Fate and tidal transport of butyltin and mercury compounds in ... - LEGOS

Marine Pollution Bulletin 64 (2012) 1789–1798

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Fate and tidal transport of butyltin and mercury compounds in the waters of the tropical Bach Dang Estuary (Haiphong, Vietnam) Patricia Navarro a, David Amouroux a,⇑, Nghi Duong Thanh b, Emma Rochelle-Newall c, Sylvain Ouillon d, Robert Arfi e, Thuoc Chu Van b, Xavier Mari c, Jean-Pascal Torréton c a Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, UMR 5254 IPREM, CNRS, Université de Pau et des Pays de l’Adour, 2 Avenue Président Pierre Angot, 64053 Pau, France b Institute of Marine Environment and Resources (Vietnam Academy of Science and Technology), 246 Da Nang Street, Haiphong City, Vietnam c IRD, Université Montpellier II, UMR 5119 ECOSYM, Place Eugène Bataillon, Bat. 24, CC 093, 34095 Montpellier, France d IRD, Université de Toulouse, UPS (OMP), UMR 5566 LEGOS, 14 Avenue Edouard Belin, 31400 Toulouse, France e IRD, Centre d’Océanologie de Marseille, UMR 6535 LOPB, 13009 Marseille, France

a r t i c l e

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a b s t r a c t

Keywords: Methylmercury Butyltin compounds Tropical estuary Surface water Tidal flux

In this work, two field campaigns were performed in July 2008 (wet season) and March 2009 (dry season) to produce original data on the concentration, partition and distribution of mercury and butyltin compounds along the tropical Bach Dang Estuary located in North Vietnam (Haiphong, Red River Delta). The results demonstrate that mercury and butyltin speciation in the surface waters of this type of tropical estuary is greatly affected by the drastic changes in the seasonal conditions. During high river discharge in the wet season, there was a large estuarine input of total Hg and tributyltin, while the longer residence time of the waters during the dry season promotes increasing MMHg formation and TBT degradation. Although most of the Hg and TBT is transported into the estuary from upstream sources, tidal cycle measurements demonstrate that this estuary is a significant source of TBT and MMHg during the wet (3 kg TBT/day) and dry (3 g MMHg/day) seasons. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction

and light stabilizers for PVC material, and DBT is also increasingly used as a binder in water-based varnishes (Antizar-Ladislao, 2008). Butyltin compounds are persistent in water and particularly in sediments, and they have extremely high toxicity and endocrine-disrupting effects at low concentration levels, e.g., imposex in gastropods or deformities in oysters (Donard et al., 2001). The toxicity, mobility and bioavailability of these compounds is closely related to their chemical form, e.g., alkylmercury is more toxic than inorganic mercury (Hg (II)) and trialkyl more than dior monoalkyltin. Therefore, the speciation of organometallic compounds is of primary interest due to their species-dependent toxicity (Leermakers et al., 2005). Although most of the methylated species are mainly formed from the inorganic tin or mercury form via biomethylation (e.g., bacteria and fungi), heavier alkylated compounds are usually of anthropogenic origin (e.g., butyltin or phenylmercury). In most cases, biomethylation occurs in biofilms and at the water– sediment interface, where the microorganisms develop (Hirner, 2006; Saniewska et al., 2010). Thus, aquatic ecosystems have been found to be the most susceptible to MMHg and butyltin contamination (Amouroux et al., 2011; Donard et al., 2001; Fitzgerald et al., 2007). Unlike the European Union, which included mercury and TBT and their degradation products in the list of priority pollutants in

Currently, the presence and pollution impact of organometallic species in the environment, especially in marine ecosystems, is well-known to be generally associated with the increase of anthropogenic activities (Amouroux et al., 2011). One of the most hazardous contaminants is mercury, which is ubiquitous and widespread; its most toxic form, monomethylmercury (MMHg), can cause severe neurological damage to humans and wildlife from even very low concentrations in water (Fitzgerald et al., 2007). There are several sources of mercury contamination in aquatic ecosystems, e.g., atmospheric deposition, erosion sources, urban discharge, agricultural sources, mining discharge and combustion and industrial discharge (Wang et al., 2004). Another organometallic pollutant present in the aquatic environment is tributyltin (TBT), and its uses, including as an antifouling agent in boat paints, in wood preservation, as an antifungal agent for textiles and in industrial water systems, have been responsible for its anthropogenic introduction into the aquatic environment (Donard et al., 2001). The TBT degradation products dibutyltin (DBT) and monobutyltin (MBT) are mainly used as heat ⇑ Corresponding author. Tel.: +33 559 40 77 56; fax: +33 559 40 77 81. E-mail address: [email protected] (D. Amouroux). 0025-326X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2012.05.036

