Environ. Sci. Technol. 2002, 36, 3671-3677
Sedimentary Records of DDT and HCH in the Pearl River Delta, South China G A N Z H A N G , * ,†,‡ A N D R E W P A R K E R , ‡ ALAN HOUSE,§ BIXIAN MAI,† XIANGDONG LI,| YUEHUI KANG,† AND ZHISHI WANG⊥ State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, Postgraduate Research Institute of Sedimentology, The University of Reading, Reading RG6 6AB, U.K., Centre for Ecology and Hydrology, NERC, Dorset, U.K., Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, and Faculty of Science and Technology, Macao University, Macao
Tropical regions in developing countries are thought to be significant sources of organochlorine pesticides in the global context, owing to high rate of use and only a recent production ban or restriction on application of these pesticides. In the present paper, DDT and HCH in eight 210Pb-dated sedimentary cores from the Pearl River Delta, South China, were analyzed in order to reconstruct the time trends of these persistent organic pollutants in this tropical region. The sedimentary inventories of ∑DDT and ∑HCH through the cores ranged from 36.6 to 1109.5 ng/ cm2 and from 11.2 to 226.3 ng/cm2, respectively, and their spatial distribution implies that the water flows from the Humen, Jiaomen, Hongqili, and Hengmen outlets rather than the Xijiang flow from Modaomen outlet, supplied the major historical input of DDT to the estuary. Although a production ban of technical HCH and DDT was imposed in China in 1983, their sedimentary fluxes display increasing trends or strong rebounds in the 1990s as recorded in the core profiles, characteristic of the increasing ratios of (DDE + DDD)/DDT and DDE/DDT. It is suggested that an enhanced land soil runoff in the process of large-scale land transform, as well as a higher river water flow in early 1990s, had mobilized these pesticides from soil to the sedimentary system in the region.
Introduction Organochlorine pesticides are typical toxic and bioaccumulative persistent organic pollutants (POPs) and have been widely used throughout the world since the 1950s. As persistent semivolatile compounds, many POPs have the unique potential of long-range atmospheric transport (1, 2). Thus, although the application of these chemicals has been banned or restricted in many countries, especially the * Corresponding author phone: +86 20 8529 0178; fax: +86 20 8529 0706; e-mail:
[email protected]. † Chinese Academy of Sciences. ‡ The University of Reading. § Centre for Ecology and Hydrology. | The Hong Kong Polytechnic University. ⊥ Macao University. 10.1021/es0102888 CCC: $22.00 Published on Web 07/20/2002
2002 American Chemical Society
developed ones, they can still be detected in environmental media, even far from the places of their production and application (3-5). More and more evidence has supported the “global distillation” model in the past decade (2, 6, 7), which indicates that the POPs can migrate from warmer to colder areas and become “fractionated” on latitudinal or altitudinal gradients (8). This implies that, in view of latitudinal migration, tropical regions tend to represent an extreme of atmospheric POPs “net emission”, in contrast to their “net sink” in polar regions. Furthermore, DDT is still in use for malaria vector control in some of the tropical countries as recommended by the World Health Organization (9, 10). So, the historical distribution of POPs in tropical environment is important for understanding the global context. However, while the time trends of POPs in mid-high latitudes have been intensively studied (3, 4, 11-13), little work has been reported on their time trends in low-latitude regions (14-17). China is the world’s second largest producer of pesticides: a pesticide production of 260 000 tonnes was recorded in 1994. HCH and DDT were widely used in China from 1950s until production of DDT, and technical HCH was banned in April 1983. Over more than 30 years, the total production of technical HCH (4.9 million tonnes) and DDT (0.4 million tonnes), respectively, in China accounted for 33% and 20% of the total world production (18). Lying in the northern tropical region of South China, the Pearl River Delta is one of the most prosperous regions in China and has a record of the highest pesticide application in the country. The average annual application from 1980 to 1995 in the region reached 37.2 kg ha-1, 4 times higher than the country’s average annual application (19). In the present paper, we report the HCH and DDT concentrations in eight sedimentary cores collected from the Pearl River Delta, which give information about the environmental fate and time trend of these compounds with a high historical application in the tropical region studied.
