Spatial trend and pollution assessment of total mercury and methylmercury pollution in the Pearl River Delta soil, South China

Chemosphere 88 (2012) 612–619

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Spatial trend and pollution assessment of total mercury and methylmercury pollution in the Pearl River Delta soil, South China Laiguo Chen a,⇑, Zhencheng Xu a,⇑, Xiaoyong Ding a,b, Weidong Zhang b, Yumei Huang a, Ruifang Fan c, Jiaren Sun a, Ming Liu a,c, Donglin Qian a, Yongbin Feng a a b c

Center for Research on Urban Environment, South China Institute of Environmental Sciences (SCIES), Ministry of Environmental Protection, Guangzhou 510655, China Chongqing Academy of Environmental Science, Chongqing 400020, China Key Laboratory of Ecology and Environmental Science in Guangdong Higher Education, College of Life Science, South China Normal University, Guangzhou 510631, China

a r t i c l e

i n f o

Article history: Received 18 December 2011 Received in revised form 6 March 2012 Accepted 9 March 2012 Available online 4 April 2012 Keywords: Mercury Methylmercury Soil The Pearl River Delta

a b s t r a c t Total mercury (THg) and methylmercury (MeHg) were measured in large number of soil samples collected from areas with different types of land use, different depth in the Pearl River Delta (PRD) of South China. THg and MeHg concentrations ranged from 16.7 to 3320 ng g1 and 0.01 to 1.34 ng g1, respectively. THg levels are highest in the top 0–20 cm soil layer, and decrease from the surface to bottom layer soil. Spatial variation was observed with different types of land use. Urban parks had the highest concentrations and the other areas tended to decrease in the order of residential areas, industrial areas, vegetable fields, cereal fields, and woodlands. Temporal variation was also noted, and two relatively high THg contamination zones located in the northwestern part of the PRD have significantly expanded over the last two decades. Both THg and MeHg concentrations were correlated significantly with soil organic matter (OM), but not with soil pH. THg pollution status was evaluated using two assessment methods. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Mercury (Hg) is one of the most toxic heavy metals and is commonly found in the global environment. It was estimated that the amount of Hg in the atmosphere has increased as much as three times since the beginning of the industrial revolution (Lindberg et al., 2007). The primary sources of Hg are natural, anthropogenic, and re-emitted sources (USEPA, 1997). Globally, about 2000–2200 tonnes of Hg were released annually into the atmosphere from anthropogenic sources, representing two-thirds of the total release (Seigneur et al., 2004). In China, the total mercury (THg) release from anthropogenic sources was estimated to be 536 ± 236 tonnes in 1999. Guangdong province has highly developed industry and huge energy demand which contributed to abundant Hg emission (Streets et al., 2005). The Pearl River Delta (PRD) covers an area of 24,440 square kilometers and has a population of 42.8 million which represents 61% of Guangdong province. In 2008, the gross domestic product (GDP) of the PRD was $434.3 billion, comprising more than 83% of the GDP of Guangdong province and about 10% of China (Guangdong Statistical Yearbook, 2010). In 2008, coal consumption of Guangdong province was approximately 132 million tonnes, which con-

⇑ Corresponding authors. E-mail addresses: [email protected] (L. Chen), [email protected] (Z. Xu). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.03.041

tinues to increase with economic development and oil price (People’s government of Guangdong province, 2009). The most important anthropogenic sources for Hg are coal-fired power, waste incineration, ceramic, and cement plants, and fluorescent lamp production (Zhang and Wong, 2007). Statistics from 2006 showed that Guangdong province had 21.7% of the waste incineration plants in China, 20% of the world’s ceramic plants, and numerous cement plants. Soil has the potential to act as a source and a sink in global Hg cycle (Kim and Lindberg, 1995). Bacteria in soils and sediments convert Hg to methylmercury (MeHg), which is of great concern because it may cause adverse health effect in wildlife and human through bioavailability and bioaccumulation (Ullrich et al., 2001). MeHg can be produced both biotically and abiotically in the environment. Methylation is typically an anaerobic process, and aerobic methylation is less common (Gilmour et al. 1998; Goulet et al., 2007). The microbially mediated processes of methylation, which is generally affected by microbial activity, mercury bioavailability, temperature, pH, redox potential, organic material and salinity, etc., are influenced by reducing conditions and microbial activity of a complex system of synergistic and antagonistic effects (Compeau and Bartha, 1985; DeLaune et al., 2004). Nevertheless, certain general trends are apparent, MeHg formation is generally favored under anaerobic conditions, whereas aerobic conditions promote demethylation processes (Ullrich et al., 2001). Hence, it is important to understand the spatial distribution of soil Hg in