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P. Navarro et al. / Marine Pollution Bulletin 64 (2012) 1789–1798

the Water Framework Directive and established limit values in water at 0.07 lg/L and 0.0015 lg/L, respectively (Directive 2008/ 105/EC), the Association of Southeast Asian Nations regulated total mercury (THg) in seawater (0.16 lg/L) in 1999, but the use of organotin compounds in antifouling paints is still permitted in many Asian countries. Only Japan and Hong Kong have regulated the use of TBT, and since 2000, South Korea has implemented partial restrictions on its use for small boats and fishing nets. Many studies on methylmercury and TBT pollution have been performed in sediments, biota and fish in Asian coastal areas (Harino et al., 2009; Jiang et al., 2010; Li et al., 2009; Midorikawa et al., 2004; Nhan et al., 2005; Oh et al., 2010; Sudaryanto et al., 2002; Suehiro et al., 2006; Yawei et al., 2005), but few works dealing with water samples have been found in the literature (Bhosle et al., 2004; Cao et al., 2009; Choi et al., 2010; Garg et al., 2010; Laurier et al., 2004), and none in Vietnam. Thus, there is a need to assess the environmental impact of these pollutants in these areas through the implementation of consolidated methodologies of analysis (Monperrus et al., 2005a,b). The Bach Dang Estuary, located on the easternmost branch of the Red River, plays an essential role in the economic development of North Vietnam. The area is subjected to increasing anthropogenic pressures on the ecosystem due to intensive agriculture, strong urbanization and the presence of one of the principal industrial ports in Vietnam. The organic pollutants used in agriculture, the organic residues and inorganic components of urban effluents, and the metals derived from urban and industrial activities are all found in the estuarine system, many of which have undergone either no or minimal treatments (Carvalho et al., 2008; Hung and Thiemann, 2002; Nhan et al., 1998, 2005). The environmental degradation resulting from these effluents can have major economic consequences through negative impacts on public health, natural resources and tourism in the affected areas, and thus these issues pose a major challenge for the sustainable economic development of the region. The present work is a part of a multidisciplinary project on the impact of anthropogenic and hydrological constraints on the biological functioning of the Bach Dang estuarine ecosystem in North Vietnam (Torréton et al., 2009). Rochelle-Newall et al. (2011) have demonstrated that the phytoplankton and bacterioplankton diversity, distribution and productivity are not only affected by the hydrological dynamics of carbon, particles and nutrients loads but most likely by the occurrence of contaminants, such as organometallic compounds of tin (butylSn) and mercury (MeHg). Thus, the distribution and variability of both autotrophic and heterotrophic species within estuarine waters was hypothesized to potentially control the speciation, fate and transport of these organometallic contaminants (Rochelle-Newall et al., 2011). In addition, Lefebvre et al. (2012) evaluated the seasonal variability of the sediment transport, aggregation and deposition in the Bach Dang Estuary. This study demonstrated that high river discharge during the wet season induces large sedimentary particle fluxes to the coastal bay while tidal pumping during the dry season favors significant particle input from the bay and promotes sedimentation within the inner estuary (Lefebvre et al., 2012). Thus, the objectives of this work are to provide, for the first time, a comprehensive distribution of the mercury compounds and butyltins in water samples from this estuary and to evaluate how both seasonal and tidal hydrodynamic features may affect their respective seaward transport. For this purpose, 18 sites were sampled to assess the spatial distribution of the organometallic compounds along the Bach Dang Estuary. This site is subjected to a sub-tropical humid climate with a wet season from May to October and a dryer, cooler season from November to April. The sampling was therefore performed during these two hydroclimatic seasons (wet and dry). Both the spatial distribution of the sampling stations and the seasonal characteristics were intended to evaluate