Materials and Methods Sampling Sites. The Pearl River is the second largest river in China, with an average annual discharge of 174 × 109 m3/year and an associated suspended load of 37.3 × 106 tonnes (20). In the Pearl River Delta (Figure 1), the river network mainly comprises the Beijiang (North River), Zhujiang (main stream), Dongjiang (East River), and Xijiang (West River), and merges into the Pearl River estuary via eight outlets. The Pearl River estuary is one of the largest in the world, covering an area of 8000 km2 (20, 21). In the estuary, the waters from the northwestern inlets tend to flow along the west coast, owing to the circulation currents affected by the Coreolis force in the Northern Hemisphere and the prevailing westward wind in the region (22). The sediment cores (Table 1) were collected in the summer of 1997. A stainless steel static gravity corer (8 cm i.d.) was employed to minimize the disturbance of the surface sediment layer. The cores were sectioned at 2-cm intervals to the depth of >50 cm and at 5-cm intervals thereafter. The sectioned sediment samples were packed into aluminum boxes and immediately stored at -20 °C until required. Dating of the Sedimentary Cores. The 210Pb activities in sediment subsamples were determined by analysis of the R radioactivity of its decay product 210Po, on the assumption that the two are in equilibrium. The Po was extracted, purified, and self-plated onto silver disks at 75-80 °C in 0.5 M HCl, with 209Po used as yield monitor and tracer in quantification. Counting was conducted by computerized multichannel R VOL. 36, NO. 17, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Sketch map showing the sapling sites and surrounding areas in the Pearl River Delta and estuary. The numbers (1-8) indicate the eight major outlets to the Pearl River Delta: (1) Humen, (2) Jiaomen, (3) Hongqili, (4) Hengmen, (5) Modaomen, (6) Jitimen, (7) Hutiaomen, (8) Yamen. spectrometry with gold-silicon surface barrier detectors. Supported 210Po was obtained by indirectly determining the R activity of the supporting parent 226Ra, which was carried by coprecipitated BaSO4. A constant rate of 210Pb supply (CRS) model was applied to date sediment cores ZJ-2, 3, 6, 7, and 10 to obtain sedimentation rates, and a constant initial 210Pb concentration (CIC) model was used for Cores ZJ-8 and 9 and ZJ-D, which gives average sedimentation rates (23). Sample Extraction and Preparation. Sediment samples were homogenized and freeze-dried. There would be volatilization losses especially of HCH isomers during freezedrying. For cores ZJ-2, 3, 8, 9, D, and 10, dried sediments were spiked with 50 ng of 2,4,5,6-tetrachloro-m-xylene (TCmX) and decachlorobiphenyl (PCB209), each as surrogates, Soxhlet-extracted with dichloromethane for 48 h. Activated copper granules were added to the collection flask to remove elemental sulfur. The extract was concentrated and solvent-exchanged to hexane and further reduced to approximately 1 mL under a gentle nitrogen stream. A 1:2 aluminum/silica gel column (1 cm i.d.) was used to clean-up and fractionate the extract. The column was eluted with 15 mL of hexane to yield aliphatic hydrocarbons and the PCB fraction and then with 5 mL of hexane and 70 mL of dichloromethane/hexane (3:7) to yield the PAH and organochlorine pesticides fraction. The second fraction was concentrated to 0.4 mL under a gentle nitrogen steam. A known quantity of pentachloronitrobenzene (PCNB) was added as an internal standard prior to gas chromatographyelectron capture detector (GC-ECD) analysis. For Cores ZJ-6 and 7, dried sediments (1-2 g) spiked with surrogates were sandwiched with glass wool and pure sand and extracted with a DIONEX 350 supercritical carbon dioxide extractor with 10% methanol dynamically added as cosolvent. The extraction was performed under 400 atm at 50 °C for 1 h, and the extract was collected in hexane at 4 °C. Cleanup was done by treating the concentrated extract with 1:1 H2SO4/silica gel packed in a glass column and eluting with dichloromethane. The dichloromethane fraction was then 3672
