An Integrated Model for Input and Migration of Mercury in Chinese

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Environmental Processes

An Integrated Model for Input and Migration of Mercury in Chinese Coastal Sediments Mei Meng, Ruoyu Sun, Hongwei Liu, Ben Yu, Yongguang Yin, Ligang Hu, Jianbo Shi, and Guibin Jiang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06329 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Environmental Science & Technology

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An Integrated Model for Input and Migration of Mercury in

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Chinese Coastal Sediments

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Mei Meng†, ‡, Ruo-yu Sun†, Hong-wei Liu‡, Ben Yu‡, Yong-guang Yin‡, Li-gang Hu‡,

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Jian-bo Shi*, ‡, §, Gui-bin Jiang‡

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† Institute

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‡

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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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§ Institute

of Surface-Earth System Science, Tianjin University, Tianjin 300072, China

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for

of Environment and Health, Jianghan University, Wuhan 430056, China

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* Corresponding author (J.B. Shi)

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Tel/fax: +86-10-62849129

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E-mail: [email protected]

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ACS Paragon Plus Environment


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TOC

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ABSTRACT

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Coastal sediments are a major sink of global mercury (Hg) biogeochemical cycle,

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bridging terrestrial Hg migration to the open ocean. It is thus of substantial interest to quantify

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the Hg contributors to coastal sediments and the extents to which the Hg sequestered into

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coastal sediments affects the ocean. Here, we measured concentrations and isotope

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compositions of Hg in Chinese coastal sediments, and found that estuary sediments had

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distinctly higher δ202Hg and lower Δ199Hg values than marine sediments. Hg isotope

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compositions of marine sediments followed a latitudinal trend where δ202Hg decreases and

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Δ199Hg increases from north to south. An integrated model was developed based on Hg

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isotope mixing model and urban distance factor (UDF), which revealed a significant

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difference in Hg source contributions among the estuary and marine sediments, and a gradual

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change of dominant Hg sources from terrestrial inputs (riverine and industrial wastewater

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discharges) to atmospheric deposition with the decrease of urban impact. An UDF value of

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306 ± 217 was established as the critical point where dominant Hg sources started to change

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from terrestrial inputs to atmospheric deposition. Our study help understand the input,

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migration of Hg in Chinese marginal seas, and provide critical insights for targeted

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environmental management.

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■ INTRODUCTION

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Ocean serves as a central role in mediating the global cycling of mercury (Hg). It receives

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atmospheric Hg dry/wet deposition (5000-6500 Mg yr-1) and riverine Hg discharges

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(1000-5500 Mg yr-1), and meanwhile emits dissolved elemental Hg0 (4000-5000 Mg yr-1) into

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the atmosphere.1-4 As estimated by Amos et al. (2014),1 about 70% of riverine Hg is buried in

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the worldwide coastal sediments, with the remainder exported to the open ocean. Therefore,

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burial of Hg in coastal sediments represents a major sink of global Hg biogeochemical cycle.

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A small fraction of accumulated Hg in sediments and oceanic water columns (typically
500 km and an average water depth of 60 m.65

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The Yangtze River, the largest inflowing river, discharges 5.0×108 tons of sediments

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annually.65 The formed Yangtze River Estuary plays a vital role in delivering various

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contaminants to the adjacent sea.66-68 Dominated by subtropical climate, the ECS has high

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temperature (20-26 C) and salinity (> 34%). The SCS is characterized by a tropical marine

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monsoon climate, with all-year-round high temperature (25-28 C), abundant rainfall 7



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(1100-2500 mm yr-1), and significant influence from typhoon. The Pearl River is the largest

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inflowing river, bringing ~9.0×107 tons of sediments annually into the Pearl River Estuary, a

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seriously contaminated estuary in China.58, 69, 70

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Sample Collection and Pretreatment. The sampling campaigns were conducted during

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two cruises in June 2011 and May 2012. A total of 141 surface sediments (0-3 cm) were

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collected using a box corer from BS (n = 29, from 12.5 m to 38 m), YS (n = 55, from 20.7 m

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to 79 m), and ECS (n = 37, from 39.5 m to 99.5 m) and its Yangtze River Estuary (n = 20)

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(Figure 1). The sedimentation rates in the studied sea area varied from 0.23 to 2.86 cm

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yr-1,71-76 and thus the collected sediments were expected to integrate deposition from 1998 to

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2012, reflecting surrounding Hg inputs from recent 1-2 decades. The surface sediments from

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SCS (n = 6, from 2918 m to 3739 m) and its Pearl River Estuary (n = 37) reported by Yin et al.