the influence of the salinity and suspended matter on the concentration of the pollutants. Moreover, the tidal flux of the pollutants was evaluated in one sampling site for 24 h to determine the upstream/downstream estuarine flux contributions. 2. Materials and methods 2.1. Sampling area and sample collection The samples were collected in the Bach Dang Estuary along three axial transects during the wet season (July 1–11, 2008) and the dry season (March 12–23, 2009), covering a wide range of salinities. During each campaign, 19 stations in 2008 and 18 stations in 2009 (see Fig. 1) were sampled from the deck of a 12 m flat-bottomed coastal vessel for the butyltin and MMHg analyses in both the aqueous and particulate fractions. At each sampling station, a CTD profiler (SeaBird SBE 19 plus, Sea-Bird Electronics, Inc., Bellevue, WA, USA) was deployed to measure the vertical profiles of temperature, salinity, photosynthetically active radiations (PAR) and in vivo fluorescence in the water column. Turbidity (in Formazin Turbidity Units, FTU) was also measured using a Seapoint turbidity meter attached to the CTD package. More details on the sampling strategy and methodology are described elsewhere (Lefebvre et al., 2012; Rochelle-Newall et al., 2011). 2.1.1. Tidal cycle (24 h) at Dinh Vu station The 24 h cycles were performed at the confluence of the Bach Dang and Cam Rivers (Site 4, Dinh Vu, see Fig. 1) to estimate the influence of the salinity and suspended particulate matter on the organometallic concentrations. For both sampling campaigns, water samples were collected every 3 h for the 24 h cycles (1 full tidal cycle) on July 5–6, 2008 for the wet season (starting at 9:30 a.m.) and on March 17–18, 2009 for the dry season (starting at 11:00 a.m.). This station in particular also allowed the concentration values obtained to be combined with the current measurements upstream and downstream of the estuary. Instantaneous river discharge was also determined from the cross-sections of the velocity profiles measured by an acoustic Döppler current profiler (ADCP) RDI Workhorse 600 kHz (Teledyne RD Instruments, Poway, CA, USA) every 3 h in Dinh Vu (site 4) for a 24 h tidal cycle. The particle size distribution along the water column was measured by a laser diffractometer (Laser In-Situ Scattering and Transmissiometry, USST-100X Type-C (Sequoia Scientific, Inc., Bellevue, WA, USA)), delivering the size distribution of the suspended particles in 32 logarithmically spaced size classes between 2.5 and 500 lm. The surface and bottom water suspended particulate matter (SPM) concentration was determined by filtration through Nuclepore 0.4 lm membranes, followed by weighing. The total net flow was calculated from the riverine outflow and marine inflow as calculated from the transversal current measurements (from bank to bank across the channel during the diurnal tidal cycle) conducted for a 24 h cycle on the river. A positive value was inferred from the net seaward flux of estuarine or riverine inputs, whereas a net negative value corresponded to an upstream flux of marine or estuarine inputs. 2.1.2. Water and sediment collection and pre-treatment The samples were directly collected by hand at the sub-surface (0.5 m depth) in clean Teflon vials while using large polyethylene gloves to avoid any contamination from any sampling device or the surface microlayer. To analyze the particulate and dissolved concentrations, each water sample (0.3–1.5 L) was filtered through a pre-cleaned, preweighted and labeled DuraporeÒ PVDF filter membrane (0.45 lm,


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Fig. 1. Sampling sites in the Bach Dang Estuary during the wet (July 2008) and dry (March 2009) seasons.

Millipore, Bedford, MA, USA). The water samples were acidified (HCl 0.1% w/v) and stored in Teflon PFA bottles at 4 °C until analysis. Each filter was rinsed with ultrapure water and immediately store at 20 °C until drying and analysis in the laboratory. Filtration blanks were also included in every analysis batch or at least each day. In this work, the aqueous solution passing through the filter (0.45 lm) has been defined as the ‘‘dissolved’’ fraction and the filter-retained solid has been defined as the ‘‘particulate’’ fraction, keeping in mind that such operationally defined fractionation does not rigorously take into account the variable colloidal distribution within the water samples that have been evaluated elsewhere (Mari et al., 2012). Sediments were collected in March 2009 (dry season) in the main channel of the Bach Dang River (at stations 10, 15, 26 and 28) and Cam River (at stations C1–C4, located between the Cam River end-member – station 2 – and the confluence between the Cam River and Bach Dang River – station BD3) using a Shipeck type grab and were kept at 20 °C until lyophilization. 2.2. Analysis of water ‘‘dissolved’’ and ‘‘particulate’’ fractions for mercury and tin speciation 2.2.1. Chemicals and instrumentation Tributyltin chloride (96%), dibutyltin dichloride (97%) and monobutyltin trichloride (95%) were obtained from Sigma–Aldrich (Seelze, Germany), and the monomethylmercury standard was obtained from Strem Chemicals (Newburyport, MA, USA). The stable isotopes and isotopically enriched species used were purchased from ISC Science (Oviedo, Spain): inorganic mercury (IHg) enriched in 199Hg (91%), MMHg enriched in 201Hg (96.5%) and a mix of MBT,

DBT and TBT enriched in 119Sn (82.4%). All stock solutions were kept in the dark at 20 °C until use and were prepared by dissolving the corresponding salt in acetic acid in the case of the butyltin compounds, in methanol for MMHg, and in ultrapure water (HCl 1% v/v) for IHg. Mixed working solutions of the butyltin compounds were prepared daily before analysis by dilution of the stock solutions with a mixture of acetic acid:methanol 3:1. In the case of mercury, the working solutions were prepared daily by dissolving the corresponding stock solution with 1% HCl in ultrapure water. Hydrochloric acid (HCl, 33–36%, ultrexÒ II ultrapure reagent) and glacial acetic acid (HAc, Instra-analyzedÒ) were purchased from J.T. Baker (Phillipsburg, NJ, USA). Methanol (MeOH, chromasolvÒ) was obtained from Sigma–Aldrich. Ammonium hydroxide (NH4OH, puriss p.a.) and tetramethylammonium hydroxide (TMAH, (25% in water, TraceSelect) were purchased from Fluka (Steinheim, Germany) and sodium acetate trihydrate (NaAc, puriss p.a.) from Riedel-de-Haën (Seelze, Germany). The extraction of the organometallic species from the particulate fraction (filter) was carried out using an Explorer focused microwave system from CEM corporation (Mathews, NC, USA), which provided an accurate control of the temperature and pressure inside the glass vial. The simultaneous determination of the Hg and butyltin species was performed in a Thermo XSeries 2 inductively coupled plasma–mass spectrometer (ICP–MS) coupled to a gas chromatograph (GC) (Thermo Fisher, Waltham, MA, USA) by a commercial GC–ICP–MS interface (Thermo Fisher, Franklin, MA, USA). 2.2.2. Sample preparation A subsample of 100 mL of water was accurately weighed in a glass headspace bottle and 5 mL of HAc/NaAc buffer (pH 5, 0.1 M)

