Microplastics in sediments from urban and suburban rivers: Influence of sediment properties.

Although sediments are considered to be a major sink for microplastics (MP), there is still a relative lack of knowledge on the factors that influence the occurrence and abundance of MP in riverine sediments. The present study investigated the occurrence and distribution of MP in riverine sediments collected at twelve sites representative of different populated and urbanized rivers (To Lich, Nhue and Day Rivers) located in the Red River Delta (RRD, Vietnam, during dry and rainy seasons. MP concentrations ranged from 1600 items kg-1 dw to 94,300 items kg-1dw. Fiber shape dominated and MP were made of polypropylene (PP) and polyethylene (PE) predominantly. An absence of seasonal effect was observed for both fragments and fibers for each rivers. Decreasing MP concentrations trend was evidenced from the To Lich River, to the Nhue River and to the Day River, coupled with a decreasing fiber length and an increasing fragment area in the surface sediment from upstream to downstream. Content of organic matter was correlated to MP concentrations suggesting that, high levels of organic matter could be MP hotspots in urban rivers. Also, high population density as well as in highly residential areas are related to higher MP concentrations in sediments. Finally, a MP high ecological risk (RI: 866 to 4711) was calculated in the RDD.

Baseline concentration of microplastics in surface water and sediment of the northern branches of the Mekong River Delta, Vietnam.

The Mekong River Delta in Vietnam, is concerned by numerous microplastic-related issues such as a lack of wastewater treatment facilities and mismanagement of plastic waste released from agriculture, aquaculture and related activities. This study aimed to examine the presence of microplastics in surface water and sediment by collecting samples from six sites along the Tien River and its distributaries in February 2019. The results showed that the average concentration of microplastics over the entire area was 53.8 ± 140.7 items m-3 in surface water and 6.0 ± 2.0 items g-1 dried weight in sediment, with a predominance of microplastic fibres rather than fragments, respectively 85 % and 98 % in surface water and sediment. In the main flow of surface water, the concentration of microplastics was stable; however, in the sediment, microplastic concentration was affected by the high dynamic flow regime rather than the sources where microplastics are released.

Selection of a density separation solution to study microplastics in tropical riverine sediment.

Microplastics (MPs) are small (< 5 mm) plastic particles that are widely found in marine, freshwater, terrestrial and atmospheric environments. Due to their prevalence and persistence, MPs are considered an emerging contaminant of environmental concern. The separation and quantitation of MPs from freshwater sediments is a challenging and critical issue. It is necessary to identify the fate and sources of MPs in the environment, minimise their release and adverse effects. Compared to marine sediments, standardised methods for extracting and estimating the amount of MPs in freshwater sediments are relatively limited. The present study focuses on MP recovery efficiency of four commonly used salt solutions (NaCl, NaI, CaCl2 and ZnCl2) for isolating MPs during the density separation step from freshwater sediment. Known combinations of artificial MP particles (PS, PE, PVC, PET, PP and HDPE) were spiked into standard river sediment. Extraction using NaI, ZnCl2 and NaCl solutions resulted in higher recovery rates from 37 to 97% compared to the CaCl2 solution (28-83%) and varied between polymer types. Low-density MPs (PE, HDPE, PP and PS) were more effectively recovered (> 87%) than the denser polymers (PET and PVC: 37 to 88.8%) using NaCl, NaI and ZnCl2 solutions. However, the effective flotation of ZnCl2 and NaI solutions is relatively expensive and unsafe to the environment, especially in the context of developing countries. Therefore, considering the efficiency, cost and environmental criteria, NaCl solution was selected. The protocol was then tested by extracting MPs from nine riverine sediment samples from the Red River Delta. Sediments collected from urban rivers were highly polluted by MPs (26,000 MPs items·kg-1 DW) compared to sediments located downstream. Using a NaCl solution was found to be effective in this case study and might also be used in long-term and large-scale MP monitoring programmes in Vietnam.

Anthropogenic fibres in white clams, Meretrix lyrata, cultivated downstream a developing megacity, Ho Chi Minh City, Viet Nam.

