Groundwater Depletion

7/31/2019 Groundwater Depletion

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Long-term impact of irrigation with sewage effluents on heavymetal content in soils, crops and groundwatera case study

R.K. Rattan a,*, S.P. Datta a, P.K. Chhonkar a, K. Suribabu a, A.K. Singh b

aDivision of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110012, Indiab Water Technology Centre, Indian Agricultural Research Institute, New Delhi 110012, India

Received 15 September 2004; received in revised form 3 February 2005; accepted 22 February 2005

Abstract

There is a gradual decline in availability of fresh water to be used for irrigation in India. As a consequence, the use of sewage

and other industrial effluents for irrigating agricultural lands is on the rise particularly in peri-urban areas of developing

countries. On the other hand, there is increasing concern regarding the exceedance of statutory and advisory food standards for

trace metals throughout the world. Hence, a case study was undertaken to assess the long-term effect of sewage irrigation on

heavy metal content in soils, plants and groundwater. For this purpose, peri-urban agricultural lands under Keshopur Effluent

Irrigation Scheme (KEIS) of Delhi, India were selected where various cereals, millets, vegetable and fodder crops have

successfully been grown. Sewage effluents, ground water, soil and plant samples were collected and analysed mainly for metal

contents. Results indicated that sewage effluents contained much higher amount of P, K, S, Zn, Cu, Fe, Mn and Ni compared togroundwater. While, there was no significant variation in Pb and Cd concentrations in these two sources of irrigation water and

metal content were within the permissible limits for its use as irrigation water. There was an increase in organic carbon content

ranging from 38 to 79% in sewage-irrigated soils as compared to tubewell water-irrigated ones. On an average, the soil pH

dropped by 0.4 unit as a result of sewage irrigation. Sewage irrigation for 20 years resulted into significant build-up of DTPA-

extractable Zn (208%), Cu (170%), Fe (170%), Ni (63%) and Pb (29%) in sewage-irrigated soils over adjacent tubewell water-

irrigated soils, whereas Mn was depleted by 31%. Soils receiving sewage irrigation for 10 years exhibited significant increase in

Zn, Fe, Ni and Pb, while only Fe in soils was positively affected by sewage irrigation for 5 years. Among these metals, only Zn in

some samples exceeded the phytotoxicity limit. Fractionation study indicated relatively higher build-up of Zn, Cu, Fe and Mn in

bioavailable pools of sewage-irrigated soils. By and large, tissue metal concentrations in all the crops were below thegeneralized

critical levels of phytotoxicity. Based on the soil to plant transfer ratio (transfer factor) of metals, relative efficiency of some

cereals, millet and vegetable crops to absorb metals from sewage and tubewell water-irrigated soils was worked out. Risk

assessment in respect of metal contents in some vegetable crops grown on these sewage-irrigated soils indicated that thesevegetables can be consumed safely by human.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Long-term sewage irrigation; Heavy metals; Soils; Groundwater; Crops; Hazard quotient

www.elsevier.com/locate/agee

Agriculture, Ecosystems and Environment 109 (2005) 310322

* Corresponding author. Tel.: +91 11 25841991; fax: +91 11 25841529.

E-mail address: [email protected] (R.K. Rattan).

0167-8809/$ see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2005.02.025


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1. Introduction

India supports more than 16% of the worlds

population with only 4% of the worlds fresh waterresources (Singh, 2003). Although agriculture sector in

this country has been major user of water, share of

water allocated to irrigation is likely to be decreased by

1015% in next two decades (CWC, 2000). In this

changing scenario, reuse of domestic and industrial

waste water in agriculture for irrigating crops appears

to be a lucrative option. Besides being source of

irrigation water, these waste waters contain appreciable

amounts of plant nutrients. In India, total waste water

generated per annum from 200 cities is about

2600 Mm3 (Kaul et al., 1989) and also the use of

sewage effluents for irrigating agricultural lands is on

the rise especially in the peri-urban area. These waste

waters carry appreciable amounts of trace toxic metals

(Feign et al., 1991; Pescod, 1992; Som et al., 1994;

Gupta et al., 1998; Brar et al., 2000; Yadav et al., 2002)

and concentrations of trace metals in sewage effluents

vary from city to city (Rattan et al., 2002). Although the

concentration of heavy metals in sewage effluents are

low, long-term use of these waste waters on agricultural

lands often results in the build-up of the elevated levels

of these metals in soils (Rattan et al., 2002). Extent of

build-up of metals in waste water-irrigated soilsdepends on the period of its application (Bansal

et al., 1992; Palaniswami and Sree Ramulu, 1994).

Crops raised on the metal-contaminated soils accu-

mulate metals in quantities excessive enough to cause

clinical problems both to animals and human beings

consuming these metal rich plants (Tiller, 1986). Since

food chain contamination is one of the major routes for

entry of metals into the animal system, monitoring the

bioavailabile pools of metals in contaminated soils has

generated a lot of interest (Datta et al., 2000; Yadav

et al., 2002). Also information on the fractionation oftrace metals in soils is potentially valuable in predicting

bioavailability, metal leaching rates, and transforma-

tion between chemical forms in agricultural and

polluted soils (Jenne and Louma, 1977; McBride,

1981; Miller et al., 1986). But such information on

agricultural lands receiving sewage irrigation for quite

a long time is meagre.

Crop species exercise differentiality in accumulat-

ing metals in their tissue (Lepp, 1981; Datta et al.,

2000) and ef ficiency of different crops in absorption of

metals is judged either by plant metal uptake or by

transfer factor of metals from soil to plants. Uptake of

metals by plants may be a good indicator of efficiency

of metal absorption of different crop species grown onsoils having uniform metal levels under controlled

conditions. Whereas, transfer factor of metal from soil

to plants indicates the efficiency of crop species better

where crops are grown on soils having variable metal

contents, e.g. farmers fields. Generally, soil to plant

transfer factor of metals is computed based on total

metal contents of soils (Hooda et al., 1997). However,

total metal content in soils does not take into account

the other soil factors that modify the bioavailability of

metals. Hence, computation of soil to plant transfer

factor of metals should be based on available soil

metal pools.