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was added. The isotopically enriched Hg and butyltin species were spiked and equilibrated for 15 min. The pH was re-adjusted to pH 5 for the simultaneous analysis of mercury and tin species. Propylation of the species was performed by adding 0.3 mL of iso-octane and 0.25 mL of NaBPr4 5% (w/v) prepared daily, and the bottle was manually shaken for 5 min. The organic phase was recovered and transferred to a chromatographic vial for the GC–ICP–MS analysis (Monperrus et al., 2005b). The filter (previously dried at room temperature under a Class 100 laminar flow hood and weighed for SPM level determination) or a subsample of 250 mg of sediment (previously freeze-dried and crushed) was accurately weighed in the extraction tube and 5 mL of HNO3 6 N for mercury analyses or HAc:MeOH (3:1) for tin analysis was added. The isotopically enriched species were spiked and left to equilibrate for 15 min. The extraction conditions were 75 °C (mercury analysis) or 70 °C (tin analysis) for 4 min with stirring (Rodríguez-González et al., 2005; Rodríguez Martín-Doimeadios et al., 2003). After cooling, the extracts were centrifuged at 3500 rpm for 5 min and an aliquot of the extracted solution was added to 22 mL glass vials in which 5 mL of HAc/NaAc buffer 0.1 mol/L (pH 4 for mercury speciation analysis or pH 5 for tin speciation analysis) was previously added. Then, the pH was re-adjusted to pH 4 or 5. The propylation of the species was performed by adding 1 mL of iso-octane and 0.25 mL of NaBPr4 5% (w/v). The organic phase was recovered and analyzed by GC–ICP–MS. 2.2.3. Sample analysis by GC–ICP–MS and isotope dilution The separation of the mercury and tin species was performed by GC using a MXTÒ SilcosteelÒ column (Restek, Bellefonte, PA, USA), and the determination was performed using an ICP–MS as previously mentioned. Detailed information on the organomercury and organotin species determination can be found elsewhere (Monperrus et al., 2005b; Rodríguez-González et al., 2005; Rodríguez Martín-Doimeadios et al., 2003). The isotope dilution analysis (IDA) was used to calculate the concentrations of the mercury and butyltin species as it is considered a definitive method that provides highly accurate and precise data and corrects for possible losses and species transformations. All concentrations are given on a dry weight basis as Hg or Sn. The accuracy and precision of this methodology have been evaluated using available certified reference materials: BCR-579 for

THg in coastal sea water (Institute of Reference Materials and Measurements, Geel, Belgium), IAEA 405 for Hg speciation in estuarine sediment (International Atomic Energy Agency, Vienna, Austria) and PACS-2 for Sn speciation in marine sediment (National Research Council Canada, NRCC, Ottawa, Canada). The precision (calculated from triplicate independent analyses as the relative standard deviation) and accuracy (calculated from triplicate independent analysis as the average deviation from the target value) obtained were 0.8% and 2.1%, respectively, for THg in BCR-579, 5.4% and 3.5%, respectively, for MMHg in IAEA-405, 3.8% and 3.4%, respectively, for THg in IAEA-405, and 2.2% and 3.7%, respectively, for TBT in PACS-2. Blanks were also analyzed with each batch to control the quality of the results, especially at low Hg species concentrations. The statistical analyses of the data were performed by linear regression, F-test or Student’s t-test using Microsoft Office Excel software (MS Office 2007, Microsoft Corporation, Redmon, WA, USA). 3. Results and discussion 3.1. Spatial and salinity distribution of organomercury and organotin compounds 3.1.1. Speciation in water and particles The average and range of concentrations obtained for the two sampling campaigns (wet and dry seasons) are listed in Table 1. The extent of the methylmercury and butyltin compounds relative to the THg and total butyltin compound concentrations (in %), respectively, are indicated to evaluate the apparent potentials of Hg methylation and TBT degradation in the aquatic environment. 3.1.1.1. Mercury compounds. The THg and MMHg concentrations found in the Bach Dang Estuary exhibit values ranging from 0.3 to 2.1 ng/g for MMHg in particles, 0.01–0.03 ng/L for dissolved MMHg, 68.4–198.4 ng/g for THg in particles and 0.2–1.6 ng/L for dissolved THg. These values are in the same range as those found in other estuaries. For example, in the Elbe Estuary, the MMHg concentrations found in the particles were in the 0.05–0.19 ng/L range, 0.8–3.3 ng/L for dissolved THg and 1.6–99.4 ng/L for THg (Coquery and Cossa, 1995). In the Mackenzie Estuary, the dissolved MMHg was at 0.02–0.45 ng/L, dissolved THg at 1.3–4.8 ng/L and THg in