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(2015)58 are also shown in Figure 1. These sampling sites cover most of Chinese marginal sea

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areas. We distinguish surface sediments as estuary sediments and marine sediments for

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samples collected from estuaries and seas, respectively. Besides, seven surface soil samples

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(0-2 cm) were collected from the industrial areas of Dalian City in the northeast coast of BS

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(Figure 1). Upon collection, all the samples were kept in a -20 C refrigerator immediately

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and transported to the laboratory, where they were freeze-dried at -45 C, ground and

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homogenized to less than 80 meshes before analysis.

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Measurement of Total Hg and Hg Isotope Ratio. A Hydra-C Hg analyzer (Teledyne

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Lemman Labs, USA) was first used for direct analysis of total Hg (THg) concentration in

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sediments and soils, following the USEPA method 7473.77 Then, all the sediments and soils

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were digested using aqua regia in a water bath of 95 C for 4 h. THg concentrations in 8


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digested sample solutions were determined with an inductively coupled plasma mass

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spectrometry (ICP-MS, Thermo iCAP-Q, Thermo Fisher Scientific, USA), which agreed well

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with THg concentrations measured by Hydra-C Hg analyzer. Before Hg isotope ratio

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measurement by a multi-collector (MC) ICP-MS (Nu-Plasma Ⅱ, Nu Instruments, UK), the

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digested sample solutions were diluted to Hg concentrations of 1-3 ng mL-1. Mass bias of

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MC-ICP-MS was corrected by internal NIST-997 Tl standard using the exponential law and

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by NIST-3133 Hg standard using the standard-sample bracketing method. Details on the

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analytical procedures for THg concentrations and Hg isotope ratios in the sediments and soils

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are provided in the Supporting Information (SI). We deem the larger 2SD uncertainties of Hg

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isotope ratios in UM-Almadén and GBW07310 (sediment reference) as the typical analytic

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uncertainties of Hg isotope ratios in sediments and soils, which are 0.14‰ for δ202Hg, 0.04‰

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for Δ199Hg, 0.03‰ for Δ200Hg, and 0.04‰ for Δ201Hg. The 2SE uncertainties of Hg isotope

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ratios in samples with multiple analysis are applied only when they are larger than the typical

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analytic uncertainties.

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Construction of An Integrated SC-UDF Model. To unravel the contributed percentage

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of possible Hg sources in the sediments of the different sea areas and to depict how the urban

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activities impact Hg migration from nearshore to offshore region, an integrated SC-UDF

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model was constructed through combining the Hg isotope source contribution (SC) model and

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urban distance factor (UDF) model.

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The source contributions were modeled by a Monte Carlo simulation approach used by

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Jiskra et al. (2015, 2017),44, 78 through the pseudorandom number generation function (i = 1 :

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1,000,000) of the MatLab software (R2016a, MathWorks) based on the following equations: 9



n

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 f * IR = IR i

i

f

1

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(1)

sample

i 1 n

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i

(2)

i 1

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where i is the number of Hg source end-member, fi is the fraction of each end-member, IRi is

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the isotope ratios of each end-member, and IRsample is the measured isotope ratios of samples.

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The isotope ratios are δ202Hg and ΔxxxHg (xxx = 199, 200, 201, 204).

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The degree of urban impact was modeled by an urban distance factor (UDF) based on the

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city population (P) and the distances (D) between sampling sites and potential source cities.