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reduced to 0.1 mL and spiked with 7.5 ng of phenanthrened10 as an internal standard prior to gas chromatographymass spectrometry (GC-MS) analysis. GC-ECD and GC-MS Analysis. An HP-5890 series II gas chromatograph was equipped with a 63Ni electron capture detector and a 60 m × 0.32 mm i.d. (0.17 µm film thickness) HP-5 fused silica capillary column. Nitrogen was used as carrier gas at 2.5 mL/min under the constant-flow mode. Nitrogen was filtered with moisture, hydrocarbon, and oxygen filters before entering the GC system. The oven temperature began at 100 °C and immediately increased to 290 °C (10 min hold time) at a rate of 4 °C/min. Split/splitless injection of a 2 µL sample was performed with a 1 min solvent delay time. Injector and detector temperatures were maintained at 250 and 300 °C, respectively. The inlet degradation of DDT was checked daily and controlled within 15%. Data were acquired and processed on HP-3365 Chemstation software. GC-MS analysis was carried out on a HP-5971 GC-MSD system operating under single ion monitoring (SIM) mode. A 30 m × 0.32 mm i.d. (0.17 µm film thickness) HP-5 fused silica capillary column was used for separation. Helium was used as carrier gas at a flow rate of 1.05 mL/min. The injector and column temperature program was similar to those in the GC-ECD analysis. A 2 µL sample was injected in the split/ splitless injector with a 1 min solvent delay. Mass spectra were acquired at electron impact (EI) mode under 70 eV. Peak confirmation and quantification were performed on a DOS-based HP ChemStation system.
Results and Discussion Sedimentation Rates and Fluxes. Sedimentation in riverine and estuarine environment is much unstable comparing to those in lakes and oceans. Uncertainties of dating may come from either natural disturbance due to flood or storm tide and human activity such as land reclamation in the studied fast developing region. The average sedimentation rates in our cores vary from site to site. Cores ZJ-2, 3, 6, 7, and ZJ-10 in which the CRS model can be applied displayed a general increasing of sedimentation rates and fluxes in the upper 20 cm. The sedimentation rates and fluxes of the cores are summarized in Table 1. Notably, ZJ-10 located at Xijiang, displays the lowest sedimentation rate (0.07-0.29 cm/a, averaged 0.17 cm/a) and flux (52.8-195 mg/(cm2 a)), despite the higher suspended load (20). This can be explained either by the sampling site not being hydrodynamically suitable for sedimentation or by a higher water flow rate in Xijiang. The average sedimentation rate in Core ZJ-7 (0.86 cm/a) on the Eatstern coast is obviously lower than ZJ-2 (1.17 cm/a), 3 (1.48 cm/a), and ZJ-6 (1.35 cm/a) on the Western coast. Total Organic Carbon (TOC). TOC in sediments is affected by the autochthonous and locochthonous organic input, as well as by the postdepositional preservation of organic matter. The average TOC of sediments in Core ZJ-6 (1.02%) was higher than ZJ-7 (0.86%). This also reflects the difference between the western and the eastern coast of the estuary. In the profiles, TOC in ZJ-6 and 7 both show a sharp decline after 1990 (Figure 2). Median Grain Size and Surface Area. The average median grain size through the profiles of ZJ-6 and ZJ-7 are 20.4 µm and 19.9 µm, respectively. Up to 61.7-78.7% (ZJ-6) and 42.484.9% (ZJ-7) of grains are within the silt range of 20-60 µm. The sediment surface areas correspond well with the median grain size. Both ZJ-6 and ZJ-7 show a time trend of an increasing median grain size and a decreasing surface area after 1990 (Figure 2) and a maximum median grain size at the top layer. Clay Minerals. The profiles of relative content of illite (I) and kaolinite (K) are drawn as I/K ratio in Figure 2. Sediment in ZJ-6 has a higher I/K ratio (1.75-2.90, average 2.34) than
TABLE 1. Site Description and Sedimentation Rates and Fluxes core station ZJ-2 ZJ-3 ZJ-6 ZJ-7 ZJ-8 ZJ-D ZJ-9 ZJ-10
core deptha (cm)
210Pb-time
longitude/latitude
water depth (m)
(from A.C.)