We attempted to study some of the above-

mentioned aspects in sewage-irrigated peri-urban

agricultural lands under Keshopur Effluent Irrigation

Scheme (KEIS) of the Delhi Government, India. Under

this scheme, sewage irrigation has been provided to the

farmers fields for more than two decades. Various

cereals, millet, leafy vegetables and fodder crops have

successfully been grown thereon. No investigation has

been carried out in sewage-irrigated soils of this peri-

urban area except one where we reported the build-up

of some of major and secondary plant nutrients in soilsas a result of sewage irrigation (Rattan et al., 2001).

The objectives of this study were: (i) to study the

chemical composition of sewage effluents emanating

from KEIS and groundwater; (ii) to assess the effects of

long-term irrigation with sewage effluents on metal

contents in soils and plants; and (iii) to assess the risk of

consuming leafy green vegetables in respect of their

heavy metal contents grown on sewage-irrigated soils

to human beings.

2. Materials and methods

2.1. Study area and collection of samples

The study area is located in the western Delhi, India,

where sewage effluents originating from Keshopur

Sewage Treatment Plant have been used for irrigation

purposes since 1979 (Fig. 1). Our study confined to six

villages, viz. Nilothi, Mundka, Ranhola, Bakarwala,

Hirankudna and Dichaonkalan covering 123, 233, 196,

R.K. Rattan et al. / Agriculture, Ecosystems and Environment 109 (2005) 310322 311


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775, 183 and 98 acres of agricultural lands,respectively.

Agricultural lands offirst four villages have been under

sewage irrigation for 20 years, while in Hirankudna and

Dichaokalan, crops have been irrigating with sewage

effluents for 10 and 5 years, respectively. Sewage

effluents were collected periodically from 13 locations

spread over these six villages under study. Similarly,

groundwater samples were also collected from eight

tubewells located in these villages. First sampling wasdone on 17 January 2002 and subsequently three more

samplingswerecarriedouteachattwomonths interval.

WithactivesupportoftheRevenueOfficialsoftheDelhi

government and local farmers, land in each village was

divided into two categories, viz. sewage and tubewell

water-irrigated. Tubewell water-irrigated landswere the

ones inaccessible for sewage irrigation because of their

slight higher elevation. In all, 431 and 106 soil samples

(015 cm) were collected from sewage and adjacent

tubewell water-irrigated lands, respectively. From

sewage-irrigated sites of Nilothi, Ranhola, Mundka,

Bakarwala, Hirankudna and Dichaokalan 37, 62, 57,

213, 39 and 23 soil samples werecollected,respectively,

the corresponding number of soil samples collected

fromadjacenttubewellwater-irrigated siteswere 11, 12,

6,51,10and16.Plantsamplesofcropsgrownonsewage

and tubewell water-irrigated lands were also collected

from the soil sampling sites wherever available.

2.2. Chemical analysis of sewage effluents,

groundwater, soil and plant samples

An aliquot of 500 mL of sewage effluents and

groundwater samples with 15 mL of HNO3 was

evaporated to near dryness on a hot plate. Then

contents were digested with 15 mL of HNO3 and

20 mL HClO4 (70%) (Brar et al., 2000). The residue

was taken in 15 mL of 6N HCl and made to the volume

(50 mL) and contents were filtered. The filtrate was

R.K. Rattan et al. / Agriculture, Ecosystems and Environment 109 (2005) 310322312

Fig. 1. Map showing the area under Keshopur Effluent Irrigation System commissioned in 1979.


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analyzed for P, K, S, Zn, Cu, Fe, Mn, Ni, Pb and Cd

using inductively coupled plasma-atomic emission

spectrophotometer (ICP-AES). Sewage effluent sam-

ples were analyzed for pH and electrical conductivityusing pH meter and solu-bridge, respectively. Carbo-

nates and bicarbonates were estimated by titrating an

aliquot of effluent samples with H2SO4. Soil samples

were dried, ground and passed through 2 mm sieve.

Soil pH was measured in suspension (soil:water::1:2)

according to Datta et al. (1997). Organic carbon

contents in soil were determined by the wet digestion

method (Walkley and Black, 1934). Further, 30 soil

samples across the villages were selected and analysed

for texture by hydrometer method (Day, 1965). For

heavy metal analysis, the soil samples were extracted

with 0.005 M DTPA according to Lindsay and Norvell

(1978) and metals in the extract were determined with

help of atomic absorption spectrophotometer (AAS;

GBC 932). In all, nine surface (015 cm) soil samples

were selected from sewage effluent-irrigated lands

receiving effluent irrigation for last 20 years and three

soil samples were also selected from adjacent tubewell

water-irrigated lands. These soil samples were used

for sequential extraction of Zn, Cu, Fe and Mn

according to the scheme of Iwasaki and Yoshikawa

(1990) which is modified form of fractionation scheme

of Miller et al. (1986). Plant samples were dried at60 8C in hot air oven, ground and digested in a diacid

mixture (HNO3:HClO4::9:4) (Jackson, 1973). Metal

contents in the plant digests were determined on

AAS.

2.3. Metal transfer factor

Soil to plant metal transfer factor was computed as

ratio of the concentration of metal in plants (on dry

weight basis) to its DTPA-extractable metal contents

in soil.