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Seven forms of UDF were calculated for each sampling site (see SI), and the UDF1 (= √P / D)

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was selected here, considering the large population base in Chinese cities. We established a

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database of UDF1 for the Chinese marginal sea areas, using the updated population data (i.e.,

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the 2010 China Census) for all the 155 potential source cities (Table S2) along the whole

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eastern and southern coastal regions in China (Figure S1). Details on the calculation process

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(Table S3) and the final UDF1 database (Table S4) are provided in the SI.

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■ RESULTS AND DISCUSSION

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Variation of THg Concentrations in Surface Sediments. THg concentrations in all the

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surface sediments ranged from 7.0 to 159.6 μg kg-1, with a mean value of 31.5 ± 19.1 μg kg-1

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(n = 141) (Table 1). One sample (B7, Figure 1) collected at the near-lowest water depth (12.5

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m) of BS was found to be highly enriched in Hg as compared to other samples (159.6 μg kg-1

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vs. 7.0 to 84.3 μg kg-1). On average, the Yangtze River Estuary sediments had the highest

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THg concentrations of 48.8 ± 18.7 μg kg-1 (n = 20), followed by BS of 38.7 ± 26.3 μg kg-1 (n 10


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= 29), ECS of 27.3 ± 15.0 μg kg-1 (n = 37), and YS of 24.2 ± 9.8 μg kg-1 (n = 55). In

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comparison, the SCS and its Pearl River Estuary sediments reported by Yin et al. (2015)58 had

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average THg concentrations of 75.5 ± 6.3 μg kg-1 (n = 6) and 98.4 ± 39.4 μg kg-1 (n = 37),

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respectively (Table 1; Figure S2-S3). This indicated that Hg levels in sediments from the

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south coast were more elevated than those from the east coast of China. The contour map of

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THg (Figure 2) clearly shows that the sediments near the estuaries (Yellow River Estuary and

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Yangtze River Estuary) and along the ECS coast had obviously higher THg concentrations

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than those of offshore areas, suggesting that terrestrial inputs by river runoffs and wastewater

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discharges were the main sources of Hg in coastal areas. As estimated by Liu et al. (2016),12

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the annual terrestrial Hg exported from mainland China into adjacent seas were 24 Mg for BS,

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25 Mg for YS, and 125 Mg for ECS in 2012. By normalizing the annual terrestrial Hg input

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by water area12 in each sea basin, we found that BS had the highest terrestrial Hg input rate of

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0.3 kg m-2 yr-1, followed by ECS of 0.16 kg m-2 yr-1 and YS of 0.07 kg m-2 yr-1. The terrestrial

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Hg input rates are positively correlated (R2 = 0.96, P = 0.12, n = 3) with mean THg

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concentrations of marine sediments in the studied three marginal seas, highlighting the

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importance of terrestrial inputs in contributing Hg enrichment in sediments.

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Among the terrestrial Hg inputs, riverine runoffs have been shown to be the dominant Hg

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sources.1, 12 The delivered Hg might combine with organic matter of river suspended loads, as

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seen from the positive correlation between THg concentrations and total organic carbon (TOC)

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contents in all sediments (R2 = 0.11, P < 0.01, n = 141). In addition, THg concentrations in

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surface sediments have a negative correlation with co-located pH values (R2 = 0.08, P < 0.01,

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n = 141), with high pH values generally corresponding to low sediment THg concentrations. 11



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The above observations corroborate previous studies demonstrating that clay sediments with

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high TOC enriched in humic acid and fulvic acid are the main Hg sink of Chinese marginal

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sea.16 However, caution should be taken to the obtained correlations, because TOC and pH

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each only explains ~10% of THg variance in sediments. Despite less affected by terrestrial

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inputs, the sediments in central YS (Y3, Y5, Y6, Y8, Y9, Y10, Y12, Y13, Y17, Y51, Y52)

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had higher (t-test, P < 0.05) THg concentrations (37.1 ± 5.7 μg kg-1) than sediments near the

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coast (21.0 ± 7.8 μg kg-1) (Figure 2; Figure S2). We attribute this contrast to the following

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reasons. First, the low terrestrial Hg inputs (absence of large rivers)12 and large surface area of

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YS could have diminished the contribution of terrestrial inputs to the Hg levels in the coastal

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areas. Second, atmospheric Hg deposition accounts for a large fraction of Hg inputs in YS,

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which has been observed to be enhanced in central YS.75 Third, an offshore mud area

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containing significant high TOC (Figure 2; Figure S4) is frequently generated in central YS

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under the influence of current circulation, serving as a scavenger of seawater Hg.

