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Residues of organochlorine pesticides in Hong Kong soils
Chemosphere 63 (2006) 633–641 www.elsevier.com/locate/chemosphere
Residues of organochlorine pesticides in Hong Kong soils H.B. Zhang a,c, Y.M. Luo a,c,*, Q.G. Zhao M.H. Wong a,b, G.L. Zhang a,c
a,c
,
a Soil and Environment Joint Open Laboratory between Institute of Soil Science, Chinese Academy of Sciences and Hong Kong Baptist University, Soil and Environmental Bioremediation Research Center, State Key Laboratory of Soil and Sustainable Agriculture, 71 East Beijing Road, Nanjing, Jiangsu 210008, China b Croucher Institute for Environmental Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong c Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
Received 2 April 2005; received in revised form 1 June 2005; accepted 8 August 2005 Available online 5 December 2005
Abstract It was short of research on the organochlorine pesticides (OCPs) residues in the soils of Hong Kong. Sixty-six representative soil samples were collected from the 46 sites covering five types of land uses in Hong Kong. Hexachlorohexanes (HCH) and 7 Stockholm Convention OCPs were analyzed by gas chromatograph (GC) equipped with a Nickel 63 electronic capture detector (lECD). The results presented that HCH and 5 Stockholm Convention pesticides were detected in Hong Kong soils although the detectable ratio varies to a great extent. The concentration sequence of the five detectable OCPs was HCH > dichlorodiphenyltrichloroethane (DDT) > hexachlorobenzene (HCB) Endrin > a-endosulfan. Among the OCPs and their homologues or isomers, b-HCH and p,p 0 -DDE were the two predominant substances according to the concentrations and detectable ratios, concentrations of which in soils were averagely 6.12 lg kg1 and 0.41 lg kg1 respectively. Soil horizon samples of 0–10 cm, 10–30 cm and >30 cm depth were selected from nine soil profiles to demonstrate the depth distributions of DDT and HCH in soil profiles. Concentrations of HCH tended to increase gradually from the topsoil to bottom layer while the lowest concentration of DDT is usually found in the subsoil (10–30 cm) in most sampling sites. In addition, close correlations of pH(KCl) and total organic carbon (TOC) with HCH indicated an effect on the residues of HCH caused by these two soils properties, but such relationships were not found with DDT or other OCPs. 2005 Elsevier Ltd. All rights reserved. Keywords: Hong Kong soil; DDT; HCH; Depth distributions
*
Corresponding author. Address: Soil and Environment Joint Open Laboratory between Institute of Soil Science, Chinese Academy of Sciences and Hong Kong Baptist University, Soil and Environmental Bioremediation Research Center, State Key Laboratory of Soil and Sustainable Agriculture, 71 East Beijing Road, Nanjing, Jiangsu 210008, China. Tel.: +86 25 86881101; fax: +86 25 86881128. E-mail address: [email protected] (Y.M. Luo).
1. Introduction Characterized by toxicity, stability, and recalcitrance to degradation in natural environments, many organochlorine pesticides (OCPs) are semivolatile and lipophilic and described as persistent organic pollutants (Meijer et al., 2001). Some OCPs such as hexachlorohexanes (HCHs), dichlorodiphenyltrichloroethane (DDT),
0045-6535/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.08.006
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hexachlorobenzene (HCB), chlordane, aldrin and dieldrin were used extensively from 1972 to 1982, with 76 000–100 000 tons annually and an application rate of 1.8–2.7 kg per metric acre in the agricultural zones around in the Pearl River Delta (Fu et al., 2003). In Hong Kong, due to the decline of agricultural activities, most of the imported pesticides had been used for non-agricultural purposes, such as domestic and outdoor (e.g., golf courses) pest control (Wong and Poon, 2003). DDT was banned for agricultural application in the Chinese Mainland and Hong Kong in 1983 and 1988, respectively (Monks, 1994). In spite of this, OCPs are still widely distributed in the environment due to their persistency, semivolatile nature resulting in long-distance transportation. In Hong Kong, OCPs such as a-HCH and HCB were found ubiquitously present in the atmosphere (Louie and Sin, 2003), and relative high concentrations of HCH and DDT were found in marine and inland water sediments (Richardson and Zheng, 1999; Zhou et al., 1999). Accumulation of OCPs in the lipid content of animals is a common phenomenon due to their hydrophobic properties (Sijm and Linde, 1995). Zhou et al. (1999) observed that tilapia (Tilapia mossambica) lived in heavily contaminated environment accumulated a substantial amount of DDT and HCHs. Accordingly, OCPs pollution in the air, water and sediments/soils would eventually give rise to food contamination. Soil was the repository of all types of chemicals including inputs of OCPs. Many OCPs had a high affinity for soil, which might be taken up by crops and by grazing animals and hence reached the human food chain. They might also be washed in run-off from the land into watercourse and emitted into atmosphere through volatilization, which indirectly resulted in water and atmospheric contamination (Fu et al., 2003; Bidleman and Leone, 2004). Residues of OCPs in soils of Hong Kong had nevertheless been focused on. The major purpose of this study was to investigate the concentrations, compositions and distributions of OCPs in Hong Kong soils. Furthermore, the edaphic factors contributing to these would also be discussed.
2. Materials and methods
that are largely unsettled and remain its natural characteristics, while the limited arable lands are mainly scattered at the alluvial plain. Most wetland can be found in the northwestern of New Territories, and half of which is mangrove swamp (Ashworth et al., 1993). Mai Po mangrove swamp is the most important wetland in Hong Kong because it has been designed as an international important Ramsar site. Along the coastline of Hong Kong, a lot of lands have been developed through reclamation from the sea, some of which are not constructed and set aside. Most soils of Hong Kong are Oxisols commonly found in the humid tropics, characterized by strong weathering and leaching (Soil and Conservation Service, 1999). Hong KongÕs climate is sub-tropical, with an annual average temperature of 24 C and relative humidity of 78% in 2000 (Hong Kong Observatory, 2001). The mean annual rainfall ranges from about 1300 mm at Waglan Island to more than 3000 mm close to Tai Mo Shan. About 80% of the rain falls between May and September, while October to April is relatively arid with frequent hill fires caused by tomb worship in the countryside (AFCD, 2002). 2.2. Reagents and instruments The n-hexane and methanol used were both High Performance Liquid Chromatography (HPLC) grade and were purchased from Fisher Scientific International Inc., USA. The anhydrous sodium sulfate (analytical grade, Nanjing Chemical Reagent Co.) and Silica gel (60 mesh, for column chromatography) were activated at 600 C for 8 h and at 110 C for 24 h, respectively. A mixture of standard solution containing a,b,c-HCH, HCB, heptachlor, Aldrin, Endrin, Dieldrin, a-Endosulfan, b-Endosulfan, p,p 0 -DDE, o,p 0 -DDE, p,p 0 -DDD, o,p 0 -DDD, o,p 0 -DDT, p,p 0 -DDT with 10.0 mg l1 per compound and a standard solution (10.0 mg l1) of pentachlorotoluene (PCT) were purchased from Labor Dr. Ehrenstorfer, Germany. Agilent6890 gas chromatograph (GC) equipped with a 63Ni electron capture detector (lECD) (Agilent Technology Co., USA) was used for the analysis. The column used was HP-5 silica capillary column with 30 m · 0.33 mm i.d · 0.25 lm film thickness. Instruments for extraction and cleanup were including ultrasonic shaking apparatus and rotary evaporation.
