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Southeast Asia (SEA) faces significant environmental challenges due to rapid population growth and economic activity. Rivers in the region are major sources of plastic waste in oceans. Concerns about their contribution have grown, but knowledge of microplastics in the area is still limited. This article compares microplastic levels in sediment and water from urban zones of three major rivers in SEA: Chao Phraya River (Thailand), Saigon River (Vietnam), and Citarum River (Indonesia). The study reveals that in all three rivers, microplastics were found, with the highest concentrations in Chao Phraya’s water (80 ± 60 items/m3) and Saigon’s sediment (9167 ± 4559 items/kg). The variations in microplastic sizes and concentrations among these rivers may be attributed to environmental factors and the exposure duration of plastic to the environment. Since these rivers are important water supply sources, rigorous land-use regulations and raising public awareness are crucial to mitigate plastic and microplastic pollution. [ABSTRACT FROM AUTHOR]

Introduction. Vietnam’s largest economic centre, Ho Chi Minh City, is facing riverbank erosion, one of the main causes of which is runoff. In this study, the flow velocity field of the Saigon River section is analyzed in different time periods.Materials and methods. Data on discharge, water level, river network topography and river channel were collected to set up simulation models in the programmes MIKE 11 and MIKE 21. These programmes are the main tools used in the study.Results. The current velocity in the middle of the river is 3–4 times higher than on the two banks. However, during the period between high and low tide, the current on both sides of the river has higher speed than the main stream, especially in the upper part of the winding banks, such as the part of the river from Ben Nghe sluice to Tan Thuan sluice and the section of the river from An Loi Dong police station to Thu Thiem 2 bridge. The velocity values in the studied river section in most cases exceed the allowable erosion-free velocity values of particles of channel material, river banks, and suspended sediment particles. The operation of the flood control sluices will slightly reduce the flow velocity values and also cause whirlpools in front of the sluices.Conclusions. The MIKE 11 and MIKE 21 software detailed the flow distribution of the Saigon River. The flow velocity on the Saigon River has a complex distribution and varies from period to period depending on the flood discharge from the Dau Tieng Reservoir and the tidal currents of the East Sea. The process of erosion on both sides of the river will occur regularly and continuously, so urgent measures are needed to protect the riverbank.

Riverine litters can negatively impact human livelihood and aquatic life especially macroplastics, which could injure animal and plant species, harm to hydraulic systems and vessels, and increase urban flooding due to clogging. There are many studies on floating macroplastics in this river, especially in Saigon river in which is recognized as one of the most polluted rivers in Vietnam and globally to date; however, studies on riverbank macroplastic pollution are still lacking so far. In the study, riverbank surveys were carried out in downstream of Saigon river nearby Thu Thiem 1 bridge between January and June 2022. The aim of this study was to quantify the abundance of macro-litters in the downstream of Saigon river. Plastic fraction and non-plastic fraction were classified according to river-OSPAR guideline, and then counted and weighted. As a result, plastics was the most abundance type of litter in term of amount, accounted for 86.54% with a total of 617 items and weight witha total of 11,490.5g). Styrofoam fragments in range of 2.5–50 cm, plastic bags and plastic cups accounted for the major parts of sampled plastic items in the meanwhile LDPE, Other, and PS were found in highest proportion in terms of standard plastic classification. Additionally, top 5 brands comprising 43% of branded litters collected in the Saigon river during the study period were PepsiCo, Acecook, Masan, Vinacafe Bien Hoa, and Nestle. This study showed the need for enhancing waste management and reducing single-use plastics to tackle plastic pollution in the Saigon river.

Water quality for the surface water along the Saigon River in Ho Chi Minh City was assessed for four groups of water samples collected at the agricultural, industrial, residential, and less impacted areas. A variety of parameters indicating water quality including physicochemical parameters, nutrients, heavy metals, and antibiotic residues were measured for both the rainy and dry seasons, two main tropical seasons in HCM City using the standard methods. The results showed that the river water in the rainy season was detected with significantly higher values of turbidity, BOD5, PO4-P, NH4-N, NO3-N; and lower values of pH, temperature, conductivity, DO, salinity, Cu, Zn, As, Ni, Hg compared to that in the dry season. Sulfamethoxazole and trimethoprim were highly detected in the industrial areas compared to the agricultural and residential areas. Multivariate analyses suggested that the industrial and residential activities were more important contributors to the pollution of the Saigon River than the agricultural activities in HCM City. HIGHLIGHTS The occurrences of nutrients, heavy metals, and selected antibiotics in the Saigon River were different between the dry and rainy seasons.; Industrial and residential activities were found to be more important contributors to the contamination in the Saigon River in comparison to the agricultural activities.; The highly polluted sites in the Saigon River were impacted by both industrial and residential activities.;

