Concentrations and chemical forms of heavy metals in urban soils of Shanghai, China

Abstract

The influence of previous irrigation of sewage water from industry and daily life on the concentrations and chemical forms of heavy metals in soils was examined in Xijia Village, Pudong District, Shanghai, China. Soil samples were taken from three regions in the upland fields that varied in distance from the abandoned open canal. To understand the background levels of heavy metals in the agricultural soils in Pudong District, soils under different land use (upland fields, greenhouses and orchards) were sampled as references from four localities with no history of sewage water irrigation. Analyses of total amounts of Cr, Mn, Ni, Cu, Zn, As, Pb, Cd and Hg indicated that the amounts in the soils from the sewage irrigated field were markedly higher than the background levels. In particular, the total amounts of Cd and Zn in the soil at distances of 2 m and 10 m from the canal exceeded the permissible value set by the Chinese Environmental Quality Standard (GB 15618–1995). Although the contamination showed a trend of decreasing with increasing distance from the canal, the total amount of Cd was still higher than the permissible value 20 m from the canal. The chemical forms of Zn and Cd in the soils taken from Xijia Village and the reference upland soils were then evaluated using a sequential extraction method. In the case of the soils 2 m and 10 m from the canal, the highest amounts of Zn and Cd were found to indicate an occlusion in Fe oxides and retention by the clay exchange sites of the soils. Based on these results, it was recommended not to use these sewage irrigated fields in Xijia Village for agricultural purposes unless the necessary remediation measures were taken to lower the heavy metal concentrations to safe levels.

Key words:

• China
• heavy metals
• sequential extraction
• Shanghai
• urban soil

INTRODUCTION

Heavy metal contamination of the environment caused by parent material in soils or human activity, including fertilization, refining and traffic, has been a worldwide phenomenon that has attracted great attention from governmental and regulatory bodies that are anxious to prevent further environmental deterioration (Kaasalainen and Yli-Halla 2003; Li et al. 2004; Vidal et al. 2004). Determining the total content of trace metals is insufficient to assess the environmental behavior of contaminated soils. As a result, sequential extraction was developed and has been widely used over recent decades to study the forms, availability, mobility and transformation of heavy metals in sludge, manure, soils and sediments (Adamo 1996; Han et al. 2001). Using this method, different chemical extractions, generally starting with the weakest and least aggressive chemicals and ending with the strongest and most aggressive, are used to quantify the different fractions of metal retention in soils (Kashem and Singh 2001). Although sequential extraction has been criticized for lack of uniformity in the procedures, lack of selectivity of the reagents used, high dependence of the results on the procedure used, sequential extraction schemes are still widely used and are considered an essential tool in establishing element fractionation in soils and sediments (Gleyzes et al. 2002).

Shanghai is an urban metropolis with a population of approximately 13 million and is an area with intensive agricultural and horticultural production surrounding the central city (Yin 2004). For a metropolis such as Shanghai, human health is strongly dependent on the status of its urban soils. However, from the 1960s Shanghai has been reported to have heavy metal pollution problems in agricultural soils. Heavy metal pollutants were mainly caused by improper industrial waste disposal and sewage irrigation (Lü 1998; Zhou et al. 1992). Many studies outlining the heavy metal pollution problems in agricultural soils in Shanghai have been reported. These studies are mainly based on investigations of total concentrations in soils. Wang et al. (1991) investigated the background level of heavy metals and the contamination situation of the vegetable cultivating area on the outskirts of the city, and pointed out that Cd caused the severest pollution and Cr, Cu, Zn and Hg caused medium pollution to the farmland. The pollutants mainly accumulated in the stratum from 0 to 20 cm, where most plant roots aggregate. Kung et al. (1990) found the presence of F, Cr, Zn, Pb, Cd and Hg contaminants in the surface soils around an industrial complex in Baoshan-Wusong area. While, Feng et al. (1991, 1993) investigated the contamination of heavy metals in vegetable and rice cultivating areas in Baoshan District, revealing that almost all the vegetable fields were subjected to heavy metal contamination to a different extent, and Cd and Hg were the main contamination elements. Wang (1989), Pang (1991) and Lu et al. (2004) pointed out that some areas of Pudong District, especially areas where sewage irrigation was once conducted, were severely polluted, and occasionally the Cd content in rice was over the permitted level of the National Sanitation Standard (0.20 mg kg⁻¹). The heavy metal concentration in soils and crops in the suburbs of Shanghai was surveyed by Pang (1994). Crops contaminated by Pb and Cd and soils contaminated by Zn and Cd were found in some areas, although the general quality of the soils was good. Yang et al. (2002) examined heavy metal distribution on a tidal flat of Chongming Island to study the deposition and transfer process, as well as the distributing character of the heavy metals.

However, very few studies have examined the chemical forms of heavy metals and their distributions in agricultural soils in Shanghai. Although most of the heavy metal contamination areas caused by industry have been changed into non-agricultural land, as a result of urban restructuring, the remaining sewage irrigated areas are receiving more and more attention. More information concerning the contamination status, especially the availability and mobility of heavy metals, in the sewage irrigated areas will be necessary for valid evaluation of the contamination and for the development of appropriate methods for controlling contamination. In the present study, single and sequential extraction procedures were used to evaluate the contamination of heavy metals in agricultural soils in Pudong District to examine the availability, mobility and distribution characteristics of the polluted metals.

