Chemosphere xxx (2014) xxx–xxx

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Chemosphere

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Assessment of the spatial and temporal distribution of legacy persistentorganic pollutants and recommendations for sample collection from thesurficial sediments of estuaries and seas in China

http://dx.doi.org/10.1016/j.chemosphere.2014.04.0040045-6535/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 13950123497; fax: +86 592 2181916.E-mail address: lhpeng@xmu.edu.cn (L. Peng).

Please cite this article in press as: Peng, L., et al. Assessment of the spatial and temporal distribution of legacy persistent organic pollutants andmendations for sample collection from the surficial sediments of estuaries and seas in China. Chemosphere (2014), http://dx.doi.org/1j.chemosphere.2014.04.004

Lihong Peng a,⇑, Xuhong Dai a, Ang Yu a,b

a College of the Environment & Ecology, Xiamen University, Xiang’an South Road, Xiang’an District, Xiamen 361102, Chinab State Key Laboratory of Marine Environmental Science, Xiamen University, Xiang’an South Road, Xiang’an District, Xiamen 361102, China

a r t i c l e i n f o

Article history:Received 28 January 2014Received in revised form 31 March 2014Accepted 4 April 2014Available online xxxx

Handling Editor: J. de Boer

Keywords:Legacy POPsSurface sedimentsSpatial distributionTemporal distributionSample collectionLong-term monitoring

a b s t r a c t

With the rapid economic development in China, environmental pollution has become a major concern,particularly pollution by persistent organic pollutants (POPs). Thus, these pollutants must be monitoredover the long term. In this study, we analyze the distribution levels and sources of POPs in the surficialsediments of Chinese estuaries and seas. Results showed that POPs in sediments significantly distributespatially and temporally. Furthermore, POPs not only concentrate in densely populated cities, bays, andindustrial areas, but also follow the natural distribution of and temporal changes in local industrial struc-tures. Hence, we recommend sampling sites and frequencies to monitor POPs in China over the long termand to defer their analysis.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

China has developed rapidly in terms of economy, industry, andagriculture in the past 20 years. Consequently, the total amount ofchemical pesticides and fertilizers used has significantly increased.These substances are considered persistent organic pollutants(POPs). POPs are chemicals that are diverse, synthetic, toxic, andhighly resistant to degradation in the environment. Thesepollutants can also be transported over long ranges. POPs tend toaccumulate in food chains (Jones and de Voogt, 1999), thusaffecting human health adversely (Harmens et al., 2013). At theStockholm Convention (UNEP, 2001), the United Nations Environ-ment Program (UNEP) highlighted 12 POPs (dirty dozen), namely,polychlorinated dibenzo-para-dioxins (PCDDs), polychlorinateddibenzofurans (PCDFs), hexachlorobenzene, polychlorinatedbiphenyls (PCBs), aldrin, chlordane, dichlorodiphenyltrichloroeth-ane (DDT), dieldrin, endrins, heptachlor, mirex, and toxaphene.Additional substances and substance groups, are covered by theprotocol of the United Nations Economic Commission for Europe

(UNECE) on POPs, including PCDDs, PCDFs, and polycyclic aromatichydrocarbons (PAHs) (UNECE, 1998).

The distribution of POPs has been widely investigated world-wide (Iwata et al., 1993; Kannan et al., 1997; Tanabe et al., 2000;Tanabe, 2006; Minh et al., 2008; Augusto et al., 2013; Zhanget al., 2013; Li et al., 2014). In water systems, POPs are hydrophobicand bind easily to particles before subsiding to the bottom throughsedimentation. Therefore, POPs in sediments can be tracked overlong term (Rawn et al., 2001). Global studies indicated that thesources of POP emission (such as DDTs and hexachlorocyclohex-anes (HCHs)) have shifted from industrialized to developing coun-tries (Loganathan and Kannan, 1994; Fu et al., 2003). In China, POPshave mainly been monitored and analyzed in economically devel-oped cities and regions in the central east, particularly PAHs, PCBs,and organic chlorinated pesticides (OCPs, such as HCHs and DDTs)(Zhou et al., 2001; Yuan et al., 2001; Jiao et al., 2012). Most of thesestudies are recent; thus, POPs must be tracked long-term to deter-mine their sources and years of entry into environment. Long-termmonitoring also verifies the novelty of pollutants, as well as theiraccumulation or degradation in the environment. Therefore, targetsamples should be collected from appropriate sites at a certain fre-quency. By assessing the spatial and temporal distribution of POPs,we can determine their major types, sources, and accumulation

recom-0.1016/

Fig. 2. Location of Xiamen Bay.