2.4. Risk assessment

Risk to human health (Hazard Quotient, HQ) for

intake of Zn, Cu and Ni through consumption spinach

(Brassica oleraceae), gobhi sarson (Brassica napus)

and Indian rape (Brassica campestris) as green leafy

vegetables grown on sewage-irrigated soils was

calculated using the following relationship (Pier-

zynski et al., 2000):

HQgv = (add/RfD), where HQgv is the hazard

quotient to a human from consumption of green

vegetables, add: the average daily dose (mg metal/kg

body weight/day) and RfD the reference dose. Thevalues of RfD for Zn and Ni were used as 0.3 and

0.02 mg/kg bw/day, respectively (IRIS, 2003). For Cu,

value of provisional maximum tolerable daily intake is

0.5 mg/kg bw/day (WHO, 1982) and the same is used

as RfD (Alam et al., 2003). Daily intake of green

vegetable was considered as 200 g/person/day which

is recommended amount from nutritional point of

view (Hassan and Ahmed, 2000). A factor of 0.085

was used to convert the fresh to dry weight of these

green vegetables. Average body weight for an adult

was considered as 70 kg. Average daily dose (add) was

computed using following relationship:

add mc cf di

bw

where mc is the metal concentrations in plant

(mg kg1) on dry weight basis, cf the fresh to dry

weight conversion factor, di the daily intake of green

vegetable (kg) and bw the body weight (kg). Assess-

ment of risk as computed here is not complete since,

metal accumulation to soil organisms, groundwater,

surface water, direct uptake of soil by human and

animal are some of the other risks which have not beenconsidered here.

2.5. Statistical analysis

The differences between DTPA-extractable metal

contents in sewage and tubewell water-irrigated soils

were statistically evaluated by applying t-test accord-

ing to Snedecor and Cochran (1967). Simple correla-

tion and multiple regression analyses were also carried

out to assess the relationships of DTPA-extractable

metal with plant metal concentration, soil pH andorganic carbon.

3. Results and discussion

3.1. Physico-chemical properties of sewage

effluents and groundwater

Plant nutrients and heavy metal contents in sewage

effluents and groundwater samples of first sampling



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are presented in Table 1. Data for second, third and

fourth sampling were not shown, since similar values of

these elements were obtained except S. In first

sampling, on an average, P, K, and S contents were

0.22, 3.58 and 14.3 mg L1, respectively in ground-

water samples, whereas, corresponding values for

sewage effluents were 2.57, 11.7 and 15.9 mg L1.

Among these plant nutrients, sewage effluents con-

tained about 12 and 3 times more P and K, respectively

compared to ground water. Sulphur contents in sewage

effluents in second, third and fourth sampling were

about two- to three-folds higher than that of ground-water. On an average, sewage effluents contained 5.5,

3.6, 2.6, 6.4 and 1.3 times higher amounts of Zn, Cu, Fe,

Mn and Ni, respectively compared to groundwater.

There were no appreciablevariations between these two

sources of irrigation water in respect of Pb and Cd. The

sewage effluents contain appreciable amounts of useful

major plant nutrients, viz. P, K and S, which was also

reflected in the appreciable build-up of these nutrients

in sewage-irrigated soils of this study area (Rattan et al.,

2001). According to Pescod (1992), threshold values of

heavy metals in irrigation water leading to crop damageare 2000 mg L1 for Zn, 200 mg L1 for Cu,

5000 mg L1 for Fe, 200 mg L1 for Mn, 200 mg L1

for Ni, 5000 mg L1 for Pb and 10 mg L1 for Cd.

Although sewage effluents had elevated concentrations

of some of the metals compared to groundwater, the

concentrations of these metals in these two sources of

irrigation water were within the permissible limits for

their use as irrigation water.

Some additional physico-chemical parameters

were determined in sewage effluent samples. Results

indicate that sewage effluents were acidic in reaction

with pH values ranging from 5.8 to 6.5 (data not

shown). The tolerance limit of pH for irrigation water

ranged from 6.0 to 9.0 (Patel et al., 1990). Thus, pH ofall the effluent samples is within the permissible limit.

Electrical conductivity of sewage effluents in all

samples exceeded 1 dS m1 (1.362.88 dS m1) indi-

cating that these effluents were saline in nature. The

carbonate and bicarbonate contents in effluent samples

varied from traces to 0.8 and 4.4 to 9.8 me L1,

respectively. Carbonate concentrations in all the

samples were much lower as compared to bicarbonate

concentrations, which is also reflected in acidic

reaction of effluents. The values of residual sodium

carbonate (RSC) varied from traces to 1.2, i.e. RSC in

all the samples were below 1.25 me L1 (safe limit for

irrigation water). These effluents can safely be used

for irrigation purpose as far as RSC is concerned.

3.2. Soil texture

Mechanical analysis of soil samples indicated that

sand and silt contents ranged from 56 to 86 and 8 to

28%, respectively, while maximum 18% clay was

recoded. Out of 30 samples analysed, 7 samples

belong to loamy sand and 23 samples were classified

as sandy loam. Thus, it is obvious that soils of thisstudy area are coarse to moderately coarse in texture.

3.3. Effect of long-term use of sewage effluents

3.3.1. Soil pH and organic carbon

Across the villages, soil pH varied from 5.1 to 9.9

and 6.2 to 9.1 in sewage and tubewell water-irrigated

soils, respectively with the corresponding average

values of 7.5 and 7.9 (data not shown). On an average,

soil pH was dropped by 0.4 unit. The organic carbon

content varied from 0.14 to 3.71% (average 0.65%) insewage-irrigated soils, the corresponding values for

tubewell water-irrigated soils were 0.140.76 (average

0.39%) (data not shown). Thus, organic carbon was

increased by 59% as result of long-term sewage

irrigation. These results concurred the findings of

Singh and Verloo (1996). After pH, soil organic

carbon (SOC) is the most important indicator of soil

quality and in addition to acting as a store-house of the

plant nutrients, plays a major role in nutrient cycling.

