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Soil Heavy Metal Pollution AssessmentNear the Largest Landfill of ChinaChang Liu a , Jun Cui a , Guofu Jiang a , Xiaofeng Chen b , Li Wang b

& Changming Fang aa Coastal Ecosystems Research Station of Yangtze River Estuary,Ministry of Education Key Laboratory for Biodiversity and EcologicalEngineering, Institute of Biodiversity Science, Fudan University,Shanghai, Chinab Research Center of Analysis and Measurement, Fudan University,Shanghai, ChinaAccepted author version posted online: 29 Nov 2012.Version ofrecord first published: 08 Feb 2013.

To cite this article: Chang Liu , Jun Cui , Guofu Jiang , Xiaofeng Chen , Li Wang & Changming Fang(2013): Soil Heavy Metal Pollution Assessment Near the Largest Landfill of China, Soil and SedimentContamination: An International Journal, 22:4, 390-403

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Soil and Sediment Contamination, 22:390–403, 2013Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320383.2013.733447

Soil Heavy Metal Pollution Assessment Near theLargest Landfill of China

CHANG LIU,1 JUN CUI,1 GUOFU JIANG,1 XIAOFENG CHEN,2

LI WANG,2 AND CHANGMING FANG1

1Coastal Ecosystems Research Station of Yangtze River Estuary, Ministry ofEducation Key Laboratory for Biodiversity and Ecological Engineering, Instituteof Biodiversity Science, Fudan University, Shanghai, China2Research Center of Analysis and Measurement, Fudan University, Shanghai,China

To assess the extent and potential hazards of heavy metal pollution at Shanghai LaogangLandfill, the largest landfill in China, surface soil samples were collected near thelandfill and concentrations of Cu, Zn, Cd, Pb, and Cr were determined. The resultsrevealed that the concentrations of heavy metals, except Pb, were higher in the surfacesoil near the landfill than in the background soil. Principal component analysis andhierarchical cluster analysis suggested that the enrichment of Cu in soil was probablyrelated to agricultural activities and Cd and Pb to landfill leachates, whereas Zn and Crconcentrations were probably controlled by soil matrix characteristics. The pollutionindices (PIs) of the metals were: Cd > Cu > Cr > Zn > Pb. Among the five measuredmetals, Cd showed the largest toxic response and might cause higher ecological hazardsthan other metals. The integrated potential eco-risk index (RI) of the five metals rangedfrom 26.0 to 104.9, suggesting a low-level eco-risk potential. This study indicated theaccumulations of Cu, Zn, Cd, Pb, and Cr did not reach high pollution levels, andtherefore posed a low eco-risk potential in surface soil near the landfill.

Keywords Heavy metal pollution, Laogang Landfill, soil quality, ecological risk as-sessment

1. Introduction

It is commonly accepted that soil contamination with heavy metals is potentially damagingto ecosystem health (Remon et al., 2005; Chai et al., 2007). Unlike organic pollutants,metals cannot be biodegraded and their residence time in the soil can be thousands ofyears. Heavy metal pollution in agricultural soils is of special concern because metals canenter the food chain via food production and threaten the health of animals and humans(Huang et al., 2007). A better understanding and evaluation of the distribution and potentialhazards of heavy metals in agricultural soils are increasingly needed to ensure food safetyand public health.

Address correspondence to Changming Fang, Coastal Ecosystems Research Station of YangtzeRiver Estuary, Ministry of Education Key Laboratory for Biodiversity and Ecological Engineering,institute of Biodiversity Science, Fudan University, Shanghai 200433, China. E-mail: cmfang@fudan.edu.cn

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Landfill Heavy Metal Pollution 391