MATERIALS AND METHODS

Study area

The study area, Heqing Town, is located in the northeast of Pudong District with a total area of 36.6 km², including 10.3 km² plowland (Li 2004). From 1969 to 1970, a 27.3-km long canal was constructed to drain wastewater, including 40% industrial sewage and 60% sanitary waste, from the central city (Lu et al. 2004; Wang 1989). Part of this canal, 17.3 km, was designed as an open canal in which the sewage flowed through Huamu, Beicai, Zhangjiang, Tang and Heqing Town. The end of the canal was just in Heqing Town (Fig. 1). Areas that the canal passed through previously used the sewage as irrigation water, and the sludge was also used in some areas as fertilizer. Although sewage irrigation was prohibited in 1979 and the open canal was rebuilt into a closed canal in 1982, high concentrations of heavy metals in the sewage and sludge had already caused contamination of, in total, 40 km² of farmland in 14 villages along the open canal (Lu et al. 2004). After the 1990s, with the development of the city, large areas of contaminated fields were changed into non-agricultural land, but an area of approximately 0.3 km² still remains as agricultural lands in Tang and Heqing Town, mainly in Heqing Town (Lu et al. 2004).

Soil sampling

The study sites were located within Heqing Town, Pudong District, Shanghai, China. According to previous research on the total heavy metal concentration in this area, soils contaminated by sewage irrigation were mainly located within the areas along the abandoned open canal (Lü 1998). Lu et al. (2004) examined areas within 25 m of the canal and showed a significant influence of distance on the concentrations of soil heavy metals. Therefore, in the present study, soils that once underwent sewage irrigation were sampled from upland fields in Xijia Village, the village in Heqing Town that is located nearest to the abandoned open canal, 2, 10 and 20 m from the abandoned open canal in September 2004, with two replicates at each site. Meanwhile, soils with different land uses (upland fields, greenhouses and orchards) were sampled as references from four localities (Qianshao, Qinyi, Yingfang and Zhaoyang Village). Sampling sites where the topsoil (0–15 cm) and the subsoil (15–30 cm) were collected are shown in Fig. 1 (Table 1).

Figure 1  Location of the sampling sites. QS, Qianshao Village; QY, Qinyi Village; XJ, Xijia Village; YF, Yingfang Village; ZY, Zhaoyang Village.

Table 1 General information about the sampling sites

Land-use type	Depth (cm)	Location	Crops
Reference soils
Greenhouse (n = 12)	0–15 (Topsoil)	Qinyi Village	Cucumber, alfalfa, lettuce, greengrocery, cabbage
	15–30 (Subsoil)	Yingfang Village
Orchard (n = 8)	0–15 (Topsoil)	Zhaoyang Village	Peach
	15–30 (Subsoil)	Qianshao Village	Peach
Upland (n = 4)	0–15 (Topsoil)	Qinyi Village	Alfalfa, cabbage, cauliflower
	15–30 (Subsoil)	Yingfang Village
Sewage irrigated soils
Upland (n = 6)	0–15 (Topsoil)	Xijia Village	Greengrocery, water convolvulus, cabbage, cucumber, cowpea
	15–30 (Subsoil)

Soil analysis

Soil samples were air-dried and crushed to pass through a 2-mm mesh sieve. Soil particle size distribution was investigated using the pipette method (Gee and Bauder 1986). The electrical conductivity (EC) and pH values were determined using a glass electrode at a soil : deionized water ratio of 1:5 after reciprocal shaking for 1 h. Exchangeable cations were extracted using 1 mol L⁻¹ ammonium acetate at pH 7.0 and determined by atomic adsorption spectrometry (AAS) (AA-6800, Shimadzu, Kyoto, Japan). After removing the excess NH⁺ ₄, the soil was extracted with 100 g L⁻¹ NaCl solution and the supernatant was used to determine cation exchangeable capacity (CEC) using the Kjeldaghl distillation and titration method (Rhoades 1982). Total carbon (T-C) and nitrogen (T-N) were determined using an NC analyzer (Sumigraph NC-80, Sumitomo Chemical, Osaka, Japan). Available phosphorus (av.P) was extracted using the Truog method and determined by inductively coupled plasma spectrometry (ICP-AES) (ICPS-1000 IV, Shimadzu).

