Mercury Export from Mainland China to Adjacent Seas and Its

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Mercury Export from Mainland China to Adjacent Seas and Its Influence on the Marine Mercury Balance Maodian Liu, Long Chen, Xuejun Wang, Wei Zhang, Yindong Tong, Langbo Ou, Han Xie, Huizhong Shen, Xuejie Ye, Chunyan Deng, and Huanhuan Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04999 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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Environmental Science & Technology

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Mercury Export from Mainland China to Adjacent Seas and Its Influence on the

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Marine Mercury Balance

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Maodian Liu,† Long Chen,† Xuejun Wang,*,† Wei Zhang,*,‡ Yindong Tong,§ Langbo

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Ou,† Han Xie,† Huizhong Shen,† Xuejie Ye,† Chunyan Deng† and Huanhuan Wang†

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†

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Environmental Sciences, Peking University, Beijing 100871, China

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‡

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100872, China

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§

Ministry of Education Laboratory of Earth Surface Process, College of Urban and

School of Environment and Natural Resources, Renmin University of China, Beijing

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072,

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China

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Corresponding authors:

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Xuejun Wang. Ministry of Education Laboratory of Earth Surface Processes, College of

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Urban and Environmental Sciences, Peking University, Beijing, 100871, China. Tel: +86-

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10-62759190. E-mail: [email protected]

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Wei Zhang. School of Environment and Natural Resources, Renmin University of China,

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Beijing, 100872, China. Tel: +86-10-62756122. E-mail: zhw326@ ruc.edu.cn

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Word count for text:

4888

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Word count for 4 figures, 1 table:

5x300=1500

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Total Word count:

6388

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ACS Paragon Plus Environment


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ABSTRACT: Exports from mainland China are a significant source of mercury (Hg) in

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the adjacent seas (Bohai Sea, Yellow Sea, East China Sea and South China Sea) near

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China. A total of 240 ± 23 Mg was contributed in 2012 (30% from natural sources and 70%

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from anthropogenic sources), including Hg from rivers, industrial wastewater, domestic

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sewage, groundwater, non-point sources and coastal erosion. Among the various sources,

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the Hg from rivers amounts to 160 ± 21 Mg and plays a dominant role. The Hg that is

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exported from mainland China increased from 1984 to 2013; the contributions from

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rivers, industrial wastewater, domestic sewage and groundwater increased, and the

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contributions from non-point sources and coastal erosion remained stable. A box model is

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constructed to simulate the mass balance of Hg in these seas and quantify the sources,

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sinks and Hg biogeochemical cycle in the seas. In total, 160 Mg of Hg was transported to

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the Pacific Ocean and other oceans from these seas through oceanic currents in 2012,

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which could have negative impacts on the marine ecosystem. A prediction of the changes

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in Hg exportation through 2030 shows that the impacts of terrestrial export might worsen

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without effective pollution reduction measures and that the Hg load in these seas will

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increase, especially in the seawater of the Bohai Sea, Yellow Sea and East China Sea and

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in the sea margin sediments of the Bohai Sea and East China Sea.

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Keywords: Hg, mainland China, river, transport, sea, mass balance

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Abstract art

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INTRODUCTION

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Methylmercury (MeHg), which is a neurotoxic form of mercury (Hg), affects human

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health through the consumption of fish from freshwater and ocean sources.1 Hg can be

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cycled rapidly in the atmosphere, hydrosphere and pedosphere, which affects the Hg

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concentration in the ocean.2 On a global scale, Hg in the ocean primarily comes from

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atmospheric deposition and riverine input. Previous studies estimated that the global Hg

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that discharges into the ocean from rivers is 1000–5500 Mg/yr,3-6 nearly 80% of which is

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deposited into sea margin sediments.3 The contribution of Hg in the open ocean from

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atmospheric deposition is approximately 5200–6600 Mg/yr.7, 8 Therefore, Hg discharges

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from rivers may significantly influence offshore and ocean environments and thus

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deserve more attention.