In China, the total domestic garbage disposal has been estimated to be about 1.5 × 109

kg in 2006, and this figure is still increasing at a rate of about 5.9% per year (Wen and Wu,2009). Almost all Chinese cities are heavily burdened with urban domestic garbage. Sucha problem has also been encountered by many other developing and developed countries,such as cities in India and Mexico (Kallipoliti, 2010). Municipal refuse landfill is themost common means to dispose of urban solid wastes in the world (Chai et al., 2007;Prechthai et al., 2008). Previous studies have showed that improper collection, segregation,and disposing practices of municipal solid waste can produce leachates that contain highconcentrations of ammonium, organic matter, and heavy metals (Tatsi and Zouboulis, 2002;Ward et al., 2005). These leachates may lead to mobilization of organic and inorganic toxicmatters into groundwater and soils, and pose potential threats to local ecosystem health(Islam et al., 2001; Mor et al., 2006).

Suburban agriculture is a vital food source for urban areas. However, suburban agri-culture is affected by application of agrochemicals or sewage irrigation (Wong et al., 2002).Continuous application of fertilizers and pesticides containing heavy metals, such as Cd,Pb, and Zn, and other soil amendments can potentially exacerbate the accumulation ofheavy metals in agricultural soils (Huang et al., 2007; Zaccone et al., 2010). In particular,there are some additional sources of heavy metal pollution in suburban agricultural soils,such as refuse landfill and cement plants near agricultural areas (Calace et al., 2005; Denget al., 2010). Previous studies showed that landfills may be an important source of heavymetal pollutants in suburban agriculture (Tesfai and Dresher, 2009). All of these suggest thatenvironmental and ecological effects of heavy metals from a landfill cannot be overlooked,particularly for the development of suburban agriculture. Currently, the lands near landfillsare not commonly used for food production, so there have been very few studies on themagnitude and signature of soil heavy metal pollution affected by landfills. With urban-ization and industrialization in developing countries, especially in China, land resourcesare becoming increasingly scarce. Hence, there is a critical need to assess potential heavymetal pollution by landfills, which may greatly influence food safety and the developmentof sustainable agriculture for these new lands.

Shanghai Laogang Landfill, the largest landfill in China, was built in 1985 and occupiesan area of about 6 km2 along the shore of the East China Sea (Zhao et al., 2000). In 2005, alarge area of about 2000 ha was reclaimed from coastal wetlands neighboring the landfill.The land has now been turned into arable land to grow crops, mainly rice, to meet anincreasing demand for high-quality food.

The purposes of this study were to: 1) determine the spatial distribution of Cu, Zn, Cd,Pb, and Cr in surface soil near the Laogang Landfill; 2) identify possible sources of heavymetals; and 3) evaluate the potential risks of heavy metal pollution along with agriculturedevelopment near the landfill of city disposals.

2. Material and Methods

2.1 Study Area

The Laogang Landfill (31.03◦N, 121.87◦E) is located in the suburb of Shanghai, China,along the shore of the East China Sea (Figure 1). There is a typical monsoon climate witha hot and rainy summer and a relatively cold and dry winter. The mean annual temperatureis 15.3◦C and annual precipitation amounts to 1004 mm. The landfill was constructed in1985 and put into use in 1989 (Zhao et al., 2000). It occupies an area of about 6 km2 andreceives about 5000 t of refuse per day, 75% of which is from the urban area of Shanghai

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Figure 1. Location of sampling sites: 1–14 = Zone I; 15–24 = Zone II; 25–35 = Zone III.

(Chai et al., 2007). About 61% of the refuse is from construction wastes, 38% fromagricultural and domestic wastes, and only 1% from industrial wastes (Yang and Zhao,2007).

To assess the risks of heavy metal pollution in the area near the Laogang Landfill,soils were extensively sampled from 35 sites (Figure 1) around the landfill. Sampling siteswere divided into three zones, depending on distances from the landfill. Zone I had 14 sites(1–14) within a distance of 0.1–1 km from the landfill, Zone II had 10 sites (15–24) within1–3 km, and Zone III had 11 sites (25–35) within 3–6 km from the landfill (Figure 1). Allsampling sites were located in paddy rice croplands, which are in rotation with wheat.