2 L. Peng et al. / Chemosphere xxx (2014) xxx–xxx

levels in the study areas. This information can then guide long-term monitoring.

This study analyzes the spatial and temporal distribution oflegacy POPs in the surficial sediments of Chinese estuaries andseas. The results indicate that POPs not only concentrate in denselypopulated cities, bays, and industrial areas in China, but also followthe natural distribution of and temporal changes in the localindustrial structure. Prompted by these findings, we propose arational process of sample collection to facilitate the long-termmonitoring and deferred analysis of POPs in sediments of Chinesewater bodies. These procedures may guide the formulation ofpolicies and precautionary measures in the country significantly.

2. Methodology

2.1. Targeted areas

To monitor POPs in China over the long term and defer theiranalysis, we compiled data regarding the concentrations of POPsin well-known estuaries and seas along various parts of theChinese coastline (Fig. 1 and Table S1). The study areas reflect mostindustrial structures; therefore, we can obtain statistically signifi-cant data regarding the spatial distribution of POPs in China.

To determine the temporal distribution of POPs, we analyzedchanges in POP concentrations in Xiamen Bay over the last20 years. Xiamen Bay is located in southeast China (Fig. 2) withinthe Xiamen Economic Special Zone. For the past 28 years, the rapidindustrial, commercial, and urban development in this zone hasgreatly stressed the environment of Xiamen Bay and its adjacentareas. These regions represent a microcosm of the progression ofChina in the last two decades. Thus, Xiamen Bay is a representativesample for the temporal distribution of POPs in China.

Fig. 1. Monitoring points for this study.

Please cite this article in press as: Peng, L., et al. Assessment of the spatial andmendations for sample collection from the surficial sediments of estuarij.chemosphere.2014.04.004

3. Results and discussion

3.1. Comprehensive assessment of the spatial distribution of legacyPOPs in Chinese sediments

3.1.1. PAHs in sedimentsPAHs have many human sources, such as the incomplete com-

bustion of fossil fuels (oil, coal, and natural gas), the combustionof plants and other hydrocarbon compounds, and oil spills (Notaret al., 2001). The main sources of PAHs in offshore environmentsinclude urban sewage, industrial wastes, and surface runoff (Tsaiet al., 2002), thus implying that regions with highly developedindustries have high PAH concentrations (Van Metre et al., 2000).

Several studies have previously determined the distribution ofPAHs in sediments of coastal and estuarine regions in China. Asshown in Table 1 and Fig. 3, estuarine regions have higher PAHconcentrations than other regions, because rivers can deliver PAHsfrom riparian areas to estuarine regions. In the Pearl River Estuary,the high average level of PAH (1863.0 ng g�1) is primarily ascribedto the high emission of PAHs by the Pearl River Delta (PRD). Thisarea is among the most agriculturally developed and economicallythriving regions in China. With respect to Chinese bays, JinzhouBay records the highest PAH level (1000.0 ng g�1) as a result ofthe high level of discharge from the Liao River, Daling River, andXiaoling River.

3.1.2. PCBs in sedimentsPCBs are composed of 209 individual chemical compounds,

which are produced through various industrial mixtures by intro-ducing different concentrations of elementary chlorine into biphe-nyl. The primary source of PCBs is industrial production, includingdischarged industrial wastewater and slag. Hence, PCB pollutionmay be prevalent in industrially active areas. As with the spatialdistribution of PAHs in sediments, overall PCB distribution followsthis pattern: estuaries > bays > seas (Table 2 and Fig. 4). The PCB

temporal distribution of legacy persistent organic pollutants and recom-es and seas in China. Chemosphere (2014), http://dx.doi.org/10.1016/

Table 1Concentrations of PAHs in sediments in different water bodies.