Besides, some estimates show that increase in the SOC


Table 1

Plant nutrients and heavy metals content in sewage effluents and

groundwater collected at first sampling

Element Sewage ef fluents Groundwater

P (mg L1

) 0.565.91 (2.57) 0.110.36 (0.22)

K (mg L1

) 9.3323.1 (11.7) 0.845.03 (3.58)

S (mg L1) 7.7122.9 (15.9) 9.8519.4 (14.3)

Zn (mg L1

) 6151 (61) 338 (11)

Cu (mg L1

) 9116 (29) 79 (8)

Fe (mg L1

) 6393793 (1464) 100745 (557)

Mn (mg L1

) 24122 (64) 115 (10)

Ni (mg L1) 3967 (49) 955 (37)

Pb (mg L1

) 2267 (33) 2241 (30)

Cd (mg L1

) 1.172.44 (1.53) 1.071.76 (1.42)

Figures in parenthesis indicate the mean values.


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content by 0.01% could lead to the carbon sequestra-

tion equal to the annual increase in the atmospheric

CO2-C (Lal et al., 1998). Such a long-term application

of the sewage effluents is a carbon-building/seques-tering and soil quality sustaining practice. Farmers of

the area have expressed that over a period of time, the

soils, which were not productive (because of the

problems of Kallarsodicity) had started giving

improved yields. Such an observation could be

explained on the basis of counteraction of alkalinity

through sewage-induced acidification and improve-

ment in soil organic carbon status.

3.3.2. Heavy metals in soils

In Nilothi, Ranhola, Mundka and Bakarwala, there

were substantial build-up of DTPA-extractable Zn,

Cu, Fe, Pb and Ni in sewage-irrigated soils over

tubewell water-irrigated ones (Table 2). Whereas, Mn

contents in soils of Nilothi, Mundka, Bakarwala were

declined by 44, 18 and 51%, respectively, as a result of

sewage irrigation. In Hirankudna, on an average,

sewage irrigation resulted in 113, 117, 29 and 81%

increase Zn, Fe, Pb and Ni, respectively, however, Cu,

Mn and Cd did not show any significant change.

Significant effect of irrigation through sewage water

was only observed in case of Fe, where 254% increase

was recorded over the tubewell water-irrigated soils ofDichaonkalan. Cadmium content was not affected by

sewage irrigation except in soils of Mundka.

To assess the impact of duration under sewage

irrigation, build-up of heavy metals in soils was

computed separately for villages receiving irrigation

for 20, 10 and 5 years (Fig. 2). In soils of Dichaokalan,

which has been under sewage irrigation for 5 years, on

average 254% build-up in Fe content was recorded and

corresponding values for Hirankudna which has been

receiving sewage irrigation for 10 years was 117%.

Whereas, as a result of sewage irrigation for 20 years,Fe increased by 170% in soils of Nilothi, Ranhola,

Mundka and Bakarwala. Zinc was not affected by

sewage irrigation for 5 years, while 113 and 208%

increases were observed in soils under sewage

irrigation for 10 and 20 years, respectively. Significant

build-up in Cu was recorded under sewage irrigation

of 20 years only, value being 170%. Sewage irrigation

for 10 and 20 years increased the Ni contents in soils

by 81 and 63%, respectively and corresponding values

for Pb in both the cases was 29%. However, sewage

irrigation for 5 years could not change the available

pool of both of these metals. Although 5 and 10 years

of sewage irrigation could not change the Mn content

in soil significantly, on an average 31% decline wasrecorded in soil which has been receiving sewage

irrigation for 20 years.

In case of Nilothi and Dichaonkalan, average

DTPA-extractable Fe content in the tubewell-irrigated

soils is below the critical limit of deficiency of

4.5 mg kg1 (as suggested by Lindsay and Norvell,

1978 and used extensively in India for delineating Fe-

deficient soil), increased more than three to five times

in the sewage-irrigated soils. It leads one to logically

conclude that the enhancement of crop productivity in

the sewage-irrigated soils of this village could partly

be due to alleviation of Fe-deficiency syndrome. In

other four villages, i.e. Mundka, Ranhola, Bakarwala

and Hirankudna, there has been an enormous build-up

in the available Fe content in the sewage-irrigated

soils. Since Fe does not cause phytotoxicity in neutral

to alkaline soils of this area, the build-up of available

Fe is not likely to limit the use of sewage effluents for

irrigation purpose. The sewage-irrigated soils are still

maintaining higher levels of DTPA-Mn than the

critical limit of Mn deficiency of 2.0 mg kg1 (being

used currently in this country for separating the Mn-

deficient soils from the non-deficient ones); however,this is a cause of concern and steps need to be

undertaken to monitor Mn deficiency and include Mn

in the fertilization schedule.

All the soils, both under tubewell and sewage

irrigation are well supplied with Zn and Cu as their

levels are generally higher than their critical levels of

deficiency of 0.6 and 0.20.5 mg kg1, respectively.

In case of the sewage-irrigated soils, some of the soils

have accumulated more than 10 mg kg1 DTPA-Zn,

which has been listed as critical level of phytotoxicity

(Rattan and Shukla, 1984). Accumulation of Zn andCu in soils coupled with decrease in soil pH has a

potential of aggravating the nutritional disorders

associated with toxicity of these metals in the long run.