2.2 Sampling and Analysis

Soil samples were taken, using a soil auger of 55 mm inner diameter, from the surface soil(0–20 cm) at each site after the rice harvest in November 2009. In each site, six samplingpoints, with a minimal distance of ca 10 m apart, were arranged in a quincunx method. Thesix subsamples were mixed to obtain a composite sample to provide a representative estimateof concentrations at that site. Samples were then sieved through a 2 mm mesh and storedat 4◦C for soil dissolved organic carbon (DOC) analysis. The remaining samples were air-dried, ground, and passed through a 0.15 µm sieve for determination of soil organic carbon(SOC) and heavy metal contents. Soil pH was measured in a deionized water suspension(1: 2.5 v/v) using a Sartorius PB-10 after 1 h agitation. About 300 mg of air-dried soilwas treated with 1N HCl for 24 hours at room temperature to completely remove inorganiccarbon (IC), and carbon (C) in the residue was considered as SOC. Total C in the soilwas determined by dry combustion with an NC element analyzer (FlashEA 1112, Thermo,

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Landfill Heavy Metal Pollution 393

Italy). DOC was extracted using a procedure modified from Bolan et al. (1996). Briefly,10 g (oven-dry basis) of soil were suspended in 40 mL of 0.5 mol L−1 K2SO4 and shakenat 250 rpm for 30 min. Then the total organic C in the extract was analyzed with a TOCanalyzer (Multi NC/3000, Analytik, Jena, Germany). For the determination of total heavymetals, about 0.1000 g of soil was hot-digested with 2:5:10 HClO4/HF/HNO3 mixture (Biet al., 2003), and diluted to 50 mL volume with 1:1 (V/V) HNO3. Concentrations of Cr, Cu,and Zn were determined using an inductively coupled plasma-atom emission spectrometer(ICP-AES; P-4010, Hitachi, Japan). Cd and Pb concentrations were determined using anatomic absorption spectrophotometer (AAS, Z-5000, Hitachi, Japan).

2.3 Quantification of Heavy Metal Pollution in the Soil

2.3.1 Pollution Index (PI). PI was used to quantify the enrichment of heavy metals incontaminated soils with respect to soil background, and was calculated as below (Hakanson,1980):

PIi = Cis

/C

in

where Cis represents the content of heavy metal i in polluted soils, Ci

n is the backgroundconcentration of heavy metal i in unpolluted natural soils, with subscripts s and n denotingcontaminated and natural background soils, respectively.

Values of the PI are interpreted as follows: PI < 1, low contamination of the soilwith examined substance; 1 ≤ PI < 3, moderate contamination; 3 ≤ PI < 6, considerablecontamination, and PI ≥ 6, very high contamination (Loska et al., 1997).

For each sampling site, a pollution load index (PLI) was calculated to indicate thedegree of integrated heavy metal pollution in affected soils (Wang et al., 2010):

PLI = 5√

PICu ×PIZn ×PICd ×PIPb ×PICr

This index provided a simple, quantitative measure of the level of heavy metal pollutionin contaminated soil. Based on PLI, the status of pollution was then categorized into fourlevels: no pollution (PLI <1), moderate pollution (1 ≤ PLI < 2), high pollution (2 ≤ PLI< 3), and very high pollution (PLI ≥ 3) (Wang et al., 2010).

2.3.2 Assessment of Potential Ecological Risk. An eco-risk index (RI) was used as adiagnostic indicator for the potential soil pollution and calculated following Hakanson(1980) and Deng et al. (2010) as:

RI =n∑

i=1

Eir =

n∑

i=1

Tir •PIi

where RI represents the potential ecological risk index of heavy metals in the soil and is thesum of potential ecological risk factor for individual metal (Ei

r ), T ir is the toxic response

factors of each heavy metal i, and subscript r denotes ecological risk. The values of T ir

and Cin in this study are given in Table 1; Table 2 shows the standard values of RIs for

assessment.