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In addition to rivers, other terrestrial pathways may play key roles in Hg inputs into the

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ocean. Hg discharges from groundwater into the ocean might be an important pathway,

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such as the Bangdu Bay in the Korean Peninsula and Waquoit Bay in the northeastern

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U.S.9, 10 Hg that is transported from coastal erosion might also be a vital source of

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particulate Hg.11, 12 In the Arctic Ocean, the Hg flux from coastal erosion reached 47

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Mg/yr, whereas riverine pathways only contributed 13 Mg/yr.11 Hg discharges from

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anthropogenic sources in coastal areas, such as industrial wastewater and domestic

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sewage discharge, may also contribute to oceanic Hg.13 In addition, non-point source

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contributions in coastal areas should be considered in the mass balance of oceanic Hg.14,

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15

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balance models of oceanic Hg have focused on limited types of terrestrial pathways, such

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as rivers, groundwater and coastal erosion.11, 16

However, most previous inventories of Hg that is exported from land to oceans or mass

4


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Regional-scale variations in rivers are among the key drivers of the global oceanic

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environment,17 especially in China, which is a critical contributor of global Hg cycling.6

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Many studies have focused on the total atmospheric Hg emission from anthropogenic

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sources in China, which amount to approximately 530–830 Mg/yr.18-21 The Hg that is

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released into the atmosphere is attracting extensive attention because of its long-range

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transportation. Hg that is exported from land to the oceans may also significantly impact

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the environment because of the transportation of Hg by ocean currents.3 A previous study

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reported that the total Hg that is discharged from rivers into the North Pacific Ocean is

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approximately 2000 Mg/yr, most of which was contributed by China, and has increased

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since the 1970s.5 Another study reported that the Yangtze River, the largest river in China

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and the third longest river in the world, has been polluted by Hg for years.22 However, no

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inventory of the Hg that is discharged into the seas from rivers and other sources in

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mainland China has been created. In addition, research on the mass balance of Hg in the

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seas near China is lacking. Therefore, the influence of the Hg that is exported from land

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to the seas is unclear.

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The objectives of this paper include the following. (1) An inventory of the Hg that is

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exported from mainland China into the adjacent seas (Bohai Sea, Yellow Sea, East China

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Sea and South China Sea) is developed. The inventory includes six pathways: rivers,

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industrial wastewater, domestic sewage, groundwater, non-point sources, and coastal

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erosion. (2) The inter-annual changes in the total Hg inventory are analyzed. (3) A marine

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box model is established to simulate the mass balance of Hg in China’s adjacent seas and

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to identify the influence of terrestrial export.

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METHODS 5



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Six pathways of Hg exportation from mainland China into the four adjacent seas are

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investigated. The pathways include rivers, industrial wastewater, domestic sewage,

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groundwater, non-point sources, and coastal erosion. The total Hg [Hg(T)] is divided into

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dissolved Hg [Hg(D)] and particle-bound Hg [Hg(P)]. The Hg transport flux into the seas

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is calculated by multiplying the Hg concentration and the volume/yr or mass/yr. The

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calculation of the six pathways is described as follows:

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F=∑

∑ × × ,

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where F (Mg) is the annual mass flux of Hg, C (ng/L, µg/L or ng/g) is the annual Hg

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concentration, E (m3/yr or Mg/yr ) is the annual exported volume per year or mass per

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year, K is the unit conversion factor, i represents different pathways, and j represents

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different sea margins.

(1)

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A marine box model of the adjacent seas (Bohai Sea, Yellow Sea, East China Sea and

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South China Sea; see Figure S1, Supporting Information) is constructed based on the Hg

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inputs from land to simulate the mass balance of Hg in these seas and identify the

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influence of terrestrial transport on the Hg balance of the seawater and sediments.

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Mercury Discharges from Rivers. An inventory of the total riverine Hg discharge

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from mainland China into the seas is constructed. The Hg data are all measurement data

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from the literature (Supporting Information) and are mainly based on CVAAS (Cold

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Vapor Atomic Absorption Spectrometry) or CVAFS (Cold Vapor Atomic Fluorescence

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Spectrometer) according to the China Environmental Quality Standards for Surface

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Water (GB 3838-2002) and Method 1631 of the EPA.23, 24 All the sampling and

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measurements were conducted by major scientific research and national monitoring 6


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institutions in China (including Chinese Academy of Sciences, Chinese Research

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Academy of Environment Sciences and major universities) and published in peer-

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reviewed journals.22, 25, 26 The flux of Hg(D) (Mg/yr) and Hg(P) (Mg/yr) in every river is

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obtained by multiplying the Hg concentration by the riverine water discharge.27, 28 The

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eight major rivers in China—the Yangtze River (1000 km3 of riverine water discharge

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and 0.16 kg/m3 of suspended sediment concentration in 2012), Pearl River (280 km3,