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Variation of Hg Isotope Ratios in Surface Sediments. Surface sediments exhibited a

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large variation in both δ202Hg (-2.43‰ to -0.09‰) and Δ199Hg (-0.08‰ to 0.31‰) (Table 1).

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Δ200Hg values in surface sediments were very small from -0.04‰ to 0.09‰, and most were

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within the measurement uncertainty. It is noted that the sample (B7) with the highest THg

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concentration (159.6 μg kg-1) had the highest δ202Hg value of -0.09‰. Different from the

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estuary sediments exhibiting small and negative Δ199Hg values, the marine sediments were

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typically characterized by positive Δ199Hg values.

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On average, δ202Hg values in surface sediments were high in BS (-0.93 ± 0.33‰, -1.48‰

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to -0.09‰) and Yangtze River Estuary (-0.98 ± 0.15‰, -1.54‰ to -0.74‰), and low in ECS 12


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(-1.63 ± 0.30‰, -2.19‰ to -1.13‰) and YS (-1.82 ± 0.45‰, -2.43‰ to -0.47‰) (Table 1). In

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contrast, Δ199Hg values in surface sediments were highest in YS (0.19 ± 0.05‰, 0.07‰ to

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0.31‰) followed by ECS (0.12 ± 0.05‰, 0.03‰ to 0.22‰), BS (0.09 ± 0.05‰, -0.01‰ to

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0.19‰) and Yangtze River Estuary (-0.02 ± 0.02‰, -0.08‰ to -0.01‰) (Table 1). Thus, it

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appears that the surface sediments with elevated Hg were more enriched in heavy Hg isotopes

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and had small or no MIF anomalies. Similar phenomenon has also been observed in SCS and

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other marginal seas.54, 57, 58 For example, Yin et al. (2015)58 showed that the sediments from

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SCS had low δ202Hg of -2.44‰ and high Δ199Hg of 0.35‰, whereas sediments from its Pearl

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River Estuary had high δ202Hg of -1.60‰ and near-zero Δ199Hg of -0.01‰.

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The linear regressions of δ202Hg versus 1/THg and Δ199Hg versus 1/THg in each estuary

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and sea were performed and shown in Figure S5. Only THg concentrations in sediments from

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Pearl River Estuary studied by Yin et al. (2015)58 showed significant correlations with both

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δ202Hg (R2 = 0.65, P < 0.01, n = 37) and Δ199Hg (R2 = 0.33, P < 0.01, n = 37), suggesting a

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mixing of binary Hg sources. THg concentrations in sediments from BS significantly

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correlated with Δ199Hg (R2 = 0.14, P < 0.05, n = 29) rather than δ202Hg. THg concentrations in

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sediments from other areas showed no correlations with either δ202Hg or Δ199Hg. Thus, we

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suggest that the variations in Hg isotope compositions of our studied sediments can not be

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convincingly explained by two sources.

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Potential Hg Sources and Influencing Factors. The Yangtze River Estuary in ECS

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primarily receives terrestrial inputs brought by the Yangtze River, the largest river in China.

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The formed “Yangtze River Delta” is surrounded by the highly-developed industrial and

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urbanized cities in Jiangsu and Zhejiang Provinces and Shanghai city. Discharges of industrial 13



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wastewater and domestic sewage into aquatic environments and soils would be carried away

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by terrestrial runoffs and accumulated into the Yangtze River Estuary. Besides, the Yangtze

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River Estuary is characterized by high total suspended particulates (0.7-51.5 g L-1),79 high

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sedimentation rate (1.36 to 4.11 cm yr-1),80 and low water clarity (< 100 FTU).81 Thus,

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Yangtze River Estuary sediments are expected to conserve the Hg isotope signatures of

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terrestrial inputs containing large amounts of anthropogenic Hg. The similarity in δ202Hg