Table 1 Concentrations obtained from the surface water collected in the Bach Dang Estuary for mercury and butyltin species during the wet (n = 30) and dry (n = 27) seasons. Wet season (July 2008)

MMHg

IHg

TBT

DBT

MBT

Total % MMHg Particulate Dissolved Total Particulate Dissolved Total % TBT Particulate Dissolved Total % DBT Particulate Dissolved Total % MBT Particulate Dissolved

Dry season (March 2009)

Average

SD

Range

Average

SD

Range

0.048 1.0 0.8 0.019 5.4 114.2 0.7 5.5 47.5 45.7 1.5 2.8 42.2 45.9 1.4 0.9 11.9 20.1 0.20

0.034 0.3 0.4 0.006 4.2 29.2 0.4 8.0 15.4 32.2 0.7 1.1 15.5 43.2 0.4 0.6 3.3 17.4 0.04

0.007–0.146 0.4–1.7 0.3–2.1 0.012–0.032 1.0–16.2 68.1–196.3 0.2–1.6 0.8–38.0 31.6–87.3 10.8–131.6 0.7–4.1 1.0–5.7 6.8–86.2 0.6–147.5 0.9–2.4 0.2–3.5 5.8–18.0 3.1–63.9 0.14–0.37

0.05 2.9 2.8 0.014 1.8 101.3 0.3 0.49 10.2 6.2 0.69 2.91 68.5 171.8 1.16 0.8 21.3 79.5 0.18

0.02 0.9 1.7 0.005 1.4 22.2 0.2 0.42 7.6 4.8 0.32 1.22 11.5 90.0 0.40 0.4 12.1 116.1 0.11

0.01–0.11 1.4–4.5 1.1–6.3 0.006–0.025 0.5–6.6 78.8–175.6 0.2–1.0 0.02–1.31 0.6–26.6 1.8–20.6 0.32–1.23 0.50–5.31 39.9–85.2 65.7–412.9 0.54–1.99 0.4–2.6 9.0–55.7 14.4–523.3 0.06–0.49

Total (ng/L); particulate fraction (ng/g); dissolved fraction (ng/L).


non-filtered water at 4.7–22.5 ng/L during 2004–2005 (Leitch et al., 2007), and similar values were also obtained by other authors in the same estuary in Canada (Graydon et al., 2009). Although the total MMHg concentrations were similar in the wet and dry seasons (0.05 ± 0.03 and 0.05 ± 0.02 ng/L, respectively; p > 0.05, Student’s t test), the total IHg concentrations were generally higher in the wet season (5.4 ± 4.2 > 1.8 ± 1.4 ng/L) (see Table 1). Regarding the apparent methylation, the MMHg extent (% MMHg) was higher during the dry season compared with the wet season (2.9 ± 0.9% > 1.0 ± 0.3% methylation, respectively; p < 0.05). Although the concentration of MMHg dissolved in the estuarine water was similar during both seasons, the maximum concentrations of dissolved MMHg and IHg were found during the wet season. IHg was found in the particles at a similar concentration during both seasons (114.2 ± 29.2 ng/g in the wet season and 101.3 ± 22.2 ng/g in the dry season; p > 0.05), but the concentration of MMHg in the particles was higher during the dry season (2.8 ± 1.7 ng/g in the dry season vs. 0.8 ± 0.4 ng/g in the wet season; p < 0.05). The main seasonal features observed can be simply depicted by the drastic changes in the hydrodynamic regime related to the river discharge. Larger Hg concentrations (mainly IHg) are related to upstream river discharge. Wetlands can also be an important source or sink of Hg during the rainy period because they are flooded and thus can release or trap Hg from or to the adjacent estuary. As the river discharge decreases during the dry season, the residence time is longer and thus methylation is most likely favored. Consequently, the concentrations of MMHg are higher under these conditions. During the dry season, the wetlands can also be a potential source of MMHg to the adjacent estuary as already observed for other floodplain and wetland areas, such as in the Amazon basin (Guimarães et al., 2000; Mauro et al., 2002). 3.1.1.2. Butyltin compounds. Our values for the concentrations of butyltins in the estuarine-coastal waters (Table 1) are in the same range but generally lower than the concentrations presented in previous works. For the butyltin compounds, there are several studies dealing with these contaminants in Asia, mainly because Asian countries have not completely restricted their use, while ship traffic has been largely developed in this region. For example, a harbor area in Korea was studied in 2007 and shown to exhibit ranges of 1.3–30 ng/L for TBT and 0.7 ± 0.3 ng/L, respectively; p < 0.05). During the dry season, the proportion of MBT and DBT was also higher compared with the wet season, thereby most likely indicating a higher rate of TBT degradation. Similarly, much larger concentrations of TBT were measured during the wet season in relation to the higher river discharge. This main source of TBT is most likely due to the pollution coming from the upper part of the estuary, where shipyards as well as large urban and industrial areas are concentrated. During the dry season, the TBT concentrations exhibited much lower levels, whereas the DBT and MBT concentrations were higher. The apparent extent of