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0.084 kg/m3), Yellow River (28 km3, 6.5 kg/m3), Minjiang River (75 km3, 0.028 kg/m3),

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Qiantang River (33 km3, 0.16 kg/m3), Luan River (3.0 km3, 0.79 kg/m3), Liao River (2.4

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km3, 0.27 kg/m3) and Hai River (0.27 km3, 0.038 kg/m3)—were included in this study to

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estimate all the riverine Hg that was discharged in 2012 because the monitoring data for

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some small rivers are limited. The locations of these eight rivers are presented in Figure

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S1, and their total riverine water discharge comprised 84% of that from all rivers in China

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in 2012. Concentration data of the Hg(T) (ng/L), Hg(D) (ng/L) and Hg(P) (ng/L) for the

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eight main rivers were collected from the literature.

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If a work provided only [Hg(D)], the partition coefficient log10KD = log10

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([Hg(P)]/[Hg(D)]) (the unit of [Hg(P)] is pg/kg, and the unit of [Hg(D)] is pg/L) is used

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to estimate [Hg(P)].5, 28 The unit conversion of [Hg(P)] (ng/L and ng/g) is based on the

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suspended sediment concentration of the corresponding river. In this study, the log10KD

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value of 3.8 ± 0.084 that was calculated by this study (based on existing data in Table S1,

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Supporting Information) is used for all rivers except the Yellow River. A log10KD value

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of 2.1 ± 0.077 is used for the Yellow River because of its extremely high suspended

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sediment concentration. The research of Chen et al. also proved this phenomenon in the

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Yellow River. In their paper, the log10KD values of Cr, Pb and Zn in the Yellow River 7



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were 2.4, 3.7 and 2.0, respectively.29 The riverine water discharge, suspended sediment

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discharge and suspended sediment concentrations are acquired from the Hydrologic Data

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Yearbook,30 China River Sediment Bulletin31 and China Water Resources Bulletin.32 The

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yearly trends of Hg concentrations could be obtained for some major rivers such as the

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Yangtze River, Pearl River and Yellow River. However, the Hg data of other small rivers

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are limited, so the Hg concentration is assumed to have remained unchanged after the

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year 2000 for these rivers when estimating the Hg flux; this assumption was also adopted

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by Amos et al.5

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Mercury Discharge from Coastal Industrial Wastewater and Domestic Sewage.

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Discharge volume data of industrial wastewater and domestic sewage are collected from

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the China Statistical Yearbook,33 China Environmental Statistics Yearbook34 and China

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Marine Statistical Yearbook.35 The data on industrial wastewater that discharges directly

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into adjacent seas are specific to every coastal city. The Hg concentrations of industrial

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wastewater are specific to the primary industrial sectors (provided in Table S2,

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Supporting Information). A weighted mean concentration of Hg is applied to calculate the

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Hg flux based on the wastewater discharge intensity of every industrial sector.

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The data on domestic sewage that is discharged into adjacent seas are specific to every

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coastal province, and the estimation method is similar to that of the Hg discharges from

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industrial wastewater. The domestic sewage discharge is divided into untreated discharge

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and treated discharge. The treatment rates of domestic sewage are collected from the

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China Environmental Statistics Yearbook.34 The variations in the Hg flux into adjacent

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seas from industrial wastewater and domestic sewage from 1984 to 2013 are estimated by

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multiplying the discharged volume and Hg concentrations. Details are provided in the

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Supporting Information.

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Mercury Discharge from Groundwater. The estimation of Hg discharge from

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groundwater into adjacent seas in 2012 and changes from 1984 to 2013 is similar to the

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riverine pathway estimation in this study and previous studies,9, 11, 36-38 except for the

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discharge intensity data. Previous studies used the submarine groundwater discharge

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(SGD) to multiply the concentration of Hg in groundwater.9, 36-38 However, the SGD

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(Figure S2, Supporting Information) includes submarine fresh groundwater discharge

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(SFGD) and recirculated saline groundwater discharge (RSGD).39, 40 The SFGD is the

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submarine fresh groundwater discharge, which originates from surface stream water in

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the mainland. The RSGD is the process of seawater penetration into the sediment and re-

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suspension of particles that originated from seawater. In addition, researchers have

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proven that the SFGD/SGD ranges from 1% to 40% in many areas. If the SGD is applied

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to calculate the Hg flux of groundwater from land into the seas, the result may be

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overestimated. Therefore, the SFGD is applied in this study to calculate the Hg

