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Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 106 (2013) 379–390 www.elsevier.com/locate/gca Influence of salinity intrusion on the speciation and partitioning of mercury in the Mekong River Delta Seam Noh a, Mijin Choi a, Eunhee Kim a, Nguyen Phuoc Dan b, Bui Xuan Thanh b, Nguyen Thi Van Ha c, Suthipong Sthiannopkao d, Seunghee Han a,⇑ a School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea b Faculty of Environment, Ho Chi Minh City University of Technology, District 10, Ho Chi Minh City, Viet Nam c Faculty of Environment, Ho Chi Minh City University of Natural Resources and Environment, Tan Binh District, Ho Chi Minh City, Viet Nam d Department of Environmental Engineering, Dong-A University, Busan 604-714, Republic of Korea Received 26 February 2012; accepted in revised form 11 December 2012; available online 29 December 2012 Abstract The lower Mekong and Saigon River Basins are dominated by distinctive monsoon seasons, dry and rainy seasons Most of the Mekong River is a freshwater region during the rainy season, whereas during the dry season, salt water intrudes approximately 70 km inland To understand the role of salinity intrusion controlling Hg behavior in the Mekong and Saigon River Basins, Hg and monomethylmercury (MMHg) in surface water and sediment of the Mekong River and in sediment of the Saigon River were investigated in the dry season Sediment Hg distribution, ranging from 0.12 to 0.76 nmol gÀ1, was mainly controlled by organic carbon distribution in the Mekong River; however, the location of point sources was more important in the Saigon River (0.21–0.65 nmol gÀ1) The MMHg concentrations in Mekong (0.16–6.1 pmol gÀ1) and Saigon (0.70–8.7 pmol gÀ1) sediment typically showed significant increases in the estuarine head, with sharp increases of acid volatile sulfide Unfiltered Hg (4.6–222 pM) and filtered Hg (1.2–14 pM) in the Mekong River increased in the estuarine zone due to enhanced particle loads Conversely, unfiltered MMHg (0.056–0.39 pM) and filtered MMHg (0.020–0.17 pM) was similar between freshwater and estuarine zones, which was associated with mixing dilution of particulate MMHg by organic- and MMHg-depleted resuspended sediment Partitioning of Hg between water and suspended particle showed tight correlation with the partitioning of organic carbon across study sites, while that of MMHg implied influences of chloride: enhanced chloride in addition to organic matter depletion decreased particulate MMHg in the estuarine zone Primary production was an important determinant of inter-annual variation of particulate Hg and sediment MMHg The bloom year showed relatively low particulate Hg with low C/N ratio, indicating biodilution of Hg In contrast, the percentage of MMHg in sediment increased significantly in the bloom year, likely due to greater availability of metabolizable fresh organic matter The overall results emphasize that Hg behavior in the lower Mekong River Basin is strongly connected to the local monsoon climate, via alterations in particle loads, biological productivity, and availability of sulfate, chloride and organic matter Ó 2012 Elsevier Ltd All rights reserved INTRODUCTION Interactions between Hg and organic matter influence the transport and bioavailability of Hg in riverine and ⇑ Corresponding author Tel.: +82 62 7152438; fax: +82 62 7152434 E-mail address: shan@gist.ac.kr (S Han) 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.gca.2012.12.018 estuarine environments (Laurier et al., 2003; Choe et al., 2003; Conaway et al., 2003; Han et al., 2007; Lee et al., 2011) Dissolved Hg distribution across watersheds is explained by dissolved organic carbon (DOC) distribution in a number of river systems (Peckenham et al., 2003; Schuster et al., 2008; Brigham et al., 2009) In estuarine systems, complexation of Hg with dissolved organic matter, coupled with colloidal coagulation, is reported to influence estuarine mixing behavior (Stordal et al., 1996; 380 S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 Choe et al., 2003; Lee et al., 2011) and bioavailability of Hg (Pan and Wang, 2004; Zhong and Wang, 2009) Monomethylmercury (MMHg) is a toxic form of Hg that biomagnifies in aquatic food chains (Chen et al., 2008; Gantner et al., 2009) Previous studies note that biogeochemical factors (e.g., sulfate, sulfide, and organic matter contents) are critical in the production of MMHg in estuarine sediments (Hammerschmidt et al., 2004; Hammerschmidt and Fitzgerald, 2006; Han et al., 2007) For example, increasing Hg(II) methylation rate along with increasing salinity is shown in estuarine sediments to be associated with sulfate availability to sulfate-reducing bacteria (Hollweg et al., 2009) The Mekong River originates at an elevation of about 5100 m in the Tibetan Plateau of western China and flows southward through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam, before discharging into the South China Sea (Edwin, 2009) It has a total length of 4800 km and drains an area of 795,000 km2, with a mean annual water discharge of 470 km3 yrÀ1, making it the longest river in Southeast Asia (Dai and Trenberth, 2002) The river flows over rock for about 80% of its length before it enters the alluvial plain of Cambodia and Vietnam (Xue et al., 2010, 2011) The Mekong River Delta in southern Vietnam is composed of Holocene alluvial sediments that were rapidly deposited beginning 8000 yr BP (144 Â 106 ton yrÀ1; Xue et al., 2010, 2011) The delta is an essential component of life for millions of South Asians residing in the vicinity of the river and its tributaries, providing the main freshwater source for urban, industrial, agricultural, and fishery uses (Osborne, 2000; Baran et al., 2007) Despite this importance, surprisingly little information is available regarding Hg levels in the Mekong River as compared to other major rivers (e.g., the Amazon and Nile Rivers) The Saigon River originates from Phum Daung in southeastern Cambodia, flows south for approximately 225 km, and empties into the Nha Be River, which discharges into the South China Sea, 20 km north of the Mekong Delta (Lambert et al., 2010) The Saigon River is joined by the Ben Cat River just upstream of Ho Chi Minh City, and it encircles Ho Chi Minh City, the largest city in Vietnam Since the 1980s, Ho Chi Minh City has experienced continually increasing population density and rapid industrial growth, which have negatively affected the environmental quality of the urban river (Thuy et al., 2007; Vo, 2007) Large volumes of untreated domestic and industrial wastewater, along with substances from accidental spills, are released directly into the river (Minh et al., 2007) Monitoring of metals pollution in the Saigon River indicates that surface river-water and sediments are severely contaminated with Cd, Cr, Cu and Zn (Thuy et al., 2007); nevertheless, no literature is available regarding levels of Hg pollution in the water and sediment of the Saigon River The lower Mekong River and Saigon River Basins are dominated by distinctive monsoon seasons The dry season occurs from November to April, with an average discharge rate of 2000 m3 sÀ1, and the rainy season occurs from May to October, with an average discharge rate of 40,000 m3 sÀ1 for the Mekong River (Hoa et al., 2007) While most of the Mekong Delta is a freshwater region during the rainy season, salt water intrudes approximately 70 km inland in the dry season, with vertical stratification in salinity (Wolanski et al., 1996, 1998; Cenci and Martin, 2004) In the wet season, salinity intrusion is observed near the river mouth, but this does not extend more than a few kilometers inland (Wolanski et al., 1996) In contrast to the Mekong River, the Saigon River has a constant flow rate (54 m3 sÀ1; People’s Committee of Ho Chi Minh City, 2006) throughout the year, due to flow regulation from the Dau Tieng Reservoir (George et al., 2004) Salinity intrusion is observed from the lower Saigon River, approximately 10 km inland from the Nha Be River (www.eng.hochiminhcity.gov.vn) Salinity intrusion influences the speciation and bioavailability of Hg in low-discharge rivers (Chiffoleau et al., 1994; Laurier et al., 2003; Covelli et al., 2006) Like permanent estuaries, interactions between Hg and organic matter have primary importance in the transport and bioavailability of Hg in these rivers (Laurier et al., 2003; Turner et al., 2004) Hg–chloro complexation could increase the mobility of Hg, even though the majority of Hg may remain in organic fraction (Ramalhosa et al., 2005) Salinity intrusion is often accompanied by formation of a high turbidity zone as a result of intense hydrodynamic energy of tidal currents (Laurier et al., 2003; Covelli et al., 2006) In a high turbidity zone, the quantity and quality of particulate organic matter influence redistribution of Hg between water and suspended particles, which may cause changes in Hg solubility and bioavailability (Laurier et al., 2003; Turner et al., 2004; Covelli et al., 2006; Cana´rio et al., 2008) A high turbidity zone often provides an ideal site for diagenetic processes of trace metals, in association with increased microbial activity and flocculation of colloidal particles (Roth et al., 2001; Laurier et al., 2003; Covelli et al., 2006) Increased MMHg concentration in the high turbidity zone at the mouth of the Isonzo River is considered to result from intense microbial methylation