L. Chen et al. / Chemosphere 88 (2012) 612–619

terrestrial ecosystems and the factors that affect its distribution therein. A few previous studies reported that the soil in some areas of the PRD was contaminated by Hg (Tao et al., 1993; Lai et al., 2005; Lin et al., 2007; Yin et al., 2009). However, these data were insufficient to support overall environmental quality management and risk assessment in PRD. This study was designed to (1) investigate and assess the soil mercury pollution status including concentrations and distribution of THg and MeHg in soils with different land use types, soil depth, and spatial and temporal variation throughout the PRD area, and (2) analyze the relationship between THg and MeHg levels with pH and organic matter (OM) in soil. 2. Materials and methods 2.1. Sample collection We collected 741 soil samples from July to September in 2009, of which 516 were only surface layer soils and the remaining 225 were surface (0–20 cm), middle (20–40 cm), and bottom layer (40– 60 cm) at 75 section sites. Stones and tree leaves were removed from the soil during sample collection. The samples were from six different land use types, three in urban areas (residential area, industrial area and park), and three in rural areas (woodland, vegetable field, and cereal field) in total nine cities of the PRD including Guangzhou (GZ), Dongguan (DG), Shenzhen (SZ), Foshan (FS), Zhuhai (ZH), Zhongshan (ZS), Huizhou (HZ), Jiangmen (JM), and Zhaoqing (ZQ). Generally, sampling sites were selected according to grid method and additional sites were added in the industrially developed areas. The coordinates of sampling sites were recorded with GPS (Venture HC, UniStrong Co., China) (Fig. 1). 2.2. Sample preparation and analysis All soil samples were dried for 24 h in a freeze-drier (FDU-2100, EYELA, Japan). The dried soil samples were homogenized with mortar and pestle, and then sieved through a 100 lm nylon mesh to remove coarse particles and biologic debris. THg concentrations in 741 soil samples were analyzed according to Feng et al. (2006). For MeHg analysis, 196 representative soil samples were pre-

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treated following the procedure developed by Liang et al. (2004) and then detected using automated methylmercury analytical system (MERX, Brooks Rand Labs, USA) according to USEPA Method 1630. Soil pH values were measured using a pH electrode with a solid: water ratio of 1:2.5 (Li et al., 2008). Soil OM concentrations were measured by modified potassium dichromate volumetric method coupled with a water heating technique (Jackson, 1982; Lu, 2000; Li et al., 2008).

2.3. Quality control and quality assurance Thirty soil samples comprised one batch. In each batch, a method blank and a randomly selected soil matrix was spiked at concentration of 400 ng g1 as quality control materials. Both the THg and MeHg contents in procedural blank were lower than method detection limits (0.01 ng g1 for THg and 0.002 ng g1 for MeHg). The recoveries in 25 spiked soil samples were within the range of 90–110% with relative standard deviations (RSDs) of 5%. Two certified reference materials (GBW07305, sediment for THg analysis, and IAEA-356, marine sediment for MeHg analysis) were analyzed every 10 samples. The results were close to the certified THg and MeHg values (GBW07305, 0.10 ± 0.02 mg kg1; IAEA356, 0.0054 ± 0.00089 mg kg1) with calculated recoveries of 85–110% and 80–120%, respectively. The RSDs were less than 10% from replicate analysis.

2.4. Data analysis Data calculation and statistical analysis were carried out in Excel (Microsoft Inc., Redmond, USA) and SPSS v.13.0 (SPSS Inc., Chicago, USA). To compare the THg pollution among cities in the PRD area, mean values were analyzed using independent-samples T test. The statistical result was considered significant if p < 0.05. In addition, a THg concentration contour map was made out based on the data from all the sampling sites by the kriging interpolation technique, using ArcGIS v.9.3. (Environmental Systems Research Institute, Inc. California, USA). In the kriging interpolation, semivariogram model, search radius setting and output cell size were assigned as spherical, 12 points and 0.00815, respectively.

Fig. 1. Map of sampling sites.

