Water Air Soil Pollut (2012) 223:1315–1330 DOI 10.1007/s11270-011-0947-x Historical Profiles of Trace Element Concentrations in Mangrove Sediments from the Ba Lat Estuary, Red River, Vietnam Nguyen Tai Tue & Tran Dang Quy & Atsuko Amano & Hideki Hamaoka & Shinsuke Tanabe & Mai Trong Nhuan & Koji Omori Received: May 2011 / Accepted: September 2011 / Published online: 15 September 2011 # Springer Science+Business Media B.V 2011 Abstract Historical profiles of trace element concentrations were reconstructed from two mangrove sediment cores collected within the Ba Lat Estuary (BLE), Red River, Vietnam Chronologies of sediment cores were determined by the 210Pb method, which showed that each respective sediment core from the south and north entrances of BLE provided a record of sediment accumulation spanning approximately 100 and 60 years The profiles of Pb, Zn, Cu, Cr, V, Co, Sb, and Sn concentrations markedly increased from the years of the 1920s–1950s, and leveled out from 1950s–1980s, and then gradually decreased from 1980s to present The profiles of Cd N T Tue (*) : H Hamaoka : S Tanabe : K Omori Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan e-mail: tuenguyentai@gmail.com N T Tue e-mail: tuent@sci.ehime-u.ac.jp T D Quy : M T Nhuan Faculty of Geology, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam A Amano Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8567, Japan and Ag concentrations increased from 1920s–1940s, and then decreased from 1940s to present The profile of Mo concentrations progressively increased from 1920s–1980s, then decreased to present The Mn concentrations failed to show a clear trend in both sediment cores Results from contamination factors, Pearson’s correlation, and hierarchical cluster analysis suggest that the trace elements were likely attributed to discharge of untreated effluents from industry, domestic sewage, as well as non-point sources Pollution Load Index (PLI) revealed levels higher than other mangrove sediment studies, and the longterm variations in PLI matched significant socioeconomic shifts and population growth in Vietnam Geoaccumulation Index showed that mangrove sediments were moderately polluted by Pb and Ag, and from unpolluted to moderately polluted by Zn, Cu, and Sb The concentrations of Pb, Zn, Cu, Cr, and Cd exceeded the threshold effect levels and effect range low concentrations of sediment quality guidelines, implying that the sediments may be occasionally associated with adverse biological effects to benthic organisms Keywords Historical profiles Trace element Mangrove sediment Ba Lat Estuary Vietnam Introduction Mangrove ecosystems act as natural filters for retaining sediments and pollutants that originate from land- 1316 derived materials and river outflows prior to entering the ocean (Harbison 1986; Lacerda et al 1991; Tam and Wong 1995; Clark et al 1998) The high density of root systems and trees reduce tidal flows, which preferentially accumulate suspended clay and silt particles As much as 80% of suspended sediments can be retained within mangrove forests from coastal waters during periods of spring tides (Furukawa et al 1997) Mangrove sediments are generally homogeneous in texture and rich in organic matter, therefore, they act as effective sinks of pollutants, particularly in the case of trace elements As a result they have a high capacity to retain trace elements from tidal and river outflows Trace elements can be retained within mangrove sediments by mechanisms of direct adsorption, forming a complex with organic matter and through the formation of insoluble sulfides (Clark et al 1998) Trace elements however are also very reactive to geochemical conditions, where factors of pH and salinity can influence their mobility within mangrove sediments (Liang and Wong 2003) Thus, trace elements have a high potential for release from the sediment–water interface when any oxidizing processes occur, such as in flooding or dredging activities From mangrove sediments, trace elements can also move by being directly absorbed into plants (MacFarlane et al 2003) and benthic organisms (Saha et al 2006; Amin et al 2009), subsequently transferring into higher trophic levels of local food webs (Jara-Marini et al 2009) In recent decades, land-use changes have resulted in high soil erosion rates and have increased pollutant yields to coastal environments (Owen and Lee 2004), and these pollutants can be subsequently retained in estuarine mangrove ecosystems of the (sub)tropical coastline However, historical concentrations of trace elements within mangrove ecosystems are poorly known, especially in developing countries Therefore, study on the historical profiles of trace element concentrations in mangrove sediments is important for not only tracing anthropogenic disturbances to the estuarine environment but also essential to sediment quality assessment The Red River (RR) is the largest river in northern Vietnam, draining a total area of 78,695 