2.1. The study area 2.3. Soil sampling Hong Kong is located at the southeast tip of the Chinese Mainland, with a total area slightly over 1100 km2 covering Hong Kong Island, Kowloon, New Territories and a number of islands, with Lantau Island being the biggest one. Three-quarters of the total land areas is countryside, in which approximately six types of land use including woodland, grassland, arable land, wetland and reclamation land were distributed (AFCD, 2002). Woodland and grassland are mainly distributed in hills
Soils were collected from the New Territories, Kowloon, Hong Kong Island, and Lantau Island in December 2000. The sampling sites are shown in Fig. 1. In total, 66 soil samples (46 surface soils of 0–10 cm and 20 different soil horizons from nine soil profiles) from 46 sites covering different types of land use patterns such as woodland, grassland, arable land, wetland and reclamation land in Hong Kong were collected, and each
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Fig. 1. Map of sampling sites.
sample consisted of 4–5 sub-samples. Among the 46 soil samples, most of which were collected from woodland and grassland, seven were from arable land, two were from wetland, namely Mi Po mangrove swamp, and one from a reclamation land located in Ma On Shan area. The nine soil profiles for demonstrating the depth distributions of OCPs were collected from four sampling sites, distributed in Ma On Shan, Tai Mao Shan, Hong Kong island and Lantau island respectively. The collected samples were air dried at room temperature (22– 25 C), sieved to <2 mm, and stored in glass bottles prior to analysis. 2.4. Sample extraction and cleanup Sample extraction, cleanup and OCPs analysis were carried out in the state Key Laboratory of Pollution Control and Resource Reuse in Nanjing University, and the procedures were developed from EPA method 3550C (US EPA, 2000). Samples (7.5 g dry weight) were weighed into centrifuge tubes, with mixture of 20 ml nhexane, 5 ml methanol, and 5 ml distilled water and sonicated for 1 h in an ultrasonic shaking apparatus. The mixture was then centrifuged and the extracts were collected. The same extraction was repeated once by adding 20 ml n-hexane to the filter residue. The two extract solutions were then combined and anhydrous sodium sulfate was added for drying, and then concentrated to around 1 ml by rotary evaporation. The concentrated extracts were cleanup through a chromatographic column filled with 1 g of activated silica gel (added with 0.07 ml of distilled water), and then eluted with 8 ml
n-hexane, after that the eluent was further concentrated to 1 ml. The solution was finally concentrated to around 0.1 ml under a gentle steam of pure nitrogen. 10 ll of PCT (1 mg l1) was added to the solution as an internal standard prior to transferring to a glass of microvial for GC injection. 2.5. OCPs analysis The analytical procedures described in Sun et al. (2002) were used. The purified extract was analyzed using GC-ECD under the splitless mode and HP-5 silica capillary column. High purity (99.99%) helium was used as carrier gas at 2.7 ml/min and nitrogen as make-up gas at 54.4 ml/min. The injector and detector temperature were 250 C and 320 C respectively. The oven temperature was initially set at 60 C, ramped at 25 C/min to 170 C, 4 C/min to 190 C, 10 C/min to 230 C and 2 C/min to 240 C, then held at 240 C for 5 min. Peak areas were calculated by Agilent Chem-Station software. OCPs measured in Hong Kong soil includes six DDT homologues, three HCH isomers, HCB, heptachlor, aldrin, endrin, P diedrin and Pa,b-endosulfan and concentrations of DDT and HCH were the sum of the six DDT homologues and three HCH isomers, respectively. 2.6. Quality control The detection limit ranged from 0.005 lg kg1 to 0.02 lg kg1 and recoveries of OCPs by this method
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Table 1 The detection limit, recovery and repeatability of the method
2.8. Statistical analysis
OCPs
Detection limit (lg kg1)
Recovery (%)
Repeatability (% RSD)
p,p 0 -DDE p,p 0 -DDD p,p 0 -DDT o,p 0 -DDT a-HCH b-HCH c-HCH Endrin HCB a-Endosulfan
0.015 0.005 0.005 0.005 0.015 0.020 0.005 0.005 0.005 0.005
90.5 96.4 110 93.3 83.2 91.7 90.4 81.0 87.4 88.3
5.6 10.4 5.4 4.7 6.0 5.4 7.5 4.6 12.2 5.4
To detect the difference of HCH and DDT concentrations among the land uses, ANOVA test companied with Least Significant Difference (LSD) method were adopted. Linear regression analyses were separately carried out for exploring the relationships of a-HCH with TOC and pH(KCl). SPSS 11.0 for windows was employed for statistical analysis.
3. Results and discussion 3.1. Main characteristics of Hong Kong soils
ranged from 81.0% to 110% (Table 1). The analysis of three blanks covering the entire analytical procedure (from the extraction to the GC analysis) was subjected to assess the interference from the reagents and glassware. GC analysis was repeated twice for each replicate sample and the relative standard deviation (RSD) of replicate samples were less than 15%. 2.7. Physical and chemical soil analysis Physical and chemical soil analysis were characterized by conventional standard procedures: Soil pH was determined in 1 M KCl at a soil to solution ratio of 1:2.5 by a potentiometric glass electrode, total organic carbon (TOC) and total nitrogen were measured with a CNS-Analyzer (Elementar Vario EL, Elementar Analysensysteme GmbH, Hanau, Germany). Particle-size distribution was carried out using the pipette method, and classification of the soil texture was based on Soil Survey Division Staff (1993). Cation exchange capacity (CEC) of mineral soils was the sum of Ca + Mg + K + Na + Fe + Al + Mn extractable with 1 M NH4acetate (Sumner and Miller, 1996).
The main characteristics of Hong Kong soils are presented in Table 2. Soil textures ranged from loam to sandy clay loam under the five land-uses, in which wetlands were found with the lowest sand content and highest clay content while reclamation land was found the opposition. In terms of pH, woodland and grassland soils ranged from strong to moderate acidic, arable land and wetland soils tended to be neutral while reclamation soils were alkaline. The soil CEC and TOC concentrations were not similar as well among all types of soils, with the lowest values found in the reclamation soils. The C/N ratios of the five kinds of soils also varied considerably, with the arable land soil having a lower C/N ratio, suggesting that the organic matter content was more nitrogen-rich. 3.2. Concentrations of OCPs in Hong Kong soils Analysis results showed that most of OCPs were found in samples except for heptachlor, aldrin, dieldrin and b-endosulfan. HCH was detected in all 46 samples, DDT in 43 samples, HCB in 5 samples, endrin in 15 samples and a-endosulfan in one sample only. In Table 3 samples were grouped according to the soils of woodland, grassland, arable land, wetland and
Table 2 The main characteristics of Hong Kong soils Soil properties
Land using Woodland
Grassland
Farmland
Wetland
Reclamation
Sand (%) Silt (%) Clay (%) Textural class USDA pH (H2O) CEC (cmol/kg) TOC (g/kg) Total nitrogen (g/kg) C/N ratio
42.9 33.0 24.1 Loam 4.62 10.2 23.8 1.25 19.0
38.3 40.8 20.9 Loam 4.70 10.9 21.0 1.18 17.8
40.9 43.9 15.2 Loam 6.23 8.57 12.3 1.07 11.5
2.3 55.7 42.0 Silty clay 6.35 9.75 29.2 1.93 15.1
54.8 34.8 10.4 Sandy loam 8.09 3.86 5.32 0.28 18.9
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Table 3 Average concentrations (lg kg1 dry weight) of organochlorine pesticides in Hong Kong soilsa OCPs 0
p,p -DDE p,p 0 -DDD p,p 0 -DDT o,p 0 -DDT P DDT a-HCH b-HCH c-HCH P HCH Endrin HCB a-Endosulfan
Target valuesb
Land-use categories Woodland
Grassland
Farmland
Wetland
Reclamationc
0.20 ± 0.19 0.01 ± 0.01 0.01 ± 0.01 0.10 ± 0.06 0.32 ± 0.20 0.05 ± 0.04 5.66 ± 1.17 0.01 ± 0.01 5.72 ± 1.18 0.01 ± 0.01 0.03 ± 0.02 n.d.