Location Range of concentration (ng g�1) Average concentration (ng g�1) Year of sampling and source of data

BayJiaozhou Bay 16.0–2164.0 559.2 2004 (Wang et al., 2006)Meizhou Bay 196.7–299.7 256.0 2000 (Lin et al., 2003)Quanzhou Bay 182.8–721.7 353.8 2009 (Zhuang et al., 2011)Xiamen Bay 330.2–716.2 436.0 2007 (Li et al., 2010)Dalian Bay 73.0–1900.0 510.0 2010 (Liu et al., 2013)

EstuaryYangtze Estuary 610.0–1390.0 947.0 1996 (Tanabe and Subramanian, 2011)Minjiang Estuary 316.0–1260.0 669.2 1996 (Yuan et al., 2001)Pearl River Estuary 323.0–2372.0 1587.4 1997 (Hewitt and Thrush, 2007)

Fig. 3. Spatial distribution of PAHs in sediments in Chinese water bodies.

L. Peng et al. / Chemosphere xxx (2014) xxx–xxx 3

concentrations of the Yangtze Estuary (maximum of 148.2 ng g�1)and Pearl River Estuary (maximum of 338.5 ng g�1) are signifi-cantly higher than those of other bodies of water, such as that of

Table 2Concentrations of PCBs in sediments in different water bodies.

Location Range of concentration (ng g�1)

BayDalian Bay 0.2–1.1Jinzhou Bay 0.2–3.7Xiamen Bay 2.3–9.3Bohai Bay 0.2–5.1

EstuaryMinjiang Estuary 15.1–57.9Pearl River Estuary 10.2–338.5Yangtze Estuary 1.9–148.2

SeasMinjiang Estuary –Mazu Seas ND-0.8Xiamen–Jinmen Seas ND-0.1

Please cite this article in press as: Peng, L., et al. Assessment of the spatial andmendations for sample collection from the surficial sediments of estuarij.chemosphere.2014.04.004

Xiamen Bay (maximum of 9.3 ng g�1). This finding was attributedto the fact that Yangtze Estuary and Pearl River Estuary are locatedin the economic zones and areas of Yangtze River Delta and PRD,respectively, thereby confirming that PCB pollution is serious inindustrially active areas.

3.1.3. OCPs in sedimentsOCPs mainly originate from improperly treated wastewater

generated by pesticide plants. These OCPs flow into rivers or lakesand are transported to water bodies through precipitation. MostOCPs adhere to the surface of suspended particles and eventually,to sediments at the bottom of the water body. Two well-knownand environmentally relevant OCPs are DDTs and HCHs, whichare widely used for their insecticidal effects. To monitor thecontent of DDTs and HCHs in food, China adopted the GlobalEnvironment Monitoring System in 1981, demonstrating the com-mitment of Chinese government to addressing OCP pollution. Themonitoring results indicated that OCP concentrations were highlydependent on the locations of their sources and their distributioncharacteristics (Tables 3 and 4 and Fig. 5). As illustrated in Fig. 5,the average DDT concentration ranges from 2.2 ng g�1 to10.5 ng g�1 in China. The highest level was detected at Pearl RiverEstuary, which also displayed high concentrations of other POPs,such as PAHs and PCBs. Surprisingly, the concentration of XiamenBay reaches 9.3 ng g�1, which is higher than that of other waterbodies, including the Minjiang Estuary (6.7 ng g�1) and the YangziEstuary (8.9 ng g�1). This observation may indicates new sources ofDDT pollution in Xiamen City (Zhang and Lin, 2004). Similar to DDTconcentration, the degree of HCH concentration in water bodiesfollows the order of estuaries > bays > seas. However, MinjiangEstuary (8.6 ng g�1) recorded the highest HCH level instead of thePearl River Estuary (3.8 ng g�1). Moreover, HCH concentration issignificantly lower than DDT concentration. This finding could beattributed to the degradation of HCHs and the widespread use ofDDT over HCH (Jones and de Voogt, 1999; Zhang and Lin, 2004).

Average concentration (ng g�1) Year of sampling and source of data

0.5 1998 (Nyati et al., 2006)0.8 1998 (Nyati et al., 2006)5.0 2007 (Li et al., 2011)1.0 1998 (Nyati et al., 2006)

34.5 1999 (Schulze et al., 2007)58.1 1997 (Kang et al., 2000)24.9 2000 (Gao et al., 2013)

0.1 1995 (Chen et al., 2000)0.1 1995 (Chen et al., 1996)

temporal distribution of legacy persistent organic pollutants and recom-es and seas in China. Chemosphere (2014), http://dx.doi.org/10.1016/

Fig. 4. Spatial distribution of PCBs in sediments in Chinese water bodies.

Table 3Concentrations of DDTs in sediments in different water bodies.