2.4 Statistics

This study employed hierarchical cluster analysis (HCA) and principal components analysis(PCA) of Cu, Zn, Cd, Pb, and Cr concentrations and soil properties (SOC, pH, and DOC).

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Table 1Natural background values (Ci

n) and toxic response factors (Tir) for heavy metals in the

sampled soils

Cu Zn Cd Pb Cr Reference

Cni ( mg kg−1) 23.6 86.1 0.13 25.5 75.0 Wang and Wang (1992)

Tir 5 1 30 5 2 Hakanson (1980)

Varimax-normalized rotation was used to maximize the variances of factor loadings in PCA.The number of significant principal factors was selected depending on the Kaiser criterion;i.e., factors with eigenvalues greater than 1 were chosen (Qishlaqi and Moore, 2007). Foreach principal factor, loadings > 0.71 were typically regarded as excellent and < 0.32 asvery poor (Han et al., 2006). Data were subjected to one-way ANOVA and Duncan’s testwas used for multiple comparisons among the three zones. All statistical analyses wereperformed using SPSS v13.0 (SPSS Inc., Chicago, IL, USA).

3. Results

3.1 The Distribution of Heavy Metals

The soils from different sampling zones varied in pH, SOC, and DOC (Table 3). The pHvalues ranged from 7.9 to 8.9, and SOC ranged from 2.6 to 14.9 g kg−1 with a median valueof 6.2 g kg−1. The mean SOC content in Zone I was 5.1 g kg−1, significantly lower (p <

0.05) than that in Zone II and Zone III. Soil DOC ranged from 3.4 to 12.9 mg kg−1, andwas significantly lower in Zones I and II than in Zone III (p < 0.05).

Concentrations of Pb in all three zones were lower than background value (25.5,Table 1). However, Cu, Zn, Cd, and Cr exhibited higher concentrations in all zones thanbackgrounds, of which Cd was about 1.5-fold higher than background. The concentrationsof Cu and Cr were higher in Zone III than in the other two zones, showing a gradual increasewith the distance from the landfill. The maximum concentration of Zn (104.1 mg kg−1)was in Zone II, but not significantly different from those in other zones. Concentrations ofCd and Pb in Zone III were significantly lower than in Zone I (p < 0.05).

Table 2Ranges of Ei

r and RI at each grade of potential ecological risk

Grades of potential ecological risk

Low Moderate Considerable High Significantly high

Eir <30 30–60 60–120 120–240 >240

RI <110 110–220 220–440 >440

Eir is the potential eco-risk factor of the heavy metal in soil; RI represents the potential ecological

risk index of soil heavy metals, and is the sum of the individual potential ecological risk factor. Gradesof Ei

r and RI are adjusted from Hakanson (1980) and Deng et al. (2010).

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Tabl

e3

Hea

vym

etal

conc

entr

atio

nan

dso

ilpr

oper

ties

(SO

C,p

H,a

ndD

OC

)ne

arth

eL

aoga

ngL

andfi

ll

All

sam

ples

(n=

35)

I(n

=14

)II

(n=

10)

III

(n=

11)

Zon

es†

Mean

±S.D

.M

ean

±S.D

.M

ean

±S.D

.M

ean

±S.D

.C

.V.(

%)

Med

ian

Ran

ge

Soil

heav

ym

etal

conc

entr

atio

ns(m

gkg

−1)

Cu

29.7

±5.

1a34

.0±

11.2

a36

.8±

9.2a

33.2

±8.

726

.134

.015

.9–4

5.0

Zn

98.3

±15

.8a

104.

1±

26.1

a99

.2±

24.1

a10

0.2

±21

.020

.999

.558

.3–1

32.6

Cd

0.23

±0.

08b

0.20

±0.

08ab

0.15

±0.

06a

0.20

±0.

0839

.40.

200.

07–0

.40

Pb14

.3±

3.2b

14.0

±3.