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discharges from groundwater. The ratio of the SFGD to the riverine water discharge into

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adjacent seas is used to estimate the flow of SFGD. A value of 6% is chosen for this

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estimation according to previous studies.39-43

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Mercury Transport from Coastal Non-Point Sources. Non-point sources are an

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important terrestrial source of Hg that is exported into the ocean. Coastal soil can be

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eroded by rain and transported into coastal seas.14, 15 Therefore, estimating the Hg that is

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exported from non-point sources is necessary. The total sediment that is transported from

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non-point sources into the seas is estimated as follows: 9



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E5=∑ × × ,

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(2)

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where E5 is the sediment that is transported from non-point sources, M is the erosion

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modulus (the mass of soil that is eroded by waterpower in a given time and area, with

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units of Mg/km2·yr; provided in Table S5, Supporting Information), L is the length of the

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coastline,44, 45 W is the width of the survey region, and j represents sea margins. In this

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study, 20 km is set as the width of the survey region, which is the average value,

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according to the features of the Chinese coastline46, 47 to estimate the contribution from

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coastal soil erosion by rainfall (Table S5, Supporting Information). The Hg

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concentrations of the coastal soil were obtained from the Chinese Soil Element

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Background Value 1990 (Table S7, Supporting Information).48 The standard error (SE) of

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the Hg concentration is estimated by SD/√ − 1 (n is the sample size). The standard

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deviation (SD) of the Hg concentrations are provided in the Chinese Soil Element

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Background Value 1990.48 The Hg flux from non-point sources into the seas from 1984 to

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2013 is estimated using the changes in the climate conditions during this period.

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According to a previous study,15 the erosion modulus is proportional to the square of the

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rainfall intensity.

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Mercury Transport from Coastal Erosion. The method of calculating the Hg that is

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transported into the ocean from coastal erosion is similar to that of previous research

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(Figure S3, Supporting Information)11, 16, 49. The total sediment flux from coastal erosion

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into the four seas is estimated as follows:

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E6=∑ × × × × ,

(3)

10


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where E6 is the sediment that is transported from coastal erosion, R is the erosion rate, L

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is the length of the coastline, P is the proportion of eroded coastline, B is the bank height,

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D is the soil density, and j represents sea margins. The erosion rates are chosen separately

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for the four seas and are provided in the Supporting Information. The bank heights of the

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four seas are estimated based on previous studies,50-53 which ranged from 2 to 6 m and

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were set at an average of 4.5 m. The length of the coastline was obtained from previous

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studies.44, 45 The soil density was estimated based on previous studies,54-57 ranging from

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1.2 to 2.0 g/cm3 and set to 1.5 g/cm3. The proportion of eroded coastline was reported by

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Ji to be 46% for the Bohai Sea, 49% for the Yellow Sea, 44% for the East China Sea and

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21% for the South China Sea.58 The Hg concentrations that are adopted in the non-point

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source estimation in this study were used for the coastal erosion estimation. The variation

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in the Hg contributions from coastal erosion from 1984 to 2013 is estimated through

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changes in the climate conditions during this period.59

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Mercury Mass Balance of Chinese Seas. A marine box model is developed to

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simulate the Hg mass balance of seawater in the four seas and sea margin sediments.

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Eight boxes are established to represent the Bohai Sea, Yellow Sea, East China Sea,

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South China Sea, and their sea margin sediments. Hg can transport between the seas,

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atmosphere, and land, and deposits ultimately in marine sediments. In the reservoirs

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representing the seas, Hg comes mainly from land export, atmospheric deposition and

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oceanic current transport from other oceans. Hg can also transport between each reservoir

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through oceanic currents and sediment transport (Figure S4, Supporting Information).

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The Hg mass flows between each box are expressed by first-order differential equations

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as follows:2 11



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dm / dt = Km + s,

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(4)

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where m is the mass of every reservoir (Mg of Hg); K is the coefficient of mass flow

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from reservoir i to reservoir j, which is represented by Kij = Fij/mi; Fij (Mg/yr of Hg) is

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the mass flow from reservoir i (source) to reservoir j (sink); and “s” is a vector that

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describes the external forcing (Mg/yr) from the deep mineral reservoir.

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Hg flows between the eight boxes (four seas and relevant sediments) are set to be

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changeable from 1984 to 2013 because of the unstable Hg input from the land. Hg inputs

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from atmospheric deposition and ocean currents from other oceans are assumed to be

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constant. This box model is established by combining the inventory that was developed in

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this study and measurement data from the literature (Table S8, Supporting Information).