Location Range of concentration (ng g�1) Average concentration (ng g�1) Year of sampling and source of data

BayDaya Bay 0.1–20.3 2.7 1999 (Qiu et al., 2002)Dalian Bay 0.7–5.7 2.2 1999 (Liu et al., 2001)Bohai Bay 0.2–22.0 4.7 2006 (Wang et al., 2008)Xiamen Bay 1.9–16.6 9.3 2003 (Zhang and Lin, 2004)

EstuaryMinjiang Estuary 1.6–13.1 6.7 1999 (Zhang et al., 2004)Pearl River Estuary 1.9–39.1 10.5 1997 (Mai et al., 2000)Yangtze Estuary 5.0–14.9 8.9 2003 (Liu et al., 2005)

SeasMinjiang Estuary –Mazu Seas 1.1–14.3 4.2 1995 (Chen et al., 2000)Xiamen–Jinmen Seas 3.0–9.6 4.7 1995 (Chen et al., 1996)

Table 4Concentrations of HCHs in sediments in different water bodies.

Location Range of concentration (ng g�1) Average concentration (ng g�1) Year of sampling and source of data

BayDaya Bay 1.1–8.0 3.2 1999 (Liu et al., 2001)Bohai Bay 2.1–4.2 3.2 2008 (Liu et al., 2012)Quanzhou Bay 0.1–3.6 0.8 2004 (Jizhou et al., 2010)Xiamen Bay 0.2–0.6 0.3 2003 (Zhang and Lin, 2004)

EstuaryMinjiang Estuary 3.0–16.2 8.6 1999 (Zhang et al., 2004)Pearl River Estuary 0.5–26.3 3.9 1997 (Mai et al., 2000)Yangtze Estuary 1.2–14.9 5.9 2003 (Liu et al., 2005)

SeasMinjiang Estuary –Mazu Seas 0.1–0.6 0.2 1995 (Chen et al., 2000)Xiamen–Jinmen Seas 0.1–0.5 0.1 1995 (Chen et al., 1996)

Fig. 5. Spatial distribution of OCPs in sediments in Chinese water bodies.

4 L. Peng et al. / Chemosphere xxx (2014) xxx–xxx

Please cite this article in press as: Peng, L., et al. Assessment of the spatial and temporal distribution of legacy persistent organic pollutants and recom-mendations for sample collection from the surficial sediments of estuaries and seas in China. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2014.04.004

L. Peng et al. / Chemosphere xxx (2014) xxx–xxx 5

3.1.4. Comparison with results from other areas in the worldThe PAH level in China is lower than that of a number of aquatic

systems in other regions worldwide (Table 5). However, its PCBand OCP levels are slightly higher. These results indicate that POPspollution is closely related to the economic and industrial develop-ment of nation.

3.2. Comprehensive assessment of the temporal distribution of POPs insediments

3.2.1. Temporal distribution of PAHs in Xiamen Bay sedimentsAccording to previous research (Table 6 and Fig. 6), PAH concen-

trations in sediments of Xiamen Bay decreased significantly after1993 and have since remained constant. This observation is consis-tent with the transformation of Xiamen’s industrial structure. Inthe early 1990s, Xiamen developed its seaside tourism industryaggressively and phased out heavy industries, thereby significantlyreducing pollution sources. Therefore, the PAH concentrations inXiamen Bay sediments dropped by more than ten orders of magni-tude in 1998 compared with those in 1993.

3.2.2. Temporal distribution of OCPs in Xiamen Bay sedimentsAs illustrated in Tables S2 and S3 and Fig. S1, OCP concentra-

tions steadily and noticeably decreased from 1982 to 2003 in thesediments of Xiamen Bay. In particular, the OCP concentrationsin 1993 were significantly lower than those in 1982. This findingis consistent with the implementation of environmental policy inChina, which banned the usage of HCH and DDT in 1989. TheOCP level in Xiamen City decreased significantly after its designa-tion as a major coastal destination for tourists.

3.2.3. Analysis of the temporal distribution of POPs in Xiamen Baysediments

According to the data (Table 6, Fig. 6, Tables S2 and S3 andFig. S1), the temporal distribution of POPs in Xiamen Bay sedi-ments is related to the transformation of Xiamen’s industrial struc-ture. Xiamen Bay has grown rapidly and aggressively in terms ofeconomy, industry, and agriculture in accordance with China’snational condition. As a result of this growth, much POPs are gen-erated, thus maintaining the high concentration of such pollutantsin the environment. In the 1990s, Xiamen developed its seasidetourism industry vigorously and phased out heavy industries. Withthis change in development concept, pollution sources were signif-icantly reduced. As a result, POP concentration in sedimentsdecreased continuously.