Total concentrations of Cr, Mn, Ni, Cu, Zn, Pb and Cd were determined by AAS after H₂SO₄, HNO₃ and HClO₄ digestion on a hotplate or after HNO₃ and HF digestion by microwave (Multiwave, Perkin-Elmer, Yokohama, Japan). Arsenic was measured using ICP equipped with a hydride generation system (HVG-1, Shimadzu) after the soils were digested by HClO₄, HNO₃ and HF on a hotplate. During the digestion, 20 g L⁻¹ KMnO₄ was added to the digesting solution to oxidize organic matter. For the determination of Hg, soils were digested by HNO₃ and H₂SO₄ in a round bottom flask equipped with a reflex condenser, and Hg was measured by reduction vapor-flameless atomic adsorption spectrometry (MVU-1A, Shimadzu). The amounts of HCl-extractable Zn and Cd (HCl-Zn, HCl-Cd) were measured by AAS after shaking with 0.1 mol L⁻¹ HCl for 1 h at a soil : solution ratio of 1:5. A sequential extraction method was carried

Table 2 Method for sequential extraction

Fraction	Extractant	Soil : solution ratio	Condition
Water soluble (Ws-)	Deionized water	1:5	Shake 1 h
Exchangeable (Ex-)	1 mol L^−1 CH3COONH4 (pH 7)	1:10	Shake 2 h
Acid soluble (Aci-)	25 g L^−1 CH3COOH (pH 2.6)	1:10	Shake 6 h
Mn-oxide occluded (MnO-)	0.1 mol L^−1 NH2OH·HCl (pH 2)	1:50	Shake 0.5 h
Organically bound (OM-)	0.1 mol L^−1 Na4P2O7 (pH 10)	1:50	Shake 24 h
Fe-oxide occluded (FeO-)	0.175 mol L^−1 (NH4)2C2O4+0.1 mol L^−1 H2C2O4+0.1 mol L^−1 ascorbic acid (pH 3.1)	1:50	Shake 4 h, then 0.5 h in boiling water bath. Stir occasionally
Residual (Res-)	HF-HNO3-HClO4 digestion

out according to the method described by Iwasaki et al. (1997) to differentiate seven fractions, including water soluble (Ws-), exchangeable (Ex-), acid soluble (Aci-), Mn oxide-occluded (MnO-), organically bound (OM-), Fe oxide-occluded (FeO-) and residual (Res-) fractions with some modifications (Table 2). The experimental details of the procedure are presented in Table 2 and all elements were measured by AAS.

Statistical analysis

For data on the physico-chemical properties and total heavy metal concentrations, multiple comparisons were made among reference soils of different land-use types according to Tukey's method. Student's t-tests were conducted between the topsoil or subsoil of upland fields in reference and the sewage irrigated soils in Xijia Village. Main effects of distance from the canal and soil depth on the amounts of 0.1 mol L⁻¹ HCl extractable Zn and Cd and those in different soil chemical fractions were subjected to two-way anova.

RESULTS

Physical and chemical soil properties

Clay content in the reference soils was 22.7–25.2% (Table 3). There was no significant difference in clay content among the reference soils of different land-use types or among the different layers, although clay content in the sewage irrigated soil was, on average, 6% higher than in the reference upland soils. The pH of the reference soils averaged 7.62–8.15, and sewage irrigated soils showed lower pH values. The EC value ranged from 0.06 to 0.26 for all soils investigated, and the EC of greenhouse soils was higher than that of upland and orchard soils. Concentrations of exchangeable bases showed the order of greenhouse > upland > orchard, and were higher in topsoil than in the subsoil for the greenhouse and upland soils. As for the soils in Xijia Village, T-N, T-C and CEC contents were higher than the values recorded in the reference upland soils. Although, exchangeable K⁺, Na⁺, Ca²⁺ and av.P contents were similar or lower than those in the upland reference soils.

Total concentration of heavy metals

Mean values of total heavy metal contents are shown in Table 4. In general, in reference soils, greenhouse and upland soils contained higher amounts of heavy metals than orchard soils. For the sewage irrigated soils, nearly all the metals (Cd, Zn, Pb Cr and Hg) showed significantly higher concentrations at a 1% level than the upland reference soils. In particular, concentrations of Cd and Zn exceeded the permissible values (0.3 and 250 mg kg⁻¹, respectively) set by the Chinese Environmental Quality Standard (GB15618-1995) for soils (Chen 2002). As shown in Fig. 2, Zn in soils 2 and 10 m from the canal and Cd in all soils exceeded the permissible values, with the highest excess of 1.4-fold and 9.1-fold, respectively. As accumulations of Zn and Cd were more marked than the other elements, single step extraction with 0.1 mol L⁻¹ HCl and sequential extraction procedures were applied to determine the status of Zn and Cd in soils of Xijia Village and the upland reference soils.

0.1 mol L⁻¹ HCl extractable Zn and Cd

Concentrations of 0.1 mol L⁻¹ HCl extractable Zn (HCl-Zn) and Cd (HCl-Cd) are shown in Fig. 3. HCl-Zn in Xijia Village soils comprised, on average, 27.1% of the total amounts. Compared with the reference soils, the average concentration of HCl-Zn increased approximately fourfold, and its distribution in the

Table 3 Mean values for the physico-chemical properties of reference soils and sewage irrigated soils from Xijia

