon content ranged from 0.5 to 1%. At Yamunapusta, Alipur, Najafgarh, Okhla and Ballabhgarh organic carbon content of soil were 1.0, 0.5, 0.6, 0.6 and 0.9%, respectively (Table II). Irrigation water of these sites had pH 7.7 to 8.5 (Table III). Among the heavy metals Zn content was the highest (83.1–196.3 μg g−1 soil) followed by Cu (28.8–129.6 μg g−1 ), Pb (32.1–59.6 μg g−1 ) and lowest with Cd (2.1–2.5 μg g−1 ) at all the five peri-urban sites (Table I). Soil samples collected from the sites showed marked variation in the level of heavy metal contamination (Table I). Ballabhgarh soils had the highest level of Cu (129.6 μg g−1 soil) and Zn (196.3 μg g−1 ) but lowest level of Cd contamination (2.1 μg g−1 ). Yamunapusta soils registered the highest levels of Pb (59.6 μg g−1 ). The differences in heavy metal level of soil was due to use of different quantity and quality of sewage and sludge, level and type of industrialization near the sampling sites, proximity of field from main road and highways, presence of thermal power plants, and use of different types of agrochemicals containing heavy metals (Rajurkar and Kulkarni, 1997; Rattan et al., 2002; Marshall et al., 2003). Higher levels of metal contamination in soil were due to thermal power plants near Yamunapusta and Okhla, and use of industrial effluents and sewage and sludge in crop fields at Yamunapusta, Okhla and Najafgarhlead. Rattan et al. (2002) reported that at Najafgarh site sewage effluent is used for irrigation for the last two decades and the DTPAextractable Zn, Cu, Fe and Ni increased by 253, 202, 337 and 157%, respectively in effluent irrigated soil as compared to that of tube well irrigated soils. Low level of contamination at Alipur site was due to its semi-rural background with least industrialization. However, Cd level in Alipur soil was at par to other sites due to application of high dose of phosphetic fertilizers and metal based pesticides in vegetable crops. Contrary to the marked differences in total metal content of soils among the different sites, small differences were observed in available metal contents in soil. The available Cu, Zn, Pb and Cd contents ranged between 1.4–11.4, 1.9 –5.5, 1.3– 2.6 and 0.06–0.09 μg g−1 soil, respectively at all the sites (Table I). Thus only 5.2–8.8, 2.4–7, 2.7–6.5 and 2.3–4.3% of total Cu, Zn, Pb and Cd contents of soil, respectively, were available (Table II). Availability of heavy metal is decided by the various physico-chemical properties of soil. Organic matter in soil may chelate these metals and reduce their availability. Cationic metals such as Zn+2 , Cu+1 , Cu+2 , Pb+2 , Cd+2 may exist as adsorbed ion on clay-humus complex. Several studies have reported strong relationships between soil pH and organic carbon with metal absorption in soil (Adriano, 1986). In the present study it was observed that

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TABLE I Heavy metal content in soils of peri-urban Delhi Total metal content (μg g−1 soil) Sampling site Yamunapusta Shakarpur Yamunapusta Ranigarden Range Mean Alipur Hamidpur Bhakhatawarpur Palla Range Mean Najafgarh Dichaukala Bakarwala Ranhola Range Mean Okhla Meethapur Madanpurkhadar Aligaon Range Mean Ballabhgarh Uchagaon Chandawali Range Mean