in the low-oxygen bottomwater (Covelli et al., 2006) In the current study, we aim to understand the role of salinity intrusion on Hg speciation in riverine water and sediment We hypothesize that: (1) salinity intrusion increases sediment MMHg production due to the enhanced availability of sulfate to Hg(II)-methylating microbes; (2) salinity intrusion increases particle solubility of Hg and MMHg due to chloride complexation and/or particulate organic matter dilution by resuspended sediment; and (3) salinity intrusion increases water column MMHg concentration, as a result of intense microbial activity in the high turbidity zone To test these hypotheses, we examine Hg and MMHg distributions in surface river water, suspended particles, and sediments in relation to relevant biogeochemical variables (e.g., salinity, suspended particulate matter (SPM), sulfate, chlorophyll-a (Chl-a), DOC, particulate organic carbon (POC), and particulate nitrogen (PN)); and of sediment (e.g., total organic carbon (TOC), total nitrogen (TN) and acid volatile sulfide (AVS)) S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 381 MATERIALS AND METHODS 2.1 Sample collection and pre-treatment Water and sediment samples were collected from 14 sites in March 2010 and seven sites in April 2011 along the Tien (Fig 1), and from eight sites along the Saigon River in March 2010 (Fig 2) Ultra-clean sample handling protocols were used to prevent sample contamination Unfiltered surface water samples were collected from a depth of approximately 0.5 m in acid-cleaned Teflon bottles, using a peristaltic pump system equipped with Teflon tubing Filtered surface water samples were collected using the same method with polyethersulfone filter capsules (MilliporeÒ, 0.45 lm) connected to the tubing outlet At each site, three independent samples were collected in Teflon, polyethylene, and borosilicate glass bottles The Teflon bottles of unfiltered and filtered water for determination of Hg and MMHg were preserved with high-purity HCl (0.2% v/v), and stored at °C within 12 h Polyethylene bottles of unfiltered water were kept at °C for measurements of SPM, POC, PN, alumina, and chlorophyll-a concentrations Borosilicate glass vials of filtered water were collected for determination of DOC and sulfate concentration Filtered water samples were collected only from the Tien River The ancillary parameters, e.g., pH, temperature, salinity, and oxidation–reduction potential (ORP), were recorded in situ using a multi-parameter probe (Thermo Scientific) The unfiltered water samples for SPM, POC, PN, alumina, and chlorophyll-a were filtered in the laboratory within 24 h of sampling Glass fiber filters (WhatmanÒ, GF/F) were used for particle collection for chlorophyll-a, alumina, POC, and PN measurements, and 0.4 lm polycarbonate membranes were used for SPM measurements The Fig Map of sampling sites in Saigon River, Vietnam filters containing particles were dried immediately for measurement of SPM and alumina, and filters for chlorophylla, POC, and PN were frozen at À20 °C until analysis Sediment samples were collected from the top-20 cm layer Fig Map of sampling sites in Tien River, Vietnam 382 S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 using a stainless steel grab sampler to determine Hg, MMHg, and TOC; and TN and AVS concentrations The collected sediments were stored in sealed polyethylene bags and frozen at À20 °C until analysis difference of duplicate analyses was typically 15 ± 14% (n = 73), whereas certified reference material (ERMCC580, IRMM, 75 ± lg kgÀ1) recovery averaged 96 ± 8% (n = 8) 2.2 Hg and MMHg analysis 2.3 Particle and sediment compositions The Hg in water samples was analyzed following EPA method 1631 (2002) Aqueous samples were oxidized with bromine monochloride (BrCl) for a minimum of 12 h After oxidation, excess oxidant was destroyed with hydroxylamine hydrochloride solution prior to analysis Divalent Hg in these samples was reduced to elemental Hg by SnCl2 solution and the elemental Hg was trapped on gold traps The Hg0 released from gold traps by thermal desorption was fed into a cold vapor atomic fluorescence spectrometer (CVAFS; Model III, Brooks Rand) An acceptable calibration curve was achieved daily with an r2 of at least 0.99 The relative differences of duplicate analyses averaged 11 ± 8% (n = 144) and recovery of the matrix spike averaged 98 ± 25% (n = 7) Recovery of certified reference material (European Commission, BCRÒ-579, coastal seawater, 1.9 ± 0.5 ng kgÀ1) averaged 105 ± 6% (n = 10) Details of the analytical protocols for MMHg analysis in an aqueous phase were as given by EPA method 1630 (2001) Acidified water samples were distilled with ammonium pyrrolidine dithiocarbamate (APDC) solution and the distillates were then converted to gaseous MMHg by aqueous-phase ethylation using tetraethylborate solution The volatile MMHg was then purged and trapped onto TenaxÒ traps, which were flash-heated in a nitrogen stream The released Hg species were thermally separated on a GC column, then detected by CVAFS (Model III, Brooks Rand) An acceptable calibration curve was achieved daily with an r2 of at least 0.99 The relative difference of duplicate analyses was typically 18 ± 5% (n = 52), whereas the matrix spike and certified reference material (ERMCE464, IRMM, 5.5 ± 0.2 mg kgÀ1) recovery averaged 99 ± 6% (n = 12) and 99 ± 6% (n = 12), respectively The concentrations of particulate Hg (PHg) and particulate MMHg (PMMHg) were determined from the differences between the unfiltered Hg (or MMHg) and filtered Hg (or MMHg), normalized to the SPM concentration The Hg found in sediment was analyzed following the appendix to EPA method 1631 (2001) The Hg in sediment (0.5–1.5 g) was digested overnight with 10 ml of aqua regia Sediment digests were then diluted with 0.07 N BrCl solution and used for Hg determination, using the same method as for aqueous Hg samples The relative difference of duplicate analyses was typically 16 ± 24% (n = 48), whereas certified reference material (ERM-CC580, IRMM, 132 ± mg kgÀ1) recovery ranges were 92 ± 6% (n = 12) The MMHg in sediment was analyzed following the procedure described by Choe and Gill (2003) The sediment MMHg (0.5–1.0 g) was digested with acidic potassium bromide solution and extracted into methylene chloride An aliquot of methylene chloride was then back-extracted to Milli-Q water by purging out methylene chloride with nitrogen gas The extracted MMHg was measured using the same method described for aqueous samples The relative The SPM was measured by filtering 0.2 L of water through a pre-weighed polycarbonate membrane (WhatmanÒ, 0.4 lm) shortly after sample collection Particles were dried at 60 °C for 12 h, and the SPM loads were then weighed For determination of organic carbon and nitrogen in SPM and sediment, the GF/F filters bearing the SPM and approximately 10 mg of sediment were freeze-dried Freeze-dried samples were acidified with HCl solution to remove inorganic carbon, then measured with an elemental analyzer interfaced to a continuous flow isotope ratio mass spectrometer (EA-IRMS, ThermoQuest) Alumina on filters was analyzed with a wavelength dispersive X-ray fluorescence spectrometer (Axios Mineral, PANalytical) The samples for chlorophyll-a were obtained by filtering 0.1– 0.4 L of water through GF/F filters Phytoplankton pigments retained on the GF/F filters were extracted in 90% acetone (Liu et al., 2007) The chlorophyll-a in the extracts was measured at 750, 665, 645, 630 and 480 nm wavelengths with an ultraviolet–visible spectrophotometer (Optizen, Mecasys) DOC and sulfate concentrations in filtered water samples were measured by TOC analyzer (Multi N/C 3100, Analytik Jena) and ion chromatography (DX-120, Dionex), respectively For determination of AVS, approximately 10 g of sediment sample was acidified with 20 ml of 6.0 M deoxygenated HCl and held under a nitrogen gas flow for h Volatilized sulfide was collected in traps filled with sulfide antioxidant buffer (SAOB; EPA, 1996), then measured with a sulfide electrode (Kim et al., 2006) Sigma PlotÒ, version 11.2 (Systat software, Inc.) was used for all statistical analyses Linear regression analysis yielded the coefficient of determination (r2) and significant probability (p) Differences between two independent groups were determined using one-way ANOVA All statistical results were reported as significant at a level of p < 0.05 Linear correlation analyses yielded the Pearson’s correlation coefficient (r) for parameters passing the normality test (Shapiro–Wilks normality test) The results were considered statistically significant if p-values were less than 0.05 RESULTS 3.1 Geochemical settings In the Tien River, the mean water temperature across sampling sites was 31 ± 0.9 °C in 2010 and 30 ± 0.8 °C in 2011, and mean pH was 7.8 ± 0.1 in 2010 and 7.4 ± 0.3 in 2011 Saline intrusion was observed from to 50 km from the coast in 2010 and from to 20 km in 2011 (Supporting Information, Table S1) In the brackish zone of the Tien River, the residence time of saline bottom-water can be quite long, as indicated by lower ORP High concentrations of SPM (>80 mg LÀ1) were found from the brack- 76 ± 65 23–160 43 ± 41 14–90 21 ± 1.4 19–22 2862 ± 3064 314–6263 5.2 ± 0.48 4.7–5.8 6.2 ± 2.4 4.5–9.0 0.92 ± 0.31 0.48–1.2 0.64 ± 0.40 0.22–1.0 4.0 ± 1.1 2.4–4.8 2.9 ± 1.1 1.7–4.0 ND 3.4 ± 0.50 3.0–4.1 4.0 ± 0.25 3.8–4.3 ND ND 24 ± 12 16–43 185 ± 106 81–293 ND ND: not