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3. Results and discussion 3.1. Concentrations of THg and MeHg In this study, the THg and MeHg concentration data were nonnormal distribution (Kolmogorov–Smirnov test, p = 0.000) and were log transformed to follow a normal distribution pattern (p > 0.05). The highest concentration of THg (3320 ng g1) was found in soils from Huangpu of GZ, while the lowest value (16.7 ng g1) was found in soil from Pinghu of SZ (Table 1). The THg mean concentrations decreased as the order: FS > GZ > JM > DG > ZQ > ZH > HZ > ZS > SZ. The concentrations of THg were significantly higher in FS and GZ (mean values of 394 and 334 ng g1) than in SZ (89.6 ng g1) (p < 0.05). FS and GZ were the most heavily polluted cities by THg in the PRD (Table 1). The mean THg concentrations in Guangdong soils were significantly higher than the background values in soil in South China (32–50 ng g1) (Wang and Wei, 1995) and throughout China (65 ng g1) (SEPAC, 1990). Compared with the soil THg level in other Chinese cities and overseas cities, as a whole, the THg concentrations in the soil of the PRD (and affiliated cities) are low or moderate in comparison to the values reported in these studies in Fig. 2. The MeHg concentrations in 196 soil samples ranged from 0.01 to 1.34 ng g1, with a mean value of 0.31 ng g1. Soil MeHg level decreased by the following order ZH  GZ > JM and FS  SZ > ZQ > HZ > DG  ZS. The MeHg concentrations in soil of the PRD are obviously lower than those in Hg mining area (0.19–20 ng g1) but higher than the background value of the soil in Guizhou province of China (0.1–0.28 ng g1), where severe Hg pollution was reported (Qiu et al., 2006). Soil MeHg levels varied with the city sites in the region of PRD. This was different from soil THg levels, which followed an obvious trends with different city sites. 3.2. Comparison of THg content with historical data The THg mean concentrations in surface soils of GZ (334 ng g1) and SZ (89.6 ng g1) were obviously higher than those previously reported in GZ soil in the 1980s (156 ng g1) (Xia et al., 1984) and the 1970s (70–100 ng g1) (Wan, 1982), and in SZ (68 ng g1) in the 1980s (Tao et al., 1993), also higher than the soil background value of Guangdong province (56 ng g1), SZ (71 ng g1), and GZ (157 ng g1) investigated in the 1980s (Editorial Department of Environmental Science, 1982; Wang and Wei, 1995). It suggested Chinese industrial development might cause an aggravating Hg contamination in these areas. 3.3. Spatial distribution of THg and MeHg in soil ArcGIS-based kriging interpolation method generated a map to show the spatial distribution of THg and MeHg in the surface soil in

the PRD (Figs. 3a and 3b). Guangzhou-Foshan area had fairly high Hg contamination (Zone 1 in (Fig. 3a). The results are consistent with those reported previously in this area (Lai et al., 2005; Lin et al., 2007). Furthermore, JM, located in the southwest of the Guangzhou–Foshan area, is becoming another new zone with relatively high Hg pollution (Zone 2 in Fig. 3a). Compared with the studies performed about ten years ago, we concluded that the Hg contamination in the PRD area were not only getting worse but also expanding to the surrounding areas. The main reason is that high energy consumption of low technology industries generates heavy environmental pollution which brings new release of mercury from the industries. The spatial distribution of MeHg (Fig. 3b) in the surface soil is obviously different from that of THg. 3.4. Vertical distribution of THg in soil THg concentrations decreased with soil depth (Fig. 4) (p = 0.000) by the following order: surface > middle > bottom soil. Mercury can be trapped in soil particles and the adsorption becomes more effective in the presence of OM. Mercury deposited on the surface would be trapped and retained even after being covered with a new deposit (Tomiyasu et al., 2003). However, in several sampling sites, the THg concentrations in middle or even bottom layer soils were higher than that in the surface layer. Agriculture activities such as plowing may result in mixing the surface soil with deep layer soil (Lai et al., 2005). Generally, Hg is mainly accumulated in the surface and middle layer of the soil. 3.5. Distribution of THg and MeHg in soil of different land use types The THg in urban soils of the PRD are significantly higher than those in rural ones (p < 0.05, Table 2). The high population density and heavy traffic in urban areas are important sources for Hg emissions to ambient air. Particularly, the industries with high Hg emission fluxes, such as waste incinerators, coal-fired power plants, and ceramic factories, are mainly situated in the urban or suburban area. Notably, THg concentrations in soil of the parks headed the list, followed by the residential and the industrial areas (Table 2) (p < 0.05). The results were consistent with a previous report (Yin et al., 2009). High soil THg concentration in the park is possibly related to the use of the municipal sludge and Hg containing pesticides. Traffic pollution may be another contributor, because most parks are located downtown. We also found exceptionally lower soil THg concentrations in the industry area since during the building period most surface soils there were removed or carried from other places, which are far from the industry area and might not be heavily polluted. In rural area, the Hg concentrations decreased from vegetable field to cereal field to woodland (p < 0.05). This is because fertilizer (Zheng et al., 2008) (e.g. dung and sludge applied

