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Analysis of HCHs and DDTs in a sediment core from the Old Yellow River Estuary, China
Ecotoxicology and Environmental Safety 100 (2014) 171–177
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Analysis of HCHs and DDTs in a sediment core from the Old Yellow River Estuary, China Chunnian Da a,b, Guijian Liu a,n, Zijiao Yuan a a CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Department of Biology & Environment Engineering, Hefei University, Hefei, Anhui 230022, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 August 2013 Received in revised form 21 October 2013 Accepted 25 October 2013 Available online 21 November 2013
The present study analyzed the distribution of HCHs and DDTs in a sediment core from the Old Yellow River Estuary, China. The OCP levels were detected by Soxhlet extraction followed by gas chromatography (GC) using a mass-selective detector. Sediment age was analyzed for 210Pb using an Ortec GWL HPGe gamma spectrometer. The results indicated that the concentrations of ∑DDT in the sediment core were much lower than those of ∑HCH. Compared with the other estuarine and coastal regions in Chinese rivers, HCHs levels in this area were higher or similar, while DDTs levels were lower. The compositional analysis indicated that β-HCH and p, p'-DDD were the predominant species. The temporal trends indicated that levels of HCHs and DDTs were related with their historical usage, emission and soil residues. According to the analysis of the ratio, HCHs in this area was mainly due to the technical historical residue and recent lindane. DDTs was mainly due to historical residue. The biodegradation conditions for DDTs were anaerobic. The dicofol-type DDTs application occurred in this area. Crown Copyright & 2013 Published by Elsevier Inc. All rights reserved.
Keywords: Sedimentary record HCH DDT Sediment core
1. Introduction Organochlorine pesticides (OCPs) have been extensively studied in the last 30 years due to their large-scale production and usage, toxicity, bioaccumulation and persistence in the environment (Yang et al., 2005). Hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs), as typical OCPs, have been extensively studied over the last 30 years because they were widely used in China from the 1950s to 1983 (Iwata et al., 1994; Lee et al., 2001). These compounds can enter marine and freshwater ecosystems through effluents release, atmospheric deposition, runoff and other means (Willett et al., 1998; Zhou et al., 2001). Sediment is one of the primary sinks of OCPs in the environment because OCPs are readily adsorbed onto suspended particulate matter due to their high hydrophobicity and low water solubility (de Boer et al., 2001; Covaci et al., 2005; Yang et al., 2005; Wang et al., 2013). The OCP residues in sediment cores may be able to reconstruct the historical input of contaminants (Hites et al., 1997; VanMetre et al., 1997; Mai et al., 2005). To the best of our knowledge, there are few studies on the contamination of OCPs in the sediment of the Old Yellow River Estuary, and no systematic research has been conducted to reconstruct the
n
Corresponding author. Fax: þ 86 551 6362 1485. E-mail address: [email protected] (G. Liu).
organochlorine pollution history or analyze possible sources of OCPs in this area (Wang et al., 2013). The Old Yellow River Estuary, located in Dongying City, Shandong Province, China, was the means by which the Yellow River fed into the Bohai Sea from 1855 to 1976. This estuary lies in the interchange of the Bohai Sea and Laizhou Bay and features a wetland ecosystem, the second-largest oilfield (Shengli Oilfield) in China, a coastal landscape, several offshore drilling platforms and other unique tourism resources. However, the rapid industrialization and urbanization around the coastal regions has resulted in severe environmental stress. The present work is a small-scale survey of the contamination status and distribution of HCHs and DDTs in the sediment core from a special marginal area in the Old Yellow River Estuary. The purpose of this study is to understand the organochlorine pollution history of HCHs and DDTs and to analyze the pollution source of these compounds in this area. 2. Materials and methods 2.1. Sampling A sediment core was collected from the Old Yellow River Estuary in July 2012. The sampling site (37139′35.7″N and 119115′47.0″E) was located near the Old Yellow River Estuary (Fig. 1). The core was collected using a gravity corer with a diameter of 10 cm and sectioned into 1-cm segments. This core was 41 cm long. Upon collection, sediments were wrapped in precleaned aluminum foil and transported to the laboratory, where they were stored at 20 1C in prewashed glass until further analysis.
0147-6513/$ - see front matter Crown Copyright & 2013 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ecoenv.2013.10.034
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Fig. 1. Map of sampling location of the sediment core. 2.2. Sample preparation and extraction The samples were freeze-dried, hand-sieved (200 mm) and homogenized. Approximately 5 g of the homogenized sample was extracted in a Soxhlet apparatus with 200 mL of dichloromethane (DCM) for 48 h. Activated copper was added for desulfurization. The extractor water bath was maintained at 49 1C. The extracts were rotary evaporated to 2 mL, 10 mL of n-hexane was added for solvent exchange, and then the extracts were further reduced to 1–2 mL by rotary evaporation. The alumina/silica (v/v¼ 1:2) gel column was eluted with 70 mL of dichloromethane/hexane (v/v ¼ 3/7) for the OCPs. The eluate was then concentrated to 1 mL by rotary evaporation.
measured at 662 keV, while 210Pb was determined via gamma emission at 46.5 keV and 226Ra at 352 keV. Dates were calculated using a constant rate of supply (CRS) dating model, yielding an average sedimentation rate of 0.5 cm a 1 in the core.
2.5. Quality control A spike recovery indicator (4,4'-dichlorobiphenyl), duplicate samples and spiked blanks were used to control the quality throughout the entire analytical procedure. The method detection limits are 0.005–0.61 ng g 1. The average recoveries of OCPs were in the range of 75–102 percent, and the relative standard deviation (RSD) was 2–5 percent.