determined Mean ± SD Range Mean ± SD Range 19–21 Brackish 10–14 Brackish 7–9 Lower freshwater 15–18 Lower freshwater 9.4 ± 3.8 4.1–14 7.1 ± 3.3 4.0–11 4.0 ± 1.1 2.8–5.3 14 ± 0.55 13–15 15 ± 1.7 13–16 946 ± 693 91–1918 5.8 ± 0.31 5.4–6.2 6.2 ± 0.24 5.9–6.4 9.6 ± 1.9 7.6–12.6 1.2 ± 0.45 0.61–1.8 0.72 ± 0.21 0.50–0.92 0.22 ± 0.050 0.16–0.29 6.0 ± 2.2 3.3–9.3 3.8 ± 1.2 2.7–5.0 1.7 ± 0.17 1.6–1.9 2.8 ± 1.9 1.4–6.4 2.0 ± 0.51 1.5–2.5 2.8 ± 0.76 2.1–4.1 1.8 ± 0.19 1.6–2.0 1.7 ± 0.21 1.5–1.9 2.4 ± 0.084 2.3–2.5 13 ± 5.0 7.8–20 16 ± 3.9 12–18 289 ± 195 82–517 1–6 Upper freshwater Mean ± SD Range Mean ± SD Range Mean ± SD Range 152 ± 27 123–195 162 ± 56 112–222 104 ± 35 61–154 Sulfate (mg LÀ1) C/N (mol/mol) PN (%) POC (%) DOC (mg LÀ1) Alumina (mmol gÀ1) SPM (mg LÀ1) 383 Tien 2011 The Hg concentrations found in Tien River sediment ranged from 0.32 to 0.76 nmol gÀ1 dry weight (mean 0.50 ± 0.14 nmol gÀ1) in 2010 and from 0.12 to 0.24 nmol gÀ1 dry weight (mean 0.18 ± 0.040 nmol gÀ1) in 2011 (Table 2); in the Saigon River, it ranged from 0.21 Tien 2010 3.3 Total Hg and MMHg in sediment ORP (mV) In the Tien River, TOC in sediment ranged from 2.2% to 3.9% (mean 2.7 ± 0.58%) in 2010, and from 1.2% to 2.4% (mean 1.7 ± 0.5%) in 2011 In the Saigon River, TOC in sediment ranged from 2.6% to 3.7% (mean 3.2 ± 0.32%) (Table 2) Little lateral variation of TOC, TN, and C/N ratios were noted in the Tien and Saigon Rivers (Supporting Information, Table S3) Temporally, atomic C/N ratios in the Tien River were significantly (p < 0.05) lower in 2011 (range 3.4–6.5) than in 2010 (range 9.6–15) AVS in sediment of the Tien River ranged from 0.053 to 6.5 lmol gÀ1 in 2010 and from 0.20 to 20 lmol gÀ1 in 2011 (Table 2) Mean AVS concentration was significantly higher (p < 0.05) in the brackish zone compared to the freshwater zone in 2010 The same distribution pattern was found in the Saigon River, where AVS ranged from 0.14 to 0.45 lmol gÀ1 in the freshwater zone, and from 5.7 to 17 lmol gÀ1 in the brackish zone (p < 0.05) The primary factors that control AVS in river and estuarine sediments are the availabilities of reactive Fe(II), dissolved sulfate, and metabolizable organic carbon (Morse et al., 2007) As the concentrations of TOC and TN, and C/N ratio were consistent across sites, increased AVS at the saline zone could be attributable to enhanced sulfate availability Site 3.2 Sediment characteristics Table Data for ORP, SPM, Alumina, DOC, POC, PN, atomic C/N, sulfate and chlorophyll-a (Chl-a) in surface waters of the Tien River, collected in March 2010 and 2011 ish zone, and were associated with hydrodynamic energy of tidal currents: the Mekong River Delta has 3.5-m semidiurnal tides from the South China Sea and irregular 1-m diurnal tides from the Gulf of Thailand (Hoa et al., 2007) In the Saigon River, the mean water temperature and pH for 2010 were 30 ± 0.9 °C and 7.7 ± 0.3, respectively, and saline intrusion was observed up to 10 km from the Nha Be River (Supporting Information, Table S2) Like the Tien River, the brackish zone showed lower ORP than the freshwater zone, and a high turbidity zone was found at sites and Concentration and compositional characteristics of the SPM (i.e., alumina, POC, PN, and atomic C/N ratio) were compared between freshwater and brackish zones of the Tien River (Table 1) Alumina content, used as a proxy for fine particles, significantly (p < 0.05) increased in the saline zone, supporting the periodic occurrence of sediment resuspension Levels of POC varied from 1.6% to 9.3% (mean 4.0 ± 2.4%) in 2010 and from 1.7% to 4.8% (mean 3.5 ± 1.2%) in 2011 POC decreased in the brackish zone in both years, implying dilution of fluvial particles by organic-depleted resuspended sediment The PN showed a similar trend to the POC The strong correlation between POC and PN (r2 = 0.95, p < 0.05, linear regression) suggests that N is predominantly bound to particulate organic matter Chl-a (lg LÀ1) S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 0.40 ± 0.063 0.33–0.46 1.2 ± 0.78 0.75–2.6 1.5 ± 1.0 0.70–2.6 5.0 ± 2.1 3.2–8.7 0.38 ± 0.24 0.21–0.65 0.47 ± 0.11 0.34–0.61 13 ± 3.2 9.1–15 11 ± 0.84 10–12 6–8 Brackish Mean ± SD Range Mean ± SD Range 1–5 Freshwater Saigon 2010 19–21 Brackish 3.1 ± 0.41 2.6–3.4 3.3 ± 0.29 2.9–3.7 0.29 ± 0.041 0.25–0.33 0.35 ± 0.017 0.33–0.38 0.26 ± 0.17 0.14–0.45 13 ± 4.6 5.7–17 1.1 ± 1.1 0.24–2.7 0.85 ± 0.51 0.40–1.4 2.2 ± 2.1 0.57–5.2 1.2 ± 0.64 0.81–2.0 0.20 ± 0.030 0.17–0.24 0.15 ± 0.040 0.12–0.20 5.2 ± 1.3 3.7–6.5 3.9 ± 0.44 3.3–4.1 Mean ± SD Range Mean ± SD Range 15–18 Lower freshwater Tien 2011 10–14 Brackish 7–9 Lower freshwater 1.9 ± 0.61 1.2–2.4 1.4 ± 0.17 1.3–1.6 0.43 ± 0.046 0.36–0.47 0.44 ± 0.041 0.39–0.47 6.3 ± 9.3 0.20–20 2.2 ± 2.4 0.81–4.9 0.64 ± 0.28 0.16–0.99 1.4 ± 1.5 0.33–3.1 1.7 ± 2.5 0.44–6.1 0.43 ± 0.11 0.32–0.63 0.54 ± 0.14 0.46–0.70 0.55 ± 0.15 0.35–0.76 12 ± 2.1 10–15 10 ± 0.7 10–11 10 ± 0.9 10–12 Tien 2010 1–6 Upper freshwater Mean ± SD Range Mean ± SD Range Mean ± SD Range 2.7 ± 0.14 2.5–2.9 3.1 ± 1.2 2.3–3.9 2.6 ± 0.70 2.2–3.8 0.26 ± 0.041 0.22–0.31 0.34 ± 0.11 0.27–0.42 0.31 ± 0.091 0.24–0.47 0.14 ± 0.11 0.053–0.31 1.6 ± 1.7 0.47–3.6 3.6 ± 2.6 1.4–6.5 MMHg (pmol gÀ1) Hg (nmol gÀ1) AVS (lmol gÀ1) C/N (mol/mol) TN (%) TOC (%) Site Table Data for TOC, TN, atomic C/N, AVS, Hg, MMHg and %MMHg in sediment of the Tien and Saigon Rivers, collected in March 2010 and 2011 0.16 ± 0.089 0.051–0.31 0.22 ± 0.20 0.072–0.44 0.25 ± 0.30 0.085–0.79 S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 %MMHg 384 to 0.65 nmol gÀ1 (mean 0.44 ± 0.16 nmol gÀ1) These ranges are comparable to those reported for urban rivers and estuaries moderately contaminated with Hg: the Patuxent River (0.29–0.79 nmol gÀ1; Benoit et al., 1998) and Bay of Fundy (0.050–0.70 nmol gÀ1; Sunderland et al., 2006) There was no statistical (p > 0.05) difference between the freshwater and saline zones in the Tien River, with weak increases at sites and 10, located close to My Tho City (Supporting Information, Table S3) The Saigon River also showed relatively constant sediment Hg, with weak peaks at sites and 7, located near Dau Tieng Reservoir and Ho Chi Minh City, respectively In the Tien River, sediment MMHg ranged from 0.16 to 6.1 pmol gÀ1 dry weight (mean 1.2 ± 1.6 pmol gÀ1) in 2010, and from 0.57 to 5.2 pmol gÀ1 dry weight (mean 1.8 ± 1.6 pmol gÀ1) in 2011 (Table 2) In the Saigon River, sediment MMHg ranged from 0.70 to 8.7 pmol gÀ1 (mean 3.7 ± 2.5 pmol gÀ1) in 2010, comparable to data reported for the Patuxent River (0.49–4.0 pmol gÀ1; Benoit et al., 1998), the Bay of Fundy (0.25–7.38 pmol gÀ1; Sunderland et al., 2006), and San Francisco Bay (0.5–5.0 pmol gÀ1; Conaway et al., 2003) In the Tien River, strong MMHg peaks were found near the estuarine head (sites 9, 10, and 18; Supporting Information, Table S3) In the Saigon River, MMHg concentrations were significantly (p < 0.05) higher in the brackish zone than in the freshwater zone, with a strong peak near the estuarine head, like the Tien (site 4) 3.4 Total Hg and MMHg in surface water In the Tien River, Hg levels in unfiltered river water (UHg) ranged from 11 to 222 pM (mean 61 ± 65 pM) in 2010 and from 4.6 to 55 pM (mean 22 ± 23 pM) in 2011 (Fig 3; Supporting Information, Table S4) These concentration ranges are similar or lower than those reported for urban rivers in China and urban estuaries in North America moderately contaminated with Hg: the Yalujiang River (154–344 pM; Zhang and Wong, 2007), East River (55– 244 pM; Liu et al., 2012), New York/New Jersey Harbor (30–550 pM; Balcom et al., 2008), and San Francisco Bay (0.73–440 pM; Conaway et al., 2003) Increased UHg levels were found from the saline zone in both years In the freshwater zone, a maximum peak of UHg was detected at site 6, which might be a local runoff effect from Cao Lanh City Temporally, UHg was about four times higher in 2010 than 2011 in the lower-river and estuarine zone DHg averaged 20% of UHg (4.7–44%), and spatial and temporal variations of DHg were similar to those of UHg PHg levels were decreased in the lower-river and estuarine zone compared to the upper river (>100 km from coast), a major contrast to the UHg and DHg distributions Temporally, PHg was higher in 2010 than in 2011 In the Tien River, MMHg concentrations in unfiltered water (UMMHg) ranged from 0.056 to 0.39 pM (mean 0.20 ± 0.085 pM) in 2010 and 0.14 to 0.28 pM (mean 0.20 ± 0.047 pM) in 2011 (Fig 3; Supporting Information, Table S4) These ranges are within the range previously reported for Hg-contaminated urban rivers and estuaries: Patuxent River (0.05–1.2 pM; Benoit et al., 1998) and San S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 385 Tien River ranged from 0.020 to 0.17 pM (mean 0.090 ± 0.052 pM) in 2010 and from 0.066 to 0.094 pM (mean 0.079 ± 0.010 pM) in 2011 In general, DMMHg was 19% to 78% of UMMHg (mean 45 ± 18%) and spatial and temporal variations in DMMHg followed those of UMMHg There was a distinct spatial variation for particulate MMHg (PMMHg), which was significantly lower (p < 0.05) in the saline zone compared to the upper- and lower-river zones DISCUSSION 4.1 Variation of sediment Hg In the Tien River, TOC was a major factor governing lateral and temporal distribution of sediment Hg (Table 3, Fig 4) A similar correlation was found in other rivers and estuaries (Hammerschmidt and Fitzgerald, 2004; Heyes et al., 2006; Hollweg et al., 2009); this may be attributed to strong affinity between Hg and thiolic binding