Heavy Metal Contamination in the Food Chain

Besides adversely influencing plant growth, the toxic effects of heavy metals are amplified along the food chain at each stage of the food web (Fig. 18.1).

Table 18.1 Worldwide input of heavy metals into soils (1,000 tons yr ) (from Nriagu and Pacyna 1988)

Source

Antimony

Arsenic

Cadmium

Chromium

Copper

Lead

Manganese

Mercury

Molybdenum

Nickel

Selenium

Vanadium

Zinc

Agricultural

4.9

5.8

2.2

82

67

26

158

0.85

34

45

4.6

19

316

and animal

wastes

Logging and

2.8

1.7

1.1

10

28

7.4

61

1.1

1.6

13

1.6

5.5

39

wood wastes

Urban refuse

0.76

0.40

4.2

20

26

40

24

0.13

2.3

6.1

0.33

0.2

60

Municipal

0.18

0.25

0.18

6.5

13

7.1

8.1

0.44

0.43

15

0.11

1.3

39

sewage and

organic waste

Solid waste

0.08

0.11

0.04

1.5

4.3

7.6

2.6

0.04

0.08

1.7

0.10

0.12

11

from metal

fabrication

coal ash

12

22

7.2

289

214

144

1076

2.6

441

68

32

39

298

Fertilizers and

0.25

0.28

0.2

0.32

1.4

2.9

12

0.01

0.46

2.2

0.27

0.97

2.5

peat

Discarded

2.4

38

1.2

458

592

292

300

0.68

1.9

19

0.15

1.7

465

manufactured

products3

Atmospheric

2.5

13

5.3

22

25

232

27

2.5

2.3

24

2.0

60

92

fallout"

Total input

26

82

22

898

971

759

1669

8.3

87

294

41

128

1322

" Metals used for industrial installations and durable goods are assumed to have a definite lifespan and to be released into the environment at a constant rate b Totals are rounded

Note that these inputs exclude mine and slags at smelter sites

" Metals used for industrial installations and durable goods are assumed to have a definite lifespan and to be released into the environment at a constant rate b Totals are rounded

Note that these inputs exclude mine and slags at smelter sites

Table 18.2 Accumulation of DTPA-extractable heavy metals in soils (mg kg'1) of various Indian cities (from Rattan et al. 2002)

City

Zinc

Copper

Iron

Manganese

Cadmium

Lead

Nickel

Chromium

Arsenic

Mercury

Kolkata

281

36.0

115

24.0

0.45

104.3

9.45

12.5

nd

nd

Ludhiana

4.38

5.50

36.0

10.9

0.07

1.88

0.37

0.57

1.02

0.51

Jalandhar

14.7

4.20

39.7

18.9

nda

nd

1.27

1.72

2.09

nd

Patna

11.4

14.5

54.9

19.4

0.21

10.2

nd

nd

nd

nd

IARI, Delhi

5.0

3.30

23.3

12.1

nd

0.40

nd

nd

nd

nd

Keshopur, Delhi

6.77

5.42

40.3

5.17

0.15

2.34

0.91

nd

nd

nd

Hyderabad

6.8

1.09

16.2

16.0

0.14

10.5

0.46

0.34

nd

nd

Madurai

5.9

5.70

32.3

39.0

0.10

3.70

4.90

2.90

nd

nd

Coimbatore

10.4

9.70

28.5

37.0

0.20

6.30

14.6

3.80

nd

nd

and-not deleted and-not deleted

Table 18.3 Plant nutrients and heavy metal contents in sewage effluents and tube well water from two villages in Delhi (from Purakayastha 2008a; Rattan et al. 2005)

N P K S Fe Mn

Zn

Cu

Cd

Pb

Ni

(mg L-1)

(Hg L-1)

Madanpur Khadar village irrigated with sewage effluents from Okhla Sewage Treatment Plant

Sewage effluents 4.65 2.63 9.86 10.6 1658 120

85.0

36.0

2.85

75.0

150

Tube well water 1.20 0.26 2.59 8.90 650 15.0

21.0

12.0

1.52

35.0

85.0

aIncrease (-fold) 3.90 10.0 3.80 1.20 2.60 8.0

4.0

3.0

1.90

2.10

1.80

Bakarwala village irrigated with sewage effluents from Keshopur Sewage Treatment Plant