3.3.3. Fractionation

In view of appreciable build-up in DTPA-

extractable Zn, Cu and Fe and depletion in Mn,

distribution of these metals in different soil pools

were studied and presented in Table 3. In tubewell

water-irrigated soils, lower proportions of Zn were



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Table 2

Effect of long-term irrigation with sewage effluent on the DTPA-extractable heavy metal status of soil (mg kg1)

Metals Sewage-irrigated Tubewell-irrigated

Range Mean S.D. # (%) Range Mean S.D.Nilothi

Zn 2.5218.6 7.31** 4.14 638 0.531.84 0.99 0.34

Cu 1.9411.9 4.91** 2.60 416 0.651.23 0.95 0.18

Fe 8.4841.6 20.1** 9.13 554 1.784.04 3.08 0.63

Mn 1.308.26 3.29** 1.45 44 4.596.95 5.87 0.99

Cd 0.110.37 0.20 0.06 0.070.44 0.14 0.10

Pb 1.252.58 1.91** 0.38 46 0.661.96 1.31 0.44

Ni 0.062.46 1.19** 0.70 284 0.090.67 0.31 0.20

Ranhola

Zn 1.5636.9 9.28** 9.27 189 1.107.50 3.21 2.01

Cu 2.2632.3 7.04** 4.96 162 1.364.13 2.69 1.16

Fe 8.6576.8 43.1** 18.57 214 4.8534.2 13.7 9.08

Mn 1.5810.7 4.19 1.87 1.524.79 3.82 1.05Cd 0.050.41 0.15 0.07 0.080.24 0.16 0.04

Pb 0.917.09 2.37** 0.99 25 1.532.37 1.90 0.24

Ni 0.0083.98 1.04** 0.77 131 0.160.86 0.45 0.17

Mundka

Zn 0.5716.17 3.68** 4.22 247 0.691.49 1.06 0.26

Cu 1.4117.03 4.39** 3.29 181 1.232.19 1.56 0.36

Fe 9.96115.5 50.2** 24.3 341 9.2017.1 11.4 2.99

Mn 4.7314.7 7.46** 2.05 18 7.7610.1 9.10 0.92

Cd 0.020.46 0.11* 0.07 38 0.050.11 0.08 0.02

Pb 1.123.79 2.60** 0.67 44 1.472.29 1.81 0.44

Ni 0.051.83 0.58** 0.40 123 0.070.39 0.26 0.12

Bakarwala

Zn 1.018.9 6.38** 5.29 186 0.757.73 2.23 1.04

Cu 1.539.7 6.53** 5.53 158 1.184.28 2.53 0.57

Fe 10.2113 62.2** 23.1 144 7.2876.2 25.4 16.1

Mn 2.6918.4 8.43** 3.03 51 4.121.2 12.7 4.3

Cd 0.040.85 0.22 0.12 0.070.85 0.19 0.11

Pb 0.507.92 2.64** 0.99 19 1.183.62 2.22 0.48

Ni 0.159.76 1.29** 1.22 14 0.061.58 0.54 0.29

Hirankudna

Zn 0.819.18 2.72* 2.00 113 0.981.51 1.28 0.20

Cu 0.494.94 1.71 0.94 1.101.79 1.41 0.18

Fe 1.8028.5 11.3** 6.67 117 2.439.75 5.22 2.58

Mn 1.0923.3 5.67 4.90 1.9113.1 6.47 3.08

Cd 0.010.90 0.14 0.18 0.010.23 0.09 0.06

Pb 0.761.46 1.21** 0.16 29 0.661.10 0.94 0.12

Ni 0.010.67 0.29* 0.19 81 0.010.35 0.16 0.13

Dichaonkala

Zn 1.1212.2 5.02 4.06 2.3118.8 6.46 4.74

Cu 0.528.59 2.65 2.47 0.995.78 3.04 1.67

Fe 2.8642.9 15.6** 12.2 254 1.7818.6 4.42 3.98

Mn 1.0320.5 6.92 5.98 1.0612.8 3.76 2.98

Cd 0.040.26 0.14 0.06 0.090.39 0.17 0.08

Pb 0.572.17 1.11 0.37 0.932.02 1.30 0.30

Ni 0.011.34 0.36 0.42 0.010.91 0.27 0.23

#(%), increase or decrease over tubewell-irrigated soils; S.D., standard deviation; (*) and (**) indicate that differences between means of sewage

and tubewell water-irrigated soils are significant at 5 and 1% probability levels, respectively.


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recorded in all the fractions except residual compared

to that of effluent-irrigated soils. Labile pool of Zn,

i.e. adsorbed fractions [(water soluble + exchange-

able) + Pb displaceable (specifically adsorbed) + a-

cid soluble] constituted higher share of total Zn

(38.3%) in sewage-irrigated soils than that in

tubewell water-irrigated ones (22.1%). Unlike Zn,

share of adsorbed Cu was more 61.2% of total Cu for

tubewell water-irrigated soils compared to effluent-

irrigated soils. However, immediately bioavailable

pools, i.e. water soluble + exchangeable Cu (Mathurand Levesque, 1983) was much higher 23.8% in

sewage-irrigated soils than that in tubewell water-

irrigated ones (1.16%). Generally, less than 10% of

total soil micronutrients are present in water soluble

and exchangeable forms (Lake et al., 1984).

Relatively higher amount of Zn and Cu in bioavail-

able forms in our study may be attributed to reduction

in soil pH as well as increase in organic carbon as a

result of long-term use of sewage effluents whichmay be a cause of concern in future.

Residual fraction of Fe constituted major share of

total soil Fe in both the categories of soils, followed by

crystalline Fe-oxide occluded and amorphous Fe-

oxide occluded fractions. Contribution of remaining

fractions to total soil Fe was meagre, ranging from

0.01 to 2.34%. Various researchers also reported that

most of the Fe resides in the amorphous Fe-oxide,

crystalline Fe-oxide occluded and residual fractions

(Hoffman and Fletcher, 1978; Shuman, 1985). Never-

theless, build-up of Fe was relatively more in first five

fractions in effluent irrigated soils indicating relatively

more lability of Fe in sewage-irrigated soils compared

to tubewell water-irrigated ones. More or less similar

pattern of distribution of Mn in different fractions was

observed in both the categories of soils, except water

soluble + exchangeable and residual fractions. Sew-

age-irrigated soils exhibited comparatively higher

share of water soluble + exchangeable Mn than that in

tubewell water-irrigated ones, while trend was reverse

for residual fractions. This may be attributed to the

decrease in pH associated with increase in organic

carbon as a result of long-term use of sewage effluents,which might have remobilized the Mn from structural

pools to more labile fractions. This probably can

explain the reason for depletion of Mn in these

sewage-irrigated light textured soils due to leaching

over the years. Takkar and Nayyar (1981) also


Fig. 2. Effect of period of sewage irrigation on the build-up/deple-

tion of DTPA-extractable metals (%) over tubewell water-irrigated

soil.