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between Yangtze River Estuary sediments and BS marine sediments and their high THg

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concentrations suggest that BS was seriously affected by terrestrial Hg inputs as well. Unlike

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other Chinese marginal seas, BS is a nearly full-enclosed sea surrounded by a large number of

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industrialized and densely populated cities in Liaoning, Hebei and Shandong Provinces, and

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receives terrestrial discharges accumulated in the Yellow River Estuary. As shown in Figure

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S4, the suspended loads along the coastlines and from the Yellow River Estuary could be well

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dispersed into offshore areas of BS under the influence of Bohai Coastal Current (BCC).

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Different from the Yangtze River Estuary sediments, the marine sediments of BS had

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small, positive Δ199Hg of 0.09 ± 0.05‰ (-0.01‰ to 0.19‰). Positive but slightly higher

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Δ199Hg values were also observed in the marine sediments of YS (0.19‰, 0.07‰ to 0.31‰)

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and ECS (0.12‰, 0.03‰ to 0.22‰) of more open water areas in this study, and deep marine

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sediments in SCS (0.35‰, 0.21‰ to 0.45‰) reported by Yin et al. (2015).58 In addition, the

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marine sediments in other marginal seas were found with positive Δ199Hg as well.53,

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Strock et al. (2015)39 showed that seawater samples from Canadian Arctic Archipelago were

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typically characterized with positive Δ199Hg (0.1‰ to 0.4‰). Thus, Δ199Hg of marine

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sediments was largely imparted by overlying seawater Δ199Hg. 14


56, 57

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As shown in Figure S6, Δ199Hg values in marine sediments of Chinese seas had the

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highest correlation (R2 = 0.26, P < 0.01, n = 141) with co-located water depths, suggesting

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that the offshore and semi-pelagic seawater had higher Δ199Hg than coastal seawater. Hg in

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offshore and semi-pelagic seawater largely derives from atmospheric wet Hg deposition with

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large positive Δ199Hg, whereas Hg in coastal seawater is seriously affected by terrestrial Hg

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with negative to near-zero Δ199Hg.57 We also found that Δ199Hg values in marine sediments

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were positively correlated with TOC and negatively correlated with pH (Figure S6),

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suggesting Hg in offshore seawater was probably sequestrated by organic matter in marine

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sediments. Notably, the sediments in central YS with the highest TOC and lowest pH values

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had very positive Δ199Hg values (Figure 2).

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The linear regressions of Δ199Hg versus Δ201Hg (Figure S7) in sediments from BS (slope =

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0.95; R2 = 0.54, P < 0.01, n = 29), YS (slope = 1.01; R2 = 0.32, P < 0.01, n = 55) and ECS

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(slope = 1.07; R2 = 0.75, P < 0.01, n = 37) in this study, and from SCS/Pearl River Estuary

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(slope = 1.05; R2 = 0.97, P < 0.01, n = 43) in Yin et al. (2015)58 all show a Δ199Hg/Δ201Hg

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slope near the unity, demonstrating that Hg had undergone photo-reduction processes before

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incorporation into the sedimentary particles in these sea areas. Photo-reduction of

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post-depositional Hg in sediments is unlikely significant, because the sedimentary Hg is

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thought to be less reactive than dissolved Hg, and has a strong complexion with organic

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matter and sulfides in sediments. Additionally, the low clarity of coastal seawater would

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greatly attenuate the intensity of light penetrated into the seawater, decreasing the extents of

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photo-reduction of sedimentary Hg.

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Integrated SC-UDF Model. Here, an integrated SC-UDF model was developed for 15



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Chinese marginal sea area using (i) Hg source contribution (SC) and (ii) urban distance factor

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(UDF) database, as follows:

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(i) Hg Source Contribution. Binary or ternary mixing model has been established to

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identify Hg sources and quantify their contributions in sediments if different Hg sources have

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characteristic and distinguishable isotope compositions.43, 58, 82 Since geochemical processes

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could alter the Hg isotope compositions of original sources, the appointment of Hg

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end-members in mixing models should reasonably correct Hg isotope fractionation attributed

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to processes occurring before and after source Hg deposition.83 For example, Lepak et al.