sedimentation rateb (cm/a)
sedimentation fluxb (mg/cm2 a)
113°22.66′ E/23°06.77′ N 113°30.94′ E/22°59.96′ N 113°40.00′ E/23°35.75′ N 113°56.35′ E/22°28.04′ N 113°37.96′ E/22°23.16′ N 113°37.98′ E/22°13.04′ N 113°33.29′ E/22°10.89′ N 113°20.53′ E/22°16.52′ N
6 4 2 5 3 4.8 3 7
44.5 62.5 66.0 32.0 58.0 90.0 66.0 14.0
1943 1950 1951 1942 1982 1937 1963 1950
0.42-4.26 (1.17) 0.48-3.85 (1.48) 0.93-3.51 (1.35) 0.26-3.30 (0.86) 3.85 1.52 1.87 0.07-0.29 (0.17)
337-5140 470-1120 836-1596 235-2880 2345 1175 1522 52.8-195
a Maximum depth subjected to pesticide analysis. b Numbers in parentheses refer to average rate. Cores ZJ-8, 9 and ZJ-D were dated with CIC models; therefore, only average sedimentation rates are available; uncertainty may exist in the date for deep slices of these cores.
FIGURE 2. Down-core variations of sediment median grain size, surface area, I/K ratio, and TOC in cores ZJ-6 (upper row) and 7 (bottom row). ZJ-7 (0.97-1.90, average 1.33), indicating the difference in sediment sources between the two sites. Concentration of ∑HCH and ∑DDT in Surface Slice. Surface slice (top 2 cm in core) represents the current sediment contamination status well. It should be noted that the HCH even in the top layers, dated to be deposited after the production ban of the technical HCH, showed no indication of the changing from technical HCH to still-inuse Lindane as reported (29). The total DDTs (∑DDT, the summation of p,p′-DDT, p,p′-DDE, p,p′-DDD, and o,p′-DDT) in the surface slices ranges from 3.8 to 31.7 ng/g (Table 2), and the total HCHs (∑HCH, summation of R-, β-, γ-, and σ-HCH) ranges from 0.4 to 6.2 ng/g. These data are well within the range of the concentrations in surface sediment (top 10 cm) from the same area we reported before (24; 2.601628.8 ng/g for ∑DDT; 0.1-17.0 ng/g for ∑HCH). However, they are higher than those reported in the Pearl River estuary (1.36-8.99 ng/g for ∑DDT, 0.29-1.23 ng/g for ∑HCH; 20). In comparison, our sampling sites were closer to the bank, while those of another group (20) were mainly concentrated in the center of the estuary. This may imply a pollution
dilution process from the edge to the center of the estuary, showing a ∑DDT concentration gradient from 20 ng/g level to mostly less than 5 ng/g level. The average ∑DDT concentration in surface sediments of the Pearl River Delta (19.7 ng/g) is well within in the range of the world coastal sediments (0-44 ng/g) as reported in the reference (25). Concentrations and Inventories of DDT and HCH in Sediment Cores. Along the Pearl River (local