2.3. Instrumental analysis The OCPs were quantified using an Agilent 6890 Series GC connected to an Agilent 5973Network mass-selective detector (MS). Samples (1 mL) were autoinjected in splitless mode, and the separation was performed with a DB-5MS fused silica capillary column (30 m 0.25 mm 0.25 mm). The carrier gas was helium gas (99.9999 percent) at a flow rate of 1.0 mL min 1. The MS was operated in EI þ mode with selected ion monitoring, and the electron energy was 70 eV. The column oven temperature was programmed to increase at a rate of 15 1C min 1 from an initial temperature of 80 1C for 1 min, at a rate of 12 1C min 1 to 200 1C, where it was held for 10 min, and at a rate of 1 1C min 1 to 220 1C. It was held at 220 1C for 5 min before being increased to the final temperature of 290 1C at 15 1C min 1, with a final holding time of 5 min. 2.4. Dating of the sedimentary core The samples were weighed and transferred to centrifuge tubes, which were sealed and allowed to stand for at least three weeks before being studied. The sediment age was analyzed for 210Pb using an Ortec GWL (a well-type series detector) HPGe gamma spectrometer (Wu et al., 2013; Yang et al., 2010). 137Cs was
3. Results and discussion 3.1. Residue levels of HCH and DDT in sediment core The ∑HCH and ∑DDT were in the range of 0.001–14.85 ng g 1 (mean¼3.23 ng g 1) and 0.04–1.07 ng g 1 (mean¼ 0.36 ng g 1) based on dry weight, respectively (see Table 1). The concentrations of ∑DDT in the sediment core samples were much lower than those of ∑HCH. This trend is not consistent with the previous observations of the contamination of OCPs in sediments in China (Ma et al., 2001; Zhou et al., 2001) but is consistent with the contamination of OCPs in sediments in Qiantang River, China (Zhou et al., 2006). The most likely explanation for the higher concentrations of HCHs in the sediment core is that significantly more HCH was used than DDT in the past in this area. As observed in Table 1, the vertical sediment exhibited a high detection rate,
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Table 1 Concentration of OCPs (ng/g dw) in sediment core. Compounds
Range
Mean
Detection rate (%)
α-HCH β-HCH γ-HCH δ-HCH o'p-DDE p'p-DDE o'p-DDT p'p-DDT p'p-DDD o'p-DDD ∑HCH ∑DDT
0.0022–0.28 0.95–14.85 0.051–0.53 0.081–4.71 0.021–0.71 0.0013–0.25 0.23–0.88 0.12–0.36 0.29–1.07 0.13–0.97 0.0014–14.85 0.041–1.07
0.11 5.58 0.27 1.42 0.31 0.05 0.51 0.32 0.78 0.51 3.23 0.36
10.31 87.05 90.24 35.58 76.47 85.61 68.81 50.97 46.47 83.31 51.37 68.51
which reflected a more extensive use of HCH and DDT in the past in this area. These compounds entered the sediment through effluent discharge, atmospheric deposition, runoff and other means. HCH and DDT were mass-produced and used in China in the 1980s. As China is an agricultural country, a large amount of OCPs were used in agricultural areas. Therefore, it is inferred that the greatest contribution of OCP contamination in this region may be runoff from farmland and agricultural activities. The detection rate of HCH (51.37 percent) is lower than that of DDT (68.51 percent). The most likely explanation for the lower detection of HCHs in the sediment core is their higher water solubility; lower lipophilicity and lower particle affinity relative to those of DDTs. DDTs tend to remain in the particulate phase longer than HCHs (Nhan et al., 2001). The detection rate of HCH is lower and the concentration of HCHs is higher, which also indicated some HCHs came from recent input. A lot of recent HCHs input led to high concentrations. The current a little lower detection rate of HCHs in sediment core is due to the degradation of most historical HCHs in the environment. 3.2. Temporal trends of HCHs and DDTs in the sediment core The temporal profiles of the OCP concentrations in the sediment core collected from this study area are shown in Fig. 2 (a: HCH; b: DDT). In the core profile, the recorded OCP concentrations varied significantly by sediment years. The HCH concentration increased until peaking in 1961, after which it decreased until peaking again in 1988. In China, a total of 4460 kt of technical HCHs were used from the 1950s until their banning in 1983 (Li et al., 2001), while lindane (γ-HCH 99.9 percent) has been used in agriculture to control pests since the 1990s (Tao et al., 1996). The increasing concentrations of HCHs in the sediment core up to the early 1960s may be related to the beginning of the use of HCHs in China during this time. The second peak was observed near 1988, which coincided with the second peak of the technical lindane production and usage. The concentrations of HCHs gradually decreased after 1988, which is consistent with the banning of the production and use of this pesticide. The usage of HCHs has influenced the residues in the sediment core; however, the most important factor affecting the HCH concentration in the sediment core may be the runoff from surface soil to the sediment pool, which was directly linked to HCH residues in soil in the area (Wang et al., 2013). In this study, the HCH residue levels in sediment core were close to zero in 1982, which is attributed to the catastrophic flooding of the Old Yellow River during this year, as HCH may have been washed away by the flood because of its high water solubility. As seen in Fig. 2(a), the HCH content in the surface sediment has rapidly decreased in recent years, which may be due to the degradation of much of the historical residue and the reduction of new input sources in recent years. According to
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reports, the time required for 95 percent HCH degradation in the environment is approximately 20 years (Gong, 2007). Therefore, it is reasonable to conclude that the HCH levels in the sediment core of the Old Yellow River have decreased in recent decades. The DDT concentration increased slightly from 1925 to the early 1950s, followed by a sharp increase in the early 1960s and a sharp decline in the surface sediment in a zigzag manner. The first peak deposition of DDTs occurred in 1964, and another peak was observed in 1990. The DDT levels in the sediment core have decreased in recent years. The usage of technical DDTs reached 270 kt before being officially banned in China in 1983. However, DDTs are still allowed to be used in mosquito repellent, dicofol production, malaria control and anti-fouling paint; in fact, from 1988 to 2002, more than 6000 t of DDTs were produced annually in China (Qiu et al., 2005). One study revealed a negative correlation between total DDT concentration in sediment core and technical DDT usage in China from 1960 to 1973 and a positive correlation from 1974 to 1984 (Wang et al., 2013). Therefore, it can be concluded that the usage of DDT affected the residues in the sediment core in this study and that the DDT concentration decreased from 1964 to 1977 despite the large-scale usage of technical DDT in this period. This finding might be due to the degradation of some of the pesticides in the environment between that period and the present. The rapid increase in DDT concentration from 1977 to 1990 corresponds to extensive organochlorine pesticide applications during that period. The usage of DDT was banned for agricultural purposes in 1983, resulting in a rapid decrease in the DDT levels in the surface sediment. In addition, the disturbance of the top layer through bioturbation can maybe influence the residue of DDT. A similar trend was also recorded in a sediment core from the Pearl River Delta, South China (Zhang et al., 2002) and Dianchi Lake, Southwest China (Guo et al., 2013). The continuously increasing input of DDTs into the coastline could be due to successive transportation of DDTs historically utilized on land (Zhang et al., 2002). However, another source of DDT could be its continued use in anti-fouling paint, especially along the coastline, where aquaculture and shipping activities are more intense (Guo et al., 2013). The government has reduced aquaculture and shipping activities in this study area in recent decades; thus, the input of DDTs into this area depended mainly upon the historical residue, soil erosion and storm runoff. According to reports, the time required for 95 percent DDT degradation in the environment is approximately 30 years (Gong, 2007). Therefore, it is reasonable that the DDT concentrations in the sediment core of the Old Yellow River have decreased in recent decades. As shown in Fig. 2, the DDT level peaked a few years later than HCH, which is due to their differing physiochemical properties. DDTs is more persistent than HCH in sediment, and the HCH residues in the sediment core responded more quickly and directly to the change in technical HCH use than that of DDT in the sediment core to the usage of technical DDT (Wang et al., 2013). In this study, it was noticeable that the HCH and DDT residues found in sediments dating before the 1950s could be attributed to the migration of DDTs to deeper sediments. A similar finding was also recorded in a sediment core from the Dianchi Lake in Southwest China (Guo et al., 2013) and Quanzhou Bay, Southeast China (Gong et al., 2007), and one study showed that organic pesticides in surface sediments can migrate downward (Gong, 2007). 3.3. Comparison of sedimentary records of HCHs and DDTs across China Sedimentary records of HCHs and DDTs across China have also been reported and are summarized in Table 2. Investigation of the sedimentary records of HCHs had previously been primarily conducted in marine sediments along China's coastline
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Fig. 2. Time trends of HCHs and DDTs concentrations in the sediment core.