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The exchange between particulate and dissolved Hg phases is not considered, but the

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Hg(T) output from seas because of fishing in coastal areas is considered. The data from

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fish catches were provided in the China Marine Statistical Yearbook,35 and the

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concentrations of Hg in fish are provided in Table S8 (Supporting Information). Hg

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atmospheric dry deposition, wet deposition and evasion from the sea surface from the

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literature are applied.16, 60, 61 The amount of Hg in the sea margin sediments is estimated

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from the sea margin area, the density of the sediment, the interaction depth and the

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concentration of Hg. The Hg in the seawater can deposit within the surface sediment,62

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and the Hg in surface sediment can release into the water volume,63 which can create

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instability in the vertical Hg concentrations in the surface sediment.64 The depth of the

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unstable layer is called the "interaction depth". The interaction depth is set by the depth at

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which Hg from the sediment could directly affect the seawater, which ranges from 20 to

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100 cm;62, 65-67 an average of 50 cm is chosen here. Details of the parameters are provided

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in Table S8 (Supporting Information).

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This marine box model is applied to identify the influence of Hg from the mainland to

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the seawater and sea margin sediments. The trends in the Hg concentrations in each sea

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and their surface sediments are simulated, and the trends in the Hg concentrations in each

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reservoir from 2014 to 2030 are predicted based on the variations in the Hg(D) and Hg(P)

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flux from the mainland into the seas from 1984 to 2013. Three scenarios (increasing,

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stable and decreasing Hg concentration) are considered. An increase is predicted based on

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the extrapolation of Hg flux changes from the mainland to the seas from 1984 to 2013.

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Stability is predicted based on a stable Hg flux from the mainland in 2013. A decrease is

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predicted based on the 12th Five-year Plan of Environmental Protection,68 in which heavy

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metals that are discharged from human activities are planned to decrease by 15% from

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2007 to 2015, with this reducing trend estimated to be maintained through 2030.

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Uncertainty Analysis. Different Hg data in various jurisdictions, sectors and

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analytical analyses result in uncertainties. The method that was used by Amos et al. is

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applied to estimate the uncertainty.5 The standard error (SE) of the Hg concentration data

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is calculated by using the bootstrapping method.5, 69 The SEs of [Hg(T)], [Hg(D)] and

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[Hg(P)] are calculated for every pathway. When the number of concentration data from

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different literature (n) is ≥ 3, the bootstrapping method is applied to repeat sampling

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1,000 times to simulate the distribution of the Hg concentration. When n < 3, the SE is

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assumed to be 65%.5 The SEs of the Hg concentrations from the other five pathways are

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calculated with the same method. The errors for all pathways are added in quadrature.5

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The uncertainties of the Hg concentrations in seawater and sediment are estimated by 13



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applying Monte Carlo simulations. The uncertainties are repeatedly calculated 1,000

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times by randomly drawing all inputs from distributions that were simulated by the

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bootstrapping method.70, 71 Some parameters do not have standard errors, so their SEs are

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assumed to be 65% of the mean, as shown in a previous study.5 The medians and 25–75

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percentiles are calculated to quantify the concentrations and characterize the uncertainties.

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RESULTS AND DISCUSSION

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Mercury Export into the Adjacent Seas and Trends. The total Hg that was exported

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into the adjacent seas from mainland China in 2012 was estimated to be 240 ± 23 Mg

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(Table 1). The export from rivers, industrial wastewater, domestic sewage, groundwater,

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non-point sources and coastal erosion were considered in this inventory. The total Hg that

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was discharged from rivers into these seas was 160 ± 21 Mg in 2012 (Table 1) (67% of

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the total exported Hg). Riverine pathways were the largest source of exported Hg but

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contributed less than previous studies in the Mediterranean (84%) and Yellow Seas

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(77%),13, 16, 72 which might be partly attributed to the inclusion of Hg from industrial

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wastewater, domestic sewage, groundwater, non-point sources and coastal erosion in our

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study (Table 1). The total Hg that was discharged into these seas from Chinese rivers in

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2012 represented 2.9% and 6.5% of the previous global riverine inventory, according to

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Amos et al. and AMAP/UNEP, respectively.5, 6 The calculation method was the same,

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and the disparity mainly resulted from the KD value. In a previous study, the log10KD

285

value was 4.7 ± 0.3;5 however, the suspended sediment concentration in the Yellow River

286

was much higher than that in the other rivers in China. Therefore, using the KD of other

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rivers to calculate the Hg discharge of suspended sediments into the Yellow River would

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result in an overestimation. In this study, 2.1 ± 0.077 was used for the Yellow River, and 14


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3.8 ± 0.084 was used for the other rivers. The Hg(P) concentration of the Yellow River

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was 12 ng/g for this river’s KD value. If the KD of other rivers were to be used to calculate

291

the Hg(P) concentration of the Yellow River, the result would be 590 ng/g.