Table 1 Soil pH, OM level and concentrations of THg and MeHg. City

THg (ng g1) a

GZ DG SZ FS ZQ HZ ZH ZS JM PRD a b

MeHg (ng g1) b

OM (mg g1)

pH

SN

Mean ± SD (range)

Median

SN

Mean ± SD (range)

Median

Range

Mean

Range

Mean

170 45 17 68 77 64 16 37 97 591

334 ± 398 (24.9–3320) 235 ± 195 (23.7–1190) 89.6 ± 47.0 (16.7–199) 394 ± 387 (24.9–1780) 229 ± 251 (24.0–1840) 190 ± 171 (16.8–835) 205 ± 197 (41.5–856) 190 ± 175 (57.0–941) 247 ± 391 (24.5–2290) 278 ± 330 (16.7–3320)

214 176 70.6 394 234 171 150 127 171 181

79 23 9 14 8 18 11 11 23 196

0.37 ± 0.28 0.20 ± 0.14 0.34 ± 0.11 0.35 ± 0.14 0.32 ± 0.27 0.23 ± 0.14 0.38 ± 0.16 0.19 ± 0.16 0.35 ± 0.26 0.31 ± 0.23

0.29 0.18 0.22 0.30 0.26 0.19 0.22 0.19 0.31 0.24

2.81–7.30 3.30–5.37 3.47–6.00 3.61–6.72 3.67–6.23 2.62–5.80 3.18–6.26 3.36–6.97 3.23–6.90 2.62–7.30

4.75 4.81 4.83 4.58 4.49 4.18 4.86 4.65 4.51 4.67

6.01–54.6 12.2–61.1 4.24–64.0 8.89–44.9 9.45–67.0 9.34–47.1 18.6–66.9 14.5–46.1 5.32–46.3 4.24–50.0

32.8 30.2 29.3 32.4 32.6 21.5 37.2 26.6 25.9 30.3

SN = sample number. SD = standard deviation.

(0.05–1.34) (0.10–0.53) (0.03–0.56) (0.18–0.57) (0.07–0.48) (0.05–0.54) (0.18–0.65) (0.14–0.43) (0.10–1.13) (0.01–1.34)

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Fig. 2. Comparison of soil THg concentrations in this study with the published studies (ng g1). Transverse short solid lines represent THg mean/median concentrations. (See above-mentioned references for further information.)

Fig. 3a. Map of THg distribution in surface soils in the PRD.

to the farmland) and Hg containing pesticides (e.g. ethyl mercuric chloride, phenyl mercuric acetate, ethyl mercuric phosphate) (Tao et al., 1993) are more frequently used in vegetable field than in cereal field. Relatively, woodland is less disturbed by human activities, such as use of pesticides and fertilizer, and the THg concentrations in woodland soil were therefore much lower. The great difference of THg levels in different land use types obviously reveals local anthropogenic Hg sources. MeHg concentrations in the urban soils were slightly higher than those in the rural ones (p < 0.05). The distribution of MeHg and THg in different land use types followed a similar order with statistical significance different (Table 2) (p < 0.05). Here cereal field included rice paddy and non-irrigated

field. Mean MeHg concentration (0.55 ± 0.31 ng g1) in paddy field was obviously higher than that of vegetable field (0.29 ± 0.18 ng g1) and woodland (0.26 ± 0. 17 ng g1) (p < 0.05). Also, MeHg percentage to THg in the paddy field (0.26 ± 0.17%) was higher than that in vegetable field (0.16 ± 0.13%) and woodland (0.24 ± 0.26%) (p < 0.05). In contrast with that in the south-east circumlittoral developed area, MeHg levels in north-west developing area were high, possibly due to a large scale of rice planting. The physicochemical conditions in the paddy environment facilitate Hg-methylation due to the presence of a flora of sulfur-reducing bacteria (SRB) (Wind and Conrad, 1995; Stubner and Conrad, 1998; Meng et al., 2011).