sites on TOC (Ravichandran, 2004; Skyllberg et al., 2007) No linear correlation was found between TOC and sediment Hg for the Saigon River, with a relatively narrow range of TOC Point source location might be more important for the Saigon River, since relatively high sediment Hg levels were found from the runoff sites of Dau Tieng Reservoir and Ho Chi Minh City Increased metal concentrations (e.g., Cd, Cr, Cu, and Zn) in urban parts of the Saigon River have been reported previously, and untreated urban and industrial wastewaters were identified as major sources of metal pollution (Thuy et al., 2007) 4.2 Variation of sediment MMHg Fig Unfiltered Hg (UHg), dissolved Hg (DHg), and particulate Hg (PHg) with distance to the coast in 2010 (a) and 2011 (b), and unfiltered MMHg (UMMHg), dissolved MMHg (DMMHg), and particulate MMHg (PMMHg) with distance to the coast in 2010 (a) and 2011 (b) in Tien River Francisco Bay (0.050–2.3 pM; Conaway et al., 2003) Except for the steep decrease in the upper river, there was no significant spatial variation for UMMHg across study sites and there were no significant (p > 0.05) differences between 2010 and 2011 Dissolved MMHg (DMMHg) in the The sediment MMHg in the Tien and Saigon Rivers showed significant linear correlation with AVS (Table 3), suggesting that biogeochemical processes that produce mineral FeS are associated with MMHg production In 2010, peak MMHg percentages were recorded at the estuarine head of the Tien (site 10) and Saigon Rivers (site 4; Fig 5) Enhanced sulfate, associated with salinity intrusion, appears to increase Hg(II) methylation rate and AVS (Hollweg et al., 2009) Relatively low MMHg percentage in the saline zone, despite large AVS, could be related to the inhibition effect of dissolved sulfide (Benoit et al., 2001; Drott et al., 2007) A number of studies report that dissolved sulfide shows strong inhibition of Hg(II) methylation rate in coastal sediments (Conaway et al., 2003; Heyes et al., 2006; Sunderland et al., 2006) Interestingly, in the present study, there is a significant positive linear correlation between TN and MMHg in both rivers (Table 3) Assuming that TN is more inorganic (e.g., ammonium) than organic, based on the lack of correlation between TOC and TN, a sediment redox condition might explain high MMHg It is commonly shown that reducing sediment provides favorable conditions for microbial Hg(II) methylation (Sunderland et al., 2006) Applying a multiple linear regression for MMHg, it was found that AVS explained 41% of variability in MMHg (p < 0.05, n = 17) and that TN explained an additional 15% of variability in MMHg ([MMHg 386 S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 Table Partial correlation matrix for Hg, MMHg, %MMHg against salinity, TOC, TN, AVS, Hg, and MMHg in sediment samples of the Tien River (a) and Saigon River (b) Hg (nmol gÀ1) MMHg (pmol gÀ1) %MMHg Salinity (ppt) À0.20 0.38 21 À0.13 0.58 21 0.083 0.72 21 TOC (%) 0.81 lg mgÀ1) from those dominated by refractory organic matter (Cifuentes et al., 1988; Liu et al., 2007; Supporting Information, Fig S1) Labile organic particles cover a narrow range of C/N, from 4.6 to 6.4, similar to the Redfield ratio For 2010, it is notable that freshwater SPM is dominated by labile organics, while brackish SPM is dominated by refractory organics For 2011, however, most particles in freshwater and brackish zones were labile; this might be associated with an excessive algal bloom in 2011 (Table 1) The log-transformed particle–water partition coefficient, Kd = [particulate Hg] (mol kgÀ1)/[dissolved Hg] (mol LÀ1), of Hg averaged 5.1 ± 0.44 in 2010 and 4.9 ± 0.21 in 2011 in the Tien River (Fig 8) These ranges were similar to those found from other rivers (e.g., 4.8–5.7 in the Patuxent River, Benoit et al., 1998; 4.5–6.5 in the St Lawrence River, Que´merais et al., 1998; and 2.8–6.6 for streams in Oregon and Wisconsin, Brigham et al., 2009) The log Kd of MMHg averaged 4.5 ± 0.77 in 2010 and 4.5 ± 0.50 in 2011 in the 388 S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 and log K0 = 13.6, Fitzgerald et al., 2007) There appears to be more than enough L with sufficient affinity with Hg to compete out Hg–chloro complexes, but this is not the case for MMHg in the estuarine zone of the Tien River SUMMARY Fig Kd (particle–water partition coefficient) of Hg, MMHg and organic carbon (OC) as a function of suspended particulate matter (SPM) concentration in Tien River Tien River We found a negative linear relationship between log Kd of Hg (MMHg and OC) and log [SPM]; this is known as a particle concentration effect (Benoit, 1995; Fig 8) The POC content in SPM may play a critical role in this correlation, as we found less POC under high SPM conditions