Experimental Study on the Role of Sedimentation and Degradation

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Experimental Study on the Role of Sedimentation and Degradation Processes on Atmospheric Deposition of Persistent Organic Pollutants in a Subtropical Water Column Yumei Huang, Ruijie Zhang, Kechang Li, Zhineng Cheng, Guangcai Zhong, Gan Zhang, and Jun Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00568 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017

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Environmental Science & Technology

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Experimental Study on the Role of Sedimentation and Degradation

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Processes on Atmospheric Deposition of Persistent Organic

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Pollutants in a Subtropical Water Column

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Yumei Huang †,‡, Ruijie Zhang §*, Kechang Li †, Zhineng Cheng †, Guangcai Zhong †,

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Gan Zhang †, Jun Li †

6 7

†

8

Academy of Sciences, Guangzhou 510640, China

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‡

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese

School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006,

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China

11

§

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Sciences, Guangxi University, Nanning 530004, China

Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, School of Marine

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ABSTRACT: The goal of this study is to experimentally assess the role of vertical

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sinking and degradation processes of persistent organic pollutants (POPs) in a

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subtropical water column. This was done by measuring the concentrations of selected

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typical organochlorine pesticides, including hexachlorocyclohexanes (HCHs),

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hexachlorobenzene (HCB), dichlorodiphenyltrichloroethanes (DDTs), trans-chlordane

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(TC) and cis-chlordane (CC), in atmosphere (gas phase), water (dissolved and

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particulate phases) and sedimentation samples simultaneously from October 2011 to

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April 2013 in a subtropical lake. The fugacity ratios suggested net deposition for

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α-HCH, γ-HCH, p,p'-DDT, p,p’-DDD, p,p’-DDE, o,p'-DDT, TC and CC, indicating 1

ACS Paragon Plus Environment


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that the subtropical lake was acting as a “sink” for these chemicals. The enantiomer

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fractions of α-HCH, o,p’-DDT, TC and CC in the dissolved phase samples were much

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more deviated from the racemic values than were those in the air samples, suggesting

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that these chemicals have suffered microbial degradation in the subtropical lake. In

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fact, 99% to 100% of atmospheric input of α-HCH and γ-HCH to the subtropical lake

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was estimated to be depleted via microbial degradation, while the role of hydrolysis

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and vertical sinking was very small. For more hydrophobic p,p'-DDT, o,p'-DDT, TC

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and CC, the role of vertical sinking was 2 to 3 orders of magnitude larger than that for

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α-HCH and γ-HCH. Microbial degradation was also very important for removing

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p,p'-DDT, o,p'-DDT, TC and CC from the water column.

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Key words: organic chlorinated pesticides, air-water exchange, sedimentation,

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degradation, enantiomer fraction

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TOC ART 2


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INTRODUCTION

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Persistent organic pollutants (POPs) have been of great concern because of their

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persistence, long-range transport ability, bioaccumulation, and negative effects on

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human beings and wildlife. Large water bodies such as oceans and seas play an

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important role in the global cycling and ultimate fate of POPs.1 Previous studies

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suggested that POPs in the aquatic environments are subject to a sequence of water

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column processes such as volatilization, degradation, particle-associated adsorption,

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phytoplankton

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particle-associated sedimentation were considered to be important processes that

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removed POPs from the water column.1-3,5-7 The relative importance of the biological

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and degradative pumps of less persistent hexachlorocyclohexanes (HCHs) in the

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North Atlantic, Arctic and Southern Oceans has been estimated by Galbán-Malagón et

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al.3,8 However, due to the difficulty in sampling, field studies on water column

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processes of POPs in marine environments are scarce. In view of the convenience in

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sample collection, remote lakes with little anthropogenic pollution are considered to

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be good fields to obtain a clearer understanding of water column processes.9

uptake,

and

vertical

sinking,2-4 of

which

degradation

and

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The distribution and fate of POPs in subtropical oceans and seas might be different

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from that in cold waters,10 as temperature is an important factor influencing many

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processes in the water. For example, there might be enhanced microbial activity and

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hydrolysis degradation in subtropical water.11 The air-water exchange of POPs in

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subtropical oceans might also be different from that in the cold area, as the Henry’s 3



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Law constant was suggested to be dependent on temperature.12-13 Investigating the

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processes in a subtropical water column is of great importance to understanding the

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transport and fate of POPs there. Several studies have been reported on the water

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column processes of POPs in subtropical areas.6,7 For example, Castro-Jiménez et al6

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investigated the atmospheric deposition and sinking fluxes of polycyclic aromatic

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hydrocarbons (PAHs) in the Mediterranean Sea, and confirmed the existence of an

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important depletion/sink of water column PAH concentrations in the photic zone,

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especially for low MW PAHs. Blooms of phytoplankton were found to be important

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for the biogeochemical processing of hydrophobic organic chemicals in Taihu Lake.7

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However, comprehensive experimental studies involving atmospheric input, vertical

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sinking and degradation processes in subtropical water bodies are very limited.