0.16 ± 0.014 0.01 ± 0.01 0.04 ± 0.03 0.03 ± 0.04 0.24 ± 0.20 0.05 ± 0.03 6.26 ± 1.52 0.01 ± 0.01 6.32 ± 1.52 0.02 ± 0.02 0.01 ± 0.01 0.0047
1.73 ± 1.53 0.05 ± 0.12 0.02 ± 0.04 0.05 ± 0.08 1.85 ± 1.64 0.14 ± 0.08 6.37 ± 0.24 0.02 ± 0.02 6.53 ± 0.17 n.d. n.d. n.d.
0.17 ± 0.21 n.d. 0.18 ± 0.15 0.08 ± 0.01 0.43 ± 0.04 0.09 ± 0.09 7.55 ± 2.05 n.d. 7.64 ± 2.14 n.d. 0.01 ± 0.01 n.d.
0.09 n.d. n.d. 0.03 0.13 0.04 6.50 0.01 6.56 n.d. 0.08 n.d.
– – – – 2.5 2.5 1 0.05 10 1 2.5 –
n.d.: Below the detection limit. a Average concentration is presented as mean ± standard deviation. b The Netherlands Soil Contamination Guidelines (Department of Soil Protection, 1994). c One sample was collected and no standard deviations is supplied.
reclamation land. P PIn general, the average concentrations of DDT and HCH of all soils were 0.52 ± 1.01 and 6.19 ± 1.31 (lg kg1 dry weight) respectively. They P were much lower than the average concentrations of DDT P (37.6 lg kg1 dry weight) and HCH (12.2 lg kg1 dry weight) found in the soils of Pearl River Delta Region (Fu et al., 2003), despite the fact that DDT and HCH have been banned from agricultural applications for more than 20 years both in Hong Kong and the Chinese Mainland. However, illegal applications of these two pesticides have been found in some areas of the Delta Region (Zhou et al., 2001; Mai et al., 2002). On the contrary, the pesticides management seems to be better in Hong Kong in addition to the substantial decline of agricultural activities, as a result that import and export of these pesticides are strictly controlled by the Pesticides Ordinance and Regulations (HKSARG, 1993). The total concentrations of DDT in arable land soils were significantly (p < 0.05) higher than those in woodland and grassland soils, suggesting that DDT was mostly used for agricultural activities in Hong Kong. However, the total concentrations of HCH in soils of the five types of land use varied only slightly, which may be contributed to the relatively higher vapor pressure of HCH (Mackay et al., 1997) causing HCH much easier to volatilize than DDT from soil to atmosphere, and return to soil through dry or wet deposition after atmospheric transportation. It will then lead to the more homogeneous distribution of HCH in soils (Fu et al., 2003; Yeo et al., 2003). Other OCPs such as endrin, HCB and a-endosulfan were also found in the soils. Endrin was detected in the New Territories and Lantau Island, ranging from 0.007 to 0.093 lg kg1 dry weight. Endrin is mostly pre-
sented in soils and sediments due to its low water solubility and vapor pressure (Mackay et al., 1997). However, a recent study indicated that no endrin was detected in the atmosphere of Hong Kong (Louie and Sin, 2003). In general, concentrations of HCB ranged from 0.007 to 0.31 lg kg1 in all soils, but it was under the detection limit in arable land soils. Although HCB has been prohibited from use as herbicides, wastes derived from chlorine related industries could be a possible HCB source to the environment (Meijer et al., 2001). The concentrations of all the detectable OCPs in the soils of Hong Kong were preliminary compared with the corresponding target values used in Netherlands (Table 3) due to the shortage of local standard for identifying soil pollution. The target value was favored to be as a critical value for division of polluted and unpolluted soils in the soil protection guideline of Netherlands (Department of Soil Protection, 1994), and they were entirely higher than the concentrations of OCPs in Hong Kong. It implied that no remediation measures were required with respect to the OCPs in soils of Hong Kong. In spite of this, the ecological and health effects of these substances through food chain at the relatively lower concentrations still need further attention in light of their possible biological magnifications in higher trophic organisms including human beings. Wong et al. (2002) demonstrated that the mean levels of p,p 0 -DDT, p,p 0 -DDE and b-HCH in human breast milk from Hong Kong were 2–15-fold higher compared to the relevant studies conducted in developed countries such as United Kingdom, Germany, Sweden, Spain, and Canada. It was noticeable that p,p 0 -DDE and b-HCH were main OCPs components in the soils of Hong Kong in terms
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of present research, the relationships between these two organochlorine pesticides in human breast milk and soils were worthwhile to exploration. 3.3. Compositions of DDT and HCH in Hong Kong soils Table 3 showed that DDT and its metabolites found in Hong Kong soils contained o,p 0 -DDT, p,p 0 -DDT, p,p 0 -DDE and p,p 0 -DDD, with the p,p 0 -DDE being the major composition of DDT (accounting for 78.8% of the total DDT concentration averagely). DDE and DDD were the two main products of DDT dechlorination. DDT was dechlorinated to DDE under aerobic conditions and reductively dechlorinated to DDD under anaerobic conditions (Heberer and Du¨nnbier, 1999; Fellenberg, 2000). The pathway of DDT to DDD could be direct and indirect, and the later was from DDT to DDE, then further to DDD (Wenzel et al., 2002). The ratios of DDD:DDE, DDD:DDT and DDE:DDT were employed to deduce the possible dechlorination pathway in Hong Kong soils (Table 4). It was probable that DDT to DDE was the main degradation route as a result that the ratios sequence was DDE:DDT > DDD:DDT > DDD:DDE. A small value of DDT:(DDE + DDD) ratio indicates aged (microbiologically degraded) DDT while a value much greater than 1.0 indicates fresh application. The average value of DDT:(DDE + DDD) was 0.81, implying the existence of aged DDT in most soils of Hong Kong. Meanwhile, the ratios with much greater than 1.0 were found in two samples from Mai Po, north of New Territory, and Discovery Bay, north east of Lantau Island respectively. However, the concentrations of DDT in these two soils were only 0.35 lg kg1 and 0.23 lg kg1 respectively, which were much lower than the target value presented in Table 3. The microbial transformation rate of DDT to its metabolites DDE and DDD depended on several factors including soil type, temperature, and moisture, organic carbon content (Hitch and Day, 1992; Boul, 1994). Some soils in the southwestern USA were observed to have a high proportion of parent DDT due to their poor metabolite capability (Hitch and Day, 1992). c-HCH was presented in technical HCH containing 10–15% c-HCH and 60–70% a-HCH as well as other