Table 2 Sedimentary record of HCHs and DDTs across China (ng/g dw). Sampling site
HCH/DDT (peak time)
Reference
Taihu Quanzhou Bay Deep Bay Sanya Bay
HCH 3.0–10.4 0–5.12 (1968) 0.16–7.35 (2004) 0.04–1.46 (1994)
Peng et al. (2005) Gong et al. (2007) Qiu et al. (2009) Zhang et al. (2010)
Pearl River Estuary Daya Bay Quanzhou Bay Deep Bay Sanya Bay
DDT 5.5–31.7 (1996) 2.07–30.7 (2001) Max (54.4 (2001) 1.0–54.5 (2000) 6.5 (1975)
Zhang et al. (2002) Wang et al. (2008) Gong et al. (2007) Qiu et al. (2009) Zhang et al. (2010)
(Peng et al., 2005; Gong et al., 2007; Qiu et al., 2009; Zhang et al., 2010). As shown in Table 2, the usage of HCHs in the agricultural activities has been banned since 1983; however, HCH is still detected in more recent sediments from China's coastline, and the HCH level peaked after 1983. For example, the peak time of HCHs was in 2004 in Deep Bay, China, and 1994 in Sanya Bay, China. The timing of the peak HCH concentration in these rivers coincided well with the use of lindane for controlling pests in agriculture beginning in the 1990 s. In this study area, the first peak time (1961) is several years earlier than the peak time (1968) of HCH in Quanzhou Bay, which indicates that the use of HCH began several years earlier in this area than reported in Quanzhou Bay in the past. The second peak time (1988) in this study area is earlier several years than the peak time (1994) of HCH in Sanya Bay, which indicates the use of lindane several years earlier in this area. Compared with the other estuarine and coastal regions of Chinese rivers reported in Table 2, the HCH levels in the sediment core in this study area were close to those of Taihu, China, and higher than those of other rivers. The sedimentary records of DDTs had previously been primarily investigated in marine sediment along China's coastline (Gong et al., 2007; Wang et al., 2008; Qiu et al., 2009; Zhang et al., 2010,
2002) (Table 2). As with the trends in HCH concentrations, despite the Chinese government’s ban on the agricultural usage of DDT beginning in 1983, DDT has also been detected in more recent sediment in the cores from China's coastline, such as cores from the Pearl River Estuary, South China (Zhang et al., 2002); Quanzhou Bay, China (Gong et al., 2007); and Daya Bay, China (Wang et al., 2008). As observed in Table 2, the second peak (1990) of DDT in the Old Yellow River is earlier than that in the Pearl River Estuary (1996), Daya Bay (2001), Quanzhou Bay (2001) or Deep Bay (2000), indicating that the use of DDT in this study area was earlier than that in the areas of the other rivers. The first peak (1964) of DDT in the Old Yellow River is similar to those dates reported in some developed countries, where DDT concentrations usually peaked in the 1960s–1970s (Venkatesan et al., 1999; Fox et al., 2001; Götz et al., 2007). Overall, the peak of the DDT concentration in this study area coincided well with China's 1983 ban of DDT for agricultural usage. Compared with other estuarine and coastal regions in Chinese rivers from Table 2, the DDT levels in the sediment core of this study area were lower. 3.4. Compositions and sources of HCH and DDT 3.4.1. HCH In China, technical HCHs and lindane have been used. Technical HCHs (α-HCH 55–80 percent, β-HCH 5–14 percent, γ-HCH 8–15 percent and δ-HCH 2–16 percent) were applied extensively in agriculture from the 1950s to the early 1980s (Li et al., 1998; Wang et al., 2013), while lindane (γ-HCH 99.9 percent) was used to control pests in agriculture beginning in the 1990s (Tao et al., 1996; Walker et al., 1999; Wang et al., 2013). These isomers have different physicochemical properties. β-HCH is more resistant to hydrolysis and environmental degradation, while α-HCH is more likely to partition into the air and be transported over long distances (Hitch and Day, 1992). Moreover, α-HCH and γ-HCH can be transformed into β-HCH in the environment (Hitch and Day, 1992), Therefore, β-HCH would be the predominant isomer in most sediment if there were no fresh inputs of technical HCH. The ratio of α-HCH/γ-HCH would be less than 3 for lindane and from 3 to 7 for
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Fig. 3. Average compositions of HCH and DDT in the sediment core.
technical HCHs (Lee et al., 2001; Yang et al., 2010). Therefore, the determination of this ratio in environmental samples can identify the sources of HCHs, namely, whether they are from lindane or technical HCHs (Kutz et al., 1991; Yang et al., 2008). The average compositions of the different isomers in the sediment core are shown in Fig. 3(a). β-HCH was still the predominant composition. The isomers of α-HCH, β-HCH, γ-HCH and δ-HCH were observed to contribute approximately 1 percent, 76 percent, 4 percent and 19 percent, respectively. Compared with the primitive components, the percentages of α-HCH decreased. The most likely explanation for the abatement of the α-HCH in the sediment core is that α-HCH transformed into β-HCH in the environment over several years (Hitch and Day, 1992). As observed in Fig. 4 (a), the values of α-HCH/γ-HCH in the sediment core were in the range of 0–6.4. Moreover, the values of α-HCH/γ-HCH in the sediment core appeared to decrease every year until 1999. From 1999 to 2005, this ratio increased until 2005, after which it decreased again and approached zero, especially in recent years, possibly indicating that the sources of HCHs in the sediment core were from the use of technical HCHs before 1988 and between 1999 and 2005. The sources of HCHs were from the use of lindane in recent years and between 1990 and 1998, consisting with the usage history of technical HCHs and lindane.