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Fig. 3b. Map of MeHg distribution in surface soils in the PRD.

Fig. 4. Variation of THg concentrations with soil depths in different PRD cities.

Table 2 Concentrations of THg and MeHg in surface soils of different land use types (ng g1). Sampling sites

Land use type

THg SN

a b c

a

MeHg Concentration

b

SNc

Concentrationa

Urban

Residential area Industrial area Park

28 38 13

43.7–3320 (480) 63–1590 (361) 24.4–2020 (626)

40 40 15

0.05–0.92 (0.32) 0.10–1.04 (0.30) 0.12–1.34 (0.41)

Rural

Woodland Vegetable field Cereal field

38 36 17

42.5–836 (155) 43.0–2100 (346) 54.0–1280 (189)

35 35 26

0.10–0.78 (0.26) 0.05–0.94 (0.36) 0.18–1.13 (0.32)

SN = sample number, these samples were collected in Guangzhou. The value in the parentheses is the mean value. These samples were collected in the PRD.

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3.6. Factors affecting concentrations of THg and MeHg in the soil Soil pH in 196 representative soil samples ranged from 2.62 to 7.03. The OM contents varied from 4.24 to 92.8 g kg1 with a mean value of 30.3 g kg1. The OM concentration and soil pH are the two principal factors which significantly influence Hg geochemistry including soil transport and transformation processes under various conditions (Semu and Singh, 1987; Yang et al., 2007). No significant correlation between lnTHg and pH was observed (R2 = 0.016, n = 196, p = 0.076) (Fig. 5), this is consistent with previous results (Yin et al., 2009). However, there was a significant correlation between lnTHg and OM (R2 = 0.466, n = 196, p < 0.01). Hg can bond to OM via the formation of Hg-OM complexes or adsorption (Wallschläge et al., 1998; Ullrich et al., 2001; Wang et al., 2003). It may explain high Hg concentrations in the soil with high OM. Similarly, no significant correlation was found between lnMeHg and pH (R2 = 0, n = 196, p = 0.998). It indicates that Hg methylation is a complex process, and affected by many other factors. On the other hand, lnMeHg, were significantly correlated with OM in this aerobic soil environment (R2 = 0.501, n = 196, p < 0.001) (Fig. 5). Qiu et al. (2006) reported the strong relationship between TOC and MeHg in soil from the Hg mining area. The presence of OM may affect the physical transfer, chemical transformation and bioavailability of Hg, and even provide adequate nutrition for methylation bacteria (Wright and Hamilton, 1982; Ullrich et al., 2001). Anaerobic methylation was stimulated in high OM soils, presum-

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ably due to active microbial growth, whereas aerobic methylation was frequently suppressed by high OM (Ullrich et al., 2001). However, the role of humic matter in the methylation of Hg remains unclear (Ullrich et al., 2001). Hg methylation is affected by many factors, such as temperature, pH, dissolved oxygen, redox potential, organic matter, microbes, and so on (Ebinghaus et al., 1994; Falter and Wilken, 1998; Ullrich et al., 2001). In this study, pH shows no significant correlation with ln(MeHg/THg) concentration ratio (R2 = 0.018, n = 196, p = 0.064) (Fig. 5). Although MeHg concentrations are obviously related to OM, no significant correlation was found between the ln(MeHg/THg) ratio and the OM (R2 = 0.022, n = 196, p = 0.042). Further study is needed to investigate the methylation mechanisms. 3.7. Assessment of THg pollution in the surface soils of the PRD China Soil Environmental Quality Standards apply pollution index method (Pi) to estimate the soil THg pollution (Huang, 1987; Chen et al., 2010) by the following functions:

Pi ¼ C i =X a ðC i < X a Þ or Pi ¼ 1 þ ðC i  X a Þ=ðX b  X a Þ ðX a < C i 6 X b Þ or Pi ¼ 2 þ ðC i  X b Þ=ðX c  X b Þ ðX b < C i 6 X c Þ or Pi ¼ 3 þ ðC i  X c Þ=ðX c  X b Þ ðC i > X c Þ

Fig. 5. Correlations between THg, MeHg and MeHg percentage to THg and pH, OM values.