isomers. Alternatively, it was available in its pure form as lindane (>99% c-HCH)(UNEP, 1995). Technical HCH has been forbidden for application in many countries, but lindane is still being used in a number of countries. The ratio of a-HCH to c-HCH in Hong Kong soil was mostly ranged from 0.3 to 5, with an average value of 6.0 (Table 4), which was close to the original ratio of technical HCH. It was assumed that historical usage of technical HCH would be the main source of HCH in soils, although no information has been found which HCH has been used in the past in Hong Kong. However, it yet cannot rule out the historical usage of lindane merely according to the HCH isomer ratios of soils when taking into account of the higher volatilization of c-HCH than a-HCH (Mackay et al., 1997). In fact, b-HCH accounting for 95.8–100% of the total HCH concentrations, is the dominant HCH isomer in most soil samples. This finding was different from that of in the of sediments river or marine and atmosphere, in which, a-HCH and c-HCH respectively rather than b-HCH predominated among the HCH isomers (Richardson and Zheng, 1999; Louie and Sin, 2003). The persistence of b-HCH in soils is mainly due to the higher Kow (log Kow = 3.78) and lower vapor pressure value (3.6 · 107 mmHg, 20 C). These will make for the b-HCH easier absorption to the soil organic matter and less evaporative loss from the soils (Hansch and Leo, 1979; Mackay et al., 1997). Furthermore, the spatial arrangement of chlorine atoms in the molecular structure of b-HCH was supposed to more resistant to microbial degradation in soils (Middeldorp et al., 1996; Kalbitz et al., 1997). 3.4. Depth distributions of OCPs in Hong Kong soils As for the depth distributions of OCPs, DDT and HCH were mainly concentrated on since concentrations of other OCPs such as HCB, endrin, heptachlor and so on were much lower in the soil samples. Fig. 2 showed the different distribution patterns of HCH and DDT along the soil profiles. Most samples had a trend of increasing from the topsoil to bottom layer for HCH concentrations and of decreasing from the topsoil to subsoil then increasing again in the
Table 4 Composition of detected DDT and HCH in Hong Kong soila Ratios of compounds
Mean value
Minimum value
Maximum value
Reference value
DDT/(DDE + DDD) DDD/DDE DDD/DDT DDE/DDT a-HCH/c-HCH
0.81 0.40 1.18 3.18 6.0
0.05 0.14 0.37 0.06 0.30
18 1.0 2.9 17 52
1.0 – – – 3–10
a
DDT, DDE and DDD means p,p 0 -DDT, p,p 0 -DDE and p,p 0 -DDD respectively.
H.B. Zhang et al. / Chemosphere 63 (2006) 633–641
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bottom layer subsoil
topsoil
3.5. Effects of pH and organic matters
5.0
5.5
6.0
6.5
7.0
7.5
8.0
bottom layer subsoil
topsoil
HCH Conc. (µg kg-1)
0.0
0.20 0.18
Tai Mao Shan Ma On Shan Hong Kong island Lantau island .1
.2
.3
.4
.5
.6
.7
.8
0.16 0.14
.9
DDT Conc. (µg kg-1) Fig. 2. Vertical distributions of HCH and DDT in soil profiles.
α-HCH
4.5
Tai Mao Shan Ma On Shan Hong Kong island Lantau island
Soil organic matters are inclined to binding with organochlorine pesticides because of their hydrophobicity. On the other hand, the increase of organic matter content in soils can supply more carbon source to facilitate microbial degradation of organochlorine pesticides (Wu et al., 1997). As a result, the content of TOC could make an impact on the residue of organochlorine pesticides in soils (Borisover and Graber, 1997; Kalbitz et al., 1997; Gong et al., 2004). In addition, pH can affect the concentrations of organochlorine pesticides in soils through modification of the structure of humus (Alawi et al., 1995; Wenzel et al., 2002). In this study, significant correlations of a-HCH with pH and TOC were observed merely (Fig. 3). A significant positive correlation (p < 0.01) of a-HCH concentrations with soil pH under acid soils was corresponded with the finding of Wenzel et al. (2002) who studied forest acidic soils in Germany. On the contrary, a significant negative correlation (p < 0.05) was found between concentrations of TOC
0.12 0.10 y=0.0638x -0.1789 R2=0.328 (p=0.0009)
0.08 0.06 0.04 0.02 0.00 3.0
3.5
4.0
4.5
5.0
5.5
pH(HCl) 1 y= -0.0014x + 0.1038 R2 = 0.142 (p=0.026)
Log(α-HCH)
bottom layer for DDT despite that no significant (p < 0.05) difference was found among the three soil stratums both for HCH and DDT. It was in line with the study carried by Piao et al. (2004), which presumably indicated the higher vertical mobility in the soil profiles for HCH than DDT due to the different solubility of HCH (5–10 mg kg1) and DDT (2 lg kg1) (Mackay et al., 1997). But yet an exception was occurred in Hong Kong island for HCH and in Lantau island for DDT, respectively. Peak concentration of HCH or DDT was found in the subsoil of these two sampling sites, respectively, and the essential reasons induced to such variations in these two sampling sites was required to be further explored. In addition, the compositions of DDT as well as HCH in the subsoil or bottom layer were nearly same as that in the topsoil, predominated by p,p 0 -DDE and b-HCH respectively in the whole soil profile. It implied that DDT and HCH of the different soil stratums were entirely undergoing the aged (microbiological degraded) stage (Harner et al., 1999).
0.1
0.01 0
10
20
30
40
50
60
TOC
Fig. 3. Correlations of a-HCH with soil pH(HCl) and TOC.
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and log(a-HCH), which was largely probably due to promotion of a-HCH biodegradation by soil microbial activity under a higher TOC (Wu et al., 1997). 4. Conclusion Five species of Stockholm Convention pesticides as well as HCH were detectable in the soils of Hong Kong, with its concentrations sequence as HCH > DDT > HCB Endrin > a-endosulfan. However, their concentrations were substantial below the counterpart target value for division of polluted and unpolluted soils in Netherlands, implying that no soil pollution was occurred presently with respect to OCPs in Hong Kong. Among these pesticides, b-HCH and p,p 0 -DDE were the two dominant substances, with the average concentrations of 6.12 and 0.41 lg kg1 respectively. It is indicative of the aged (microbiologically degraded) HCH and DDT in the soils. As for the depth distributions of HCH and DDT in soil profiles, concentrations of HCH tended to increase with the soils depth while that of DDT tended to decrease from the topsoil to subsoil and increase down to the bottom layer in the soil profiles of most sampling sites, but such depth variations was not significant statistically. The analyses of linear regression indicated that pH and TOC were the two major soil properties playing an important role in the residues of HCH, but not for DDT and other OCPs of Hong Kong soils. Acknowledgments The authors are grateful to the financial support given by the Major State Basic Research Development Program of China (973 Program), No. 2002CB410810/09, and by the Strategic Research Fund of the Hong Kong Baptist University.