3.4.2. DDT Although the usage of DDTs in agricultural activities was officially banned in 1983, it has been reported that this practice was not terminated until the end of 2000 (Tao et al., 2007). After the ban of technical DDT in China, the pesticide dicofol, with a high impurity of DDT compounds, has been applied (Liu et al., 2008). The ratios between the parent DDT compounds and its metabolites can provide important information on the source of DDT (Zhang et al., 2011). The ratio of (DDE þDDD)/DDTs has been used to judge whether DDT emission was from recent input or historical residue (Wang et al., 2013). A ratio of (DDE þDDD)/ DDTs 40.5 can be thought to be indicative of aged (microbially degraded) DDTs, whereas a ratio much lower than 0.5 indicates newly inputted DDTs (Zhang et al., 1999). In addition, DDT can be microbially degraded into DDD under anaerobic conditions and into DDE under aerobic conditions; therefore, the ratio of DDE/ DDD could distinguish between aerobic and anaerobic conditions in the sediment environment (Hitch and Day, 1992). A ratio of DDD/DDE greater than 1 indicates that anaerobic degradation is the main pathway of DDT loss, while a ratio of less than 1 implies that the occurrence of aerobic degradation was predominant (Wu et al., 2013). The ratio of o, p'–DDT/p, p'–DDT can also indicate
whether the DDT pollution was from dicofol or technical DDTs. The ratios of o, p'–DDT/p, p'–DDT range from 0.2 to 0.3 in technical DDTs and from 1.3 to 9.3 in dicofol (Qiu et al., 2005; Yang et al., 2010). The average compositions of DDTs in the sediment core can be seen in Fig. 3(b). p, p'–DDD was the predominant composition, which may be because DDT degraded into DDD under anaerobic conditions in the sediment core over several years. The variations of DDT in the sediment core are shown in Fig. 4(b) and (c). The ratio of (DDE þDDD)/DDTs in the sediment core ranged from 0 to 0.87. However, a turning point occurred in the sediment core in approximately 1988. The (DDE þDDD)/DDTs ratio was in the range of 0.51–0.76 before 1988 and reduced to 0–0.49 from 1988 to 1999. The ratio was then in the range of 0–0.76 from 2006 to 2012. This finding may indicate new inputs of parent DDT compounds around this area over the past thirty decades and that DDT deposited after the production ban was more likely to be “aged” DDT. This new source of parent DDTs could be indicated by the ratios of o, p'–DDT/p, p'–DDT, as shown in Fig. 4(c), which were below one before 1988 but have increased sharply since 1988, possibly indicating that the source of DDTs in the sediment was the use of dicofol after 1988 and the use of technical DDT before 1988. Over 60 manufacturers produce dicofol in China, and some studies have found that the input of DDTs to agricultural land from dicofol was approximately 8000 t from 1988 to 2002 in China (Qiu et al., 2004; Qiu et al., 2005). In addition, the ratio of DDE/DDD in most samples was below one, which indicated that the sediment core was dominated by anaerobic biodegradation, which was consistent with findings presented elsewhere (Hitch and Day, 1992; Liu et al., 2006).
4. Conclusions Sedimentary record of HCH and DDT in a sediment core from the Old Yellow River Estuary, China has been studied. The concentrations of ∑HCH and ∑DDT ranged from 0.001 to 14.85 ng g 1 (mean: 3.23 ng g 1 dw) and 0.04 to 1.07 ng g 1 (mean: 0.36 ng g 1 dw), respectively. The variation profiles of the concentrations showed that OCPs were widely used in this study area in the past. The use of OCPs in this study area is earlier than that elsewhere in China. Compared with the other estuarine and coastal regions in Chinese rivers, the HCH levels in this study area were higher or similar, whereas the DDT levels in this study area were lower. β-HCH and p, p'–DDD were the predominant species in the sediment core. The temporal trends of HCHs and DDTs in sediment core were related with historical usage, emission and soil residues. According to the analysis of the ratio, HCHs
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Fig. 4. Down-core variation of α/γ-HCHs, (DDE þDDD)/DDTs, o',p-DDT/p',p-DDT and DDE/DDD in sediment core.
in this area was mainly due to the technical historical residue and recent lindane. DDTs was mainly due to historical residue. DDT biodegradation occurred in anaerobic conditions. Dicofol-type DDT application was present in the study area.
5. Expectation Since the Yellow river estuary is a wildlife refuge, and DDTs can affect both eggshell thinning and reproductive failure in many bird species (Ge et al., 2013). DDTs should be paid more attention in this area; therefore, ecological risk assessment should be further investigated. In addition, previous research showed that fish species could be potentially contaminated by POPs (Ben Ameur et al., 2013). We will determine OCPs in fish and estimate risks to the ecosystems and human health in the next work. In addition, we will also explore whether the OCPs have an impact on surrounding agricultural land.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 41173032 and 41373110); National Science and Technology Support Program (1012BAC10B02), and Special thanks were given to anonymous reviewers for their useful suggestions and comments. References Ben Ameur, W., El Megdiche, Y., Eljarrat, E., Ben Hassine, S., Badreddine, B., Souad, T., Bechir, H., Barcelo, D., Driss, M.R., 2013. Organochlorine and organobromine compounds in a benthic fish (Solea solea) from Bizerte Lagoon (northern Tunisia): implications for human exposure. Ecotox. Environ. Saf. 88, 55–64. Covaci, A., Gheorghe, A., Voorspoels, S., Maervoet, J., Steen Redeker, E., Blust, R., Schepens, P., 2005. Polybrominated diphenyl ethers, polychlorinated biphenyls and organochlorine pesticides in sediment cores from the Western Scheldt river (Belgium), analytical aspects and depth profiles. Environ. Int. 31, 367–375. de Boer, J., van der Zande, T.E., Pieters, H., Ariese, F., Schipper, C.A., van Brummelen, T., Vethaak, A.D., 2001. Organic contaminants and trace metals in flounder liver and sediment from the Amsterdam and Rotterdam harbours and off the Dutch coast. J. Environ. Monit. 3, 386–393. Fox, W.M., Connor, L., Copplestone, D., Johnson, M.S., Leah, R.T., 2001. The organochlorine contamination history of the Mersey estuary, UK, revealed by analysis of sediment cores from salt marshes. Mar. Environ. Res. 51, 213–227.