Cu

Zn

35.5 67.3 125.1 32–129 76.0

103.5 155.4 253.4 95–262 170.7

46.6 63.3 68.8 3.2–7.0 59.6

2.7 1.4 2.4 4.4 2.4 16.4 2.0–3.0 1–16 2.5 7.4

20.8 26.6 39.1 16–43 28.8

65.1 82.8 101.5 60–111 83.1

18.4 23.5 54.3 0.9–6.5 32.1

2.4 2.2 2.5 2.0–2.7 2.4

1.0 1.1 2.1 0.8–2.5 1.4

23.4 50.4 57.2 17–92 43.7

79.5 126.0 134.1 65–202 113.2

18.7 58.5 47.4 1.0–6.5 41.5

2.3 2.5 2.3 2.0–3.0 2.4

1.2 4.1 2.8 0.7–9.4 2.7

16.8 64.8 53.6 124.8 127.5 98.6 14–120 40–214 66.0 96.1

16.7 56.0 47.2 1.4–9.6 40.0

127.8 131.4 120–132 129.6

56.2 61.8 49–68 59.0

204.1 188.5 167–213 196.3

Pb

Available metal content (μg g−1 soil) Cd

Cu

Zn

Pb

Cd

1.3 4.3 10.3 1–11 5.3

1.2 2.0 1.5 1.0–2.0 1.6

0.05 0.06 0.08 0.04–0.1 0.06

1.9 2.0 1.9 1.6–2.3 1.9

1.6 1.4 1.1 0.9–1.8 1.4

0.06 0.07 0.04 0.04–.07 0.06

2.4 5.3 5.1 2–10 4.3

1.5 1.2 1.1 0.9–1.6 1.3

0.05 0.07 0.09 0.04–.15 0.07

2.4 1.0 2.2 4.3 2.3 10.3 1.7–2.8 0.9–12 2.3 5.2

1.7 7.2 6.6 1.7–7.0 5.2

0.9 1.0 2.2 0.7–2.4 1.4

0.07 0.06 0.08 0.04–.10 0.07

2.0 2.1 1.8–2.3 2.1

5.6 5.4 4.8–6.8 5.5

2.6 2.6 2.1–2.9 2.6

0.09 0.08 0.07–0.1 0.09

11.7 11.1 11–13 11.4

the availability of Cu and Zn in soil decreased with increasing pH (Figure 2). This could be due to the formation of insoluble complex of ZnCO3 or CuCO3 . On the contrary for Pb and Cd, increased soil pH influenced little to the availability as they follow a different path of fixation. A positive relationship between available heavy metal content and organic carbon content of soil was observed (Figure 2). Similar positive relationship between total Cu and Zn contents with their available

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TABLE II Physico-chemical properties and metal availability in soils of peri-urban Delhi Metal availability (%) Sampling site Yamunapusta Shakarpur Yamunapusta Ranigarden Range Mean Alipur Hamidpur Bhakhatawarpur Palla Range Mean Najafgarh Dichaukala Bakarwala Ranhola Range Mean Okhla Meethapur Madanpurkhadar Aligaon Range Mean Ballabhgarh Uchagaon Chandawali Range Mean

Cu

Zn

Pb

Soil character Cd

pH

Org. C (%)