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Residues of organochlorine pesticides in Hong Kong soils H.B. Zhang a,c, Y.M. Luo a,c,*, Q.G. Zhao M.H. Wong a,b, G.L. Zhang a,c
a,c
,
a Soil and Environment Joint Open Laboratory between Institute of Soil Science, Chinese Academy of Sciences and Hong Kong Baptist University, Soil and Environmental Bioremediation Research Center, State Key Laboratory of Soil and Sustainable Agriculture, 71 East Beijing Road, Nanjing, Jiangsu 210008, China b Croucher Institute for Environmental Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong c Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
Received 2 April 2005; received in revised form 1 June 2005; accepted 8 August 2005 Available online 5 December 2005
Abstract It was short of research on the organochlorine pesticides (OCPs) residues in the soils of Hong Kong. Sixty-six representative soil samples were collected from the 46 sites covering five types of land uses in Hong Kong. Hexachlorohexanes (HCH) and 7 Stockholm Convention OCPs were analyzed by gas chromatograph (GC) equipped with a Nickel 63 electronic capture detector (lECD). The results presented that HCH and 5 Stockholm Convention pesticides were detected in Hong Kong soils although the detectable ratio varies to a great extent. The concentration sequence of the five detectable OCPs was HCH > dichlorodiphenyltrichloroethane (DDT) > hexachlorobenzene (HCB) Endrin > a-endosulfan. Among the OCPs and their homologues or isomers, b-HCH and p,p 0 -DDE were the two predominant substances according to the concentrations and detectable ratios, concentrations of which in soils were averagely 6.12 lg kg1 and 0.41 lg kg1 respectively. Soil horizon samples of 0–10 cm, 10–30 cm and >30 cm depth were selected from nine soil profiles to demonstrate the depth distributions of DDT and HCH in soil profiles. Concentrations of HCH tended to increase gradually from the topsoil to bottom layer while the lowest concentration of DDT is usually found in the subsoil (10–30 cm) in most sampling sites. In addition, close correlations of pH(KCl) and total organic carbon (TOC) with HCH indicated an effect on the residues of HCH caused by these two soils properties, but such relationships were not found with DDT or other OCPs. 2005 Elsevier Ltd. All rights reserved. Keywords: Hong Kong soil; DDT; HCH; Depth distributions
*
Corresponding author. Address: Soil and Environment Joint Open Laboratory between Institute of Soil Science, Chinese Academy of Sciences and Hong Kong Baptist University, Soil and Environmental Bioremediation Research Center, State Key Laboratory of Soil and Sustainable Agriculture, 71 East Beijing Road, Nanjing, Jiangsu 210008, China. Tel.: +86 25 86881101; fax: +86 25 86881128. E-mail address: [email protected] (Y.M. Luo).
1. Introduction Characterized by toxicity, stability, and recalcitrance to degradation in natural environments, many organochlorine pesticides (OCPs) are semivolatile and lipophilic and described as persistent organic pollutants (Meijer et al., 2001). Some OCPs such as hexachlorohexanes (HCHs), dichlorodiphenyltrichloroethane (DDT),
0045-6535/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.08.006
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hexachlorobenzene (HCB), chlordane, aldrin and dieldrin were used extensively from 1972 to 1982, with 76 000–100 000 tons annually and an application rate of 1.8–2.7 kg per metric acre in the agricultural zones around in the Pearl River Delta (Fu et al., 2003). In Hong Kong, due to the decline of agricultural activities, most of the imported pesticides had been used for non-agricultural purposes, such as domestic and outdoor (e.g., golf courses) pest control (Wong and Poon, 2003). DDT was banned for agricultural application in the Chinese Mainland and Hong Kong in 1983 and 1988, respectively (Monks, 1994). In spite of this, OCPs are still widely distributed in the environment due to their persistency, semivolatile nature resulting in long-distance transportation. In Hong Kong, OCPs such as a-HCH and HCB were found ubiquitously present in the atmosphere (Louie and Sin, 2003), and relative high concentrations of HCH and DDT were found in marine and inland water sediments (Richardson and Zheng, 1999; Zhou et al., 1999). Accumulation of OCPs in the lipid content of animals is a common phenomenon due to their hydrophobic properties (Sijm and Linde, 1995). Zhou et al. (1999) observed that tilapia (Tilapia mossambica) lived in heavily contaminated environment accumulated a substantial amount of DDT and HCHs. Accordingly, OCPs pollution in the air, water and sediments/soils would eventually give rise to food contamination. Soil was the repository of all types of chemicals including inputs of OCPs. Many OCPs had a high affinity for soil, which might be taken up by crops and by grazing animals and hence reached the human food chain. They might also be washed in run-off from the land into watercourse and emitted into atmosphere through volatilization, which indirectly resulted in water and atmospheric contamination (Fu et al., 2003; Bidleman and Leone, 2004). Residues of OCPs in soils of Hong Kong had nevertheless been focused on. The major purpose of this study was to investigate the concentrations, compositions and distributions of OCPs in Hong Kong soils. Furthermore, the edaphic factors contributing to these would also be discussed.
2. Materials and methods
that are largely unsettled and remain its natural characteristics, while the limited arable lands are mainly scattered at the alluvial plain. Most wetland can be found in the northwestern of New Territories, and half of which is mangrove swamp (Ashworth et al., 1993). Mai Po mangrove swamp is the most important wetland in Hong Kong because it has been designed as an international important Ramsar site. Along the coastline of Hong Kong, a lot of lands have been developed through reclamation from the sea, some of which are not constructed and set aside. Most soils of Hong Kong are Oxisols commonly found in the humid tropics, characterized by strong weathering and leaching (Soil and Conservation Service, 1999). Hong KongÕs climate is sub-tropical, with an annual average temperature of 24 C and relative humidity of 78% in 2000 (Hong Kong Observatory, 2001). The mean annual rainfall ranges from about 1300 mm at Waglan Island to more than 3000 mm close to Tai Mo Shan. About 80% of the rain falls between May and September, while October to April is relatively arid with frequent hill fires caused by tomb worship in the countryside (AFCD, 2002). 2.2. Reagents and instruments The n-hexane and methanol used were both High Performance Liquid Chromatography (HPLC) grade and were purchased from Fisher Scientific International Inc., USA. The anhydrous sodium sulfate (analytical grade, Nanjing Chemical Reagent Co.) and Silica gel (60 mesh, for column chromatography) were activated at 600 C for 8 h and at 110 C for 24 h, respectively. A mixture of standard solution containing a,b,c-HCH, HCB, heptachlor, Aldrin, Endrin, Dieldrin, a-Endosulfan, b-Endosulfan, p,p 0 -DDE, o,p 0 -DDE, p,p 0 -DDD, o,p 0 -DDD, o,p 0 -DDT, p,p 0 -DDT with 10.0 mg l1 per compound and a standard solution (10.0 mg l1) of pentachlorotoluene (PCT) were purchased from Labor Dr. Ehrenstorfer, Germany. Agilent6890 gas chromatograph (GC) equipped with a 63Ni electron capture detector (lECD) (Agilent Technology Co., USA) was used for the analysis. The column used was HP-5 silica capillary column with 30 m · 0.33 mm i.d · 0.25 lm film thickness. Instruments for extraction and cleanup were including ultrasonic shaking apparatus and rotary evaporation.