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Table 3 The sample numbers and percentage of different polluted classes in cities of the PRD using two evaluation methods. City

Pi sample number (%) a

GZ DG SZ FS ZH ZS HZ JM ZQ PRD a

Fi sample number (%)

High con.

Moderate con.

Low con.

No con.

High con.

Moderate con.

Low con.

No con.

5 (2.95) 0 (0) 0 (0) 1 (1.59) 0 (0) 0 (0) 0 (0) 4 (3.92) 1 (1.29) 11 (1.86)

3 (1.76) 1 (2.22) 0 (0) 5 (7.94) 0 (0) 0 (0) 0 (0) 2 (1.96) 0 (0) 11 (1.86)

112 (65.9) 26 (57.8) 2 (11.8) 40 (63.5) 9 (56.3) 19 (51.4) 28 (43.8) 61 (59.8) 42 (54.6) 339 (57.4)

50 (29.4) 18 (40.0) 15 (88.2) 17 (27.0) 7 (43.8) 18 (48.7) 36 (56.3) 35 (34.3) 34 (44.2) 230 (38.9)

110 (64.7) 24 (53.3) 2 (11.8) 39 (61.9) 8 (50.0) 11 (29.7) 26 (40.6) 57 (55.9) 39(50.6) 316 (53.5)

29 (17.1) 10 (22.2) 4 (23.5) 12 (19.0) 3 (18.8) 18 (48.6) 13 (20.3) 19 (18.6) 24 (31.2) 132 (22.3)

18 (10.6) 7 (15.6) 9 (52.9) 7 (11.1) 1 (6.3) 8 (21.6) 17 (26.6) 18 (17.6) 10 (13.0) 95 (16.1)

13 (7.6) 4 (8.9) 2 (11.8) 5 (7.9) 4 (25.0) 0 (0) 8 (12.5) 8 (7.8) 4 (5.2) 48 (8.1)

Abbr. of contamination.

where Ci was the determined soil THg concentrations, and Xa, Xb and Xc are assigned with 0.15 mg kg1, 1.0 mg kg1 and 1.5 mg kg1 according to Soil Environmental Quality Standards in China, and represented suitable for the threshold values of natural background value, human health and plant growth, respectively (GB 15618, 1995). If calculated Pi was less than or equal to 1 (Pi 6 1), the surface soil was classified as no contamination. Similarly, if 1 < Pi 6 2 then classified as light contamination; if 2 < Pi 6 3 then moderate contamination; if Pi > 3 then high contamination. The calculated results (Table 3) showed that among 591 surface soil samples in the PRD, 230 samples (38.9%) were uncontaminated, 339 (57.4%) were lightly contaminated, 11 (1.86%) were moderately contaminated, and 11 (1.86%) were highly contaminated. Single factor contaminant index was also used to estimate the Hg pollution status (Li, 1994; Yin et al., 2009) in the PRD soils by the following function:

F i ¼ C i =Si where Fi is the pollution index of THg, Ci is the measured concentration of THg in surface soil, and Si is the standard value of THg. For THg pollution assessment, the background soil value (56 ng g1) in Guangdong province (Wang and Wei, 1995) was assigned as the standard value. If Fi < 1, the soil was classified into no contamination. Similarly, 1 6 Fi < 2, light contamination; 2 6 Fi < 3, moderate contamination and Fi P 3, high contamination. Of the 591 surface soil samples estimated, the percentage of uncontaminated, lightly, moderately and highly contaminated soils were 8.1%, 16.1%, 22.3%, and 53.5%, respectively (Table 3). Pollution index (Pi) and single factor contaminant index (Fi) are two totally different soil Hg contamination evaluation methods. Pi method underlined the human health and ecological risk via exposure to THg in soil, while Fi method based on local soil background value stressed human activity’s cumulative impact to ambient soil. The tremendous difference between the two assessment results may be attributed to the assessment method itself and the assessment standard as well. 4. Conclusions Rapid economic and industrial development caused relatively high levels of Hg contamination in these zones and the contamination area significantly expanded during the last decades. Highly contaminated areas are around Guangzhou–Foshan and Jiangmen, in the northwestern part of the PRD. THg levels are highest in the top 0–20 cm soil layer, and decrease from the surface to bottom layer soil. THg concentrations varied with different land use types and tended to decrease from urban area to rural area. The highest levels were in urban parks and decreased in the order, park to residential area to industrial area to vegetable field to cereal field to

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