3.8 6.6 13.3 2.7–14.3 7.9

1.2 2.7 4.1 1.0–4.2 2.7

2.8 3.1 2.3 1.8–4.6 2.7

1.9 2.7 3.2 1.5–4.0 2.6

7.9 7.6 6.9 6.6–8.1 7.5

0.5 1.1 1.4 0.4–1.4 1.0

5.0 5.3 5.3 4.0–7.0 5.2

2.9 2.5 1.9 1.5–3.6 2.4

10.1 7.5 2.0 1.9–14.0 6.5

2.3 3.0 1.7 1.6–3.1 2.3

7.8 7.8 7.9 7.1–8.3 7.8

0.5 0.5 0.5 0.02–0.7 0.5

5.0 6.7 5.7 2.7–12.0 5.8

3.0 3.6 4.1 2.2–5.6 3.6

10.1 2.1 2.5 1.6–6.8 4.9

2.4 2.7 3.7 1.6–6.2 2.9

7.8 7.7 7.2 6.8–8.2 7.6

0.4 0.7 0.6 0.03–1.0 0.6

6.3 8.4 8.1 4.6–9.5 7.6

2.6 6.1 12.2 2.5–17.0 7.0

5.8 2.3 7.2 1.2–10.0 5.1

2.7 2.9 3.5 1.7–5.0 3.0

8.2 7.7 7.9 7.7–8.3 8.0

0.5 0.5 0.8 0.3–0.9 0.6

9.2 8.5 8.3–9.8 8.8

2.8 2.9 2.3–3.4 2.8

4.7 4.2 3.5–5.3 4.5

4.4 4.1 3.2–5.0 4.3

8.1 8.1 8.0–8.2 8.1

0.9 0.9 0.8–1.0 0.9

contents was recorded. But for Pb and Cd such relationship did not exist indicating that their availability depends upon soil pH rather than total contents in soil. In earlier studies also pH was reported to be one of the important factors of heavy metal availability (Sanders et al., 1987; Chang et al., 1987; Anderson and Christensen, 1988). But clay content (Herms and Burmmer, 1984), ion interaction (Baste and Tabatabai, 1992), redox potential (Brown et al., 1989) and organic matter content (McGrath et al., 1988) are also significant. Additionally,

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Figure 2. Relationship between soil pH and organic carbon content and available metal content.

heavy metal availability can be directly affected by plant itself (Zhang et al., 1991). The average Cu, Zn, Pb and Cd loads of irrigation water ranged between 0.16– 0.20, 0.02–0.11, 0.26–0.60 and 0.02–0.03 μg mL−1 , respectively (Table III). The

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TABLE III Heavy metal content and pH in irrigation water of peri-urban Delhi Metal content in irrigation water (μg ml−1 ) Sampling site Yamunapusta Yamunapusta Sakarpur Ranigarden Range Mean Najafgarh Dichukala Bakarwala Ranhola Range Mean Alipur Bhakthawarpur Hamidpur Palla Range Mean Okhla Madanpurkhadar Aligaon Meethapur Range Mean Ballabhgarh Chandanwali Unchagaon Range Mean

Cu

Zn

Pb

Cd

pH

0.17 0.18 0.14 0.1–0.3 0.16

0.04 0.01 000 0–0.05 0.02

0.26 0.45 0.03 0.03–0.7 0.26

0.03 0.02 0.03 .02–0.03 0.03

8.2 8.0 7.9 7.7–8.4 8.0

0.15 0.18 0.27 0.1–0.3 0.20

0.09 0.03 0.05 .03–0.06 0.06

0.33 0.35 0.40 0.23–0.5 0.36

0.03 0.03 0.03 .03–0.03 0.03

8.2 8.1 8.5 8.0–8.5 8.3

0.14 0.26 0.16 0.1–0.3 0.19

0.04 0.06 0.03 .03–.07 0.04

0.16 0.49 0.23 .03–0.8 0.29

0.03 0.03 0.03 .02–0.04 0.03

8.0 8.0 7.9 7.7–8.1 8.0

0.13 0.23 0.18 0.08–0.30 0.18

0.04 0.07 0.07 0–0.10 0.06

0.30 0.70 0.80 0.40–0.80 0.60

0.03 0.03 0.03 0.02–0.03 0.03

8.0 8.2 8.2 8.0–8.2 8.1

0.09 0.24 0.08–.31 0.16

0.16 0.07 0.08–.24 0.11

000 0.52 0–0.7 0.26

0.03 0.03 0.03–0.04 0.03

8.2 8.1 7.7–8.3 8.1

variation in metal load was due to the variation in quality of sewage used and background level of metal contamination. 3.2. H EAVY

METAL CONTENTS IN VEGETABLES

Loads of Cu, Zn, Pb and Cd in okra collected from different sites ranged between 15.3–25.0, 79.4–150.0, 1.1–5.6 and 1.1–7.0 μg g−1 , respectively (Table IV). Most (90–100%) of the okra samples registered Cu contamination level below its safe

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TABLE IV Heavy metal content in vegetables grown in peri-urban Delhi Spinach (μg g−1 ) Sampling site Yamunapusta Yamunapusta Ranigarden Sakarpur Range Mean Alipur Bhakhatawarpur Palla Garhimajra Range Mean Najafgarh Bakarwala Dichukala-1 Ranhola Range Mean Okhla Madanpurkhadar Aligaon Methapur Range Mean Ballabhgarh Chandawali Muchgar Uchagaon Range Mean

Cu

Zn

Okra (μg g−1 )