sense) and the southward circular current in the west coast of the estuary, ZJ-2, 3, 8, D, and ZJ-9 display a decreasing trend of average ∑DDT and ∑HCH concentrations and inventories (Table 3), although the highest average concentration (22.7 ng/g) and inventory of ∑DDT were observed in ZJ-6, at the upper top of the estuary directly receiving freshwater flow from rivers. Located in Xijiang, ZJ-10 showed a narrow concentration range of ∑DDT (4.8-8.9 ng/g), lower average concentration of ∑DDT (6.7 ng/g), and lower inventories of ∑DDT and ∑HCH (36.6 and 12.1 ng/cm2; Table 2) as compared with the west-coast cores. This suggests that the water flows from the Humen, Jiaomen, Hongqili, and Hengmen outlets (Figure 1) rather than the West River flow from Modaomen outlet, VOL. 36, NO. 17, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Concentrations and Inventories of ∑DDT and ∑HCH core concn rangea (ng/g dry wt)
present layer concnb (ng/g dry wt)
inventoriesc (ng/cm2)
core
∑DDT
∑HCH
∑DDT
∑HCH
∑DDT
∑HCH
ZJ-2 ZJ-3 ZJ-6 ZJ-8 ZJ-D ZJ-9 ZJ-10 ZJ-7
3.6-31.0 (17.6) 5.5-31.7 (15.7) 1.6-41.5(22.7) 6.9-19.2 (11.8) 2.1-26.5 (9.5) 2-21.2 (7.9) 4.8-8.9 (6.7) 0.8-4.5 (2.5)
1.7-11.2 (5.9) 2.7-5.5 (4.0) 1.2-4.1 (2.6) 1.8-6.4 (3.8) 0.4-5.6 (2.4) 0.5-9 (2.3) 1.4-4.2 (2.6) 0.1-0.7 (0.4)
22.0 31.7 24.1 19.2 26.5 21.2 8.9 3.8
6.2 5.0 2.9 5.6 3.9 1.0 3.7 0.4
695.3 558.3 1109.5 416.3 399.6 408.4 36.6 67.2
226.3 141.4 134.3 135.5 135.1 131.3 12.1 11.2
a Numbers in parentheses are down-core average concentrations. b Top 2-cm layer. c Calculated by integrating the target compound flux (ng/ cm2/a) by the time (refer to Table 1 for initial time), the mass of contaminant for unanalyzed intervals was estimated by linear interpolation of adjacent measured intervals.
TABLE 3. Main Features of Sedimentary Records of Fluxes of ∑DDT and ∑HCH ∑DDT flux core
first increase
ZJ-2
1965
ZJ-3 ZJ-6
1955-1968a 1964
ZJ-8
b
ZJ-D
1964
ZJ-9
1963
ZJ-10 1965-1973a 1961-1964a
ZJ-7 a
peak time 1997 1990-1997 1980-1984 1993-1995 1986 1992-1997 1971 1995 1977-1979 1993-1996 1973 1995 1969-1971 1993-1996
∑HCH flux recent trend
first increase
peak time
increase
1955-1965a
increase sharp rebounds
1956-1968a
increase
1957-1964a
sharp rebounds
1963
increase
1950
1980 1997 1994-1997 1982-1984 1994-1997 1986 1992-1997 1964 1984 1963 1984 1995
decrease with rebounds
1964
1969-1971 eventually decreased
1964
increase
As time range for no data available between.
b
b
9
increase increase sharp rebounds increase slowly decrease eventually decreased after rebound in 1993 sharp rebound
No data.