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Analysis of HCHs and DDTs in a sediment core from the Old Yellow River Estuary, China Chunnian Da a,b, Guijian Liu a,n, Zijiao Yuan a a CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Department of Biology & Environment Engineering, Hefei University, Hefei, Anhui 230022, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 August 2013 Received in revised form 21 October 2013 Accepted 25 October 2013 Available online 21 November 2013
The present study analyzed the distribution of HCHs and DDTs in a sediment core from the Old Yellow River Estuary, China. The OCP levels were detected by Soxhlet extraction followed by gas chromatography (GC) using a mass-selective detector. Sediment age was analyzed for 210Pb using an Ortec GWL HPGe gamma spectrometer. The results indicated that the concentrations of ∑DDT in the sediment core were much lower than those of ∑HCH. Compared with the other estuarine and coastal regions in Chinese rivers, HCHs levels in this area were higher or similar, while DDTs levels were lower. The compositional analysis indicated that β-HCH and p, p'-DDD were the predominant species. The temporal trends indicated that levels of HCHs and DDTs were related with their historical usage, emission and soil residues. According to the analysis of the ratio, HCHs in this area was mainly due to the technical historical residue and recent lindane. DDTs was mainly due to historical residue. The biodegradation conditions for DDTs were anaerobic. The dicofol-type DDTs application occurred in this area. Crown Copyright & 2013 Published by Elsevier Inc. All rights reserved.
Keywords: Sedimentary record HCH DDT Sediment core
1. Introduction Organochlorine pesticides (OCPs) have been extensively studied in the last 30 years due to their large-scale production and usage, toxicity, bioaccumulation and persistence in the environment (Yang et al., 2005). Hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs), as typical OCPs, have been extensively studied over the last 30 years because they were widely used in China from the 1950s to 1983 (Iwata et al., 1994; Lee et al., 2001). These compounds can enter marine and freshwater ecosystems through effluents release, atmospheric deposition, runoff and other means (Willett et al., 1998; Zhou et al., 2001). Sediment is one of the primary sinks of OCPs in the environment because OCPs are readily adsorbed onto suspended particulate matter due to their high hydrophobicity and low water solubility (de Boer et al., 2001; Covaci et al., 2005; Yang et al., 2005; Wang et al., 2013). The OCP residues in sediment cores may be able to reconstruct the historical input of contaminants (Hites et al., 1997; VanMetre et al., 1997; Mai et al., 2005). To the best of our knowledge, there are few studies on the contamination of OCPs in the sediment of the Old Yellow River Estuary, and no systematic research has been conducted to reconstruct the
n
Corresponding author. Fax: þ 86 551 6362 1485. E-mail address: [email protected] (G. Liu).
organochlorine pollution history or analyze possible sources of OCPs in this area (Wang et al., 2013). The Old Yellow River Estuary, located in Dongying City, Shandong Province, China, was the means by which the Yellow River fed into the Bohai Sea from 1855 to 1976. This estuary lies in the interchange of the Bohai Sea and Laizhou Bay and features a wetland ecosystem, the second-largest oilfield (Shengli Oilfield) in China, a coastal landscape, several offshore drilling platforms and other unique tourism resources. However, the rapid industrialization and urbanization around the coastal regions has resulted in severe environmental stress. The present work is a small-scale survey of the contamination status and distribution of HCHs and DDTs in the sediment core from a special marginal area in the Old Yellow River Estuary. The purpose of this study is to understand the organochlorine pollution history of HCHs and DDTs and to analyze the pollution source of these compounds in this area. 2. Materials and methods 2.1. Sampling A sediment core was collected from the Old Yellow River Estuary in July 2012. The sampling site (37139′35.7″N and 119115′47.0″E) was located near the Old Yellow River Estuary (Fig. 1). The core was collected using a gravity corer with a diameter of 10 cm and sectioned into 1-cm segments. This core was 41 cm long. Upon collection, sediments were wrapped in precleaned aluminum foil and transported to the laboratory, where they were stored at 20 1C in prewashed glass until further analysis.
0147-6513/$ - see front matter Crown Copyright & 2013 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ecoenv.2013.10.034
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Fig. 1. Map of sampling location of the sediment core. 2.2. Sample preparation and extraction The samples were freeze-dried, hand-sieved (200 mm) and homogenized. Approximately 5 g of the homogenized sample was extracted in a Soxhlet apparatus with 200 mL of dichloromethane (DCM) for 48 h. Activated copper was added for desulfurization. The extractor water bath was maintained at 49 1C. The extracts were rotary evaporated to 2 mL, 10 mL of n-hexane was added for solvent exchange, and then the extracts were further reduced to 1–2 mL by rotary evaporation. The alumina/silica (v/v¼ 1:2) gel column was eluted with 70 mL of dichloromethane/hexane (v/v ¼ 3/7) for the OCPs. The eluate was then concentrated to 1 mL by rotary evaporation.
measured at 662 keV, while 210Pb was determined via gamma emission at 46.5 keV and 226Ra at 352 keV. Dates were calculated using a constant rate of supply (CRS) dating model, yielding an average sedimentation rate of 0.5 cm a 1 in the core.
2.5. Quality control A spike recovery indicator (4,4'-dichlorobiphenyl), duplicate samples and spiked blanks were used to control the quality throughout the entire analytical procedure. The method detection limits are 0.005–0.61 ng g 1. The average recoveries of OCPs were in the range of 75–102 percent, and the relative standard deviation (RSD) was 2–5 percent.