Pb

Cd

11.5 99.0 39.2 255.2 18.9 110.5 7–50 98–282 23.2 154.9

1.8 1.5 1.8 1.4–2.2 1.7

2.0 20.6 1.9 20.3 2.1 20.6 1.7–2.2 19–23 2.0 20.4

20.8 22.3 21.9 18–23 21.7

3.6 4.9 5.0 3–6 4.5

3.1 17.0 102.2 1.7 1.0 2.9 18.5 139.5 2.9 1.2 2.3 18.5 135.8 2.3 1.0 1.7–3.5 16–29 103–154 1.4–4.4 0.8–1.2 2.8 18.0 125.8 2.3 1.1

69.7 69.3 74.3 68–78 71.1

Cu

Zn

Pb

Cd

154.0 1.8 1.2 151.2 1.6 1.3 144.5 1.4 1.2 39–156 0.9–2.2 1.1–1.4 150.0 1.6 1.2

27.8 74.1 6.3 3.4 18.1 31.4 104.7 6.1 3.7 14.7 29.9 76.8 7.4 4.1 12.9 28–32 69–109 5.5–9.0 3.2–9.2 12–18 29.7 85.2 6.6 3.7 15.3

124.5 1.1 4.3 98.5 1.6 4.4 84.2 0.6 4.3 81–182 0.4–1.7 4.1–4.5 102.4 1.1 4.3

20.2 18.8 15.8 18–21 18.3

94.5 2.6 5.2 21.6 59.0 3.3 5.6 25.8 55.3 4.2 5.8 27.6 51–108 2.4–4.7 5.1–6.0 20–32 69.6 3.4 5.5 25.0

98.4 1.4 4.7 103.3 2.2 4.9 100.0 2.9 5.1 88–110 1.1–3.1 4.5–5.2 100.6 2.2 4.9

25.0 26.3 25.2 24–29 25.5

64.0 69.7 72.7 60–84 68.8

7.1 6.9 24.8 6.8 7.1 22.8 7.1 7.4 23.6 6.3–7.5 6.5–7.5 22–24 7.0 7.1 23.7

78.3 85.0 75.0 68–89 79.4

5.2 6.9 5.5 7.1 5.8 7.1 4.7–6.0 6.7–7.3 5.6 7.0

Safe limits: Cu (30 μg g−1 ); Zn (50 μg g−1 ); Pb (2.5 μg g−1 ); Cd (1.5 μg g−1 ). Source: Prevention of Food Adulteration Act of India (1954).

limit (30 μg g−1 ) while 100, 33 and 60% samples crossed the safe limits of Zn (50 μg g−1 ), Pb (2.5 μg g−1 ) and Cd (1.5 μg g−1 ) (Table V). The contents of Cu, Zn, Pb and Cd in spinach samples ranged between 18.3– 29.7, 69.6–154.9, 1.7–7.0 and 2.0–7.1 μg g−1 , respectively (Table IV) and 13, 95, 78 and 100% samples, respectively exceeded the safe limits (Table V).

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TABLE V Mean content of heavy metals in okra and spinach collected from fields Metal content in vegetable samples (μg g−1 ) Sampling site Okra Yamunapusta Alipur Najafgarh Okhla. Ballabhgarh Mean Spinach Yamunapusta Alipur Najafgarh Okhla. Ballabhgarh Mean Sampling site Yamunapusta Alipur Najafgarh Okhla. Ballabhgarh Mean