supplied the major historical input of DDT to the estuary. A former study in the Pearl River estuary also found that organochlorine pesticide concentrations of sediment were influenced by the runoff of freshwater (20). Similarly, the transport of suspended sediments from the SacramentoSan Joaquin drainage basin strongly influenced the concentration and distribution of sediment-associated pesticides entering the San Francisco Bay in America (26). Core ZJ-7, situated in Shenzhen Bay on the east side of the Pearl River estuary, has the lowest average ∑DDT and ∑HCH concentrations (2.5 and 0.4 ng/g) and lower inventories (67.2 and 11.2 ng/cm2). In addition, we also found (27) that the concentration of pyrolytic polycyclic aromatic hydrocarbons (PAHs), incomplete combustion products of fossil fuel, in surface sediment collected at the same site (184 ng/g) was much lower than those at other sites (>415 ng/g). Although there was a previous study suggesting that the discharge of Zhujiang may be a main source of chlorinated pesticides in this area (28), our result implies that the Shenzhen Bay appears to be less affected by the river inflow from Humen outlet to the Pearl River estuary. This result is also supported by the difference illustrated by clay mineral composition between Core ZJ-6 and ZJ-7 as mentioned previously (Figure 2). Time Trends of DDT and HCH. The main features of sedimentary records of fluxes of ∑DDT and ∑HCH are summarized in Table 3, and their down-core variations are shown in Figure 3. The first increase in DDT at around 19631965 can be identified in all of the cores, especially in Core ZJ-6 (in 1964) which has good time resolution. A similar trend can also be observed with HCH records. Though China began 3674
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to apply organochlorine pesticides in 1950s, this date may indicate the critical time when China began to use them extensively. Two peak-time periods of flux were observed in the profiles. For DDT, the period was identified in sediments dated between the 1970s (ZJ-7, ZJ-10, and ZJ-9) and 1980s (ZJ-6 and ZJ-8: Table 3). For HCH, the first period was found in the 1960s (ZJ-9 and ZJ-D), 1970s (ZJ-7), and 1980s (ZJ-2, 6, 8, D, and ZJ-9), respectively. These may correspond to the extensive organochlorine pesticide application period (1960s1980s) in China (18, 29). However, the lack of a peak of DDT in the 1960s, compared with HCH, may indicate that the major application time of HCH was earlier than DDT in China. The second flux peak was observed in most of the profiles after 1992, with a recent increasing trend or sharp rebounds of sedimentary fluxes of ∑DDT and ∑HCH. As shown in Table 3 and Figure 4, the fluxes of ∑DDT display the recent increases in ZJ-2, 3, 8, D, and ZJ-10 after 1992 and sharp rebounds in around 1993-1996 in ZJ-6 and ZJ-7. In the meantime, the fluxes of ∑HCH show recent increasing trends in ZJ-2, 3 and ZJ-8 and sharp rebounds in ZJ-6 (1994-1997), ZJ-9 (1993), and ZJ-10 (1995), while in ZJ-D they slowly decrease from the stabilization period and eventually decrease to a low level in ZJ-7. On the basis of a comparison of historical surfacesediment concentration data, Zheng et al. (28) also addressed the observation that there is little sign of any declining trend in concentrations of DDT following its ban in China in 1983. These recent increasing trends or sharp rebounds are interesting, though not identically synchronous. The production of DDT and technical HCH was banned in 1983 (18,
FIGURE 3. Down-core variations of sediment fluxes of ∑DDT and ∑HCH. 29), while the production of their organophosphate substitutes increased sharply from 1986 and contributed more than 50% of the total pesticide application in 1989 and thereafter (18). A similar recent increasing trend of ∑DDT concentrations was observed for sediment cores collected from Mississippi River Delta and Galveston Bay in the United States
(16). In North Africa, the γ-HCH and p,p′-DDE sediment fluxes were reported to reach a maximum in subsurface and surface layers deposited after 1990 in sediments from Moroccan and Tunisian wetland lakes (17). Recently, in the third largest freshwater lake in China, Taihu Lake (East China), we also found ∑HCH (10.2 ng/g) and ∑DDT (24.9 ng/g) VOL. 36, NO. 17, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Down-core variations of (DDE + DDD)/DDT and DDE/DDT ratios in cores ZJ-6, 7, 9, and D. concentration maxima in subsurface sediment layers corresponding to a deposition time in 1998 (Peng and Zhang, unpublished data). These sediments are all within the lowmid latitude zone (below 32° N in latitude). The difference between the Chinese