km2 (Le et al 2007), with the Red River Delta (RRD) being one of the largest deltas in Southeast Asia (Tanabe et al 2003) The total human population of the RRD is Water Air Soil Pollut (2012) 223:1315–1330 approximately 19.6×106 people (http://www.gso.gov vn), which is the most populous region of Vietnam As a result, the RRD drains a very large area consisting of high urban and industrial development, trade villages, agriculture, and ventures in aquaculture However, very little in terms of effluent treatment occur within this vast and complex system (Marcussen et al 2008), with runoff from urban, industrial, and agricultural activities being directly discharged into surface-flowing channels and rivers The RRD has therefore been considered as one of the top environmental hotspots of Vietnam (http://www.nea.gov.vn), yet there still remains a major deficiency of information on pollutant concentrations (e.g., trace elements) from the region (i.e., estuaries and coastal ecosystems) The purpose of this study was to examine the historical profiles of 12 trace element concentrations (Pb, Zn, Cu, Cr, V, Mn, Cd, Co, Sb, Sn, Ag, and Mo) in age-dated sediment cores, in order to understand the temporal variations of these trace element concentrations, and to assess sediment quality of mangrove ecosystems from the Ba Lat Estuary (BLE) Our results provide information on trace element concentrations in mangrove sediments in the context of anthropogenic disturbances to the system, both during the past and in forecasting future risk This study presents the first record of historical profiles of trace element concentrations in the mangrove ecosystems from the BLE The trace element concentrations reported in this work are highly valuable as baselines for comparison in future sediment quality studies Materials and Methods 2.1 Study Area The BLE is the largest estuary of the RR system, consisting of two major mangrove wetland sites (Xuan Thuy National Park and Tien Hai Nature Reserve; Fig 1) The mangrove forests are dominated by trees of Sonneratia caseolaris, Bruguiera gymnorhiza, Kandelia candel, Aegiceras corniculatum, and Acanthus ilicifolius, which play important roles in the filtering and containment of terrestrial-derived materials and various pollutants, and act as a physical buffer against erosion and surge from major storm events Moreover, the BLE is of great regional importance as a Water Air Soil Pollut (2012) 223:1315–1330 1317 Fig Sampling sites showing location of cores R2 and R4 within mangrove forests from the Ba Lat Estuary, Vietnam major breeding and stopover for migratory birds along the East-Asian and Australian flyways, and locally as essential habitat for a diversity of benthic organisms and vertebrate animals and other wildlife The BLE is located within a distinct monsoon climate zone with a rainy season from May to October and a dry season from November to April The temperature and rainfall annually vary from 15.9 to 29°C and from 1,300 to 1,800 mm, respectively Tides at the BLE are diurnal with a mesotidal regime and a tidal range from 2.5 m at spring tide to 0.5 m at the neap tide Waves approach the BLE from the south in the rainy season and from the northeast in the dry season 2.2 Sample Collection and Storage Sampling was conducted from 28 January to 10 February, 2008 during low tide from two dense natural mangrove forests, one adjacent to the south entrance (Xuan Thuy National Park, core R2) and one to the north entrance (Tien Hai Nature Reserve, core R4) of the BLE (Fig 1) Cores R2 and R4 were located in the high and low intertidal zone, respectively Core R4 was located approximately 200 m from a tidal creek Cores were taken by hand corer with a PVC inner tube (1.5 m in length and 10 cm in diameter), with lengths of cores R2 and R4 being 68 and 75 cm, respectively Immediately following collection, cores were capped and stored in an upright position and maintained cool Cores were processed within 12 h of collection by first removing the outer layers (0.5 cm in thickness) and then slicing by a plastic knife into cm intervals Sediment samples were packed in labeled polyethylene bags for further analysis Sections of the sediment slices were also placed in plastic cubes (1 cm3) for porosity analysis Samples were immediately stored in iceboxes and transported to the laboratory where they were frozen at −20°C until further processing and analysis 2.3 Analytical Methods Sediment samples were dried at 60°C for 48 h in an electric oven and subsequently pulverized using a 1318 Water Air Soil Pollut (2012) 223:1315–1330 mortar and pestle For 210Pb analysis, approximately 20 g of the pulverized sample was sealed in a plastic jar and equilibrated with 226Ra, 222Rn, and 214Pb for 30 days Based on characteristics of the gamma peaks, the activities of 210 Pb (46.5 KeV) and 214 Pb (351.9 KeV) were measured using a Ge detector (GXM25P, ORTEC Co.) 210Pb excess (210Pbex) was calculated by subtracting 214Pb