Anthropogenic fibres are an emerging pollutant worldwide. The Can Gio mangrove area is located downstream of the Saigon River, and is characterised by high level of anthropogenic fibres originating from domestic and industrial textile and apparel manufacturing. In this area, biota is thus subjected to a high potential risk of anthropogenic fibre contamination. This study aims to characterise the accumulation of anthropogenic fibres in different tissues, i.e. gills, digestive systems, and remaining tissues, of white clams (Meretrix lyrata) cultivated in the Can Gio beach sand, during a seven-month sampling period. The results showed an average concentration of 3.6 ± 2.1 fibres individual-1 or 2.7 ± 2.4 fibres g-1 ww. Higher fibre accumulation was observed in remaining tissues than in gills and digestive systems, and no temporal variation was observed in all clam tissues. The intake of fibres by humans consuming clams was estimated to be 324 fibres inhabitant-1 yr-1.

Phytoplankton characterization in a tropical tidal river impacted by a megacity: the case of the Saigon River (Southern Vietnam).

The spatiotemporal variation of phytoplankton and their relationship with environmental variables were analyzed in the Saigon River-a tropical river in Southern Vietnam. Two longitudinal profiles were conducted during dry and rainy season at 18 sampling sites covering more than 60 km long in the river. Besides, a bi-weekly monitoring conducted in the upstream, urban area (Ho Chi Minh City-HCMC), and downstream of Saigon River was organized from December 2016 to November 2017. The major phytoplankton were diatoms (e.g., Cyclotella cf. meneghiniana, Leptocylindrus danicus, Aulacoseira granulata), cyanobacteria (Microcystis spp., Raphidiopsis raciborskii, Pseudanabaena sp.), and euglenoids (Trachelomonas volvocina). Commonly freshwater phytoplankton species and sometimes brackish water species were dominant during the monitoring. Phytoplankton abundances in dry season were much higher than in rainy season (>100 times) which was explained by a shorter riverine water residence time and higher flushing capacity during the dry season. There was a clear separation of phytoplankton abundance between the urban area and the remaining area of Saigon River because of polluted urban emissions of HCMC. Redundancy analysis shows that the environmental variables (TOC, nitrogen, pH, salinity, Mo, Mn) were the driving factors related to the dominance of L. danicus and Cyclotella cf. meneghiniana in the upstream river and urban section of Saigon River. The dominance of cyanobacterium Microcystis spp. in the downstream of Saigon River was related to higher salinity, Mg, Cu concentrations, and lower concentrations of nutrients, Mn, Co, and Mo. The dominance of potentially toxic cyanobacteria in Saigon River possesses health risk to local residents especially upon the increasing temperature context and nutrient loading into the river in the next decades.

Microplastics in Asian freshwater ecosystems: Current knowledge and perspectives.

Plastic pollution in freshwater ecosystems, including microplastics (MPs) smaller than 5 mm, has become an emerging global concern. Asia is considered a "hot spot" for plastic pollution due to rapid economic and demographic growth, together with rapid urbanization. Here, we provide an overview of the current knowledge on MP abundance, sources, fate, and transfer in Asian freshwater ecosystems based on publications from January 2014 to May 2021. MP contamination in freshwater compartments, including water, sediment, and biota, was found to vary strongly. In water, it ranged from 0.004 items m-3 in a moderately urbanized region to more than 500,000 items m-3 in a dumping river in a highly populated watershed. In the sediment, MP abundance ranged from 1 to more than 30,000 items kg-1 dry weight. Polyethylene (PE) and polypropylene (PP) were predominant in both water and sediment compartments. MP was detected in biota samples from all the studied species, but their abundance depended on the locations and species studied. Overall, MP characteristics (form, size, color, and polymer type) depended on sources and natural constraints (mainly hydrodynamics). This study also revealed that MP in Asian freshwater ecosystems mainly originated from domestic wastewater/runoff, followed by industrial emissions, fisheries and aquaculture wastewater. Plastic waste is not efficiently recycled or incinerated in Asia, leading to MP transfer and accumulation in the aquatic environment, and, more importantly, to ingestion by low to high trophic level organisms. This work highlights several knowledge gaps to guides future research to improve MP pollution management for the sustainable development of highly populated regions such as Asia.

Microplastic in atmospheric fallouts of a developing Southeast Asian megacity under tropical climate.