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the TBT degradation was thus more important, most likely due to the longer residence times in the estuary that promote both bacterial and photochemical degradation processes (Antizar-Ladislao et al., 2011). Considering mercury and butyltin contamination, IHg and TBT were the principal pollutant species imported into the Bach Dang estuarine system, and they were mainly controlled by the origin of the transported material and the turbidity, which was indeed higher during the wet season than the dry season. In terms of water quality, chronic toxic effects are found at levels of only a few nanograms of TBT per liter (Sudaryanto et al., 2002). Consequently, the total TBT concentrations during the wet season could present a significant environmental risk. The levels of mercury compounds were not alarming, but conducting food chain studies, especially among fish, is necessary to assess whether a significant impact of mercury contamination is occurring. 3.1.2. Distribution as a function of salinity and suspended particulate matter The concentration variations and the proportion of mercury compounds (MMHg and IHg) and butyltin species are presented as a function of salinity (practical salinity unit) in Fig. 2. The total MMHg varied during the wet season from 0.07 to 0.01 ng/L, from salinity 0 to 13 and during the dry season from 0.11 to 0.02 ng/L (0–32 salinity). The total IHg varied during the wet season from 7.1 to 1.2 ng/L (salinity 0–13) and during the dry season from 6.6 to 0.5 ng/L (salinity 0–32). In the case of TBT, during the dry season, the total concentrations decreased from 1.0 to 0.03 ng/L, the salinity from 0 to 32, and DBT from 5.0 to 0.5 ng/L. During the wet season, the butyltin concentrations did not show any important variation (p > 0.05). A decrease in total concentrations as salinity increased can be noted, particularly for MMHg and IHg during both seasons and for TBT during the dry season (p < 0.05). This observation indicates that the source of Hg and TBT pollution in the Bach Dang Estuary was mainly located upstream (i.e., at low salinity), as shown by the dilution of these pollutants by the estuarine mixing process. However, the decreasing trends were not strictly linear, suggesting that such estuarine transport and mixing was not conservative. A number of exchange and/or disposal processes may thus take place during the estuarine transport. These conclusions suggest that the estuarine mass balance of each contaminant could be further evaluated, assuming steady state conditions. However, such mass balance was not applied in this study because specific freshwater and marine end-members could not be collected during the field campaign and the complexity of hydrology in the Red River deltaic systems (several fresh water end-members and wetlands) means that assuming steady state conditions within the Bach Dang and Cam branches would be incorrect. The total concentrations of the main pollutants studied were also strongly linked, and even correlated in the case of Hg, with the suspended particulate matter concentrations (Fig. 3). During the dry season, the suspended particles were more enriched with IHg than during the wet season, as shown by the slopes of the regressions. Conversely, TBT was more strongly associated with suspended particles during the wet season, supporting the hypothesis that TBT can be readily transported by particles from upstream sources and did not have enough time to be degraded within the estuary due to the short water residence times. This result shows that particles were a primary source of these contaminants released from upstream inputs, runoff from urban and industrial areas and re-suspension of contaminated sediments. The composition of these particles also did not change during the estuarine mixing, except in the outer estuary, which was dominated by plankton during the dry season. The MMHg proportions were higher in the dry season at lower salinities and at higher suspended matter concentrations. Fig. 4

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Fig. 2. Variations in the total MMHg, IHg and butyltin species concentrations (ng/L) along the estuarine mixing during the wet and dry seasons.

Fig. 3. Distribution of IHg and TBT as a function of SPM during the wet and dry seasons.

shows that the apparent mercury methylation, as expressed by the total methylmercury extent (i.e., dissolved and particulate fractions), was significantly higher during the dry season. This result confirms that mercury was most likely methylated during its transfer across the estuarine gradient and its proportion subsequently increased during the dry season as the residence time was longer. However, the selective bioaccumulation by phytoplankton during the dry season could also be an efficient scavenging pathway for MMHg and its enrichment in ‘‘biogenic particles’’. During the same campaigns, Rochelle-Newall et al. (2011) performed an extensive plankton study and mainly concluded that while diatoms were the dominant taxa during both seasons, the phytoplankton abundance, primary production and biomass

Fig. 4. Apparent mercury methylation and TBT degradation potentials during the wet and dry seasons.