Land-use type	Depth (cm)	EC (dS m^−1)	pH	Exchangeable bases				CEC (cmolc kg^−1)	T-N	T-C	av.P (mg kg^−1)	Sand	Silt	Clay	Texture
				K^+	Na^+	Ca^2+	Mg^2+
				(cmolc kg^−1)					(g kg^−1)			(%)
Reference soils
Greenhouse (n = 12)	0–15	0.26 a	7.62 b	2.09 a	0.66 a	18.8 b	5.33 a	14.6 a	2.77 a	24.5 a	1805 a	37.9 a	37.4 a	24.7a	L
	15–30	0.25 A	7.85 B	1.80 A	0.74 A	18.7 B	5.38 A	14.2 A	1.87 A	17.0 A	1182 A	38.8 A	36.8 A	24.4 A	L
Orchard (n = 8)	0–15	0.09 a	8.09 a	0.63 b	0.14 b	22.6 a	2.73 b	12.6 a	1.28 b	13.2 b	123 b	37.0 a	40.6 a	22.7 a	L
	15–30	0.10 A	8.15 A	0.28 B	0.25 B	23.3 A	3.03 B	13.2 A	1.50 A	14.4 A	86.7 B	36.6 A	40.8 A	22.9 A	L
Upland (n = 4)	0–15	0.09 a	7.91 ab	0.58 b	0.43 ab	18.7 b	3.58 b	13.3 a	2.01 a	16.7 b	577 b	37.8 a	37.0 a	25.2 a	L
	15–30	0.08 A	7.98 AB	0.40 B	0.37 B	19.5 B	3.51 B	13.6 A	1.37 A	13.2 A	462 B	39.3 A	36.0 A	24.8 A	L
Sewage irrigated soils (Xijia Village)
Upland (n = 6)	0–15	0.08	7.09	0.43	0.24**	14.8*	4.86*	18.5*	2.59	21.7	285*	25.5	43.4	31.6	CL
	15–30	0.06**	7.10**	0.46	0.32	13.1**	4.70*	18.4**	1.61*	16.5**	284*	27.3	42.6	30.4	CL
Means followed by the same small letter are not significantly different at a 5% level for topsoil and those followed by the same capitalized letter are not significantly different at a 5% level for subsoil according to a Tukey's test. Means followed by * and ** are significantly different from the mean values for the top and subsoil of reference upland soils at 5% and 1% levels, respectively. CEC, cation exchangeable capacity; T-C, total carbon; T-N, total nitrogen; av.P, available phosphorus.

Table 4 Mean values of total heavy metal concentrations for reference soils and sewage irrigated soils from Xijia Village

Land-use type	Depth (cm)	Cd	Zn	Cu	Pb	Cr	Mn	Ni	As	Hg
		(mg kg^−1)
Reference soils
Greenhouse (n = 12)	0–15	0.19 b	150 a	39.0 a	27.0 a	88.2 a	576 a	29.4 a	9.72 a	0.19 a
	15–30	0.14 AB	127 A	32.3 A	28.0 A	82.8 A	550 B	29.9 A	9.66 B	0.17 AB
Orchard (n = 8)	0–15	0.10 c	93.0 b	26.9 b	27.0 a	74.7 b	638 a	31.0 a	11.3 a	0.09 b
	15–30	0.10 B	91.6 B	27.6 A	26.6 A	78.6 A	649 A	32.1 A	12.5 A	0.13 B
Upland (n = 4)	0–15	0.26 a	123 ab	34.2 ab	29.4 a	84.8 ab	565 a	32.1 a	10.1 a	0.17 ab
	15–30	0.20 A	122 AB	33.7 A	29.8 A	81.4 A	581 AB	32.7 A	10.1 AB	0.26 A
Sewage irrigated soils (Xijia Village)
Upland (n = 6)	0–15	2.15**	455**	78.0*	45.0**	159**	531	41.6*	13.4*	0.51**
	15–30	1.98**	414**	69.5*	41.0*	140**	504	40.8*	12.4	0.51**
Means followed by the same small letter are not significantly different at a 5% level for topsoil and those followed by the same capitalized letter are not significantly different at a 5% level for subsoil according to a Tukey's test. Means followed by * and ** are significantly different from the mean values for the top and subsoil of reference upland soils at 5% and 1% levels, respectively.

Figure 2  Total concentrations of Zn and Cd in sewage irrigated soils from Xijia Village. 2, 10, 20 and R refer to soils sampled at 2, 10 and 20 m from the open canal and reference samples. (▒), topsoil; (□) subsoil, and the dotted line represents the permissible value set by the Chinese Environmental Quality Standard (GB15618-1995). Bars on the top right represent the least significant differences (LSD) at the 95% level for comparison between distance from the abandoned canal (left) and depth (right).

Figure 3  The 0.1 mol L−1 HCl extractable fraction of Zn and Cd in sewage irrigated soils from Xijia Village and reference upland soils. 2, 10, 20 and R refer to soils sampled at 2, 10 and 20 m from the open canal and reference samples. (▒), topsoil; (□) subsoil. Bars on the top right represent the least significant differences (LSD) at the 95% level for comparison between distance from the abandoned canal (left) and depth (right).

total amount increased by 7.4%. In addition, higher amounts of HCl-Zn accumulated in the soils near the canal compared with soils further away. The concentration of HCl-Zn also showed a significant negative correlation with pH, and a significant positive correlation with av.P content (Table 5).

HCl-Cd comprised, on average, 88.3% of the total amount. Compared with the upland soils, the average concentration of HCl-Cd in Xijia Village soil has been elevated approximately 10-fold, and its distribution in the total amount has increased by 15.6%. Heavy metal content showed a trend of decreasing with increasing distance from the canal. The HCl-Cd content also showed a significant correlation with pH and av.P content (Table 5).