6b9.

2±

1.3a

12.6

±3.

628

.411

.67.

5–20

.1C

r87

.1±

24.8

a90

.7±

22.6

a91

.6±

9.4a

89.5

±19

.722

.186

.059

.2–1

55.2

Soil

char

acte

rist

ics

SOC

(gkg

−1)

5.1

±1.

4a8.

5±

3.5b

9.1

±3.

3b7.

3±

3.2

43.9

6.2

2.6–

14.9

pH8.

4±

0.1b

8.5

±0.

2b8.

2±

0.2a

8.4

±0.

22.

48.

47.

9–8.

9D

OC

(mg

kg−1

)8.

3±

1.3b

9.5

±2.

0b5.

6±

1.5a

7.8

±2.

228

.27.

93.

4–12

.9

† See

Figu

re1

for

Zon

esI,

II,a

ndII

I.L

ower

case

lette

rs(a

,b)

show

that

ther

ear

esi

gnifi

cant

diff

eren

ces

amon

gth

eth

ree

zone

sat

0.05

leve

ls.

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Table 4Pearson’s correlation matrix between heavy metals and soil properties (n = 35)

Element Cu Zn Cd Pb Cr SOC pH DOC

Cu 1.000Zn 0.878∗∗ 1.000Cd 0.376∗ 0.547∗∗ 1.000Pb 0.255 0.495∗∗ 0.732∗∗ 1.000Cr 0.426∗ 0.392∗ −0.051 0.022 1.000SOC 0.713∗∗ 0.505∗∗ 0.260 −0.045 0.220 1.000pH −0.507∗∗ −0.329 −0.204 0.216 −0.095 −0.624∗∗ 1.000DOC 0.093 0.337∗ 0.622∗∗ 0.610∗∗ −0.034 0.252 0.166 1.000

∗:p < 0.05; ∗∗:p < 0.01.

3.2 Pollution Sources

Table 4 shows the Pearson coefficients between soil variables. There were significantcorrelations between Cu and Zn (r = 0.878, p < 0.01), Cu and Cr (r = 0.426, p < 0.05), andCd and Pb (r = 0.732, p < 0.01). Cu and Zn were positively correlated to SOC (p < 0.01).Zn, Cd, and Pb were significantly correlated with pH (p < 0.05). Cr was not significantlycorrelated to SOC, pH, or DOC (p > 0.05).

The results of PCA analysis on the five metals and soil properties are reported inTable 5. The first three factors accounted for 82.7% of total variance. According to theinitial component matrix, Cu, Zn, Cd, and SOC all displayed large values on Factor 1,suggesting that they were highly associated with each other. However, Cu, Cd, and SOCwere also partially represented in Factor 2. In Factor 3, only Cr showed great loading value,but the value of pH could not be ignored. Hence, the component matrix was rotated. Afterthe rotation, Factor 1 accounted for 33.2% of total variance and was mainly correlated toPb, Cd, and DOC; Factor 2 accounted for 30.1% and was mainly correlated to Cu, SOC,and pH; and Factor 3 accounted for 19.4% and was best correlated to Cr and moderately toCu and Zn.

Figure 2. Clustering of chemical analysis parameters using Ward’s method.

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Tabl

e5

Tota

lvar

ianc

eex

plai

ned

and

com

pone

ntm

atri

ces

ofal

lfac

tors

Ext

ract

ion

Sum

sof

Rot

atio

nSu

ms

ofIn

itial

Eig

enva

lues

Squa

red

Loa

ding

sSq

uare

dL

oadi

ngs

%of

Cum

ulat

ive

%of

Cum

ulat

ive

%C

umul

ativ

eFa

ctor

Tota

lV

aria

nce

%To

tal

Var

ianc

e%

Tota

lV

aria

nce

%

Tota

lVar

ianc

eE

xpla

ined

13.

474

43.4

3043

.430

3.47

443

.430

43.4

302.

653

33.1

6033

.160

22.