Table 3

Mean values of Zn, Cu, Fe and Mn fractions in sewage and tubewell water-irrigated soils

Fractions Zn Cu Fe Mn

Sewage Tubewell Sewage Tubewell Sewage Tubewell Sewage Tubewell

Water soluble +

exchangeable

2.91 (3.8) 1.77 (3.4) 9.69 (23.8) 0.30 (1.16) 7.27 (0.07) 1.8 (0.01) 44.9 (12.1) 2.55 (0.77)

Pb-displaceable 16.9 (22.2) 5.92 (11.3) 7.34 (18.04) 11.7 (45.3) 5.48 (0.06) 4.33 (0.04) 42.1 (11.3) 43.2 (13.1)

Acid soluble 9.32 (12.3) 3.88 (7.4) 6.12 (15.04) 3.81 (14.7) 27.4 (0.30) 18.9 (0.18) 47.8 (12.8) 40.7 (12.3)

Mn-occluded 2.64 (3.47) 0.46 (0.88) ND ND 33.4 (0.37) 23.6 (0.23) 31.5 (8.46) 26.4 (8.0)

Organically bound 22.6 (29.7) 19.1 (36.6) 1.72 (4.22) 0.35 (1.33) 215 (2.34) 128 (1.26) 8.87 (2.38) 6.04 (1.83)

Amorphous Fe-oxide

occluded

6.84 (8.99) 4.68 (9.0) 3.96 (9.73) 0.73 (2.82) 566 (6.16) 396 (3.9) 5.78 (1.55) 5.82 (1.76)

Crystalline Fe-oxide

occluded

8.20 (10.7) 4.74 (9.1) 0.94 (2.31) 0.053 (0.20) 1000 (10.9) 1071 (10.6) 18.3 (4.91) 17.5 (5.31)

Residual 6.63 (8.7) 11.5 (22.0) 10.9 (26.8) 8.90 (34.4) 7304 (79.7) 8492 (83.8) 173 (46.5) 187 (56.9)

Figures in parenthesis indicate percentage of total metal content in soils; ND: not detectable in flame AAS.


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reported Mn deficiency in some light textured soils of

Punjab because of the leaching losses of Mn from the

surface soil layer.

3.3.4. Metal contents in plants

Mean contents of metals in the dry matter of

various crops grown on sewage effluent and tubewell

water-irrigated soils are summarized in Table 4. Rice

grain accumulated much higher amount of Zn and Cu

grown on sewage-irrigated soils compared to tubewell

water-irrigated soils, while slight increase in Ni

content was recorded. Manganese content in rice grain

for sewage-irrigated soils was much lower than that

for tubewell water-irrigated soils. In general, this trend

is in concurrence with the levels DTPA-extractableMn in both categories of soils. Although sewage

effluent-irrigated soils exhibited much higher amount

of DTPA-Fe, it was not reflected in Fe content of rice

grain. From this study, it is difficult to elucidate the

reasons for this anomaly. Iron content in rice straw to

some extent reflected the variations in DTPA-Fe

between sewage and tubewell water-irrigated soils.

Rice straw accumulated almost two times more Ni

produced with sewage water over that of tubewell

water irrigation. However, Zn and Cu contents in the

rice straw produced on the sewage-irrigated soils

appeared to be rather marginally lower than its

background level. Wheat had elevated contents of Zn,

Cu, Fe, Mn and Ni grown on sewage-irrigated soilscompared to that produced with tubewell irrigation.

Sorghum accumulated higher amount of Fe, Cu and Ni

on sewage effluent irrigated soils, while Zn and Mn

contents were lower than its background level. Maize

showed higher accumulation of all the elements on

sewage effluent irrigated soils. Similar trends were

observed in gobhi sarson and oats with only exception

of Mn, where plant contents of Mn conformed to the

DTPA-Mn in both the categories of soils. Spinach

grown on sewage-irrigated soils accumulated higher

amount of Zn, Cu and Ni than those grown on tubewellwater-irrigated soils, whereas reverse were the trends

for Fe and Mn. Comparison of metal contents in Indian

rape, cauliflower, cucumber, Egyptian clover and

radish for both of these soils could not be made as

these crops were not grown with tubewell irrigation in

this study area at the time of sampling. Cadmium and

lead contents in tissue of all the crops under study,

were below the analytical detection limits, hence no

data were reported. Similar were the observations of

Datta et al. (2000) and Yadav et al. (2002) for some


Table 4

Heavy metal contents in crops grown on sewage effluents and tubewell water-irrigated soils

Crops Metal contents (mg kg1

)

Zn Cu Fe Mn NiS T S T S T S T S T

Rice (Oryza sativa L.)

Grain 49.6 (27) 29.6 (4) 51.6 23.0 122 186 53.3 88.1 10.1 9.85

Straw 58.9 (27) 61.9 (4) 57.2 59.8 233 202 208 229 10.4 5.27

Wheat (Triticum aestivum L.) 65.3 (88) 47.5 (27) 9.39 7.45 404 336 15.3 13.6 20.0 19.7

Sorghum (Sorghum vulgare Pers.) 54.2 (38) 73.4 (9) 16.9 115.5 526 485 40.6 44.8 14.8 11.6

Maize (Zea mays L.) 78.8 (15) 67.6 (1) 14.9 13.3 531 99.0 26.0 15.3 16.5 5.20

Oats (Avena sativa L.) 59.0 (7) 44.3 (2) 8.71 6.35 458 400 23.8 29.2 18.3 37.3

Gobhi sarson (Brassica napus) 66.9 (17) 38.7 (3) 23.1 14.1 454 401 69.0 104 12.0 3.73

Spinach (Spinacea oleraceae L.) 77.1 (17) 38.4 (2) 20.6 16.1 711 734 39.3 87.8 18.4 13.2

Indian rape (Brassica campestris

var. toria Dutch)

52.5 (10) 5.42 475 42.6 8.78

Cauliflower (Brassica oleracea L.) 46.7 (11) 10.8 328 31.8 14.4 Cucumber (Cucumis sativus L.) 79.4 (5) 19.3 932 19.9 21.5

Radish (Raphanus sativus L.)