Sequential extraction of Zn and Cd

Recovery values of the two heavy metals Zn and Cd varied from 90.5 to 117.7%. Correlation coefficients

Table 5 Correlation coefficients between concentrations of metal fractions and soil properties of sewage irrigated soils from Xijia Village

Metal	Fractions	Total metal	pH	K^+	T-N	T-C	av.P
Zn	0.1 mol L^−1 HCl extractable	0.97**	−0.84*	−0.80	NS	0.81	0.87*
	Ws-	NS	NS	NS	0.85*	0.77	NS
	Ex-	0.89*	−0.83*	NS	0.81	NS	0.77
	Aci-	0.94**	−0.77	−0.81	NS	NS	0.86*
	MnO-	0.98**	−0.79	−0.81	NS	NS	NS
	OM-	0.89**	NS	−0.82*	NS	NS	0.91*
	FeO-	0.99**	−0.83*	−0.83*	NS	NS	0.82*
Cd	0.1 mol L^−1 HCl extractable	0.99**	−0.81*	−0.86	NS	NS	0.85*
	Ws-	0.79	−0.83*	NS	NS	NS	NS
	Ex-	0.93**	−0.80	−0.86*	NS	NS	0.88*
	Aci-	0.93**	NS	−0.89*	NS	NS	0.83*
	MnO-	0.95**	−0.78	−0.81	NS	NS	NS
	OM-	NS	NS	NS	NS	NS	NS
	FeO-	0.91*	NS	−0.86*	NS	NS	0.92**
*P < 0.05;
**P < 0.01; NS, not significant at a 10% level (n = 6). T-C, total carbon; T-N, total nitrogen; av.P, available phosphorus.

between the metal content in each fraction and the soil properties are shown in Table 5.

Zn

Obviously, concentrations of all the fractions of Zn in the sewage irrigated soils at a distance of 2 m from the canal were higher than at 20 m, and were much higher than those in the reference upland soils (Fig. 4). The distribution pattern of the different fractions was different in different positions. The average percentage distribution for soils at 2 and 10 m from the canal followed the order: FeO- > Res- > OM- > MnO- > Aci- > Ex- > Ws-fraction. While for soils at 20 m, it followed the order: Res- > FeO- > OM- > MnO- > Aci- > Ex- > Ws-fraction, which showed a similar distribution pattern to the reference soils. Moreover, Zn found in Ex-, Aci-, MnO-, OM-, and FeO-fractions showed high correlations with total content (Table 5) in Xijia Village soils. The pH, K⁺ and av.P showed high correlations with Ex-, Aci-, OM- and FeO-fractions.

In the soils of Xijia Village, on average, 9.4% of total Zn was distributed into the Ws-, Ex- and Aci-fractions, nearly twofold the values recorded in the reference upland soils. Among these three fractions, Ws-Zn was noticeably lower in concentration than the other fractions, and Ws-Zn at 20 m was even under the detection limit (0.01 mg kg⁻¹). The percentage of Ws-Zn in total concentration was no higher than 0.1%, similar to the values recorded in the reference soils. However, Ex- and Aci-Zn were of higher content and proportion than those in the reference upland soils, especially for Ex-Zn, which showed a 12-fold increase. Most Zn was distributed in oxide-occluded fractions (43.7% on average), followed by residual fractions (40.7% on average). For MnO-Zn and FeO-Zn, the concentration generally decreased with increasing distance from the canal and with soil depth. For Res-Zn, there was no clear trend. Moreover, in soils 2 and 10 m from the canal, the main fraction was oxide occluded (FeO- and MnO-), while in soil 20 m from the canal, Res-Zn became the dominant fraction.

Cd

Concentrations of all fractions of Cd were also higher in the soils 2 m away from the canal than 20 m away, and the Cd distributed in Ex-, Aci-, MnO- and FeO-fractions was greatly increased in the sewage irrigated soils compared with the reference upland soils (Fig. 5). In particular, the average concentration of Ex-Cd in the sewage irrigated soils was approximately 13-fold higher than that in the reference soils. The average percentage distribution of Cd followed the order: Ex- > MnO- > Aci- > FeO- > OM- > Ws- > Res-fraction for soils 2 and 10 m away, and MnO- > Aci- > Ex- > FeO- > Ws- > OM- > Res-fraction for soils 20 m away. Ex-, Aci-, MnO- and FeO-Cd had high correlations with total content (Table 5). The fractions Ws-, Ex-, Aci- and FeO-Cd also showed high correlations with pH value, K⁺and av.P concentrations.

On average, 61.0% of the total Cd was distributed into the Ws-, Ex- and Aci-fractions, and the total concentrations of these three fractions were elevated nearly ninefold compared with the reference upland soils. The concentration of Ws-Cd was very low, showing a value similar to the reference upland soils, while Ex- and Aci-Cd were much higher compared with the reference soil.

Figure 4  Fractionation of Zn in sewage irrigated soils from Xijia Village and reference upland soils. *Concentration below the detection limit (0.01 mg kg−1). 2, 10, 20 and R refer to soils sampled at 2, 10 and 20 m from the open canal and reference samples. (▒), topsoil; (□) subsoil. Bars on the top right represent the least significant differences (LSD) at the 95% level for comparison between distance from the abandoned canal (left) and depth (right). An anova was not conducted for the Ws-fraction because of a lack of data.