(assessed by Chlorophyll a concentrations) were generally lower or similar during the dry season, even taking into account the higher SPM load during the wet period. Although the phytoplankton bioaccumulation of MMHg cannot be higher during the dry season (Rochelle-Newall et al., 2011), mainly the new production or inputs of MMHg in the estuary can thus promote its larger extent. In contrast, the TBT proportions were lower during the dry season and decreased with increasing salinity and suspended matter. The TBT degradation was also higher during the dry season (Fig. 4). This result confirms that TBT can be efficiently degraded during its

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transfer along the estuary via either bacterial or photochemical pathways, especially when its residence time is longer (dry season). Interestingly, Rochelle-Newall et al. (2011) reported that bacterial activity did not significantly differ between the two seasons in this estuary, and thus residence time rather than instantaneous bacterial activity rates appear to control the degradation of these pollutants. Because the variation of the total extent of MMHg and TBT between the wet and dry seasons is mainly controlled by the particulate fraction, the findings from Lefebvre et al. (2012) also suggest that the particle load may undergo significant input from the bay, which thus affects their contamination level. Even if these particles are originally transported to the bay from river discharge during the wet season, their long residence time in the bay may again promote both Hg methylation and TBT degradation. 3.1.3. Concentration levels in sediments from the navigation channel Similar and typical estuarine values of MMHg were found in the sediments of the Bach Dang Estuary (see Table 2), and no significant differences in the concentrations were noted between the Cam River and the rest of the estuary, similar to the THg distribution. The MMHg concentrations in the sediments exhibited values ranging between 0.15 and 0.28 lg/kg, which are approximately 10 times lower than MMHg concentrations found in the particulate fraction (Table 1, 2.8 ± 1.2 lg/kg in the dry season). The MMHg present in fine suspended particles is suspected to be more concentrated than in bulk sediment due to grain size partitioning between the water column and the sediment. In addition, the production of

MMHg most likely occurs predominantly in the continuous adjacent wetlands more than in bottom sediments of the main navigation channel (Roulet et al., 2001). The TBT concentrations were higher in the Cam River than in the downstream estuarine section (Table 2), which can be clearly explained by intense port activity and the existence of several shipyards (Nhan et al., 2005). However, relatively low concentrations were measured in the main channel compared with the river bank sediments collected and analyzed by Nhan et al. (2005), suggesting that the Cam River sediments located in the navigation channel were neither a sink of upstream TBT inputs nor a source of downstream estuarine waters via particle resuspension. Generally, the sediment values ranged within the background levels and were comparable with previous investigations in some other estuaries (Garg et al., 2010; Üveges et al., 2007). Similar to Hg, the TBT in particles was more concentrated than in the surface sediments (up to more than 15 times), especially during the wet season when local urban discharge and upstream inputs are expected to mainly occur. Interestingly, the major sediment deposition in the main channel of the Cam and Bach Bang channels was clearly identified to occur during the dry season when most particles are originating from the downstream coastal bay due to tidal pumping (Lefebvre et al., 2012). As discussed previously for suspended particles, we may suggest that most surface sediment in the main channels originates from the settling of particles less contaminated with TBT. Overall, our superficial sediment data mainly showed that the major sources and sinks of MMHg and TBT were not located in the sediments of the main channel but were most likely from

Table 2 Concentrations (lg/kg dry weight) of MMHg, THg and butyltin species found in sediments collected along the estuary (n = 4) during the dry season.

Bach Dang (estuary) Cam River

MMHg

THg

TBT

DBT

MBT

0.23 ± 0.05 (0.15–0.28) 0.22 ± 0.03 (0.20–0.25)

72.1 ± 17.2 (57.2–97.7) 71.2 ± 8.2 (61.4–83.2)

1.4 ± 0.2 (1.2–1.8) 3.1 ± 0.6 (2.4–4.0)

3.6 ± 0.6 (2.6–4.4) 3.8 ± 0.3 (3.3–4.3)

2.3 ± 0.5 (1.4–3.1) 2.2 ± 0.3 (1.8–2.8)

Fig. 5. Variations in the salinity, water height, SPM and mercury and butyltin species concentrations along the 24 h tidal cycle during the wet and dry seasons.

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Table 3 Integrated flux values (g/day) of mercury and butyltin species for a 24 h tidal cycle at the Dinh Vu station during the wet and dry seasons. The positive flow means the downstream direction and the negative flow means the upstream direction. Wet season

Dry season

TBT d TBT p TBT t

432.0 2801.8 3233.8

16.1 1.8 17.9

DBT d DBT p DBT t

267.9 117.1 385.0

43.1 29.3 13.8

MBT d MBT p MBT t

25.2 166.2 191.4

9.1 650.3 659.4

RBuSn d RBuSn p RBuSn t

725.2 3085.1 3810.3

50.1 677.8 627.7

MMHg d MMHg p MMHg t

2.1 2.5 0.4

0.1 2.8 2.7

IHg d IHg p IHg t

23.9 2588.3 2612.2

12.3 105.2 93.0

THg d THg p THg t

26.0 2585.9 2611.8

12.2 102.5 90.3

BuSn = butyltin species; d = dissolved; p = particulate; t = total.