292

Correspondingly, the Hg(P) flux of the Yellow River would be 2.2 Mg/yr or 110 Mg/yr;

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notably, the latter value is 50 times higher than the former. Previous studies have reported

294

that the [Hg(P)] concentration of the Yellow River ranged from 12 to 200 ng/L (1.8 to 31

295

ng/g; the suspended sediment transport rate of the Yellow River was 6.5 kg/m3 in 2012)73,

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74

297

Hg(P) in rivers with high suspended sediment concentrations, such as the Yellow River.

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Table 1. Export of Hg into Adjacent Seas from Mainland China in 2012

. Clearly, the KD values of other rivers cannot be used to calculate the concentration of

Pathway

Hg(T) export

Hg(D) export

Hg(P) export

(Mg/yr)

(Mg/yr)

(Mg/yr)

Rivers

160 ± 21 a

71 ± 12

86 ± 9.6

Industrial wastewater

25 ± 5.0

8.0 ± 3.0

17 ± 7.0

Domestic sewage

1.0 ± 0.15

0.34 ± 4.6×10-2

0.71 ± 0.10

Groundwater

8.7 ± 3.5

6.1 ± 2.4

2.6 ± 1.1

Non-point source erosion

36 ± 7.5 b

n/a c

36 ± 7.5

Coastal erosion

8.2 ± 1.8 d

n/a

8.2 ± 1.8

Total

240 ± 23 e

86 ± 13

150 ± 14

a

All the data are expressed as the mean ± SE (standard error) and retain two significant digits. The errors

of every pathway are added in quadrature.5 bHg is assumed to remain in particle form and flow offshore directly from the non-point source. cNot available. dThe exported Hg by coastal erosion is estimated in a particulate state.11 eThe SE of the total Hg flux, which is added from all six pathways, is also estimated by adding in quadrature.

15



299

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Figure 1 provides the seasonal variation in Hg(T) discharges from the Yangtze River

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and Yellow River and the variation in riverine water discharge in the two rivers in 2012.

301

The Hg(T) flux was remarkably affected by the riverine water discharge.

302 303

Figure 1. Seasonal variation of total Hg discharged from the Yellow River (A) and Yangtze River (B), and

304

the variation of riverine water discharge in 2012.

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Figure 2 (A) shows the variation in the Hg(T) that is discharged from the Yangtze

306

River, Pearl River and Yellow River from 1984 to 2013. The Hg fluxes of these rivers are

307

remarkably influenced by the Hg concentration and riverine water discharge.27, 28 The

308

annual Hg(T) discharge from the Yangtze River increased over the last 30 years, and the

309

annual Hg(T) discharge from the Pearl River decreased in recent years. Because the

310

annual water runoff in the Yangtze River and Pearl River showed no obvious variations

311

during this period, the changes in the Hg flux could be attributed to the concentrations.

312

This observation is verified in the literature. The Hg pollution in the Yangtze river 16


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became increasingly serious from the 1980s to the 2000s,75-77 whereas the Hg pollution in

314

the Pearl River Basin was well controlled.26, 78 The annual Hg(T) discharge from the

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Yellow River decreased from 1984 to 2002 and increased from 2003 to 2012 with the

316

variation in the riverine water discharge,79 whereas the Hg concentration exhibited no

317

obvious variation tendencies.73, 80

318

The Hg that was exported from other pathways cannot be ignored (33% of the total Hg

319

that was exported from mainland China in 2012). The contributions of the other five

320

terrestrial pathways are presented in Figure 2 (B). The Hg that was transported from non-

321

point sources was the second largest source, amounting to 36 ± 7.5 Mg (15% in 2012),

322

but exhibited no obvious tendencies from 1984 to 2013 (Figure 2 (B)). The contribution

323

of Hg from industrial wastewater was slightly less than that from non-point sources,