Microplastics in atmospheric fallouts from a developing megacity influenced by a tropical monsoon climate were investigated during a year. Three sites were selected according to the surrounding population density, the land use and occupation. The microplastic deposition fluxes varied between of 71-917 items m-2 d-1, for an observation size range of 300-5000 µm. Fibers predominated while fragments were observed occasionally. Unexpectedly, contrary to available scientific literature, deposition fluxes did not vary temporally with rainfall and wind intensity or direction, showing no effect of the tropical climate. Variations were observed between sites and were attributed to their environmental characteristics: population density and occupation space. The median length of fibers also differed between sites and could be related to in-situ fragmentation processes due to occupation space (solid waste treatment facility). Those first results from tropical climate region are showing interesting insights and are opening new perspectives on the understanding of microplastics fate from atmospheric fallouts.

Baseline assessment of microplastic concentrations in marine and freshwater environments of a developing Southeast Asian country, Viet Nam.

In aquatic environments, assessment of microplastic concentrations is increasing worldwide but environments from developing countries remain under-evaluated. Due to disparities of facilities, financial resources and human resources between countries, protocols of sampling, analysis and observations used in developed countries cannot be fully adapted in developing ones, and required specific adaptations. In Viet Nam, an adapted methodology was developed and commonly adopted by local researchers to implement a microplastic monitoring in sediments and surface waters of 21 environments (rivers, lakes, bays, beaches) of eight cities or provinces. Microplastic concentrations in surface waters varied from 0.35 to 2522 items m-3, with the lowest concentrations recorded in the bays and the highest in the rivers. Fibers dominated over fragments in most environments (from 47% to 97%). The microplastic concentrations were related to the anthropogenic pressure on the environment, pointing out the necessity in a near future to identify the local sources of microplastics.

Characteristics of microplastics in shoreline sediments from a tropical and urbanized beach (Da Nang, Vietnam).

Microplastics in shoreline sediments were investigated from Da Nang beach for the first time. Sediment samples at the two depth strata (0-5 cm and 5-10 cm) at eight sites along the entire coast were collected for identifying the characteristics of microplastics, including their concentration, size, shape, color, and nature. The synthetic fiber was the predominant type of microplastics, accounting for 99.2% of the total items. Blue (59.9%) and white (22.9%) were the most common colors of the fibers. Synthetic fibers showed a homogenous distribution at all sampling sites with a mean concentration of 9238 ± 2097 items kg-1 d.w. Meanwhile, the fibers tended to concentrate much more at the surface stratum than the deeper stratum. A large number of synthetic fibers (81.9%) were in the size range of 300-2600 µm, which might pose a threat to marine biota and human health.

Bioaccumulation of trace elements in the hard clam, Meretrix lyrata, reared downstream of a developing megacity, the Saigon-Dongnai River Estuary, Vietnam.

A large number of white hard clam farms are in the estuary shoreline of Saigon-Dongnai Rivers, which flow through Ho Chi Minh City, a megacity, and numerous industrial zones in the basin catchment area. In this study, eleven trace elements (Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Hg, and Pb) in the hard clam Meretrix lyrata and its habitats including surface water, suspended particulate matter, and sediment were evaluated to understand the bioaccumulation of trace metals from the environment into the whole tissues of the hard clam as well as its different organs. The samples were collected monthly in dry, transition, and wet seasons of the southern part of Vietnam from March to September 2016. The results showed that seasonal and spatial variations of the studied metal concentrations in the hard clam M. lyrata might be influenced by the sea current as well as the surface runoff in the rainy season. The relationship between condition index and the element concentrations in M. lyrata might be affected by the living environment conditions and farming methods. In addition, the hazard index values of all trace elements in the hard clam M. lyrata harvested in the sampling time show that the hard clams farmed in the study area were safe for local consumers.

Sedimentological and geochemical data in bed sediments from a tropical river-estuary system impacted by a developing megacity, Ho Chi Minh City - Vietnam.

Sedimentological and geochemical data were obtained for bed sediments from a tropical estuary environment in Vietnam in October 2014, January 2016, and November 2016. The data include grain-size distribution, percentage of clay, silt and sand, percentage of organic matter, concentration of total particulate phosphorus (TPP), concentration of particulate inorganic phosphorus (PIP), concentration of particulate organic phosphorus (POP), percentage of total nitrogen (TN), percentage of total carbon (TC), trace metals concentrations (V, Cr, Co, Ni, Cu, Zn, As, Mo, Cd, Pb) and major elements (Al, Fe, Mn). Geochemical indexes (Enrichment factor EF and Geo-accumulation Index I-geo) and sediment quality guideline (mean Effect Range Median quotients) were calculated.