080

26.0

0269

.432

2.08

026

.002

69.4

322.

408

30.1

0663

.266

31.

059

13.2

3182

.663

1.05

913

.231

82.6

631.

552

19.3

9782

.663

40.

616

7.70

090

.363

50.

416

5.19

795

.560

60.

182

2.27

697

.836

70.

139

1.73

699

.572

80.

034

0.42

810

0.00

0

Com

pone

ntM

atri

xR

otat

edC

ompo

nent

Mat

rix

Ele

men

tFa

ctor

1Fa

ctor

2Fa

ctor

3Fa

ctor

1Fa

ctor

2Fa

ctor

3

Com

pone

ntM

atri

xC

u0.

862

−0.3

800.

121

0.25

70.

719

0.56

5Z

n0.

906

−0.0

540.

204

0.52

60.

512

0.57

1C

d0.

728

0.49

6−0

.254

0.86

70.

295

−0.0

46Pb

0.56

20.

728

0.14

30.

904

−0.1

430.

169

Cr

0.35

4−0

.348

0.78

1−0

.100

0.05

30.

918

SOC

0.68

9−0

.460

−0.2

780.

117

0.85

00.

167

pH−0

.453

0.63

10.

468

0.15

0−0

.893

0.05

2D

OC

0.50

90.

652

−0.1

110.

833

−0.0

08−0

.054

397

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398 C. Liu et al.

Hierarchical cluster analysis (Figure 2) for all soil samples suggests they can beclassified into three groups by a rescaled distance of 0 and 5: group i was mainly related toCd, Pb, DOC, pH, and SOC; group ii to Cu only and group iii to Zn and Cr.

3.3 Indices of Heavy Metal Pollution

Table 6 shows PIs of the heavy metals in the soils. The PIs of Cd ranged from 0.5 to 3.0,with an average of 1.5, which was the highest among the five metals studied. The PIs ofPb were the lowest, less than 1.0 in all zones. Zn also exhibited relatively low PIs, varyingfrom 0.7 to 1.5 and with a mean of 1.2 across zones. Low to moderate PIs were observedfor Cu and Cr, varying from 0.7 to 2.1 and from 0.8 to 2.1, respectively.

Calculated PLI varied from 0.7 to 1.4, with an average of 1.1, across the sampling sites(Table 6). There were no significant differences in PLIs among the three zones (p > 0.05),although the mean PLI in Zone III, less than 1.0, was lower than in the other zones.

For Cd, the values of Eir ranged from 15.8 to 90.4, with a mean 44.8 (Table 6), which

was much higher than that of all other metals. For Cd, 69% of our sampling sites showedmoderate ecological risk and 14% showed a considerable risk. For the other four metals,ecological risks were low in all sites.

The potential RI for the five measured metals (Cu, Zn, Cr, Pb, and Cd) ranged from26.0 to 104.9 across all sites (Table 6), suggesting a low potential eco-risk of the five metals.The mean RI of Zone I was significantly greater (p < 0.05) than that of Zone III.

4. Discussion

4.1 Enrichment of Heavy Metals in Soils Affected by the Laogang Landfill

Previous studies suggested that solid waste disposal sites or refuse landfills in suburbanareas might have a negative impact on the quality of nearby soil (Islam et al., 2001; Wardet al., 2005). In this study, however, all of the heavy metals except for Pb showed onlyslight enrichment in the soil. For example, Pb and Cu showed relatively low concentrations(14.3 mg kg−1 and 29.7 mg kg−1, Table 3) even within 0.1–1 km from the landfill, whichwere much lower than that found in soils around landfills in other areas such as Turkey(267 mg kg−1 and 222 mg kg−1; Tumuklu et al., 2007) and Tijuana (74.2 mg kg−1 and304.4 mg kg−1; Nava-Martı́nez et al., 2011). Similarly, concentrations of Zn and Cd wererelatively low compared to previous studies (Nduka et al., 2006; Nava-Martı́nez et al.,2011). Only Cr concentration in one of the three zones (87.1 mg kg−1 in Zone I) was higherthan that reported for soils around a large landfill in Nigeria (17 mg kg−1; Nduka et al.,2006). Overall, we did not find serious accumulation of heavy metals near the LaogangLandfill.