For soils 2 and 10 m from the canal, Ex-Cd showed the highest concentration among these three fractions, and the concentration in the subsoil was higher than that in the topsoil. The content of Aci-Cd was next to that of Ex-Cd in these two soils. While for soils 20 m from the canal, Aci-Cd concentration was higher than the Ws- and Ex-fractions. The MnO-fraction was similar to the Ex-fraction for soils 2 and 10 m from the canal, while for soil 20 m from the canal this fraction had the highest concentration among all the fractions. A smaller percentage of Cd was found in the OM- and Res-fractions, whose contents were mostly below the detection limit (0.01 mg kg⁻¹).

DISCUSSION

Physico-chemical properties

The soil texture of the reference soils was classified as loam according to the United States Department of Agriculture (USDA) soil textural triangle. The relatively similar parent material developed from alluvial deposit, as shown in Hou (1992), and frequent plowing meant

Figure 5  Fractionation of Cd in sewage irrigated soils from Xijia Village and reference upland soils. *Concentration below the detection limit (2 mg kg−1 for Ws-Cd, and 0.01 mg kg−1 for OM- and Res-Cd). 2, 10, 20 and R refer to soils sampled at 2, 10 and 20 m from the open canal and reference samples. (▒), topsoil; (□) subsoil. Bars on the top right represent the least significant differences (LSD) at the 95% level for comparison between distance from the abandoned canal (left) and depth (right). An anova was not conducted for the Ws-, OM-, FeO- and Res-fractions because of a lack of data.

that the soil texture tended to be homogenous among the reference soils in different positions. Soils in Xijia Village were clay loam in soil texture, probably because of the addition of sludge in the sewage. The pH of soils was neutral to alkaline. It has frequently been observed that soils under long-term sewage sludge application become acidified from mineralization reactions involving N and S (McBride 2003). A lower pH was also found in the soils in Xijia Village. However, sewage irrigation in this area was prohibited more than 25 years ago, which has not had a significant acidification effect on the soils. Moreover, the calcareous nature of Shanghai soils provided buffering against acidification and gradually made the soil pH neutral (Burt et al. 2003; Hou 1992).

The average EC value of soils in this study was generally at a safe level (below 0.25–0.8 dS m⁻¹) according to He et al. (2004), although the value in greenhouse soils was higher than that in upland and orchard soils. Sewage irrigated soils contained higher amounts of T-N and T-C than upland reference soils. Higher contents of T-C as well as clay resulted in a significantly higher CEC value in Xijia Village soils, implying a higher adsorption ability of cations, including heavy metals, in these soils (Kabata-Pendias and Pendias 1992). However, exchangeable K⁺, Na⁺, Ca²⁺ and av.P content in Xijia Village soils was similar or lower than the upland reference soil. Although sewage irrigation is a valuable source of essential nutritional elements, such as N, P, K, and organic matter for agriculture, sewage irrigation in Xijia Village ceased over 25 years ago, and large part of the available mineral nutrients introduced by sewage may have already been depleted.

Heavy metal content

An investigation of the total content of heavy metals showed that greenhouse and upland soils accumulated more heavy metals than orchard soils, probably because of the heavy application of fertilizers for vegetable production. Cd and Zn may be introduced in phosphate fertilizers, and As, Cr, Ni and Pb may be introduced in poultry manure (Han et al. 2001; Kabata-Pendias and Pendias 1992).

For the sewage irrigated soils, greater amounts of heavy metals were observed, especially for Cd, Zn, Pb, Cr and Hg. According to Hou (1992), soils of Pudong generally developed on alluvial deposits from Yangtze River and the East Sea. Hydromica as well as a small amount of kaolinite, montmorillonite, chlorite and vermiculite dominate the clay minerals over the whole region. Thus, there is no big difference in the parent material of soils. In addition, Wang et al. (1991) reported that there was no correlation between soil type and heavy metal concentrations in the soil of Shanghai. Therefore, the great elevation of heavy metal concentrations in soils of Xijia Village can be considered to result from sewage irrigation.

Among all the elements investigated, Cd and Zn caused the severest contamination because their contents exceeded the permissible values set by the Chinese Environmental Quality Standard (GB15618-1995). According to an investigation by the Shanghai Environmental Science Institution in 1976, soil contamination in an area within 50 m along the canal was the severest, especially by Cd and Zn. Wang et al. (1991) reported that the range of total concentrations of Zn and Cd in Heqing Town were 181.5–420.0 mg kg⁻¹ and 0.53–1.98 mg kg⁻¹, respectively. It is obvious from our results that even 30 years after the application of sewage, large amounts of Zn and Cd still remained in the soils. Moreover, soils near the canal accumulated more Zn and Cd than soils further away (Fig. 2). Lu et al. (2004) also indicated that the distribution of heavy metal contents changed according to the distance from the canal in the sewage irrigated area in Pudong. The nearer to the canal the soil, the more sewage irrigation the site would receive, and the severer the contamination.