2.3. Instrumental analysis The OCPs were quantified using an Agilent 6890 Series GC connected to an Agilent 5973Network mass-selective detector (MS). Samples (1 mL) were autoinjected in splitless mode, and the separation was performed with a DB-5MS fused silica capillary column (30 m 0.25 mm 0.25 mm). The carrier gas was helium gas (99.9999 percent) at a flow rate of 1.0 mL min 1. The MS was operated in EI þ mode with selected ion monitoring, and the electron energy was 70 eV. The column oven temperature was programmed to increase at a rate of 15 1C min 1 from an initial temperature of 80 1C for 1 min, at a rate of 12 1C min 1 to 200 1C, where it was held for 10 min, and at a rate of 1 1C min 1 to 220 1C. It was held at 220 1C for 5 min before being increased to the final temperature of 290 1C at 15 1C min 1, with a final holding time of 5 min. 2.4. Dating of the sedimentary core The samples were weighed and transferred to centrifuge tubes, which were sealed and allowed to stand for at least three weeks before being studied. The sediment age was analyzed for 210Pb using an Ortec GWL (a well-type series detector) HPGe gamma spectrometer (Wu et al., 2013; Yang et al., 2010). 137Cs was
3. Results and discussion 3.1. Residue levels of HCH and DDT in sediment core The ∑HCH and ∑DDT were in the range of 0.001–14.85 ng g 1 (mean¼3.23 ng g 1) and 0.04–1.07 ng g 1 (mean¼ 0.36 ng g 1) based on dry weight, respectively (see Table 1). The concentrations of ∑DDT in the sediment core samples were much lower than those of ∑HCH. This trend is not consistent with the previous observations of the contamination of OCPs in sediments in China (Ma et al., 2001; Zhou et al., 2001) but is consistent with the contamination of OCPs in sediments in Qiantang River, China (Zhou et al., 2006). The most likely explanation for the higher concentrations of HCHs in the sediment core is that significantly more HCH was used than DDT in the past in this area. As observed in Table 1, the vertical sediment exhibited a high detection rate,
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Table 1 Concentration of OCPs (ng/g dw) in sediment core. Compounds
Range
Mean
Detection rate (%)
α-HCH β-HCH γ-HCH δ-HCH o'p-DDE p'p-DDE o'p-DDT p'p-DDT p'p-DDD o'p-DDD ∑HCH ∑DDT
0.0022–0.28 0.95–14.85 0.051–0.53 0.081–4.71 0.021–0.71 0.0013–0.25 0.23–0.88 0.12–0.36 0.29–1.07 0.13–0.97 0.0014–14.85 0.041–1.07
0.11 5.58 0.27 1.42 0.31 0.05 0.51 0.32 0.78 0.51 3.23 0.36
10.31 87.05 90.24 35.58 76.47 85.61 68.81 50.97 46.47 83.31 51.37 68.51
which reflected a more extensive use of HCH and DDT in the past in this area. These compounds entered the sediment through effluent discharge, atmospheric deposition, runoff and other means. HCH and DDT were mass-produced and used in China in the 1980s. As China is an agricultural country, a large amount of OCPs were used in agricultural areas. Therefore, it is inferred that the greatest contribution of OCP contamination in this region may be runoff from farmland and agricultural activities. The detection rate of HCH (51.37 percent) is lower than that of DDT (68.51 percent). The most likely explanation for the lower detection of HCHs in the sediment core is their higher water solubility; lower lipophilicity and lower particle affinity relative to those of DDTs. DDTs tend to remain in the particulate phase longer than HCHs (Nhan et al., 2001). The detection rate of HCH is lower and the concentration of HCHs is higher, which also indicated some HCHs came from recent input. A lot of recent HCHs input led to high concentrations. The current a little lower detection rate of HCHs in sediment core is due to the degradation of most historical HCHs in the environment. 3.2. Temporal trends of HCHs and DDTs in the sediment core The temporal profiles of the OCP concentrations in the sediment core collected from this study area are shown in Fig. 2 (a: HCH; b: DDT). In the core profile, the recorded OCP concentrations varied significantly by sediment years. The HCH concentration increased until peaking in 1961, after which it decreased until peaking again in 1988. In China, a total of 4460 kt of technical HCHs were used from the 1950s until their banning in 1983 (Li et al., 2001), while lindane (γ-HCH 99.9 percent) has been used in agriculture to control pests since the 1990s (Tao et al., 1996). The increasing concentrations of HCHs in the sediment core up to the early 1960s may be related to the beginning of the use of HCHs in China during this time. The second peak was observed near 1988, which coincided with the second peak of the technical lindane production and usage. The concentrations of HCHs gradually decreased after 1988, which is consistent with the banning of the production and use of this pesticide. The usage of HCHs has influenced the residues in the sediment core; however, the most important factor affecting the HCH concentration in the sediment core may be the runoff from surface soil to the sediment pool, which was directly linked to HCH residues in soil in the area (Wang et al., 2013). In this study, the HCH residue levels in sediment core were close to zero in 1982, which is attributed to the catastrophic flooding of the Old Yellow River during this year, as HCH may have been washed away by the flood because of its high water solubility. As seen in Fig. 2(a), the HCH content in the surface sediment has rapidly decreased in recent years, which may be due to the degradation of much of the historical residue and the reduction of new input sources in recent years. According to
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reports, the time required for 95 percent HCH degradation in the environment is approximately 20 years (Gong, 2007). Therefore, it is reasonable to conclude that the HCH levels in the sediment core of the Old Yellow River have decreased in recent decades. The DDT concentration increased slightly from 1925 to the early 1950s, followed by a sharp increase in the early 1960s and a sharp decline in the surface sediment in a zigzag manner. The first peak deposition of DDTs occurred in 1964, and another peak was observed in 1990. The DDT levels in the sediment core have decreased in recent years. The usage of technical DDTs reached 270 kt before being officially banned in China in 1983. However, DDTs are still allowed to be used in mosquito repellent, dicofol production, malaria control and anti-fouling paint; in fact, from 1988 to 2002, more than 6000 t of DDTs were produced annually in China (Qiu et al., 2005). One study revealed a negative correlation between total DDT concentration in sediment core and technical DDT usage in China from 1960 to 1973 and a positive correlation from 1974 to 1984 (Wang et al., 2013). Therefore, it can be concluded that the usage of DDT affected the residues in the sediment core in this study and that the DDT concentration decreased from 1964 to 1977 despite the large-scale usage of technical DDT in this period. This finding might be due