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Thus, the goal of the present work was to experimentally study the processes in a

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subtropical water system and attempt to determine the coupling of these processes. A

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subtropical maar lake with little anthropogenic pollution was selected as the study

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field. Typical organochlorine pesticides (OCPs) such as HCHs, hexachlorobenzene

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(HCB),

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cis-chlordane (CC) were chosen as target compounds because these chemicals have

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different physical-chemical properties and high detection frequencies in the

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environment. Furthermore, α-HCH, o,p’-DDT, TC and CC are chiral compounds, and

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the enantiomeric compositions of these chemicals can be used to indicate the

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microbial degradation process.5,11,14-15

dichlorodiphenyltrichloroethanes

(DDTs),

trans-chlordane

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4


(TC)

and

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EXPERIMENTAL SECTION

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Study Site. Lake Huguangyan (21.15°N, 110.28°E) is a subtropical volcanic maar

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lake located on the Leizhou Peninsula, South China (Figure 1). The lake is the largest

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maar lake worldwide, with a surface area of ~2.3 km2 and a maximum depth of ~20

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m.16 The lake is influenced by the East Asian winter monsoon from the Siberian

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anticyclone in the winter and is influenced by the East Asian summer monsoon from

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the western Pacific and the Indian summer monsoon in the summer.17 The lake is

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located in a national geo-park with an area of 38 km2. The lake has neither an inﬂow

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stream nor an outﬂow stream, and its water mainly comes from rainfall, surface

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runoffs and underground water from the catchment.18 A relatively remote location and

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simple hydrology make the lake a good field to study the water column processes of

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POPs.

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Sample Collection. Passive Air Sampler (PAS).Ten air samples were continuously

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collected by pre-cleaned polyurethane foam (PUF) disks using a PAS apparatus19-22

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deployed aside Lake Huguangyan from 05/15/12 (15 May 2012) to 04/08/13 (08 Apr

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2013), with sampling periods ranging from 30 to 44 days (Figure 1,Table S1). All the

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PUF disks were prepared as described in the Supporting Information (SI). At the end

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of each sampling period, the PUF disk was retrieved and resealed in the original

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aluminum packet and then sent back to the laboratory and stored at -20 °C until

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analysis.

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Polyethylene (PE) Sheet Passive Sampler. Three PE samplers were deployed at

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depths of 0.5 m, 6 m, and 12 m in the middle of Lake Huguangyan, from 03/15/12 to 5



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02/23/13 (Figure 1,Table S2). Each PE sampler contained three pieces of PE sheets.

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At the beginning of sampling, pre-cleaned PE sheets fixed on stainless steel frames

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were installed in a stainless steel chain and then deployed in the lake. A total of 30 PE

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samples were collected, with sampling periods ranging from 30 to 31 days (Table S2).

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PE sheets were prepared as described in the SI. At the end of each sampling period,

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PE sheets were unfixed and sealed in clean aluminum packets and then sent back to

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the laboratory and stored at -20 °C until analysis.

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Suspended Particle Matter (SPM) Sampling. A total of 39 SPM samples were

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collected from Lake Huguangyan from 12/18/11 to 02/23/13 (Figure 1,Table S3). On

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the sampling date, 25 L of water was collected at depths of 0.1 m, 6 m, and 11.5 m

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and then filtered through pre-baked glass fiber filters (GFFs) (I.D., 0.75 µm) to obtain

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SPM. SPM samples were sent back to the laboratory in clean aluminum packets and

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then freeze-dried and stored at -20 °C until analysis.

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Sediment Trap Sampler. A sediment trap sampler that collects settling particles in

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the water column was used in the present work.23-24 Close to the PE sampler

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deployment, three sediment trap samplers were deployed at depths of 0.5 m, 6 m, and

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12 m. Each sediment trap sampler contained four polycarbonate tubes. The collecting

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area of each sediment trap sampler was approximately 0.038 m2. A total of 42 trap

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samples were collected from 10/16/11 to 02/23/13, with sampling periods ranging

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from 28 to 34 days (Table S4). Each trap sample contained sediment and SPM in the

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tubes. SPM in the water of the trap tubes was collected by filtering in situ through

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pre-baked GFFs. The trap samples were sent back to the laboratory in clean aluminum 6


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packets and then freeze-dried and stored at -20 °C until analysis.