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(2015)43 assumed that adsorption of dissolved Hg onto particles could cause an enrichment of

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lighter Hg isotopes in particles, and applied a negative MDF offset (-0.40‰ in δ202Hg) to

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correct precipitation Hg deposited into the Great Lakes sediments in a ternary mixing model.

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This MDF offset is based on a laboratory-controlled experiment on dissolved Hg adsorption

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onto goethite reported by Jiskra et al. (2012).26 However, Washburn et al. (2017)84 have

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recently observed a consistent and positive MDF offset (0.28‰ in δ202Hg) from dissolved Hg

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to suspended particulate bound Hg in a contaminated river system of Virginia. This indicates

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that the correction of source Hg isotope compositions might involve with considerable

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uncertainty. For this reason, the use of the samples that integrate isotope signatures of both

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potential Hg sources and the subsequent transport and transformation processes as the

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end-members is more plausible than the use of sources itself or sources only corrected by a

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process offset.85 Here, we constructed an improved ternary mixing model for surface

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sediments of Chinese marginal seas (Figure 3), with the three end-members determined based

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on soil samples in this study (representing direct discharge of industrial Hg into seawater), 16


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riverine sediments taken from a contaminated river49 (representing riverine Hg inputs), and

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deep-sea sediments58 (representing atmospheric Hg deposition) (Table S6).

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(1) Direct Discharge of Industrial Hg. THg concentrations in studied surface soils near

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the coastal city varied over a wide range of 23.4-912.7 μg kg-1, with a mean value of 295.5 ±

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386.9 μg kg-1 (Table 1). Two (So3 and So4, in bold, Table S5) of these soil samples with

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extremely high THg levels (897.5 μg kg-1; 912.7 μg kg-1) were sampled near large wastewater

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outflows of an industrial complex, which were probably severely contaminated by Hg in the

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discharged wastewater. These surface coastal soils had a narrow variation range between

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-0.67‰ and -0.37‰ in δ202Hg, and negligible MIF (-0.02‰ to 0.04‰ for Δ199Hg; -0.03‰ to

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0.02‰ for Δ201Hg) (Table 1). The mean Hg isotope compositions (δ202Hg = -0.50 ± 0.10‰;

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Δ199Hg = 0.01 ± 0.02‰) of these soils are comparable to those of commercial liquid Hg

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(δ202Hg = -0.40‰; Δ199Hg = -0.02‰), Hg ores (δ202Hg = -0.59‰; Δ199Hg = 0.03‰), and

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other non-ferrous metal ores (δ202Hg = -0.48‰, Δ199Hg = 0.01‰) as summarized in Sun et al.

339

(2016).86 We thus used Hg isotope compositions of these measured coastal soils to represent

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those of discharged wastewater from industries.

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(2) Riverine Hg Inputs. Hg isotope compositions in dissolved and particulate-bound Hg

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in Chinese rivers are currently not available. Because particulate-bound Hg is the primary

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form of Hg in rivers and most of particulate-bound Hg is buried in the coastal sediments,1 we

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used Hg isotope compositions (δ202Hg = -1.65 ± 0.54‰; Δ199Hg = -0.11 ± 0.09‰) of surface

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sediments from a contaminated river reported by Liu et al. (2011)49 to represent riverine Hg

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inputs. The chosen river flows through urban, industrial, and background regions and is the

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main inflowing river to Pearl River Estuary. Its sediments can be deemed as the best 17



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integrators of isotope signatures of mixed anthropogenic Hg sources. We expect that rivers

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flowing into other Chinese estuaries were contaminated by similar anthropogenic Hg sources.