to the degradation of some of the pesticides in the environment between that period and the present. The rapid increase in DDT concentration from 1977 to 1990 corresponds to extensive organochlorine pesticide applications during that period. The usage of DDT was banned for agricultural purposes in 1983, resulting in a rapid decrease in the DDT levels in the surface sediment. In addition, the disturbance of the top layer through bioturbation can maybe influence the residue of DDT. A similar trend was also recorded in a sediment core from the Pearl River Delta, South China (Zhang et al., 2002) and Dianchi Lake, Southwest China (Guo et al., 2013). The continuously increasing input of DDTs into the coastline could be due to successive transportation of DDTs historically utilized on land (Zhang et al., 2002). However, another source of DDT could be its continued use in anti-fouling paint, especially along the coastline, where aquaculture and shipping activities are more intense (Guo et al., 2013). The government has reduced aquaculture and shipping activities in this study area in recent decades; thus, the input of DDTs into this area depended mainly upon the historical residue, soil erosion and storm runoff. According to reports, the time required for 95 percent DDT degradation in the environment is approximately 30 years (Gong, 2007). Therefore, it is reasonable that the DDT concentrations in the sediment core of the Old Yellow River have decreased in recent decades. As shown in Fig. 2, the DDT level peaked a few years later than HCH, which is due to their differing physiochemical properties. DDTs is more persistent than HCH in sediment, and the HCH residues in the sediment core responded more quickly and directly to the change in technical HCH use than that of DDT in the sediment core to the usage of technical DDT (Wang et al., 2013). In this study, it was noticeable that the HCH and DDT residues found in sediments dating before the 1950s could be attributed to the migration of DDTs to deeper sediments. A similar finding was also recorded in a sediment core from the Dianchi Lake in Southwest China (Guo et al., 2013) and Quanzhou Bay, Southeast China (Gong et al., 2007), and one study showed that organic pesticides in surface sediments can migrate downward (Gong, 2007). 3.3. Comparison of sedimentary records of HCHs and DDTs across China Sedimentary records of HCHs and DDTs across China have also been reported and are summarized in Table 2. Investigation of the sedimentary records of HCHs had previously been primarily conducted in marine sediments along China's coastline
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Fig. 2. Time trends of HCHs and DDTs concentrations in the sediment core.
Table 2 Sedimentary record of HCHs and DDTs across China (ng/g dw). Sampling site
HCH/DDT (peak time)
Reference
Taihu Quanzhou Bay Deep Bay Sanya Bay
HCH 3.0–10.4 0–5.12 (1968) 0.16–7.35 (2004) 0.04–1.46 (1994)
Peng et al. (2005) Gong et al. (2007) Qiu et al. (2009) Zhang et al. (2010)
Pearl River Estuary Daya Bay Quanzhou Bay Deep Bay Sanya Bay
DDT 5.5–31.7 (1996) 2.07–30.7 (2001) Max (54.4 (2001) 1.0–54.5 (2000) 6.5 (1975)
Zhang et al. (2002) Wang et al. (2008) Gong et al. (2007) Qiu et al. (2009) Zhang et al. (2010)
(Peng et al., 2005; Gong et al., 2007; Qiu et al., 2009; Zhang et al., 2010). As shown in Table 2, the usage of HCHs in the agricultural activities has been banned since 1983; however, HCH is still detected in more recent sediments from China's coastline, and the HCH level peaked after 1983. For example, the peak time of HCHs was in 2004 in Deep Bay, China, and 1994 in Sanya Bay, China. The timing of the peak HCH concentration in these rivers coincided well with the use of lindane for controlling pests in agriculture beginning in the 1990 s. In this study area, the first peak time (1961) is several years earlier than the peak time (1968) of HCH in Quanzhou Bay, which indicates that the use of HCH began several years earlier in this area than reported in Quanzhou Bay in the past. The second peak time (1988) in this study area is earlier several years than the peak time (1994) of HCH in Sanya Bay, which indicates the use of lindane several years earlier in this area. Compared with the other estuarine and coastal regions of Chinese rivers reported in Table 2, the HCH levels in the sediment core in this study area were close to those of Taihu, China, and higher than those of other rivers. The sedimentary records of DDTs had previously been primarily investigated in marine sediment along China's coastline (Gong et al., 2007; Wang et al., 2008; Qiu et al., 2009; Zhang et al., 2010,
2002) (Table 2). As with the trends in HCH concentrations, despite the Chinese government’s ban on the agricultural usage of DDT beginning in 1983, DDT has also been detected in more recent sediment in the cores from China's coastline, such as cores from the Pearl River Estuary, South China (Zhang et al., 2002); Quanzhou Bay, China (Gong et al., 2007); and Daya Bay, China (Wang et al., 2008). As observed in Table 2, the second peak (1990) of DDT in the Old Yellow River is earlier than that in the Pearl River Estuary (1996), Daya Bay (2001), Quanzhou Bay (2001) or Deep Bay (2000), indicating that the use of DDT in this study area was earlier than that in the areas of the other rivers. The first peak (1964) of DDT in the Old Yellow River is similar to those dates reported in some developed countries, where DDT concentrations usually peaked in the 1960s–1970s (Venkatesan et al., 1999; Fox et al., 2001; Götz et al., 2007). Overall, the peak of the DDT concentration in this study area coincided well with China's 1983 ban of DDT for agricultural usage. Compared with other estuarine and coastal regions in Chinese rivers from Table 2, the DDT levels in the sediment core of this study area were lower. 3.4. Compositions and sources of HCH and DDT 3.4.1. HCH In China, technical HCHs and lindane have been used. Technical HCHs (α-HCH 55–80 percent, β-HCH 5–14 percent, γ-HCH 8–15 percent and δ-HCH 2–16 percent) were applied extensively in agriculture from the 1950s to the early 1980s (Li et al., 1998; Wang et al., 2013), while lindane (γ-HCH 99.9 percent) was used to control pests in agriculture beginning in the 1990s (Tao et al., 1996; Walker et al., 1999; Wang et al., 2013). These isomers have different physicochemical properties. β-HCH is more resistant to hydrolysis and environmental degradation, while α-HCH is more likely to partition into the air and be transported over long distances (Hitch and Day, 1992). Moreover, α-HCH and γ-HCH can be transformed into β-HCH in the environment (Hitch and Day, 1992), Therefore, β-HCH would be the predominant isomer in most sediment if there were no fresh inputs of technical HCH. The ratio of α-HCH/γ-HCH would be less than 3 for lindane and from 3 to 7 for
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Fig. 3. Average compositions of HCH and DDT in the sediment core.