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Chemical Analysis. PUF Samples. PUF samples were spiked with recovery

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surrogates tetrachloro-m-xylene (TCmX) and decachlorinated biphenyl (PCB209) and

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then Soxhlet extracted with dichloromethane (DCM) for 48 h and further purified on a

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multilayer silica gel-alumina column and an acid silica gel column in sequence. The

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solvent for the elution was changed to hexane and then concentrated to a volume of

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approximately 50 µL under subtle nitrogen stream.

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internal standard for instrumental analysis. Details of the preparation can be found in

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the SI.

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C-PCB141 was added as an

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PE Sheet Samples. The PE sheets were cut into small pieces, spiked with TCmX

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and PCB209, and then extracted with DCM. After 12 h, the PE sheets were removed

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and further extracted twice with DCM for 12 h. The extraction solvents were

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combined, concentrated, and further prepared as the PUF samples.

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Trap and SPM samples. Trap and SPM samples spiked with recovery surrogates

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were Soxhlet extracted with DCM/methanol (v:v 1:1) for 48 h. The extract was

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concentrated and purified on a neutral alumina column. The elution was further

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prepared on a multilayer silica gel-alumina column and an acid silica gel column as

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the PUF and PE samples. A more detailed description of the purification of the trap

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and SPM samples is provided in the SI.

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Achiral OCPs were measured on an Agilent 7890/7000 GC-MS/MS with an

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HP-5MS capillary column (30 m × 0.25 cm × 0.25 mm, Agilent, CA, USA) as

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described by Huang et al.11 After achiral OCP determination, the same extracts were 7



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analyzed for enantiomers of α-HCH, TC, CC and o,p'-DDT on the Agilent

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GC-MS/MS with a BGB-172 column (15 m × 0.25 cm × 0.25 mm, BGB Analytik AG,

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Switzerland). More information on the procedures was given in the SI. The results of

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chiral analysis were expressed as enantiomer fractions (EFs, EFs = peak areas of the

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(+) / ((+) + (-)) enantiomers).25

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Quality Assurance and Quality Control (QA/QC). Two laboratory blanks, one

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PUF field blank, two GFF field blanks and nine PE sheet field blanks were prepared

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and analyzed. The instrumental detection limits (IDLs) were calculated from the

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corresponding amount of compounds that would generate a signal-to-noise ratio of

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3:1. The method detection limits (MDLs) were assigned as average values of the

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blanks plus three times the standard deviations of the blank values. When the

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compounds were not detected in the blanks, MDLs were assigned as three times the

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IDLs (Table S5). Target OCPs were undetectable in all the laboratory blanks and field

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blanks. The average recoveries of all the samples were 49±13% and 77±25%,

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respectively, for TCmX and PCB209. The reported values of α-HCH, γ-HCH and

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HCB were corrected according to the recoveries of TCmX. The reported values of

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p,p'-DDT, p,p'-DDD, p,p'-DDE, o,p'-DDT, TC and CC were corrected according to

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the recoveries of PCB209.

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For chiral analysis, racemic standards were injected repeatedly (n = 6) to determine

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the reproducibility of the EF measurements. The average EF values of the standards

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were 0.501±0.001, 0.492±0.004, 0.495±0.002, and 0.501±0.002, respectively, for

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α-HCH, TC, CC and o,p’-DDT. An EF of 0.5 indicates a racemic composition. If the 8


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EF of a particular sample fell outside the 95% confidence interval of the EF values of

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the standards (the range was 0.500-0.503 for α-HCH and o,p’-DDT, 0.487-0.497 for

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TC, and 0.492-0.497 for CC), it was concluded that the sample has a non-racemic

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composition.26

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Statistical Analysis. All statistical analyses were performed using SPSS v 13.0 for

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windows (SPSS Inc., Chicago, USA). OCP concentrations in the air and dissolved

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phase were compared with other studies using one-sample t test, while OCP

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concentrations in the dissolved phase of different depths and vertical sinking fluxes of

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OCPs at different depths were compared using paired-samples t test. The results of the

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statistical tests were considered statistically significant when p

Experimental Study on the Role of Sedimentation and Degradation

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