Several reasons may be given for the relatively low enrichment of heavy metals. Firstly,this might be due to a relatively low content of heavy metal in solid wastes dumped into thelandfill. Chai et al. (2007) found that in the Laogang Landfill, the concentrations of Cu, Cr,Cd, and Pb of solid wastes were all within the third class of the National Soil EnvironmentalStandard of China (GB15168-1995), and that in leachate were below the limits of theIntegrated Wastewater Discharge Standard of China (GB8978-1996). In addition, domesticgarbage, which constituted 38% of all wastes in the Laogang Landfill, generally containedminor amounts of heavy metals and its leachate was often characterized by low heavy metalconcentrations (Xie et al., 2009). Secondly, the accumulation of heavy metals can be alsoaffected by soil properties, such as SOC and pH (Remon et al., 2005; Chai et al., 2007).

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Tabl

e6

The

pollu

tion

inde

x(P

I),p

ollu

tion

load

inde

x(P

LI)

,pot

entia

leco

logi

calr

isk

fact

or(E

i r)an

dpo

tent

iale

colo

gica

lris

kin

dex

(RI)

ofhe

avy

met

als

inth

eso

ilsa

mpl

es

Prop

ortio

nsof

tota

lsam

ples

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399

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07:

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400 C. Liu et al.

Average SOC concentration (< 1%, Table 3) in studied area was low for agricultural soils,suggesting a low absorbing capacity of surface soil to heavy metals (Huang et al., 2007). Inthis study, soil pH values ranged from 7.9 to 8.9 at all sites (Table 3). Alkaline soil (pH >

8.0) limits the mobilization of heavy metals, thus reducing heavy metal concentrations inlandfills (Sharma et al., 2007). Coastal wetland reclamation could cause substantial changesin soil properties, particularly decreases in soil pH and carbonate content (Santin et al.,2007). The solubility and transport of most heavy metals in soils, such as Cd and Pb, arelikely to be enhanced at low pH (Schubauer-Berigan et al., 1993). Previous studies showedthat most agricultural soils tend to become more acidic due to agricultural managementpractices; for example, continuous application of nitrogen fertilizers (Barak et al., 1997).With a continuous agricultural cultivation in this area, reclaimed soil may receive a highinput of anthropogenic heavy metals. Therefore, monitoring the accumulation of heavymetals in a soil-crop system surrounding a landfill is needed, as the accumulation processesof heavy metals may be affected by the landfill. Additionally, the extent of migration andenrichment of heavy metals was also influenced by climate conditions; e.g., temperatureand precipitation (Clark et al., 1998). Rainfall is abundant in the study area. Although highprecipitation in this sub-tropical coastal climate would accelerate the leaching of heavymetals from the landfill, the accumulation of heavy metals in the soil has been insignificantto the time of sampling.

4.2 Spatial Distribution of Soil Heavy Metal in the Vicinity of the Landfill

It is generally believed that the accumulation of heavy metals in soils is closely relatedto the distance from the source of pollution (Bhuiyan et al., 2010; Wang et al., 2010).Tumuklu et al. (2007) found that the concentrations of heavy metals in soils decreased withthe distance from a garbage dump. In our study, the concentrations of Cd and Pb clearlyshowed a decreasing trend with distance from the landfill (p < 0.05), whereas Cu, Zn, andCr did not (Table 3). The result suggested that only the enrichment of Cd and Pb in studiedsoils might have been caused by the landfill. A likely explanation is the higher migrationrate of Cd than for the other metals. Similarly, Prechthai et al. (2008) reported that Cd hadthe highest mobility and was more susceptible to release from the Nonthaburi dumpsite inThailand than Zn, Cu, Cr, or Pb. Xiao et al. (2005) also found a much higher migration rateof Cd than that for Cu, Pb, or Zn in the Tianziling Landfill of Hangzhou, China.