2.1. The study area 2.3. Soil sampling Hong Kong is located at the southeast tip of the Chinese Mainland, with a total area slightly over 1100 km2 covering Hong Kong Island, Kowloon, New Territories and a number of islands, with Lantau Island being the biggest one. Three-quarters of the total land areas is countryside, in which approximately six types of land use including woodland, grassland, arable land, wetland and reclamation land were distributed (AFCD, 2002). Woodland and grassland are mainly distributed in hills
Soils were collected from the New Territories, Kowloon, Hong Kong Island, and Lantau Island in December 2000. The sampling sites are shown in Fig. 1. In total, 66 soil samples (46 surface soils of 0–10 cm and 20 different soil horizons from nine soil profiles) from 46 sites covering different types of land use patterns such as woodland, grassland, arable land, wetland and reclamation land in Hong Kong were collected, and each
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Fig. 1. Map of sampling sites.
sample consisted of 4–5 sub-samples. Among the 46 soil samples, most of which were collected from woodland and grassland, seven were from arable land, two were from wetland, namely Mi Po mangrove swamp, and one from a reclamation land located in Ma On Shan area. The nine soil profiles for demonstrating the depth distributions of OCPs were collected from four sampling sites, distributed in Ma On Shan, Tai Mao Shan, Hong Kong island and Lantau island respectively. The collected samples were air dried at room temperature (22– 25 C), sieved to <2 mm, and stored in glass bottles prior to analysis. 2.4. Sample extraction and cleanup Sample extraction, cleanup and OCPs analysis were carried out in the state Key Laboratory of Pollution Control and Resource Reuse in Nanjing University, and the procedures were developed from EPA method 3550C (US EPA, 2000). Samples (7.5 g dry weight) were weighed into centrifuge tubes, with mixture of 20 ml nhexane, 5 ml methanol, and 5 ml distilled water and sonicated for 1 h in an ultrasonic shaking apparatus. The mixture was then centrifuged and the extracts were collected. The same extraction was repeated once by adding 20 ml n-hexane to the filter residue. The two extract solutions were then combined and anhydrous sodium sulfate was added for drying, and then concentrated to around 1 ml by rotary evaporation. The concentrated extracts were cleanup through a chromatographic column filled with 1 g of activated silica gel (added with 0.07 ml of distilled water), and then eluted with 8 ml
n-hexane, after that the eluent was further concentrated to 1 ml. The solution was finally concentrated to around 0.1 ml under a gentle steam of pure nitrogen. 10 ll of PCT (1 mg l1) was added to the solution as an internal standard prior to transferring to a glass of microvial for GC injection. 2.5. OCPs analysis The analytical procedures described in Sun et al. (2002) were used. The purified extract was analyzed using GC-ECD under the splitless mode and HP-5 silica capillary column. High purity (99.99%) helium was used as carrier gas at 2.7 ml/min and nitrogen as make-up gas at 54.4 ml/min. The injector and detector temperature were 250 C and 320 C respectively. The oven temperature was initially set at 60 C, ramped at 25 C/min to 170 C, 4 C/min to 190 C, 10 C/min to 230 C and 2 C/min to 240 C, then held at 240 C for 5 min. Peak areas were calculated by Agilent Chem-Station software. OCPs measured in Hong Kong soil includes six DDT homologues, three HCH isomers, HCB, heptachlor, aldrin, endrin, P diedrin and Pa,b-endosulfan and concentrations of DDT and HCH were the sum of the six DDT homologues and three HCH isomers, respectively. 2.6. Quality control The detection limit ranged from 0.005 lg kg1 to 0.02 lg kg1 and recoveries of OCPs by this method
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Table 1 The detection limit, recovery and repeatability of the method
2.8. Statistical analysis
OCPs
Detection limit (lg kg1)
Recovery (%)
Repeatability (% RSD)
p,p 0 -DDE p,p 0 -DDD p,p 0 -DDT o,p 0 -DDT a-HCH b-HCH c-HCH Endrin HCB a-Endosulfan
0.015 0.005 0.005 0.005 0.015 0.020 0.005 0.005 0.005 0.005
90.5 96.4 110 93.3 83.2 91.7 90.4 81.0 87.4 88.3
5.6 10.4 5.4 4.7 6.0 5.4 7.5 4.6 12.2 5.4
To detect the difference of HCH and DDT concentrations among the land uses, ANOVA test companied with Least Significant Difference (LSD) method were adopted. Linear regression analyses were separately carried out for exploring the relationships of a-HCH with TOC and pH(KCl). SPSS 11.0 for windows was employed for statistical analysis.
3. Results and discussion 3.1. Main characteristics of Hong Kong soils
ranged from 81.0% to 110% (Table 1). The analysis of three blanks covering the entire analytical procedure (from the extraction to the GC analysis) was subjected to assess the interference from the reagents and glassware. GC analysis was repeated twice for each replicate sample and the relative standard deviation (RSD) of replicate samples were less than 15%. 2.7. Physical and chemical soil analysis Physical and chemical soil analysis were characterized by conventional standard procedures: Soil pH was determined in 1 M KCl at a soil to solution ratio of 1:2.5 by a potentiometric glass electrode, total organic carbon (TOC) and total nitrogen were measured with a CNS-Analyzer (Elementar Vario EL, Elementar Analysensysteme GmbH, Hanau, Germany). Particle-size distribution was carried out using the pipette method, and classification of the soil texture was based on Soil Survey Division Staff (1993). Cation exchange capacity (CEC) of mineral soils was the sum of Ca + Mg + K + Na + Fe + Al + Mn extractable with 1 M NH4acetate (Sumner and Miller, 1996).
The main characteristics of Hong Kong soils are presented in Table 2. Soil textures ranged from loam to sandy clay loam under the five land-uses, in which wetlands were found with the lowest sand content and highest clay content while reclamation land was found the opposition. In terms of pH, woodland and grassland soils ranged from strong to moderate acidic, arable land and wetland soils tended to be neutral while reclamation soils were alkaline. The soil CEC and TOC concentrations were not similar as well among all types of soils, with the lowest values found in the reclamation soils. The C/N ratios of the five kinds of soils also varied considerably, with the arable land soil having a lower C/N ratio, suggesting that the organic matter content was more nitrogen-rich. 3.2. Concentrations of OCPs in Hong Kong soils Analysis results showed that most of OCPs were found in samples except for heptachlor, aldrin, dieldrin and b-endosulfan. HCH was detected in all 46 samples, DDT in 43 samples, HCB in 5 samples, endrin in 15 samples and a-endosulfan in one sample only. In Table 3 samples were grouped according to the soils of woodland, grassland, arable land, wetland and
Table 2 The main characteristics of Hong Kong soils Soil properties
Land using Woodland
Grassland
Farmland
Wetland
Reclamation
Sand (%) Silt (%) Clay (%) Textural class USDA pH (H2O) CEC (cmol/kg) TOC (g/kg) Total nitrogen (g/kg) C/N ratio
42.9 33.0 24.1 Loam 4.62 10.2 23.8 1.25 19.0
38.3 40.8 20.9 Loam 4.70 10.9 21.0 1.18 17.8
40.9 43.9 15.2 Loam 6.23 8.57 12.3 1.07 11.5
2.3 55.7 42.0 Silty clay 6.35 9.75 29.2 1.93 15.1
54.8 34.8 10.4 Sandy loam 8.09 3.86 5.32 0.28 18.9
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Table 3 Average concentrations (lg kg1 dry weight) of organochlorine pesticides in Hong Kong soilsa OCPs 0
p,p -DDE p,p 0 -DDD p,p 0 -DDT o,p 0 -DDT P DDT a-HCH b-HCH c-HCH P HCH Endrin HCB a-Endosulfan
Target valuesb
Land-use categories Woodland
Grassland
Farmland
Wetland
Reclamationc
0.20 ± 0.19 0.01 ± 0.01 0.01 ± 0.01 0.10 ± 0.06 0.32 ± 0.20 0.05 ± 0.04 5.66 ± 1.17 0.01 ± 0.01 5.72 ± 1.18 0.01 ± 0.01 0.03 ± 0.02 n.d.