Leaf 60.8 (7) 14.6 358 41.9 9.26

Root 58.9 (7) 10.4 166 16.7 11.5

Egyptian clover (Trifolium

alexandrinum L.)

91.9 (15) 18.3 623 20.7 20.6

Figures in parenthesis indicate number of samples analysed; S: sewage-irrigated; T: tubewell water-irrigated.


10/13

sewage-irrigated soils of India. By and large,

concentrations of metals in all the crops grown on

sewage effluent-irrigated soils were below the general-

ized critical levels of phytotoxicity (as summarized by

Datta et al., 2000) except Fe. Although no toxicity

symptoms were noticed, Fe contents in sorghum,

maize, spinach, cucumber and Egyptian clover

exceeded the phytotoxicity limit (> 500 mg kg1;

on dry weight basis). Crop species exercised

differentiality in accumulating metals in their tissues.

Similar were the observations of Lepp (1981),

Sauerback (1991), Smith (1994), Hooda et al.(1997), Datta et al. (2000) and Yadav et al. (2002).

Since DTPA is the most commonly used soil test

for assessing bioavailability of metals in contaminated

soils (Hooda et al., 1997; Datta et al., 2000; Yadav

et al., 2002; Patel et al., 2004), simple correlation

coefficients were worked out between plant metal

concentrations and DTPA-extractable metals in soil.

Results indicate that DTPA-Zn had most consistent

positive relationships with tissue metal concentrations

of all the crops as listed in Table 5, except sorghum.

DTPA-extractable Cu positively contributed to thetissue concentration of Cu in case of wheat, sorghum

and cucumber. DTPA-extractable Fe contents were not

related to the Fe contents in any of the crops.

Manganese contents in plants did not exhibit any

relationship with soil Mn, except rice grain. Nickel

contents in sorghum, Indian rape and cauliflower were

positively related with DTPA-Ni in soils. Failure of

DTPA extractant in some cases in the present study

may be attributed lesser number of observations and

variations in the management practices among the

farmers of the study area. Even under controlled

conditions, ability of extractants to predict plant

available metals depends on the crop species, the

metal and extractant used (Hooda et al., 1997). In

several studies, it is established that solubility of

metals in soils mainly depends on soil pH and organic

carbon (Ma and Lindsay, 1993; Jopony and Young,

1994; Hough et al., 2003; Tye et al., 2003). Hence, in

the present investigation, effect of these two important

soil properties on the lability of metals was studied

(Table 6). Results show that soil pH had negative

influence on the extractability of metals from soils byDTPA. While increase in soil organic carbon content

enhanced the DTPA-extractable metals, except Mn.

However, inclusion of pH and organic carbon in

multiple regression equation along with DTPA-

extractable metals as predictor variables to predict

plant metal concentrations, could not improve the

values of prediction coefficients except spinach (data

not shown). It appears that available metal content in

soils is the principal factor predicting its concentration


Table 6

Simple correlation coefficients of DTPA-extractable metals with soil

pH and organic carbon

Metals pH Organic carbon

Zn 0.37** 0.55**

Cu 0.37**

0.71**

Fe 0.50**

0.54**

Mn 0.31**

0.02

Ni 0.46**

0.73**

Pb 0.09**

0.32**

Cd 0.29**

0.42**

(**) significant at 1% probability level.

Table 5

Simple correlation coefficients (r) between DTPA-extractable metals and metal contents in crops

Crops Metal contents in crops

Zn Cu Fe Mn NiRice (Oryza sativa L.)

Grain 0.58**

0.05 0.04 0.38**

0.20

Straw 0.58**

0.06 0.25 0.02 0.15

Wheat (Triticum aestivum L.) 0.35**

0.43**

0.14 0.04 0.12

Sorghum (Sorghum vulgare Pers.) 0.25 0.38**

0.25 0.04 0.28*

Gobhi sarson (Brassica napus) 0.45* 0.12 0.26 0.27 0.38

Spinach (Spinacea oleraceae L.) 0.51*

0.43 0.03 0.09 0.03

Indian rape (Brassica campestris var. toria Dutch) 0.86**

0.21 0.25 0.35 0.61**

Cauliflower (Brassica oleracea L.) 0.72* 0.67* 0.41 0.34 0.66*

Egyptian clover (Trifolium alexandrinum L.) 0.57*

0.28 0.01 0.027 0.03

(*) and (**) significant at 5 and 1% probability levels, respectively.


11/13

in plants. Regression analysis according to Snedecor

and Cochran (1967) revealed that DTPA-Zn, soil pH

and organic carbon could explain the variability in

Zn content in spinach to the extent of 79% [Zncontent in spinach (mg kg1) = 241 + 2.11 (DTPA-Zn

mg kg1) 25.9 (pH) + 54.7 (organic carbon %);

R2 = 0.79; significant at 1% probability level].

Transfer factors (TF), i.e. the ratio of metal

concentration in plants to DTPA-extractable metal

in soil, varied considerably among the crops (Table 7).

In case of Zn and Fe, TFs for all the crops grown on

sewage effluent-irrigated soils were lower than that for

crops produced with tubewell water irrigation.

However, for other metals, such consistent variations

in TFs were not obtained. This indicates that uptake ofmetals by crops does not increase linearly with

increasing concentrations of metals in soils. This is in

concurrence with the findings ofHooda et al. (1997).