activity from 210Pb activity During the 210Pb analysis process, 137Cs activity was simultaneously analyzed However, the 137 Cs activities ranged from undetectable to very low in sediment cores Thus, 137Cs activities were not used to confirm the chronologies of sediment cores Based on 210Pbex activities, the constant initial concentration (CIC) model was used to calculate sedimentation rates The CIC model has been successfully applied in other studies of estuaries (e.g., Bonotto and de Lima 2006), intertidal mudflats (e.g., Andersen et al 2000), and mangrove ecosystems (e.g., Sanders et al 2010) Ages of sediment cores were calculated with the assumption of a constant sedimentation rate (Appleby and Oldfield 1978) For the CIC model, the 210Pbex activity (Cx) at any sediment layer (x) with age (t) is simply expressed as: Cx ¼ Co»eÀlt ¼ Co»eÀlx=So ; ð1Þ in which Co presents the 210Pbex at the sediment– water interface, is the radioactive decay constant for 210 Pb (0.03114 year−1), and So is the sedimentation rate at the sediment–water interface (cm year−1) From Eq 1, the sedimentation rate (So) was determined by the slope of 210Pbex profiles using least squares regression However, a number of studies have shown that the effect of compaction on sediment layers may cause an incorrect depth (x) (e.g., Lu 2007) An alternative method, which expresses 210Pbex as a function of the cumulative weight of sediment (w: g cm−2) removes the compaction effect (Lu 2007) The Eq can be rewritten as: Cm ¼ Co»eÀlt ¼ Co»eÀlðw=rÞ ; ð2Þ where w is the cumulative dry weight (g cm−2) at the sediment layer with bulk weight (m: g cm−3), r is the sediment accumulation rate (g cm−2 year−1), Cm is the 210Pbex at the sediment layer (m), and Co is the 210 Pbex at the sediment–water interface layer From Eq 2, the sediment accumulation rate (r) was determined by the slope of 210Pbex profiles using least squares regression The ages of sediments (year) was calculated based on the equation: t ẳ w=r: 3ị Finally, the 210Pb chronologies of both methods were compared to derive the final chronologies The results of both methods were agreed, thus the compaction effects did not show significantly in the 210Pb chronologies in the both sediment cores Therefore, the results of sediment chronologies were unique and reported by the least squares regression from the Eq Sediment grain size was measured by using a laser diffraction particle size analyzer (SALD-2100, Shimadzu Co.) according to the procedure described by Amano et al (2006) Sediment grain size was assigned to the median diameter based on the scale (Md8) To examine water content and porosity of sediment, the wet sediment in a plastic cube (1 cm3) was weighed and dried in an electric oven at 40°C until obtaining a constant weight The relative water content was determined after drying Sediment porosity was calculated based on a sediment density of 2.65 gcm−3, the sample cube volume (1 cm3) and the bulk dry sediment weight (Baskaran and Naidu 1995; Bonotto and de Lima 2006) For total organic carbon (TOC) analysis, a total of g of pulverized sediment sample was placed in a glass tube and approximately ml of N HCl was added and thoroughly mixed using a vibrating mixer, and then left at room temperature for 24 h to remove carbonates After acid treatment, the samples were thoroughly rinsed with MILLI-Q water (Millipore), and then dried at 60°C for 48 h in an electric oven Total organic carbon was analyzed with an element analyzer CN corder Yanaco, MT-700 Hippuric acid (C6H5CONHCH2COOH; for the CN coder, Co Ltd Kishida chemicals) was used as the certified reference material for calibration of the organic carbon For trace element analysis, 0.2 g of pulverized sample was treated in a microwave Teflon vessel with an acid mixture (5 ml HNO3, and ml HF) The mixture was heated in a microwave system (Ethos D, Milestone S.r.l., Sorisole, BG, Italy) with the following programs: 2, 3, 5, 5, 5, and under 250, 0, 250, 400, 500, and 400 W power, respectively; this was followed by ventilation for To remove HF, digested sample solutions were evaporated by heat- Water Air Soil Pollut (2012) 223:1315–1330 1319 ing After digestion and cooling, the samples were diluted with ultrapure MILLI-Q water to 50 ml for further analysis Concentrations of 12 trace elements (Pb, Zn, Cu, Cr, V, Mn, Cd, Co, Sb, Sn, Ag, and Mo) were analyzed with an inductively coupled plasmamass spectrometer (ICP-MS, HP-4,500, Avondale, PA, USA) with rhodium as the internal standard Accuracy and precision of the methods were assessed using the certified marine sediment reference material PACS-2 (National Research Council Canada), and recoveries of all the trace elements ranged from 89.3% to 111.6% of the certified values (Table 1) In addition, triplicate analyses were applied for each sediment sample, and the concentrations of trace elements were displayed by the average values One half of the value of the respective limits was substituted for those