0.16 ± 0.014 0.01 ± 0.01 0.04 ± 0.03 0.03 ± 0.04 0.24 ± 0.20 0.05 ± 0.03 6.26 ± 1.52 0.01 ± 0.01 6.32 ± 1.52 0.02 ± 0.02 0.01 ± 0.01 0.0047
1.73 ± 1.53 0.05 ± 0.12 0.02 ± 0.04 0.05 ± 0.08 1.85 ± 1.64 0.14 ± 0.08 6.37 ± 0.24 0.02 ± 0.02 6.53 ± 0.17 n.d. n.d. n.d.
0.17 ± 0.21 n.d. 0.18 ± 0.15 0.08 ± 0.01 0.43 ± 0.04 0.09 ± 0.09 7.55 ± 2.05 n.d. 7.64 ± 2.14 n.d. 0.01 ± 0.01 n.d.
0.09 n.d. n.d. 0.03 0.13 0.04 6.50 0.01 6.56 n.d. 0.08 n.d.
– – – – 2.5 2.5 1 0.05 10 1 2.5 –
n.d.: Below the detection limit. a Average concentration is presented as mean ± standard deviation. b The Netherlands Soil Contamination Guidelines (Department of Soil Protection, 1994). c One sample was collected and no standard deviations is supplied.
reclamation land. P PIn general, the average concentrations of DDT and HCH of all soils were 0.52 ± 1.01 and 6.19 ± 1.31 (lg kg1 dry weight) respectively. They P were much lower than the average concentrations of DDT P (37.6 lg kg1 dry weight) and HCH (12.2 lg kg1 dry weight) found in the soils of Pearl River Delta Region (Fu et al., 2003), despite the fact that DDT and HCH have been banned from agricultural applications for more than 20 years both in Hong Kong and the Chinese Mainland. However, illegal applications of these two pesticides have been found in some areas of the Delta Region (Zhou et al., 2001; Mai et al., 2002). On the contrary, the pesticides management seems to be better in Hong Kong in addition to the substantial decline of agricultural activities, as a result that import and export of these pesticides are strictly controlled by the Pesticides Ordinance and Regulations (HKSARG, 1993). The total concentrations of DDT in arable land soils were significantly (p < 0.05) higher than those in woodland and grassland soils, suggesting that DDT was mostly used for agricultural activities in Hong Kong. However, the total concentrations of HCH in soils of the five types of land use varied only slightly, which may be contributed to the relatively higher vapor pressure of HCH (Mackay et al., 1997) causing HCH much easier to volatilize than DDT from soil to atmosphere, and return to soil through dry or wet deposition after atmospheric transportation. It will then lead to the more homogeneous distribution of HCH in soils (Fu et al., 2003; Yeo et al., 2003). Other OCPs such as endrin, HCB and a-endosulfan were also found in the soils. Endrin was detected in the New Territories and Lantau Island, ranging from 0.007 to 0.093 lg kg1 dry weight. Endrin is mostly pre-
sented in soils and sediments due to its low water solubility and vapor pressure (Mackay et al., 1997). However, a recent study indicated that no endrin was detected in the atmosphere of Hong Kong (Louie and Sin, 2003). In general, concentrations of HCB ranged from 0.007 to 0.31 lg kg1 in all soils, but it was under the detection limit in arable land soils. Although HCB has been prohibited from use as herbicides, wastes derived from chlorine related industries could be a possible HCB source to the environment (Meijer et al., 2001). The concentrations of all the detectable OCPs in the soils of Hong Kong were preliminary compared with the corresponding target values used in Netherlands (Table 3) due to the shortage of local standard for identifying soil pollution. The target value was favored to be as a critical value for division of polluted and unpolluted soils in the soil protection guideline of Netherlands (Department of Soil Protection, 1994), and they were entirely higher than the concentrations of OCPs in Hong Kong. It implied that no remediation measures were required with respect to the OCPs in soils of Hong Kong. In spite of this, the ecological and health effects of these substances through food chain at the relatively lower concentrations still need further attention in light of their possible biological magnifications in higher trophic organisms including human beings. Wong et al. (2002) demonstrated that the mean levels of p,p 0 -DDT, p,p 0 -DDE and b-HCH in human breast milk from Hong Kong were 2–15-fold higher compared to the relevant studies conducted in developed countries such as United Kingdom, Germany, Sweden, Spain, and Canada. It was noticeable that p,p 0 -DDE and b-HCH were main OCPs components in the soils of Hong Kong in terms
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of present research, the relationships between these two organochlorine pesticides in human breast milk and soils were worthwhile to exploration. 3.3. Compositions of DDT and HCH in Hong Kong soils Table 3 showed that DDT and its metabolites found in Hong Kong soils contained o,p 0 -DDT, p,p 0 -DDT, p,p 0 -DDE and p,p 0 -DDD, with the p,p 0 -DDE being the major composition of DDT (accounting for 78.8% of the total DDT concentration averagely). DDE and DDD were the two main products of DDT dechlorination. DDT was dechlorinated to DDE under aerobic conditions and reductively dechlorinated to DDD under anaerobic conditions (Heberer and Du¨nnbier, 1999; Fellenberg, 2000). The pathway of DDT to DDD could be direct and indirect, and the later was from DDT to DDE, then further to DDD (Wenzel et al., 2002). The ratios of DDD:DDE, DDD:DDT and DDE:DDT were employed to deduce the possible dechlorination pathway in Hong Kong soils (Table 4). It was probable that DDT to DDE was the main degradation route as a result that the ratios sequence was DDE:DDT > DDD:DDT > DDD:DDE. A small value of DDT:(DDE + DDD) ratio indicates aged (microbiologically degraded) DDT while a value much greater than 1.0 indicates fresh application. The average value of DDT:(DDE + DDD) was 0.81, implying the existence of aged DDT in most soils of Hong Kong. Meanwhile, the ratios with much greater than 1.0 were found in two samples from Mai Po, north of New Territory, and Discovery Bay, north east of Lantau Island respectively. However, the concentrations of DDT in these two soils were only 0.35 lg kg1 and 0.23 lg kg1 respectively, which were much lower than the target value presented in Table 3. The microbial transformation rate of DDT to its metabolites DDE and DDD depended on several factors including soil type, temperature, and moisture, organic carbon content (Hitch and Day, 1992; Boul, 1994). Some soils in the southwestern USA were observed to have a high proportion of parent DDT due to their poor metabolite capability (Hitch and Day, 1992). c-HCH was presented in technical HCH containing 10–15% c-HCH and 60–70% a-HCH as well as other