0.1 mol L⁻¹ HCl extractable Zn and Cd

Although the 0.1 mol L⁻¹ HCl extraction method is not suitable for predicting Zn availability in soils above pH 7.0 (Baker and Amacher 1987), this method has been widely used as an evaluation method for heavy metal contamination in soils. Therefore, we determined concentrations of HCl-Zn and HCl-Cd to examine the potential mobility of Zn and Cd in the sewage irrigated soils.

Concentrations of HCl-Zn and HCl-Cd in sewage irrigated areas were higher than those in reference soils. Obviously, they tended to accumulate more in soils near the canal compared with soils further away. With increasing distance from the sewage canal, the influence of the sewage water decreased, resulting in a decrease in the contaminants. Both HCl-Zn and HCl-Cd showed a significant negative correlation with pH, confirming the influence of pH on controlling mobility of trace metals in soils. That is, the increase in pH has made the Zn and Cd distribute into less labile fractions (Table 5) (Han et al. 2001; Kashem and Singh 2001; Shuman 1991). HCl-Zn and HCl-Cd also showed a significantly positive correlation with av.P content, probably because added P may redistribute Zn and Cd from the Mn-oxide and crystalline Fe-oxide fractions to the exchangeable fraction, presumably making it more labile (Shuman 1991).

When the concentrations of HCl-Zn and HCl-Cd were compared, it was obvious that the concentration of Cd was much lower than that of Zn. However, a much higher percentage distribution of HCl-Cd in the total amount compared with Zn revealed the relatively higher mobility of Cd. Cd has been always considered to be an extremely significant pollutant because of its high toxicity and greater availability (McLaughlin and Singh 1999). Although sewage irrigation has been prohibited for more than 25 years, Cd added through sewage water still remained in the potentially mobile fractions.

Chemical fractionation of Zn and Cd

According to the results of the sequential extraction shown in Figs 4,5, the concentrations of all the fractions of Zn and Ex-, Aci-, MnO- and FeO-fractions of Cd were much higher than those in the reference upland soils, and all the fractions of both Zn and Cd were higher in the soils 2 m away from the canal than 20 m away. Zinc introduced by sewage irrigation mainly distributed into Ex-, Aci-, MnO-, OM- and FeO-fractions, according to their high correlation with total content (Table 5), while Cd was introduced mainly in Ex-, Aci-, MnO- and FeO-fractions. Distance from the canal appeared to have a great influence on the distribution pattern of different fractions.

The proportion of Zn in the Ex-, Aci-, MnO-, OM- and FeO- was higher in the contaminated soil, and the Res-fraction was lower. The proportion of Ws-, Ex-, Aci-, MnO-, OM- and FeO-Zn tended to decrease with increasing distance from the canal and that of Res-Zn increased, which was similar to a report from Kashem and Singh (2001) examining the city sewage-contaminated soil in Bangladesh.

Most Zn occurred in FeO-fraction, which was the dominant fraction in soils 2 and 10 m from the canal, followed by the residual fraction. The association of large amounts of Zn with oxides, especially Fe oxide, may be because of the high stability constants of Zn oxides or the ability of Zn to substitute for Fe in the structure of oxide minerals (Banerjee 2003; Burton et al. 2005). The concentration of Res-Zn in Xijia Village was higher than that in the reference upland soil. According to Shuman (1991), added metals will revert to the less soluble oxide and residual fractions. Considering that the input of heavy metal by sewage irrigation ceased 25 years ago, some added Zn might be transferred into the residual fraction, resulting in the higher amount of Res-Zn in Xijia Village soils. However, the percentage distribution of Zn in the Res-fraction of these soils was smaller than that in the soil 20 m away, where Res-Zn became the dominant fraction, showing a similar distribution pattern to the reference soils. Moreover, Burt et al. (2003) and Kaasalainen and Yli-Halla (2003) have reported that an increased proportion of Zn resided in the Res-fraction with depth, which was also true in Xijia Village soils. The residual fraction was mainly constituted by primary and secondary minerals containing metals in the crystalline lattice (Gleyzes et al. 2002). This fraction is considered relatively inactive and could reflect the native metal concentration in soil (Burt et al. 2003; Kaasalainen and Yli-Halla 2003). Therefore, metals associated with non-residual fractions have been used as an indicator of anthropogenic enrichment. The decreased distribution of non-residual fractions and the increased distribution of residual fractions with increasing distance from the canal and the deepening of the soil horizon can be explained by the decreased anthropogenic enrichment of Zn owing to distance and depth.

According to Kashem and Singh (2001) and Relićet al. (2005), bioavailability of metals decreases in the order: water soluble > exchangeable > carbonate and Mn-oxide > Fe-oxide > organic > residual, and the first three fractions represent forms of high mobility and potential bioavailability. In particular, the Ws- and Ex-fractions are considered readily mobile and available to plants (Burt et al. 2003). In the present study, the amount of Zn distributed in the Ws-, Ex- and Aci-fractions, defined as mobile fractions according to Kashem and Singh (2001), was lower than that in the FeO- and Res-fractions. However, a significant amount of mobile Zn, as a result of sewage irrigation, was still found because the amount of Zn distributed in the mobile fractions was nearly twofold that of the reference upland soils. Ws-Zn was relatively lower among the mobile fractions, probably because of the neutrality of soil that has decreased the solubility of Zn, or because of the distribution of Ws-Zn into other less soluble fractions with time as Zn is easily adsorbed by mineral and organic matter components (Kabata-Pendias and Pendias 1992).