The apparent advantage of this phenomenon is that

although long-term sewage irrigation resulted into

elevated concentration of metal in soil, the same

would not be proportionately transferred to food

chain. Taking all the crops together, relative orders of

transfer of metals from soil to plants grown on sewage-

irrigated soils were Ni > Zn > Fe > Mn > Cu. These

results show that as far as entry of these metals to food

chain plants is concerned, Ni has the greatest potential,

followed by Zn, Fe, Mn and Cu. Based on the TFs,

relative efficiency of crops to absorb metals fromsewage-irrigated soil could be arranged in the

following order:

Zinc: Egyptian clover> spinach> sorghum> rice

leaf> Indian rape > wheat > rice grain > gobhi sar-

son> oats > radish root > maize > cucumber> rad-

radish leaf.

Copper: Rice leaf> rice grain> sorghum > spi-

spinach > gobhi sarson> Indian rape> Egyptian

clover > cucumber > radish leaf> maize > wheat

>

radish root>

cauliflower>

oats.Iron: Sorghum > spinach > gobhi sarson > cucum-

ber > radish leaf> Indian rape > cauliflower > mai-

ze > Egyptian clover > wheat> oat > radish root >

rice leaf> rice grain.

Manganese: Rice leaf> gobhi sarson > Indian ra-

pe> radish leaf> sorghum > spinach > rice grain >

cauliflower > radish root> cucumber > maize > oa-

ts > Egyptian clover > wheat.

Nickel: Sorghum > cucumber > spinach > wheat >

rice leaf> Egyptian clover> Indian rape > radish


Table 7

Transfer factor (TF) for crops grown on sewage effluents and tubewell water-irrigated soils

Crops Transfer factor of metals

Zn Cu Fe Mn NiS T S T S T S T S T

Rice (Oryza sativa L.)

Grain 16.5 21.2 12.9 7.46 2.30 4.43 8.69 10.9 15.9 20.2

Straw 20.8 47.5 15.2 21.9 4.46 5.92 31.5 26.8 23.1 12.1

Wheat (Triticum aestivum L.) 18.3 22.6 2.35 16.7 7.91 16.2 1.93 1.27 24.3 47.8

Sorghum (Sorghum vulgare Pers.) 23.7 33.4 8.48 6.80 57.8 104 10.9 16.3 145 457

Maize (Zea mays L.) 14.4 32.0 2.81 8.59 11.8 19.6 3.58 3.85 18.9 11.4

Oats (Avena sativa L.) 15.5 29.6 1.69 2.48 7.17 15.3 2.65 1.90 15.2 37.3

Gobhi sarson (Brassica napus) 16.1 43.5 5.57 14.3 25.6 123 21.2 23.2 16.0 31.7

Spinach (Spinacea oleraceae L.) 24.6 27.7 6.18 16.1 26.0 255 9.07 15.3 32.3 52.2

Indian rape (Brassica campestris

var. toria Dutch)

19.1 55.6 5.42 18.1 15.8 200 15.6 20.2 22.1 94.3

Cauliflower (Brassica oleracea L.) 7.25 1.91 14.7 8.59 15.6 Cucumber (Cucumis sativus L.) 13.1 3.16 22.5 3.98 37.3

Radish (Raphanus sativus L.)

Leaf 9.55 3.05 22.4 13.7 9.00

Root 15.0 2.32 6.00 4.77 19.7

Egyptian clover (Trifolium

alexandrinum L.)

25.1 4.71 11.8 2.60 22.5

S: sewage-irrigated; T: tubewell water-irrigated.


12/13

root > maize > gobhi sarson> rice grain > cauli-

flower > oats > radish leaf.

This information will be very useful in selecting thesuitable crops to be grown on metal-contaminated

soils.

3.4. Risk assessment

In this study area, spinach, gobhi sarson and Indian

rape are produced and marketed as leafy green

vegetables. Also there was a substantial build-up of

Zn, Cu and Ni sewage-irrigated soils. Hence, hazard

quotients (HQgv), i.e. the ratio of average daily dose to

the reference dose (Pierzynski et al., 2000) for intake

of metals by human through the consumption of these

leafy green vegetables were worked out. The values of

HQgv for gobhi sarson varied from 0.040 to 0.068 for

Zn, 0.004 to 0.021 for Cu and 0.027 to 0.442 for Ni

(data not shown). In case of spinach, HQgv varied from

0.035 to 0.152, 0.008 to 0.015 and 0.046 to 0.502 for

Zn, Cu and Ni, respectively. The values of HQgv for

Indian rape ranged from 0.027 to 0.053, 0.004 to 0.014

and 0.016 to 0.429 for Zn, Cu and Ni, respectively.

Although, Ni exhibited relatively higher HQ for all the

crops compared to other two metals, most of the values

were far less than 1. Hence, these green vegetables arenot likely to induce any health hazard to consumers

(human) as far as its metal contents are concerned.

4. Conclusions

From this study, it can be concluded that besides

use as irrigation water, these sewage effluents are also

a potential source of plant nutrients. Build-up of heavy

metals, particularly Zn, Cu and Ni in sewage-irrigated

soils needs to be monitored periodically in view oftheir significant accumulation in bioavailable pool

associated with decline in pH. Appreciable depletion

in available Mn under these intensively cultivated

sewage-irrigated soils is likely to induce unsustain-

ability in soil productivity and thus, Mn needs to be

included in balanced fertilization programme. There is

a great respite that accumulation of dreaded metals

like Cd and Pb has not posed any threat even after such

long-term use of these sewage effluents. As far as

metal contents are concerned, the leafy green

vegetables grown on these sewage-irrigated soils are

still safe to be consumed by human beings.

Acknowledgement

Authors would like to place on record their sincere

thanks to Indian Council of Agricultural Research,

New Delhi, India for funding this study as a part of AP

Cess Fund project.

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