3.3. Recommendations for the collection of samples for long-term POPmonitoring

In long-term monitoring and deferred analysis, samplecollection and storage are essential. To preserve the integrity ofsamples, standard operating procedures establish a system ofhigh-quality assurance with regard to sampling, handling, storage,

Table 5The level of POPs in surface sediments around the world.

Site PAHs PCBs

San Francisco Bay, USA 80.0–20000.0 <0.1–8.Casco Bay, USA 16.0–21000.0 0.4–485Liverpool Bay, UK N/A 0.1–38.Manukau Harbor, New Zealand 16.0–5300.0 0.2–1.5Osaka Bay, Japan N/A 63.0–24Kyeonggi Bay, Korea 9.1–1400.0 <1.0–58Sava River, Slovenia 4000.0 4.0Alexandria harbor, Egypt 0.9–1210.0 <0.3–88Black Sea, Ukraine 5.7–6.8 35.0–65

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documentation, sample inventory, and specimen tracking (Schulzeet al., 2007).

Standardized sampling relies on a rigid system in which a fixednumber of specimens are collected at certain intervals, seasons,locations, and densities. During sampling, an adequate number ofindividual samples are obtained as archived materials for futureanalysis. This analysis suggests that appropriate sampling can beguided and ensured by ongoing monitoring programs, such asthe spatial or temporal distribution of POPs.

3.3.1. Recommendations for the selection of sampling sitesA key step in long-term monitoring is the selection of suitable

sampling sites (Caeiro et al., 2003; Hewitt and Thrush, 2007;Owlia et al., 2011). In the selection of sampling sites, the followingfactors should be considered: the type of information desired, typeof specimen to be collected, characteristics of the study area, andabundance of the study specimen. In relation to the spatial distri-bution of POPs in Chinese sediments, regional characteristics aresignificant. Based on the analysis and assessment, POP pollutionis severe in estuaries or bays and is negligible in open seas. Thisphenomenon suggests that sampling sites should be chosen inaccordance with the criterion of economic development level alongthe entire Chinese coast.

To monitor POPs in China over the long term, we must identifyat least one group of sampling sites or a total of five sampling sites(Fang et al., 2003) for each coastal area in the country, includingharbors, bays, and estuaries. Subsequently, additional samplingsites should be developed for each area depending on the level ofPOP pollution. The sites from which samples of POP concentration(PAH, PCB, and DDT) can be obtained should be decreased insemi-closed bays (such as Jiaozhou Bay and Bohai Bay) becausethe POP concentration in the sediments of these areas is stableand homogenous given the unique geographical feature of thesetwo bays. Thus, we propose the following criteria for the selectionof sampling sites:

1. Any two adjacent sampling sites should be at least 1 km apart.2. When a single group of sampling sites is located in an area with

a low concentration of POPs, a grid-shaped network of samplingstations may be set up randomly.

3. When more than one group of sampling sites is located in anarea with a high concentration of POPs, the sites should be scat-tered at the edges of domestic, agricultural, and industrialoutfalls.

4. Sampling sites should be adjacent to the shoreline. Fewsampling sites should be set up at areas that are distant fromthe shore.

5. A single group of sampling sites should be set up in a semi-closed bay.

3.3.2. Recommendations for sampling frequencyGiven strict storage conditions to prevent the unnecessary

influence of confounding factors, an optimal sampling frequency

DDTs Source of data

1 N/A Pereira et al. (1994).0 <0.3–20 Kennicutt et al. (1994)

0 N/A Camacho-Ibar and McEvoy (1996)0.1–22.0 Holland et al. (1993)

0.0 0.2–12.0 Iwata et al. (1994)0.0 <0.1–32.0 Lee et al. (2001)

0.5–1.0 Heath et al. (2010)5.0 <0.3–2.1 Barakat et al. (2002).0 1.3–2.3 Fillmann et al. (2002)

temporal distribution of legacy persistent organic pollutants and recom-es and seas in China. Chemosphere (2014), http://dx.doi.org/10.1016/

Table 6Concentrations of PAHs in sediments in Xiamen Bay.