isomers. Alternatively, it was available in its pure form as lindane (>99% c-HCH)(UNEP, 1995). Technical HCH has been forbidden for application in many countries, but lindane is still being used in a number of countries. The ratio of a-HCH to c-HCH in Hong Kong soil was mostly ranged from 0.3 to 5, with an average value of 6.0 (Table 4), which was close to the original ratio of technical HCH. It was assumed that historical usage of technical HCH would be the main source of HCH in soils, although no information has been found which HCH has been used in the past in Hong Kong. However, it yet cannot rule out the historical usage of lindane merely according to the HCH isomer ratios of soils when taking into account of the higher volatilization of c-HCH than a-HCH (Mackay et al., 1997). In fact, b-HCH accounting for 95.8–100% of the total HCH concentrations, is the dominant HCH isomer in most soil samples. This finding was different from that of in the of sediments river or marine and atmosphere, in which, a-HCH and c-HCH respectively rather than b-HCH predominated among the HCH isomers (Richardson and Zheng, 1999; Louie and Sin, 2003). The persistence of b-HCH in soils is mainly due to the higher Kow (log Kow = 3.78) and lower vapor pressure value (3.6 · 107 mmHg, 20 C). These will make for the b-HCH easier absorption to the soil organic matter and less evaporative loss from the soils (Hansch and Leo, 1979; Mackay et al., 1997). Furthermore, the spatial arrangement of chlorine atoms in the molecular structure of b-HCH was supposed to more resistant to microbial degradation in soils (Middeldorp et al., 1996; Kalbitz et al., 1997). 3.4. Depth distributions of OCPs in Hong Kong soils As for the depth distributions of OCPs, DDT and HCH were mainly concentrated on since concentrations of other OCPs such as HCB, endrin, heptachlor and so on were much lower in the soil samples. Fig. 2 showed the different distribution patterns of HCH and DDT along the soil profiles. Most samples had a trend of increasing from the topsoil to bottom layer for HCH concentrations and of decreasing from the topsoil to subsoil then increasing again in the
Table 4 Composition of detected DDT and HCH in Hong Kong soila Ratios of compounds
Mean value
Minimum value
Maximum value
Reference value
DDT/(DDE + DDD) DDD/DDE DDD/DDT DDE/DDT a-HCH/c-HCH
0.81 0.40 1.18 3.18 6.0
0.05 0.14 0.37 0.06 0.30
18 1.0 2.9 17 52
1.0 – – – 3–10
a
DDT, DDE and DDD means p,p 0 -DDT, p,p 0 -DDE and p,p 0 -DDD respectively.
H.B. Zhang et al. / Chemosphere 63 (2006) 633–641
639
bottom layer subsoil
topsoil
3.5. Effects of pH and organic matters
5.0
5.5
6.0
6.5
7.0
7.5
8.0
bottom layer subsoil
topsoil
HCH Conc. (µg kg-1)
0.0
0.20 0.18
Tai Mao Shan Ma On Shan Hong Kong island Lantau island .1
.2
.3
.4
.5
.6
.7
.8
0.16 0.14
.9
DDT Conc. (µg kg-1) Fig. 2. Vertical distributions of HCH and DDT in soil profiles.
α-HCH
4.5
Tai Mao Shan Ma On Shan Hong Kong island Lantau island
Soil organic matters are inclined to binding with organochlorine pesticides because of their hydrophobicity. On the other hand, the increase of organic matter content in soils can supply more carbon source to facilitate microbial degradation of organochlorine pesticides (Wu et al., 1997). As a result, the content of TOC could make an impact on the residue of organochlorine pesticides in soils (Borisover and Graber, 1997; Kalbitz et al., 1997; Gong et al., 2004). In addition, pH can affect the concentrations of organochlorine pesticides in soils through modification of the structure of humus (Alawi et al., 1995; Wenzel et al., 2002). In this study, significant correlations of a-HCH with pH and TOC were observed merely (Fig. 3). A significant positive correlation (p < 0.01) of a-HCH concentrations with soil pH under acid soils was corresponded with the finding of Wenzel et al. (2002) who studied forest acidic soils in Germany. On the contrary, a significant negative correlation (p < 0.05) was found between concentrations of TOC
0.12 0.10 y=0.0638x -0.1789 R2=0.328 (p=0.0009)
0.08 0.06 0.04 0.02 0.00 3.0
3.5
4.0
4.5
5.0
5.5
pH(HCl) 1 y= -0.0014x + 0.1038 R2 = 0.142 (p=0.026)
Log(α-HCH)
bottom layer for DDT despite that no significant (p < 0.05) difference was found among the three soil stratums both for HCH and DDT. It was in line with the study carried by Piao et al. (2004), which presumably indicated the higher vertical mobility in the soil profiles for HCH than DDT due to the different solubility of HCH (5–10 mg kg1) and DDT (2 lg kg1) (Mackay et al., 1997). But yet an exception was occurred in Hong Kong island for HCH and in Lantau island for DDT, respectively. Peak concentration of HCH or DDT was found in the subsoil of these two sampling sites, respectively, and the essential reasons induced to such variations in these two sampling sites was required to be further explored. In addition, the compositions of DDT as well as HCH in the subsoil or bottom layer were nearly same as that in the topsoil, predominated by p,p 0 -DDE and b-HCH respectively in the whole soil profile. It implied that DDT and HCH of the different soil stratums were entirely undergoing the aged (microbiological degraded) stage (Harner et al., 1999).
0.1
0.01 0
10
20
30
40
50
60
TOC
Fig. 3. Correlations of a-HCH with soil pH(HCl) and TOC.
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and log(a-HCH), which was largely probably due to promotion of a-HCH biodegradation by soil microbial activity under a higher TOC (Wu et al., 1997). 4. Conclusion Five species of Stockholm Convention pesticides as well as HCH were detectable in the soils of Hong Kong, with its concentrations sequence as HCH > DDT > HCB Endrin > a-endosulfan. However, their concentrations were substantial below the counterpart target value for division of polluted and unpolluted soils in Netherlands, implying that no soil pollution was occurred presently with respect to OCPs in Hong Kong. Among these pesticides, b-HCH and p,p 0 -DDE were the two dominant substances, with the average concentrations of 6.12 and 0.41 lg kg1 respectively. It is indicative of the aged (microbiologically degraded) HCH and DDT in the soils. As for the depth distributions of HCH and DDT in soil profiles, concentrations of HCH tended to increase with the soils depth while that of DDT tended to decrease from the topsoil to subsoil and increase down to the bottom layer in the soil profiles of most sampling sites, but such depth variations was not significant statistically. The analyses of linear regression indicated that pH and TOC were the two major soil properties playing an important role in the residues of HCH, but not for DDT and other OCPs of Hong Kong soils. Acknowledgments The authors are grateful to the financial support given by the Major State Basic Research Development Program of China (973 Program), No. 2002CB410810/09, and by the Strategic Research Fund of the Hong Kong Baptist University.
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