For Cd, the Ex-fraction showed the highest concentration. Cd has been considered to be an extremely significant pollutant because of its high toxicity as well as mobility (Das et al. 1997). In the present study, more than 50% of the total Cd was found in mobile fractions (including Ws-, Ex- and Aci-fractions), indicating its high mobility. And highly elevated amounts of mobile fractions, especially for Ex- and Aci-fractions, compared with the reference upland soils, implied that the input of sewage had increased the mobile Cd by a large amount. Low concentrations of Ws-Cd, similar to the reference upland soils, implied that the Ws-Cd was again insolubilized by time and the neutral soil pH. For soils 2 and 10 m from the canal, Ex-Cd was the dominant fraction among the three mobile fractions. Content of Aci-Cd was a little less than Ex-Cd in these two soils, similar to the study of Lucho-Constantino et al. (2005), which indicated that the easily mobile heavy metal cations, such as Cd, are mainly bound to Ex-fraction and to a lesser extent to Aci-fraction. While for soils 20 m from the canal, Aci-Cd dominated the mobile fractions, followed by Ex-Cd, showing the same distribution pattern as reference upland soils. Instead of Ex-fraction, MnO-fraction dominated in the soil 20 m from the canal. It appears that with increasing distance from the canal, which results in a decrease in pollutant input, the main forms of Cd change from labile forms (Ws-, Ex- and Aci-forms) to the relatively unlabile MnO-occluded form, indicating a trend of stabilization. Moreover, the distribution pattern of the soils 20 m from the canal showed a similar trend to that of reference upland soils, whose dominant fraction was also occluded by MnO. Content level of OM- and Res-Cd was similar to the reference upland soils. The same results were also reported by Banerjee (2003), and Kashem and Singh (2001) found that the anthropogenically added Cd remained in the mobile fractions and did not become incorporated within the crystal lattice of minerals, and OM-Cd complexes were only loosely bound and easily removed. It is also very clear that the parent material of Shanghai soil contained very low amounts of Cd, and most Cd in the soils came from anthropogenic input.

In addition to the distance effect, soil properties such as pH value and av.P concentrations also had an impact on the distribution pattern of Cd and Zn in the different fractions. According to the correlation coefficients shown in Table 5, pH value and exchangeable K⁺content showed negative correlations with Ex- and FeO-fractions and with OM- and FeO-fractions, respectively. A decrease in Ex-Zn with an increase in pH has also been reported by Iwasaki et al. (1993), Han et al. (2001) and Lu et al. (2003). The decrease of exchangeable K⁺ can be explained by a decrease in specific adsorption of heavy metal cations because of the occupation of adsorption sites on soil surface by heavy metal cations (Sun 1993). While for av.P, it seemed to have promoted the occlusion of Zn into Aci-, OM- and FeO-fractions. Phosphate was reported to affect the retention of heavy metals because sorbed P can lower the zero point of charge on oxide surfaces, with a resultant increase of negative charge with which metal ions may react (Harter 1991). In addition, sorption of phosphate onto oxide surfaces provided either additional negative charges or complexation sites with which Zn reacted (Barrow 1987; Harter 1991).

Furthermore, significant correlation at a 5% level between Ws-Cd and Ws-Zn was indicated in this study (r = 0.85*). Cd and Zn have similar geochemical and environmental properties, and the mining, processing and subsequent release of Zn to the environment is normally accompanied by Cd environmental pollution (Ye et al. 2003). As Lu et al. (2004) reported, a significant correlation between the total amount of Zn and Cd was found (r = 0.94**), showing the concomitant pollution of Zn and Cd in the sewage irrigation area. It has been shown that at low Cd : Zn ratios, a synergistic effect may take place between Zn and Cd, which involves an increase in Cd solubility in soil as Zn can effectively compete for the sites of Cd fixation in soil and consequently results in the translocation of Cd to plant shoots (Srivastava and Gupta 1996). Therefore, under the low Cd : Zn ratio in the soil of Xijia Village, a synergistic effect may also happen between these two elements, aggravating the contamination status.

In general, sewage irrigation causes an accumulation of higher amounts of soil heavy meals than the accumulation in soils with no sewage irrigation history. In particular, concentrations of Zn and Cd in the studied area have exceeded the permissible limit of the Chinese Environmental Quality Standard. Although sewage irrigation has been prohibited for nearly 30 years, no clear alleviation of contamination was found, at least for Zn and Cd. Generally, the percentage distribution of Cd into the Ws-, Ex-, Aci- and MnO-fractions, which are readily or potentially available to plants, was 0.2, 35.5, 25.4 and 35.3% of the total Cd in the sewage irrigated soils, respectively, much higher than those of Zn. The high concentrations of total Zn and Cd, the high availability and mobility of Cd, and the possible synergistic effect between these two elements in sewage irrigated soils makes them unsuitable for agricultural purposes, unless necessary remediation measures are taken.