Sewage effluents - 2.57 11.7 15.9 1464 64.0

61.0

29.0

1.53

33.0

49.0

Tube well water - 0.22 3.58 14.3 557 10.0

11.0

8.0

1.42

30.0

37.0

aIncrease (-fold) - 11.7 3.30 1.10 2.60 6.40

5.50

3.60

1.10

1.10

1.30

" Increase over tube well irrigated soils

" Increase over tube well irrigated soils

Table 18.4 Long-term effects of sewage irrigation on available zinc, copper, iron, manganese, cadmium, lead, and nickel concentrations in soils of two villages of Delhi (from Purakayastha 2008a; Rattan et al. 2005)

Available metal (mg kg-1)

Table 18.4 Long-term effects of sewage irrigation on available zinc, copper, iron, manganese, cadmium, lead, and nickel concentrations in soils of two villages of Delhi (from Purakayastha 2008a; Rattan et al. 2005)

Available metal (mg kg-1)

Zinc

Copper

Iron

Manganese

Cadmium

Lead

Nickel

Madanpur Khadar village

Sewage-irrigated

3.S1**

6.44**

6S.G*

22.3**

G.27**

4.G9*

1.59**

Tube well

1.34

3.12

26.2

1G.3

G.13

2.59

G.S7

irrigated

a(%)

1S4

1G6

16G

117

1GS

5S

S3

Mundka village

Sewage-irrigated

6.3S**

6.53**

62.2**

S.43**

G.22*

2.64**

1.29**

Tube well

2.23

2.53

25.4

12.7

G.19

2.22

G.54

irrigated

a (%)

247

1S1

341

-1S

3S

44

123

a (%), increase or decrease over tube well irrigated soils, " * i-tests showing significance at the 5% and 1% levels, respectively

Industrial products Burned fuel Fertilizers Pesticides

Rocks in Earth's crust

Plants

Soil

Domestic animals "

Fig. 18.1 Biomagnification of heavy metals along the food chain

The heavy metals gain entry into the human and animal food chain through crops grown on soils contaminated with them. Such soils are often used to cultivate leafy vegetables and tuber crops to meet the demands of nearby urban populations. These crops are known for their capacity to accumulate heavy metals in their edible parts. The entry of heavy metals such as Cd, Zn, Pb, Cu, Ni, Mn, and Fe into the food chain has been widely reported.

In an Indian context, Rattan et al. (2005) observed increasing accumulations of Zn, Ni, Cu, and Fe in different fields containing vegetable and fruit crops such as maize, mustard, rice, jowar, spinach, cauliflower, brinjal, radish, guavas, citrus, etc., which were grown under sewage irrigation from the Keshopur Effluent Irrigation System in Western Delhi. The agricultural sustainability of such production system depends to a large extent upon maintaining or enhancing the soil quality, which is rapidly deteriorating due to the disposal of untreated effluents onto it. About 9.5% of rice paddy soils have been rendered unsuitable for growing rice for human consumption because of excessive metal contamination.

Different doses of heavy metals can cause undetectable, therapeutic, toxic, or even lethal effects. Selenium, copper, and zinc often become toxic as the dose of the metal and exposure to it increase. These metals enter livestock as well as our own bodies through the food chain. Zinc toxicosis is manifested as gastrointestinal distress, decreased food consumption, anorexia, hemoglobinuria, anemia, poor bone mineralization, and arthritis. Lead poisoning is the most frequently diagnosed toxicological condition in veterinary medicine. Its occurrence has been reported in all domestic species.

Many trace elements are essential for normal metabolism, but most can also be toxic if the intake is much above the required level. For example, the ingestion of Zn in large amounts can cause vomiting, diarrhoea, and neurological damage. Wilson's disease is an autosomal recessive disorder in which the inherited metabolic defect is associated with the gradual and progressive accumulation of Cu in the liver. Symptoms of Cu toxicity include hemolysis, hepatic necrosis, and renal damage. A study conducted in 1990 in Bangkok, Thailand, showed that by age seven the average child had lost six points in IQ tests because of Pb poisoning from the air (Gupta and Gupta 1998). Average blood Pb levels in Thailand were 40-45 jmg dL-1, which is ten times the US standard.

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