324

which were 25 Mg (10% in 2012). However, the contribution of Hg from industrial

325

wastewater increased rapidly from 1984 to 2013 (Figure 2 (B)) because of the

326

development of industry.34 The Hg discharge from domestic sewage was similar to that

327

from industrial wastewater and increased rapidly from 1984 to 2013 because of rapid

328

increases in population and urbanization. However, the contribution from domestic

329

sewage was small, which were 1.0 Mg (0.42% in 2012). The contribution of Hg from

330

groundwater was 8.7 Mg in 2012 (3.6%) and increased from 1984 to 2013 because Hg

331

pollution in groundwater in China has become more serious in recent years.81, 82

332

Wastewater irrigation was the most important source for Hg contamination in soil and

333

groundwater in China.83 The contribution of Hg from coastal erosion was 8.2 Mg (3.4%

334

in 2012), which remained unchanged from 1984 to 2013.

17



335

Page 18 of 29

Hg that is exported from anthropogenic and natural sources of mainland China can be

336

differentiated based on the results of Mason et al.84 According to their model of the

337

preindustrial global mercury cycle and current global mercury cycle, 80% of the Hg that

338

is discharged from rivers, 10% of the Hg that is discharged from surface soil erosion, and

339

80% of the Hg that is discharged from groundwater originated from human activities.

340

Based on these assumptions, the Hg that was exported into adjacent seas from all the

341

natural sources that are considered in this study from mainland China was estimated to be

342

75 Mg in 2012 (30% of all exported Hg). Anthropogenic sources contribute 170 Mg

343

(70%), approximately 1/3 of the Hg emissions into the air from anthropogenic sources in

344

China, which were estimated to be 538 t in 2010.20

345 18


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346

Figure 2. Total exported Hg from mainland China to the seas and changes from 1984 to 2013. (A)

347

Contributions of total Hg from the three main rivers of China (Yangtze River, Pearl River and Yellow

348

River); (B) Contributions of total Hg from the other five terrestrial pathways.

349

Mass Balance of Hg in China’s Adjacent Seas and the Influence of Exported

350

Mercury. A marine box model was developed to simulate the Hg mass balance in the

351

four adjacent seas of China. We found that the total Hg that was exported from various

352

sources in mainland China (53% of the total Hg input) was the dominant Hg input into

353

the seas. The significant terrestrial inputs in this study could be attributed to strong

354

anthropogenic activities, high sediment transport in rivers, and the involvement of more

355

input sources in this study. The amount of Hg from atmospheric deposition in China’s

356

adjacent seas was 140 Mg in 2012 (30%). The proportion of Hg that was deposited into

357

sediment was 50% of the total Hg output. Hg evasion from surface sediment and surface

358

water comprised 2.4% of the total Hg input and 26% of the total Hg output of the seas.

359

The amount of Hg that was transported from China’s adjacent seas into the Pacific Ocean

360

and other open oceans (Figure 3) through oceanic currents was 160 Mg in 2012 (40% of

361

the total Hg output from the water body). This process may influence the Hg mass

362

balance and aquatic ecosystem of the Pacific Ocean.

19



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363 364

Figure 3. Hg mass balance in China’s adjacent seas and sea margin sediments in 2012 (unit: Mg). Hg(D): Hg in dissolved phase; Hg(P): Hg in particulate phase.

20


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365


The total Hg concentrations in the Bohai Sea, Yellow Sea, East China Sea and South

366

China Sea were 55 (uncertainty from -16% to 23%), 2.1 (-25% to 50%), 2.4 (-35% to

367

75%) and 1.0 (-35% to 62%) ng/L in 2012, respectively. The result for the Bohai Sea was

368

remarkably higher than those for the other seas, ranging from 25 to 100 ng/L.85-88 This

369

value could be attributed to the well-developed industry in surrounding provinces87 and

370

the small, shallow (average depth of 20 m) and relatively enclosed waters of the Bohai

371

Sea.85-87 However, the Hg concentration in the sediment of the Bohai Sea was not

372

significantly higher than those in the other seas. The Yellow River, which has the second

373

largest mass of suspended sediment discharge in the world,89 contributes abundant

374

sediment to the Bohai Sea. The literature (Table 1, Supporting Information) proved that

375

the Hg(P) concentrations in the sediment of the Yellow River are lower than other major

376

rivers,26, 74, 90 which might explain the relatively low Hg concentrations in the sediment of

377

the Bohai Sea.