% Samples crossed the safe limits

Cu

Zn

Pb

Cd

Cu

Zn

Pb

Cd

20.4 18.0 15.3 25.0 23.7 20.5

149.9 126.5 102.4 100.6 79.7 111.8

1.6 2.3 1.0 2.2 5.6 2.5

1.2 1.1 4.3 4.9 7.0 3.7

0 0 0 11 0 2

100 100 100 100 100 100

0 33 0 33 100 33

0 0 100 100 100 60

23.2 21.7 29.4 20.5 25.5 24.1

154.9 71.1 85.2 70.9 68.8 90.2

1.7 4.5 6.6 3.4 6.9 4.6

2.0 2.8 3.7 5.5 7.1 4.2

8 0 58 0 0 13

100 100 100 100 75 95

0 100 100 90 100 78

100 100 100 100 100 100

21.8 19.8 22.3 22.7 24.6 22.2

157.4 98.8 93.8 85.7 74.3 102.0

1.7 3.4 3.8 2.8 6.2 3.6

1.6 1.9 4.0 5.2 7.1 4.0

4 0 29 6 0 8

100 100 100 100 78 98

0 69 50 60 100 55

50 50 100 100 100 80

TABLE VI Effect of washing on metal removal from vegetable samples Metal content in vegetable samples (μg g−1 ) Cu

Zn

Pb

Cd

Crop

U.W.

W

U.W.

W

U.W.

W

U.W.

W

Okra Spinach Cauliflower Mean

11.3 11.5 3.7 8.8

9.2 8.5 1.7 6.5

37.3 39.2 28.3 34.9

23.9 23.7 21.5 23.0

2.3 2.0 0.0 1.4

0.3 0.3 0 0.2

1.0 1.9 1.0 1.3

0.1 0.2 0 0.1

U.W: Unwashed; W: Washed.

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Figure 3. Relationship between total and available metal content in soil and okra and palak (spinach).

Spatial difference in heavy metal contamination load of okra as well as spinach was due to different levels of heavy metal contamination of soil, water and air through various polluting sources prevailed in these areas (Tables I and III). Apart from heavy metal content in soil, the pysico–chemical properties of soil also affect the degree of contamination of vegetable crop. The cultural practices adopted by

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farmers at different sites, like use of industrial and municipal effluents for irrigation, application of sewage-sludge and application of agro- chemicals might have caused differences in the level of contamination. The results showed higher level of Pb contamination in spinach (4.6 μg g−1 ) compared to okra (2.5 μg g−1 ) (Table V). Several workers have reported higher level of metal contamination in foliage leafy vegetables than in non-foliage crops (Bingham et al., 1979). The difference in level of heavy metal contamination between okra and spinach was due to their morpho-physiological differences in terms of heavy metal uptake, exclusion, accumulation, foliage deposition and retention efficiency (Carlton Smith and Davis, 1983). When the vegetable samples were washed 2–3 times with clean tap water, the level of heavy metals reduced drastically (Table VI). Removal of contamination load by washing was greater for Pb and Cd (75–100%) than those for Cu and Zn (27– 55%). This indicated that the Pb and Cd contamination of vegetables was mainly through foliar deposition, while Cu and Zn contamination was mainly through uptake from soil. Moreover, a higher positive correlation between available metal contents in soil and their corresponding contents in okra as compared to that of spinach (Figure 3) indicated that okra took up more metals from soil. Spinach being a leafy vegetable with large exposed leaf area than okra fruits, might have led to higher deposition of heavy metal on its foliage especially.

4. Conclusion The soil, irrigation water and okra and spinach vegetables samples collected from five peri-urban sites of New Delhi showed substantial level of contamination with heavy metal such as Cu, Zn, Pb and Cd. Most of the soil and irrigated water samples showed higher levels of Pb and Cd loads as compared to Cu and Zn loads. Most of the vegetable samples crossed the safe limits of Zn, Pb and Cd while Cu content was below the safe limit. The level of contamination was higher in spinach than in okra. Thermal power plants, industrial effluent, sewage water and pesticides contributed the metal contamination of soil and vegetables. Thus peri-urban lands may be utilized with caution for growing vegetables. These lands may be used for growing ornamental and timber plants rather than leafy, root and tuber vegetables to minimize the transfer of heavy metals from soil to human beings through food chain.

References Adriano, D. C.: 1986, Trace Elements in the Terrestrial Environment, Springer-Verrlag Inc., New York, pp. 1–533. Anderson, P. R. and Christensen, T. H.: 1998, ‘Distribution of coeffieceant of Cd, Co, Ni and Zn in soils’, Journal of Soil Science 39, 15–22.

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