technical HCHs (Lee et al., 2001; Yang et al., 2010). Therefore, the determination of this ratio in environmental samples can identify the sources of HCHs, namely, whether they are from lindane or technical HCHs (Kutz et al., 1991; Yang et al., 2008). The average compositions of the different isomers in the sediment core are shown in Fig. 3(a). β-HCH was still the predominant composition. The isomers of α-HCH, β-HCH, γ-HCH and δ-HCH were observed to contribute approximately 1 percent, 76 percent, 4 percent and 19 percent, respectively. Compared with the primitive components, the percentages of α-HCH decreased. The most likely explanation for the abatement of the α-HCH in the sediment core is that α-HCH transformed into β-HCH in the environment over several years (Hitch and Day, 1992). As observed in Fig. 4 (a), the values of α-HCH/γ-HCH in the sediment core were in the range of 0–6.4. Moreover, the values of α-HCH/γ-HCH in the sediment core appeared to decrease every year until 1999. From 1999 to 2005, this ratio increased until 2005, after which it decreased again and approached zero, especially in recent years, possibly indicating that the sources of HCHs in the sediment core were from the use of technical HCHs before 1988 and between 1999 and 2005. The sources of HCHs were from the use of lindane in recent years and between 1990 and 1998, consisting with the usage history of technical HCHs and lindane.
3.4.2. DDT Although the usage of DDTs in agricultural activities was officially banned in 1983, it has been reported that this practice was not terminated until the end of 2000 (Tao et al., 2007). After the ban of technical DDT in China, the pesticide dicofol, with a high impurity of DDT compounds, has been applied (Liu et al., 2008). The ratios between the parent DDT compounds and its metabolites can provide important information on the source of DDT (Zhang et al., 2011). The ratio of (DDE þDDD)/DDTs has been used to judge whether DDT emission was from recent input or historical residue (Wang et al., 2013). A ratio of (DDE þDDD)/ DDTs 40.5 can be thought to be indicative of aged (microbially degraded) DDTs, whereas a ratio much lower than 0.5 indicates newly inputted DDTs (Zhang et al., 1999). In addition, DDT can be microbially degraded into DDD under anaerobic conditions and into DDE under aerobic conditions; therefore, the ratio of DDE/ DDD could distinguish between aerobic and anaerobic conditions in the sediment environment (Hitch and Day, 1992). A ratio of DDD/DDE greater than 1 indicates that anaerobic degradation is the main pathway of DDT loss, while a ratio of less than 1 implies that the occurrence of aerobic degradation was predominant (Wu et al., 2013). The ratio of o, p'–DDT/p, p'–DDT can also indicate
whether the DDT pollution was from dicofol or technical DDTs. The ratios of o, p'–DDT/p, p'–DDT range from 0.2 to 0.3 in technical DDTs and from 1.3 to 9.3 in dicofol (Qiu et al., 2005; Yang et al., 2010). The average compositions of DDTs in the sediment core can be seen in Fig. 3(b). p, p'–DDD was the predominant composition, which may be because DDT degraded into DDD under anaerobic conditions in the sediment core over several years. The variations of DDT in the sediment core are shown in Fig. 4(b) and (c). The ratio of (DDE þDDD)/DDTs in the sediment core ranged from 0 to 0.87. However, a turning point occurred in the sediment core in approximately 1988. The (DDE þDDD)/DDTs ratio was in the range of 0.51–0.76 before 1988 and reduced to 0–0.49 from 1988 to 1999. The ratio was then in the range of 0–0.76 from 2006 to 2012. This finding may indicate new inputs of parent DDT compounds around this area over the past thirty decades and that DDT deposited after the production ban was more likely to be “aged” DDT. This new source of parent DDTs could be indicated by the ratios of o, p'–DDT/p, p'–DDT, as shown in Fig. 4(c), which were below one before 1988 but have increased sharply since 1988, possibly indicating that the source of DDTs in the sediment was the use of dicofol after 1988 and the use of technical DDT before 1988. Over 60 manufacturers produce dicofol in China, and some studies have found that the input of DDTs to agricultural land from dicofol was approximately 8000 t from 1988 to 2002 in China (Qiu et al., 2004; Qiu et al., 2005). In addition, the ratio of DDE/DDD in most samples was below one, which indicated that the sediment core was dominated by anaerobic biodegradation, which was consistent with findings presented elsewhere (Hitch and Day, 1992; Liu et al., 2006).
4. Conclusions Sedimentary record of HCH and DDT in a sediment core from the Old Yellow River Estuary, China has been studied. The concentrations of ∑HCH and ∑DDT ranged from 0.001 to 14.85 ng g 1 (mean: 3.23 ng g 1 dw) and 0.04 to 1.07 ng g 1 (mean: 0.36 ng g 1 dw), respectively. The variation profiles of the concentrations showed that OCPs were widely used in this study area in the past. The use of OCPs in this study area is earlier than that elsewhere in China. Compared with the other estuarine and coastal regions in Chinese rivers, the HCH levels in this study area were higher or similar, whereas the DDT levels in this study area were lower. β-HCH and p, p'–DDD were the predominant species in the sediment core. The temporal trends of HCHs and DDTs in sediment core were related with historical usage, emission and soil residues. According to the analysis of the ratio, HCHs
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Fig. 4. Down-core variation of α/γ-HCHs, (DDE þDDD)/DDTs, o',p-DDT/p',p-DDT and DDE/DDD in sediment core.
in this area was mainly due to the technical historical residue and recent lindane. DDTs was mainly due to historical residue. DDT biodegradation occurred in anaerobic conditions. Dicofol-type DDT application was present in the study area.
5. Expectation Since the Yellow river estuary is a wildlife refuge, and DDTs can affect both eggshell thinning and reproductive failure in many bird species (Ge et al., 2013). DDTs should be paid more attention in this area; therefore, ecological risk assessment should be further investigated. In addition, previous research showed that fish species could be potentially contaminated by POPs (Ben Ameur et al., 2013). We will determine OCPs in fish and estimate risks to the ecosystems and human health in the next work. In addition, we will also explore whether the OCPs have an impact on surrounding agricultural land.
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