The PCA results could be used to identify sources of other heavy metals in the soils.Factor 1 in PCA analysis was well correlated with Pb and Cd, suggesting that thesevariables had a common source, probably the landfill leachate. Factor 2 in PCA was mainlyassociated with Cu, SOC, and pH, indicating that the spatial distribution of Cu was relatedto soil pH. As SOC was subject to the influences of cultivation, Cu would likely be affectedby agricultural activities, as suggested by Bhuiyan et al. (2010). Factor 3 was best correlatedwith Cr and moderately with Cu and Zn, which suggests that they have similar sources,presumably soil minerals.

4.3 Ecological Impact Assessment of Soil Heavy Metals

There is an increasing concern about possible ecological consequences of heavy metalpollution in agricultural soils (Calace et al., 2005; Deng et al., 2010; Wang et al., 2010). Asurvey in 2002 by the Chinese Research Academy of Environmental Sciences showed thatalmost every landfill in China had discharged large amounts of leachate into the environment

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(Chen et al., 2004), and heavy metals in leachate might cause serious damage to nearbyagricultural soils and ecosystems.

Our results showed that although there is a slight accumulation of heavy metals inthe soil near the Laogang Landfill, heavy metal concentrations within the surface soilwere generally below the nationwide natural background threshold values, according tothe Environmental Quality Standard for Soils (GB15618-1995; National EnvironmentalProtection Agency of China, 1995). The PIs of Cr, Cu, Cd, and Zn were all within a rangeof 1–3 (Table 6), indicating soils near the landfill were moderately contaminated by thesemetals. Soils in the nearest area adjacent to the landfill (i.e., Zone I) had the highest RIindex, suggesting that the landfill increased ecological risks in its vicinity. Therefore, closemonitoring of Cr, Cu, Cd, and Zn is needed to determine whether croplands around thelandfill are suitable for safe food production in the future.

To date, however, contamination of the studied metals is not serious, according to thepollution load index (PLI) and ecological risk index (RI). PLIs for all three zones wereless than 2, indicating only moderate pollution. Only Ei

r for Cd indicated that 69% ofsampling sites had a moderate ecological risk, while 14% showed considerable risk. Apossible explanation was that the toxic-response factor of Cd was relatively high while thisfactor was much lower for Cr, Cu, Pb, or Zn (Table 1), and that the mean PI for Cd washigher than for other metals. The RIs of all samples were also relatively low (Table 6),below 110, indicating ecological risks of the five metals were at low levels around theLaogang Landfill. Therefore, we predict that, at least in the near future, adverse biologicaleffects caused by heavy metals Cu, Pb, Zn, Cd, and Cr were unlikely to occur in areassurrounding the Laogang Landfill. However, to avoid possible risks for food productionin this area, agricultural activities should be kept a safe distance (>ca. 100 m) from thelandfill.

5. Conclusions

This study revealed that the concentrations of five common heavy metals (Cu, Zn, Cd, Pb,and Cr) in cropland soils near the Laogang Landfill, the largest landfill in China, variedwith distance to the landfill. Except for Pb, heavy metal concentrations did not exceedbackground levels, suggesting that heavy metal pollution had occurred at relatively lowlevels in the soil nearby the landfill. Cd showed a different spatial distribution and highermigration rate than other metals and its potential ecological hazards should be given moreattention by local land managers. Overall, slight pollution of heavy metals was detected insoils, and a minimal safe distance of 100 m from landfill is required for food production inthis moist, sub-tropical and warm temperate zone.

Acknowledgements

This study was supported by the National Basic Research Program of China(2010CB950604) and Science and Technology Commission of Shanghai Municipality(Grant No. 09DZ1900106).

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