values below the limit of detection Results and Discussion 3.1 210 Pbex Geochronology The plots of 210Pbex and depth for both cores R2 and R4 are shown in Fig The 210Pbex increased from the core bottom to the depth of 11 and 18 cm, and then decreased to surface sediments in cores R2 and R4, respectively The decrease in 210Pbex activities at the surface sediments may have resulted from perturbation processes (Farmer 1991) In mangrove ecosystems of the BLE, there are high densities of crabs and mollusks that can cause significant remixing of sediments (Smoak and Patchineelam 1999) In addition, during operation of the hand corer, the mechanical mixing and subsequent diffusion of 210Pb may occur in the first layers of the sediment cores (Bonotto and de Lima 2006) Thus, the 210Pbex activities at these depths were not used in the least squares regressions for calculating sedimentation rates (Fig 2) The results showed that the sedimentation rate for core R2 was 0.78 cm year−1, suggesting core R2 provided a record of sediment accumulation spanning approximately 100 years The sedimentation rate for core R4 was 1.2 cm year−1, nearly twice that of core R2, providing a record of sediment accumulation approximately 60 years These sedimentation rates were consistent with those of a previous study (range 0.81 to 1.46 cm year−1) within mangrove forests from the BLE (Van Santen et al 2007) The sedimentation rates were also confirmed from our observation of a distinctive, very thin layer of bivalve shells, and a fine sand layer from 29 to 30 cm of depth in core R2, which was likely formed during a major storm and flooding event that occurred in 1971 in the RRD (van Maren 2007) The chronologies of the sediment cores therefore provided reliable histories of sediment accumulation in the mangrove ecosystems of the BLE 2.4 Statistical Analysis The representative average concentrations of trace elements were log transformed prior to statistical analysis to meet assumptions of a normal distribution Pearson’s correlation was used to examine correlations among trace elements and sediment parameters (TOC, porosity, and sediment grain size (Md8)) Hierarchical cluster analysis is a multivariate technique, which is used to classify the variables into categories based on their similarity The objective of the hierarchical cluster analysis is to find an optimal grouping for which the variables with each cluster are similar, but the clusters are dissimilar to each other Hierarchical cluster analysis was applied to the representative mean concentrations of trace elements and sedimentary parameters (TOC, porosity, and Mdφ) The distance metric was based on the Euclidean distance completed linkage method All statistical analyses were performed using a SPSS statistical software package 17 (SPSS 17.0) Table Marine sediment reference material (PACS-2) for trace element values, analytical values, and recovery (n=16) Elements Pb Zn Cu Cr V Mn Cd Co Sb Sn Ag Mo Analytical value (μg/g dry wt.) PACS-2 reference value (μg/g dry wt.) Recovery (%) 187±9 393±18 323±13 81.0±4.0 140±6.0 417±20.0 2.35±0.21 12.0±0.6 12.5±0.5 21.3±0.6 1.28±0.12 6.06±0.60 183±8 364±23 310±12 90.7±4.6 133±5.0 440±19.0 2.11±0.15 11.5±0.3 11.3±2.6 19.8±2.5 1.22±0.14 5.43±0.28 102.19 107.97 104.19 89.31 105.26 94.77 111.37 104.35 110.62 107.58 105 111.6 The trace element concentrations are shown by the mean ± 1SD 1320 Water Air Soil Pollut (2012) 223:1315–1330 markedly increased from 0.23% to 2.65% between the core bottom and 26 cm of depth The TOC content remained high to15 cm of depth, and then decreased to 1.2% at the surface sediment For core R4, the Md8 decreased from 29.85 to 10.6 μm between the core bottom and 61 cm of depth The Md8 was then invariant from 61 cm of depth to the surface sediment The porosity showed a progressive increase from the bottom of core to the surface sediment The TOC content varied from 0.61% to 1.43%, with a mean of 0.93% The TOC content showed a progressive increase from core bottom to surface sediment 3.3 Historical Profiles of Trace Element Concentrations Fig Plots of 210Pbex activity with depth in the sediment cores used to determine sedimentation rates in this study Error bars denote the standard error of mean 210Pbex Shaded areas indicate parts of the cores that were affected by perturbation processes Data from filled squares were therefore not included in least squares regressions for calculating sedimentation rates (see text for detail) Top figure: Core R2, bottom figure: Core R4 3.2 Sediment Characteristics For both cores, the sediment characteristics were homogeneous, muddy, and rich in organic matter The sediment colors changed from light olivinebrown to dark grayish brown, indicating the dominant reducing conditions Sediment grain sizes (Mdφ), porosity, and TOC contents (%) of both cores R2 and R4 are shown in Fig According to the Mdφ values, the sediments of both cores R2 and R4 were mainly composed of very fine grain sizes (