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Our used values for riverine Hg inputs are comparable to Hg isotope compositions of

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watershed soils (δ202Hg: -1.82 ± 0.39‰; Δ199Hg: -0.29 ± 0.12‰) that were compiled globally

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and used in a ternary mixing model for Hg tracing in Chinese coastal sediment cores.82

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(3) Atmospheric Hg Deposition. Atmospheric Hg deposition is an important Hg source

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to oceans. However, the atmospheric Hg incorporated into sediments should have quite

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different Hg isotope compositions from the original atmospheric Hg, because atmospheric

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dry/wet deposition processes, the following Hg transformation (e.g., photo-reduction and

357

adsorption) in seawater columns and post-depositional processes are expected to cause

358

significant Hg isotope fractionation. Thus, we used Hg isotope compositions (δ202Hg = -2.44

359

± 0.25‰; Δ199Hg = 0.35 ± 0.08‰) in the deep-sea offshore sediments (2918-3739 m) of

360

SCS58 to represent those of atmospheric Hg deposition. The sediments in SCS were collected

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from deep-sea offshore regions, and were mainly affected by Hg from atmospheric deposition

362

with limited terrestrial Hg contribution.

363 364

According to the isotope mixing model presented in equations 1-2, the fractions of the above three end-members (sources) were simulated according to the following equations:

365

f1 * δ202Hg1 + f2 * δ202Hg2 + f3 * δ202Hg3 = δ202Hgsample

(3)

366

f1 * Δ199Hg1 + f2 * Δ199Hg2 + f3 * Δ199Hg3 = Δ199Hgsample

(4)

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f1 * Δ201Hg1 + f2 * Δ201Hg2 + f3 * Δ201Hg3 = Δ201Hgsample

(5)

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f1 + f2 + f3 = 1

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The simulated results (mean ± 1SD) for fractions of direct industrial Hg discharge (f1),

(6)

18


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riverine Hg (f2), and atmospheric Hg (f3) were summarized in Table S7 and shown in Figure

371

S8. Terrestrial inputs were the main Hg sources in BS and Yangtze River Estuary and Pearl

372

River Estuary (Figure S8), with direct industrial wastewater Hg dominated in BS and riverine

373

Hg dominated in these two estuaries. SCS sediments were nearly exclusively contributed by

374

atmospheric Hg deposition (Figure S8), because we used them as the atmospheric Hg

375

end-member. Atmospheric Hg deposition was also the largest Hg source in most of the

376

sediments from YS, although Hg contribution from riverine inputs was significant as well

377

(Figure S8). Notably, some sediments in YS near Shandong/Liaoning Provinces had the

378

largest Hg contribution from industrial discharge. For ECS, the largest contributors changed

379

from terrestrial inputs in estuary sediments (on average, 92.7% in Yangtze River Estuary) to

380

atmospheric deposition in offshore sediments (on average, 50.9% in sites: E8, E9, E13, E14,

381

E18, E19, E26, E27, Figure 1; in bold, Table S5) (Figure S8). A transition of dominant Hg

382

sources from terrestrial inputs to atmospheric deposition in ECS was estimated to be 367 ± 24

383

km (Figure S8).

384

(ii) UDF Database. The Pearl River Estuary sampling sites showed the highest UDF1

385

values (738.1, 640.4 to 795.5), suggesting the greatest urban impact. The BS sampling sites

386

had the next high UDF1 values (473.3, 434.3 to 499.9), followed by sampling sites from

387

Yangtze River Estuary (425.2, 390.1 to 476.3), YS (407.0, 342.8 to 517.8), ECS (357.4, 276.2

388

to 456.9), and SCS (261.0, 228.0 to 283.7).

389

Based on the calculation results of SC and UDF1, an integrated SC-UDF model was

390

developed and shown in Figure 4. The left side of Figure 4 displayed both the Hg source

391

contribution percentage (pie-map) and the urban impact degree (UDF1, contour-map). In 19



392

general, a decreasing degree of urban impact corresponds to a change in dominant Hg sources

393

from terrestrial inputs (i.e., the sum of industrial wastewater discharge and riverine inputs) to

394

atmospheric deposition. The right side of Figure 4 showed the relationships of UDF1 with

395

fractions of terrestrial Hg and atmospheric Hg. With the decrease of UDF1, the contribution

396

of terrestrial Hg showed a significantly decreasing trend (y = 0.163x + 0.10, R2 = 0.26, P

An Integrated Model for Input and Migration of Mercury in Chinese

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