Temporal dynamic of anthropogenic fibers in a tropical river-estuarine system.

Anthropogenic fibers, gathering synthetic fibers, artificial fibers and natural fibers are ubiquitous in the natural environment. Tremendous concentrations of anthropogenic fibers were previously measured in the tropical Saigon River (Vietnam), i.e. a river impacted by textile and apparel industries. In the present study, we want to examine the role of contrasted seasonal variation (e.g., dry and rainy seasons), via the rainfall and monthly water discharges, and of water's physico-chemical conditions on the concentrations of anthropogenic fibers in the surface water. The one year and half monthly survey evidenced that concentrations of anthropogenic fibers varied from 22 to 251 items L-1 and their variations were not related to rainfall, water discharge or abiotic factors. However, their color and length distribution varied monthly suggesting variations in sources and sinks. Based on the 2017 survey, we estimated an annual emission of anthropogenic fibers from the river to the downstream coastal zone of 115-164 × 1012 items yr-1.

Seasonality of riverine macroplastic transport.

Marine plastic pollution is an increasing environmental threat. Although it is assumed that most marine plastics are transported from land to the ocean through rivers, only limited data on riverine plastic transport exists. Recently, new methods have been introduced to characterize riverine plastics consistently through time and space. For example, combining visual counting observations and plastic debris sampling can provide order of magnitude estimations of plastic transport through a river. In this paper, we present findings from multi-season measurement campaign in the Saigon River, Vietnam. For the first time, we demonstrate that macroplastic transport exhibits strong temporal variation. The monthly averaged plastic transport changes up to a factor five within the measurement period. As it is unclear what drives the variation in plastic transport, relations between rainfall, river discharge, presence of organic material and plastic transport have been explored. Furthermore, we present new findings on the cross-sectional and vertical distribution of riverine plastic transport. With this paper we present new insights in the origin and fate of riverine plastic transport, emphasizing the severity of the emerging thread of plastic pollution on riverine ecosystems.

Patterns of trace metal bioaccumulation and trophic transfer in a phytoplankton-zooplankton-small pelagic fish marine food web.

Trace metal contamination in the European sardine and anchovy food web was investigated in the Gulf of Lions, NW Mediterranean Sea, including seawater and size fractions of plankton. The results highlighted: i) higher and more variable concentrations in the smaller plankton size classes for all metals except cadmium; ii) higher concentrations in anchovy versus sardine for all elements except lead; iii) different patterns of metal bioaccumulation through the food web: cobalt, nickel, copper, silver, lead and zinc displayed continuously decreasing concentrations (with the exception of increased zinc in fish only), while mercury concentrations dropped considerably in larger plankton size classes and rose significantly in fish. Lastly, cadmium concentrations were found to be highest in intermediate plankton size classes, with very low levels in fish. The need to efficiently characterize the biological composition of plankton in order to fully identify its role in the mobilization and transfer of metals was highlighted.

Bioaccumulation of some trace elements in tropical mangrove plants and snails (Can Gio, Vietnam).

Mangrove sediments can store high amount of pollutants that can be more or less bioavailable depending on environmental conditions. When in available forms, these elements can be subject to an uptake by mangrove biota, and can thus become a problem for human health. The main objective of this study was to assess the distribution of some trace elements (Fe, Mn, Co, Ni, Cr, As, and Cu) in tissues of different plants and snails in a tropical mangrove (Can Gio mangrove Biosphere Reserve) developing downstream a megacity (Ho Chi Minh City, Vietnam). In addition, we were interested in the relationships between mangrove habitats, sediment quality and bioaccumulation in the different tissues studied. Roots and leaves of main mangrove trees (Avicennia alba and Rhizophora apiculata) were collected, as well as different snail species: Chicoreus capucinus, Littoraria melanostoma, Cerithidea obtusa, Nerita articulata. Trace elements concentrations in the different tissues were determined by ICP-MS after digestion with concentrated HNO3 and H2O2. Concentrations differed between stands and tissues, showing the influence of sediment geochemistry, species specific requirements, and eventually adaptation abilities. Regarding plants tissues, the formation of iron plaque on roots may play a key role in preventing Fe and As translocation to the aerial parts of the mangrove trees. Mn presented higher concentrations in the leaves than in the roots, possibly because of physiological requirements. Non-essential elements (Ni, Cr and Co) showed low bioconcentration factors (BCF) in both roots and leaves, probably resulting from their low bioavailability in sediments. Regarding snails, essential elements (Fe, Mn, and Cu) were the dominant ones in their tissues. Most of snails were "macroconcentrators" for Cu, with BCF values reaching up to 42.8 for Cerithidea obtusa. We suggest that high quantity of As in all snails may result from its high bioavailability and from their ability to metabolize As.