cases and the American and North African cases lies in the ca. 1 order of magnitude higher concentration of ∑DDT in Chinese sediments than in the others. DDT is degraded into DDD under anaerobic conditions and into DDE in aerobic environments (30). It was reported that river sediments and waters in developed countries, where DDT has been banned for long time, contain a higher amount of DDE than DDT and DDD, showing a (DDE + DDD)/DDT ratio above 10 (e.g., ref 31). On the other hand, in archived soils in the United Kingdom, DDE was found to be higher in concentration than DDT, while DDD can hardly be detected (32). These results may probably indicate that the process of on-land weathering tends to favor the formation and preservation of DDE, as compared to DDD. As shown in Figure 4, similar shifts or “jumps” of (DDE + DDD)/DDT from lower values to higher values can be observed after the DDT production ban in cores ZJ-6 (1992), ZJ-7 (1992), ZJ-9 (1986), and ZJ-D (1984), with corresponding shifts or increases of DDE/DDT ratio. These ratio variations, mostly concurrent with the increasing or rebounds of ∑DDT and ∑HCH fluxes (Figure 3), suggest that the DDT deposited after the production ban were more likely to be “weathered” DDT derived from soil residues other than continuous applied “fresh” DDT which might contain less metabolites. This is also supported by the decreasing TOC and sediment grain surface area and the increasing sediment grain size after 1990 in Cores ZJ-6 and ZJ-7 (Figure 2). In addition, we also observed a slight increase in the I/K ratio in the period in ZJ-6 (Figure 2). Both in the reported Northern African wetland profiles (17) and in Mississippi Delta profiles (16), wash-out of DDTs from soils was considered to be the possible process which 3676
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caused the increasing concentrations of DDTs in the recent years. The Pearl River Delta has been a paradigm of China’s economic development in the last two decades, characteristic of fast regional urbanization, industrialization, and intensifying population. This process was enhanced after 1992, followed by a latterly recognized economic “overheat” period until 1995. Remote-sensing studies on the history of regional soil runoff in the Pearl River Delta by thematic mapping data (33), and water turbidity in Macao estuary by Sea-viewing wide field-of-view sensor data (34), also indicated a largescale land transformation taking place in the early 1990s. In addition, for the last 50 years, the water quantity of the Pearl River has been at a maximum and caused the most severe flood in the Pearl River Delta in 1994 (35). In a study in the Oder estuary in Germany, elevated PAH concentrations in the flood period were found in suspended particulates (36). An analysis of a total of 63 soils collected from the Pearl River Delta (Zhang et al., unpublished data) found high residues of ∑DDT (averaged 68.5 ng/g) and ∑HCH (averaged 16.2 ng/g) in crop soils. And the average concentrations of ∑DDT and ∑HCH in the raw soils are 6.7 and 8.2 ng/g, respectively. DDE dominates the compositions of DDTs in most of the soil samples with an average DDE/DDT ratio of 4.0 for raw soils and 14.0 for crop soils. On the basis of this, it is assumed that the excessive soil runoff enhanced by the large-scale land transform and regional flooding in the region might have contributed to the transport of organochlorine pesticides from soil to the sedimentary system in the early 1990s in the Pearl River Delta. In the meantime, the development resulted in the increasing or rebounding of DDT and HCH concentrations in the sedimentary cores and the significant changes in sediment TOC, grain size, surface area and clay mineral composition.
Acknowledgments G.Z. wishes to thank the Royal Society, U.K., for supporting his work in PRIS, The University of Reading. Special thanks
to Dr. A. J. Peters in CEH Dorset (U.K.) and Ms. Z. Lin in SKLOG (China) for their help in GC-ECD and GC-MSD analysis. This project is supported by the Ministry of Science and Technology of China (“973” Project No. 1999045706), Guangdong Bureau of Science and Technology (ResearchTeam Grant No. 20003046), and the Hong Kong Polytechnic University (G-V854) and the Research Grant Council of Hong Kong SAR Government (POLYU5075/99E).
Supporting Information Available Figure of the variations of excess 210Pb versus cumulative dry weight in sediment cores Zj-2, 3, 6, 7, 8, 9, and 10. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review November 7, 2001. Revised manuscript received June 10, 2002. Accepted June 10, 2002. ES0102888
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