378

The Hg concentrations of the other three Chinese seas were also larger than those in

379

the other major oceans around the world.3 The total Hg concentrations in the sea margin

380

sediments of the four seas were 31 (-23% to 35%), 27 (-22% to 43%), 37 (-44% to 110%)

381

and 47 (-42% to 71%) ng/g in 2012, respectively. These values were within the ranges of

382

other seas, such as the Mediterranean Sea (8.0–40 ng/g)91 and Greenland Sea (6.0–260

383

ng/g).92 The result of Monte Carlo simulations indicate that the uncertainties in the Hg

384

concentrations mainly came from ocean currents, but the uncertainties in the Hg

385

concentrations of sediments were higher than those of seawater.

386

The temporal variations in the Hg mass balance in the four seas and their sediment

387

margin areas from 1984 to 2013 were determined according to the box model results and 21



Page 22 of 29

388

are depicted in Figure 4. In Figure 4, the Hg(T) concentrations in the seas changed with

389

the Hg inputs. The increase in the Hg(T) concentrations in the Bohai Sea and Yellow Sea

390

over the last 30 years was significant (Figure 4 (B1) and Figure 4 (B2)) because their sea

391

reservoirs of Hg were relatively small, which were 84 Mg and 36 Mg, respectively

392

(Figure 3) in 2012. The increase in the Hg(T) concentration in the East China Sea was

393

also significant (Figure 4 (B3)) because of the effect of the Yangtze River. The variation

394

in the Hg(T) concentration in the South China Sea was insignificant because the sea

395

reservoir of Hg in the South China Sea was 5100 Mg in 2012 and the Hg input from

396

mainland China was relatively small. In addition, no significant increase in Hg input was

397

observed in the South China Sea (Figure 4 (A4)).

398

Small changes were observed for the concentrations of Hg(T) in the sea margin

399

sediments from 1984 to 2013, which were significantly different from the seawater

400

(Figure 4(C)). Only 40% of the Hg(P) from mainland China was deposited into the sea

401

margin;93 this proportion in a previous study3 was approximately 72% in the North

402

Pacific Ocean. Sediment transport was considered in the box model93 because the Hg(P)

403

in sea margin sediments could be transported to the open ocean. Sea margin sediments

404

cannot be recognized as the most important sink in the marine biogeochemical cycle in

405

this area.5

406

The Hg concentrations in China’s adjacent seas could remain stable or slightly increase

407

from 2014 to 2030 (Figure 4 (C)) according to the stable and decreasing scenarios if

408

effective measures are implemented to control the Hg that is exported from

409

anthropogenic sources in mainland China. However, the Hg concentrations in the

410

seawater of the Bohai Sea, Yellow Sea and East China Sea and the sea margin sediments 22


Page 23 of 29


411

of the Bohai Sea and East China Sea could rise to a relatively high level in 2030

412

according to the increasing scenario if the Hg that is exported from mainland China

413

continuously increases. Due to the limit of measurement data of Chinese adjacent seas,

414

the model gives a relatively rough prediction from 2014 to 2030. In order to provide a

415

better prediction in the future, more studies should be carried out with the support of

416

extensive sampling and monitoring, as well as the improvement of the model.

417 418

Figure 4. Variation in the Hg that was exported from 1984 to 2013 and predictions of Hg in the seawater

419

and sea margin sediments from the box model. (A) is the flux of particulate Hg and dissolved Hg from

420

mainland China from 1984 to 2013, (B) is the total Hg concentration in the seawater and uncertainty, and

421

(C) is the total Hg concentration in the sea margin sediments and uncertainty. (1) is the Bohai Sea, (2) is the

422

Yellow Sea, (3) is the East China Sea, and (4) is the South China Sea. Predictions to 2030 were included in

23



Page 24 of 29

423

Figures (B) and (C). The observational data in the figure are all from previous studies and are provided in

424

Table S8 (Supporting Information).

425

ASSOCIATED CONTENT

426

Supporting Information

427

The Supporting Information includes the complete Hg concentration data of every

428

pathway and detailed descriptions of the data that were used to estimate the Hg that was

429

exported from mainland China.

430

ACKNOWLEDGEMENTS

431

This work was funded by the National Natural Science Foundation of China (41571484,

432

41130535, 41471403). The authors would like to thank the editor and three reviewers for

433

their insight comments and improvements on the manuscript.

434

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Mercury Export from Mainland China to Adjacent Seas and Its

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