Metals geochemistry and ecological risk assessment in a tropical mangrove (Can Gio, Vietnam).

Mangrove sediments act as natural biogeochemical reactors, modifying metals partitioning after their deposition. The objectives of the present study were: to determine distribution and partitioning of metals (Fe, Mn, Ni, Cr, Cu, Co and As) in sediments and pore-waters of Can Gio Mangrove; and to assess their ecological risks based on Risk Assessment Code. Three cores were collected within a mudflat, beneath Avicennia alba and Rhizophora apiculata stands. We suggest that most metals had a natural origin, being deposited in the mangrove mainly as oxyhydroxides derived from the upstream lateritic soils. This hypothesis could be supported by the high proportion of metals in the residual fraction (mean values (%): 71.9, 30.7, 80.7, 80.9, 67.9, 53.4 and 66.5 for Fe, Mn, Ni, Cr, Cu, Co, and As respectively, in the mudflat). The enrichment of mangrove-derived organic matter from the mudflat to the Rhizophora stand (i.e. up to 4.6% of TOC) played a key role in controlling metals partitioning. We suggest that dissolution of Fe and Mn oxyhydroxides in reducing condition during decomposition of organic matter may be a major source of dissolved metals in pore-waters. Only Mn exhibited a potential high risk to the ecosystem. Most metals stocks in the sediments were higher in the Avicennia stand than the Rhizophora stand, possibly because of enhanced dissolution of metal bearing phases beneath later one. In a context of enhanced mangrove forests destruction, this study provides insights on the effects of perturbation and oxidation of sediments on metal release to the environment.

Nutrient dynamics and eutrophication assessment in the tropical river system of Saigon - Dongnai (southern Vietnam).

Saigon-Dongnai Rivers in Southern Vietnam is a complex lowland hydrological network of tributaries that is strongly influenced by the tidal cycles. The increasing economic, industrial and domestic developments in and around Ho Chi Minh City (HCMC) have led to serious impacts on water quality due to lack of appropriate wastewaters treatment. Drinking water production is impacted and the large aquaculture production areas may also be affected. We analyzed spatial and seasonal variability of nutrient concentrations (Phosphorus, Nitrogen and Silica) and eutrophication indicators (Organic Carbon, Chlorophyll-a and Dissolved Oxygen) based on bi-monthly monitoring during two hydrological cycles (July 2015-December 2017). Four monitoring sites were selected to assess the impact of HCMC: two upstream stations on the Saigon River and Dongnai River branches to provide the reference water quality status before reaching the urbanized area of HCMC; one monitoring station in the city center to highlight Saigon River water quality within the heart of the megacity; the fourth station downstream of the confluence to evaluate the impact of HCMC on the estuarine waters. This study points to excess nutrients in HCMC's water body with concentrations of NH4+ and PO43- averaging to 0.7 ±â€¯0.6 mgN L-1 and 0.07 ±â€¯0.06 mgP L-1, respectively in mean over the monitored period and rising up to 3 mgN L-1 and 0.2 mgP L-1, in extreme conditions. During the dry season, we evidenced that untreated domestic discharges leads to degradation of the Saigon River's water quality with extreme values of algal biomass (up 150 µChl-a L-1) and hypoxic conditions occurring episodically (DO < 2 mg L-1) in the heart of the megacity. Until now, eutrophication in the urban center has had no clear effect downstream because eutrophic water mass from the Saigon River is efficiently mixed with the Dongnai River and sea water masses during the successive semi-diurnal tidal cycles.