either upstream rivers (both Hg and TBT), point discharges such as ports (TBT source) or adjacent wetland inputs (Hg source). 3.2. Tidal transport at Dinh Vu between the upstream and downstream sectors of the estuary in the wet and dry seasons 3.2.1. Variation of mercury and organotin species along a 24 h tidal cycle The concentrations obtained during the 24 h tidal cycles are represented in Fig. 5. Following the concentration over a full tidal cycle can be useful for establishing how the tidal processes affect the distribution, transport modes and direction (upstream or downstream) of a pollutant. During the wet season tidal cycle, the maximum concentrations of butyltin species and IHg, along the time series measurements, were systematically related to the maxima of SPM. This result was coincident with the ebb tide, at which time current velocities reach a maximum and thus generate a downstream export of turbid waters, enhancing fine sediment re-suspension (Lefebvre et al., 2012). A first assessment shows that during the wet season, the

upper part of the estuary is a clear and significant source of organometallic contaminants toward the downstream section. During the dry season, this source seems to be considerably reduced, and tidal exchanges of these pollutants remain very limited. Under these conditions, the hydrodynamic feature favors the retention of the compounds in both particulate and dissolved fractions and their subsequent in situ transformation (e.g., Hg methylation and TBT degradation) within the estuarine mixing zone. 3.2.2. Tidal fluxes of mercury and organotin species within the estuary The instantaneous flux (g/h) of the pollutants in the dissolved and particulate fractions and their respective daily integrated flux (g/day) were estimated by combining the depth-profiles of the instantaneous fluxes (m3/s) obtained by ADCP, the turbidity profile (linearly related to SPM) and the concentrations measured at Dinh Vu station in the particulate and dissolved fractions (Table 1). During the wet season, the water fluxes during the tidal cycle ranged from 4809 to +5085 m3/s, while the suspended particle fluxes ranged from 2168 to +1907 kg/s. During the dry season, the water fluxes during the tidal cycle were between 3378 and +3309 m3/s, while the suspended particle fluxes ranged between 284 and +142 kg/s. Positive flux values reflect a downstream direction (seaward or ebb tide maximum flow), whereas negative values indicate an upstream flow. The net integrated daily fluxes (g/day) of Hg and Sn compounds based on the instantaneous solid and water flux (g/h) measurements at Dinh Vu station are detailed in Table 3. The variation of the major Hg and Sn compound fluxes along the tidal cycle is also displayed in Fig. 6. The integrated fluxes along the tidal cycle showed that during the wet season, important contributions of THg (mainly as IHg) and TBT occurred from the Bach Dang and Cam Rivers to the South China Sea (Fig. 6), exhibiting a net seaward flux of 2.6 kg and 3.2 kg per day, respectively (Table 3). These total fluxes are mainly due to the particulate fluxes. The DBT and MBT contributions were lower than that of TBT, which most likely came from both upstream river inputs (Red River drainage basin) and shipyards and harbor discharges into the Cam River (Haiphong industrial and urban areas). However, during the dry season, almost no significant exchange can be evaluated for both IHg and TBT. MBT is the most significant seaward flux (0.7 kg/day), as exported by suspended particles, whereas DBT and TBT exhibit low upstream fluxes. MMHg transport was very limited during the wet season campaign, whereas a significant seaward export flux of 2.7 g/day was measured during the dry season. Even if the order of magnitude of the MMHg flux remained much lower than for IHg, this estuarine input can be considered to be significant for such highly biomagnified contaminants.

Fig. 6. Instantaneous fluxes (g/h) of the mercury and butyltin species along the 24 h tidal cycle in both the wet and dry seasons.


4. Conclusions IHg and TBT are the principal mercury and butyltin species transported into the Bach Dang estuarine waters. These contributions are directly correlated with the turbidity and origin of the transported material, which are more important during the wet season than the dry season. Total concentrations are strongly linked with the suspended particulate matter, showing that particles are the primary source of these contaminants in this estuary. MMHg and TBT seem to be methylated and degraded, respectively, during their transfer along the estuary, particularly during the dry season. Indeed, Hg methylation and TBT degradation seems to increase along with the estuarine residence time of the water and particles. Taking into account flux values, the estuary appears to be an important source of IHg and TBT for the coastal area during the wet season and of MMHg during the dry season. This remarkable result is, to our knowledge, one of the first estimations conducted within a tropical estuary. During the wet season, the TBT concentrations could present a significant environmental risk, and its estuarine flux to the coastal zone also seems very significant. In contrast, the levels of mercury compounds are not alarming, but a study extended to the food chain is necessary to evaluate the impact of mercury contamination. Therefore, these results are a first assessment of the major organometallic contamination for the implementation of coastal water policies and management in the Bach Dang Estuary in Vietnam.

Acknowledgements This work was funded by the EC2CO French National Program through the HAIPHONG project and by the ANR CES program through the IDEA project. P. Navarro is grateful to the Basque Government (Education, Universities and Research Department) for her postdoctoral financial support. N. Duong Thanh thanks the IRD (France) for support provided by the BEST grant.

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