Year of sampling Average concentration (ng g�1) Range of concentration (ng g�1) Source of data

1993 20333.0 2900.0–60000.0 Hong et al. (1995)1998 367.0 247.0–480.0 Zhang et al. (2001)1999 334.0 59.0–1177.0 Maskaoui et al. (2002)2002 254.0 133.0–308.0 Ou et al. (2004)2007 436.0 330.2–716.2 Li et al. (2010)

Fig. 6. Temporal distribution of PAHs in Xiamen Bay.

6 L. Peng et al. / Chemosphere xxx (2014) xxx–xxx

increases the total amount of samples and improves collection effi-ciency (Hewitt and Thrush, 2007). For long-term monitoring,therefore, sampling frequency should be determined in accordancewith the following principles: the collection of environmental datathrough effective sampling, technical feasibility, and the exhibitionof characteristics of environmental change. According to theseprinciples, we propose sample collection at different frequenciesbased on POP distribution.

POP concentration in Xiamen Bay varies temporally; thus, thediversity of POP concentration is highly consistent with thetransformation of Xiamen’s industrial structure. This consistencysuggests that sampling frequency should be determined inaccordance with the transformation of the industrial structure inthe sampling area. In most regions, the industrial structureremains stable for a period, thereby indicating that POP concentra-tion may be constant. Therefore, sampling frequency shouldremain low during this period. Once the industrial structure isaltered, POP concentration may change dramatically. Thus, a low-frequency sampling level is insufficient for accurate long-termmonitoring. Sampling frequency should therefore be increased,and this frequency should remain high in areas with serious POPpollution to improve monitoring and to test the rationality of theenvironmental policy. Hence, we propose the following criteria todetermine sampling frequency:

1. Sampling frequency should consistent with either industrialtransformation or the direction of development in the samplingareas (The Five-Year Plan on the National Economic and SocialDevelopment). In areas with stable industrial structures, sam-pling frequency should be maintained at a low level once everyfive years.

2. When the industrial structure changes, particularly the indus-trial and agricultural densities, sampling frequency should beincreased to once per year.

3. To enhance monitoring and to test the rationality of the envi-ronmental policy in areas with serious POP pollution, samplingfrequency should be increased to once every two years.

Please cite this article in press as: Peng, L., et al. Assessment of the spatial andmendations for sample collection from the surficial sediments of estuarij.chemosphere.2014.04.004

4. Conclusions and outlook

The pollution of Chinese water bodies by legacy POPs in sedi-ments follows certain patterns of spatial and temporal distribution.Spatially, the POP distribution pattern is consistent with thelocation of main POP sources, such that POP pollution is a seriousproblem in estuaries or bays near densely populated areas withflourishing industries or agriculture (including the Pearl RiverEstuary and Jinzhou Bay). By contrast, POP concentration is gener-ally low in open seas that are distant from human activities (suchas the Xiamen–Jinmen Seas). Temporally, the variation in POP con-centration is consistent with the degradation characteristics ofPOPs and the changes in the industrial structure of the monitoringarea. For instance, the concentrations of POPs in the sediments ofXiamen Bay were significantly lower during the 1990s than duringthe 1980s. These concentrations gradually declined after the late1990s and have since remained constant. Thus, the pattern of tem-poral changes is largely consistent with the variations in the city’sindustrial structure and POP characteristics.

Hence, spatial and temporal sampling should be sound toenhance the long-term monitoring of POPs in China based on exist-ing regulations for marine monitoring. Spatially, sampling sitesshould be developed according to the abundance of POPs in thearea. Temporally, sampling frequency should be determined basedon the nature of these pollutants and the changes in the industrialstructure of the study area. The frequency of sample collectionshould be adjusted based on the development level of the indus-trial structure. Therefore, our study proposes a method that canguide the long-term monitoring and deferred analysis of POPs inChina.

Acknowledgments

We would like to thank senior expert in the field ofenvironmental science, Professor Jianfu Zhao (Director of StateKey Laboratory of Pollution Control and Resources Reuse & ViceChancellor of Tongji University), for his constructive commentsfor this paper. The authors also are grateful to senior expert inthe field of environmental engineering, Professor Chang-TangChang (from Department of Environmental Engineering at NationalI-Lan University), for providing writing assistance for this paper.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.chemosphere.2014.04.004.

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