Where has the pollution gone? A survey of organic contaminants in Ho Chi Minh city / Saigon River (Vietnam) bed sediments.

A wide range of persistent organic chemicals, including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), some insecticides, as well as polybrominated diphenyl ethers (PBDEs) and some perfluoroalkyl substances (PFASs) were analyzed in 17 bed sediments collected along the Saigon River and at adjacent canal mouths from upstream to downstream in Ho Chi Minh City (Vietnam). Concentrations were rather low for PAHs, as well as for legacy PCBs and dichloro-diphenyl-trichlorethane and metabolites (DDTs), or below detection limits for several PFASs and all PBDEs measured. Several insecticides (chlorpyrifos-ethyl, and the pyrethroids cypermethrin and λ-cyhalothrin) displayed rather high concentrations at a few sites within the city. There was no distinct upstream - downstream trend for PAHs, (DDTs) or PCBs. Although adjacent canal sediments tended to be more contaminated than Saigon River sediments, the differences were not significant. Emissions are almost certainly substantial for PAHs, and probably also for other contaminants such as PBDEs and some PFASs. During the dry season, contaminants are presumably stored in the city, either in canals or on urban surfaces. Heavy rainfall during the monsoon period carries away contaminated particle flows into the canals and then the Saigon River. The strong tidal influence in the river channel hinders the accumulation of contaminated particles. Contaminated deposits should accordingly be investigated further downstream in depositional environments, such as the mangrove.

Macroplastic and microplastic contamination assessment of a tropical river (Saigon River, Vietnam) transversed by a developing megacity.

Both macroplastic and microplastic contamination levels were assessed for the first time in a tropical river estuary system, i.e. the Saigon River, that traverses a developing South East Asian megacity, i.e. Ho Chi Minh City, Vietnam. The analysis of floating debris collected daily on the Nhieu Loc - Thi Nghe canal by the municipal waste management service shows that the plastic mass percentage represents 11-43%, and the land-based plastic debris entering the river was estimated from 0.96 to 19.91 g inhabitant-1 d-1, namely 350 to 7270 g inhabitant-1 yr-1. Microplastics were assessed in the Saigon River and in four urban canals by sampling bulk water for anthropogenic fiber analysis and 300 µm mesh size plankton net exposition for fragment analysis. Fibers and fragments are highly concentrated in this system, respectively 172,000 to 519,000 items m-3 and 10 to 223 items m-3. They were found in various colors and shapes with smallest size and surface classes being predominant. The macroplastics and fragments were mainly made of polyethylene and polypropylene while the anthropogenic fibers were mainly made of polyester. The relation between macroplastic and microplastic concentrations, waste management, population density and water treatment are further discussed.

Trace metals partitioning between particulate and dissolved phases along a tropical mangrove estuary (Can Gio, Vietnam).

Mangroves can be considered as biogeochemical reactors along (sub)tropical coastlines, acting both as sinks or sources for trace metals depending on environmental factors. In this study, we characterized the role of a mangrove estuary, developing downstream a densely populated megacity (Ho Chi Minh City, Vietnam), on the fate and partitioning of trace metals. Surface water and suspended particulate matter were collected at four sites along the estuarine salinity gradient during 24 h cycling in dry and rainy seasons. Salinity, pH, DO, TSS, POC, DOC, dissolved and particulate Fe, Mn, Cr, As, Cu, Ni, Co and Pb were measured. TSS was the main trace metals carrier during their transit in the estuary. However, TSS variations did not explain the whole variability of metals distribution. Mn, Cr and As were highly reactive metals while the other metals (Fe, Ni, Cu, Co and Pb) presented stable log KD values along the estuary. Organic matter dynamic appeared to play a key role in metals fractioning. Its decomposition during water transit in the estuary induced metal desorption, especially for Cr and As. Conversely, dissolved Mn concentrations decreased along the estuary, which was suggested to result from Mn oxidative precipitation onto solid phase due to oxidation and pH changes. Extra sources as pore-water release, runoff from adjacent soils, or aquaculture effluents were suggested to be involved in trace metal dynamic in this estuary. In addition, the monsoon increased metal loads, notably dissolved and particulate Fe, Cr, Ni and Pb.