However, there should be additional factors to explain the close negative correlation, since POC contents were relatively constant when log [SPM] > 1.9 (Supporting Information, Table S1) Increasing colloidal organic matter along with increasing SPM may be associated with decreases in Kd of Hg, MMHg, and OC as a function of SPM (Honeyman and Santschi, 1991; Benoit and Rozan, 1999; Lee et al., 2011) The slope (À0.52) of the linear regression between log Kd of Hg and log [SPM] was similar to that (À0.38) between log Kd of OC and log [SPM], indicating the critical role of organic matter in governing phase-partitioning of Hg (Benoit et al., 1998; Choe and Gill, 2003; Conaway et al., 2003; Laurier et al., 2003; Sunderland et al., 2006) Interactions between organic matter and Hg, attributable to strong Hg binding with reduced sulfur-containing functional groups (e.g., thiol), appear to control particle–water partition of Hg On the other hand, the slope (À0.97) of the linear regression between log Kd of MMHg and log [SPM] was substantially lower than those for Hg and OC We speculate that chloro-complexation of MMHg provides additional decreases in log Kd of MMHg in the estuarine zone (log [SPM] > 1.9) It is known that interactions between MMHg and organic ligand (L) are less significant compared to those between Hg and L (Zhong and Wang, 2009) According to the Hg–Cl–L ligand complexation model, HgL complex dominates dissolved Hg pool under the Tien’s estuarine conditions ([Cl] = 0.02–0.3 M, [DOC] = 180–360 lM), assuming L/DOC = (5–50) Â 10À6 and 22 < log K0 < 25, Fitzgerald et al., 2007) On the contrary, the CH3Hg–Cl–L ligand complexation model predicts that CH3HgL competes with CH3HgCl under our estuarine conditions, assuming L/DOC = (5–50) Â 10À6 The Mekong River Delta, located in Vietnam, is a flat, low-lying area of highly complex rivers, channels, and flood plains Although there are more than 1000 man-made canals in the Mekong Delta for inhibition of saltwater, transport, and land reclamation, saline intrusion still occurs and causes severe human suffering during dry seasons (Hoa et al., 2007) In surface sediment, enhanced %MMHg was found at the estuarine head, along with increased AVS, emphasizing the importance of sulfate availability In surface water, UHg concentrations were greater in the estuarine high turbidity zone compared to the upper and lower rivers, due to enhanced particle load Fractions of particulate Hg appear to be remobilized in the estuarine high turbidity zone, since DHg also increased in the high turbidity zone On the contrary, UMMHg and DMMHg did not increase in the estuarine high turbidity zone compared to the upper and lower rivers, likely due to large decreases in PMMHg Although the high turbidity zone appears to have elevated microbial activity, which manifests as low ORP, sulfide may play a critical role in constraining active Hg(II) methylation Regarding particle–water distribution of Hg and MMHg, we found increased solubility of Hg and MMHg in the estuarine zone, with a negative linear relationship between log Kd of Hg (MMHg) and log [SPM] Between 2010 and 2011, sediment and water conditions were highly variable in terms of Hg and organic quality Hg in suspended particles was depleted in the bloom year (2011), highlighting the importance of the biodilution effect In contrast, sediment %MMHg increased in the bloom year, perhaps due to enrichment of labile fresh organic matter in sediment, and subsequent enhancement of microbial activity Taken as a whole, Hg speciation and partitioning in the lower Mekong River Basin were strongly influenced by salinity intrusion, and major biogeochemical factors affecting Hg behaviors were particle loads, biological productivity, and concentrations of sulfate, chloride and organic matter ACKNOWLEDGEMENTS We are grateful for the support of the Hanyang Research and Industry Cluster at Hanyang University This study was supported by the Ministry of Science and Technology, Korea, through the Institute of Science and Technology for Sustainability (UNU & GIST Joint Program); and by the Ministry of Land, Transport, and Maritime Affairs through “Impacts of ocean acidification on the bioaccumulation and release of mercury by microbes” APPENDIX A SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.gca.2012.12.018 S Noh et al / Geochimica et Cosmochimica Acta 106 (2013) 379–390 REFERENCES Balcom 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distinctive monsoon seasons The dry season occurs from November... Salinity intrusion is observed from the lower Saigon River, approximately 10 km inland from the Nha Be River (www.eng.hochiminhcity.gov.vn) Salinity intrusion in uences the speciation and bioavailability