Antibiotic resistance causes serious health problems associated with an increase in hospitalization time, treatment cost, and mortality. In this study, the contamination levels of antibiotic resistant bacteria were determined in the Saigon River, which is impacted by different anthropogenic activities in Ho Chi Minh City. Five target antibiotics were supplemented to the R2A medium at the concentration of amoxicillin (9.6 µg/ml) / clavulanic acid (4.8 µg/ml), amikacin (19.2 µg/ml), cefixime (1.2 µg/ml), ciprofloxacin (1.2 µg/ml), and trimethoprim (2.4 µg/ml)/ sulfamethoxazole (45.6 µg/ml). As the results, the antibiotic resistant bacteria were found in all the sampling sites and at the highest concentration at the sites impacted by wastewater from both industrial and residential activities. Enterobacteriaceae was the most abundant family with 23.8% of total isolates of the antibiotic resistant bacteria. Human opportunistic pathogens and pathogens such as Klebsiella pneumoniae (6.74%), Enterobacter sp. (4.15%), Escherichia coli (6.63%), Stenotrophomonas maltophilia (3.11%), Aeromonas Caviae (4.66%) were identified in all surface water samples. Correlation analysis suggested that the pollutions of dissolved salt, organic matter, and wastewaters may facilitate the antibiotic resistant bacteria in the aquatic environments. [ABSTRACT FROM AUTHOR]

The metal concentration in surface water of a river could be affected by season, position, and oceanic process such as tide. The current study aimed to (1) examine the heavy metal(loid) concentration in surface water from the Saigon River as affected by the combination of season, tide, and position and (2) apportion and quantify pollution sources. Ninety-six surface water samples were collected from 13 sites on the River in four campaigns (rainy season + ebb tide, rainy season + flood tide, dry season + ebb tide, and dry season + flood tide). Eight heavy metal(loid)s (Al, B, Bi, Fe, Mn, Pb, Sr, and Zn) were measured and subjected to multivariate analyses. Three-way ANOVA showed that in the rainy season, the total concentration of the metal(loid)s (TCM) in two tides was not clearly different from each other while in the dry season the TCM was significantly higher during the ebb tide than during the flood tide. Principal component analysis/factor analysis and Pearson correlation matrix showed that the TCM could be derived from three main sources, grouped into anthropogenic activities such as industrial, agricultural, and domestic wastes from inside Ho Chi Minh city, and natural origins from lowland area and acid sulfate soil. Three pollution sources explained 70% and 68% of the total variance of TCM in the rainy and dry seasons, respectively. In brief, the metal(loid) concentration was significantly affected by the season and tide and the pollution sources could be derived from inside Ho Chi Minh City and from lowland areas beyond the river estuary. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • Saigon River sediments displayed low concentrations of persistent organic contaminants. • For PAHs, emissions appear similar to those of other cities in South-East Asia. • Tidal influence in Saigon River, water management and monsoon control PAH fate. • Similar patterns are applicable to other contaminants such as PBDEs or PFASs. • Ecotoxicological impacts might be expected in the mangrove where sediment deposit. [ABSTRACT FROM AUTHOR]

Sub-tidal free-living nematodes in 12 stations along the Saigon River were investigated in both dry and rainy seasons during 2014 and 2015. In total, 157 nematode genera were found to belong to 59 families, and 11 orders of two classes of Enoplea and Chromadorea were identified. The trophic structure of free-living nematode communities was analysed spanning the overall seasons to see that deposit-feeders developed dominantly in the Saigon River indicating their association with food types including particles, bacteria, diatom, etc.. The dominance of deposit-feeders was contributed to a high abundance of genera in each station, including Theristus, Teschellingia, Monhystera, Thalassomonhystera, Paraplectonema, Daptonema, Aphanonchus, Sphaerotheristus, and Geomonhystera. Epistrate-feeder types were found as the second most abundant group. The remaining feeding types, such as chewers and suction-feeders, were in rather low proportion at most stations.

Estuaries are considered as important sources of the global emission of greenhouse gases (GHGs). Urbanized estuaries often experience eutrophication under strong anthropogenic activities. Eutrophication can enhance phytoplankton abundance, leading to carbon dioxide (CO 2) consumption in the water column. Only a few studies have evaluated the relationship between GHGs and eutrophication in estuaries. In this study, we assessed the concentrations and fluxes of CO 2 , methane (CH 4) and nitrous oxide (N 2 O) in combination with a suite of biogeochemical variables in four sampling campaigns over two years in a highly urbanized tropical estuary in Southeast Asia (the Saigon River Estuary, Vietnam). The impact of eutrophication on GHGs was evaluated through several statistical methods and interpreted by biological processes. The average concentrations of CO 2 , CH 4 and N 2 O at the Saigon River in 2019–2020 were 3174 ± 1725 μgC-CO 2 L−1, 5.9 ± 16.8 μgC-CH 4 L−1 and 3.0 ± 4.8 μgN-N 2 O L−1, respectively. Their concentrations were 13–18 times, 52–332 times, and 9–37 times higher than the global mean concentrations of GHGs, respectively. While CO 2 concentration had no clear seasonal pattern, N 2 O and CH 4 concentrations significantly differed between the dry and the rainy seasons. The increase in eutrophication status along the dense urban area was linearly correlated with the increase in GHGs concentrations. The presence of both nitrification and denitrification resulted in elevated N 2 O concentrations in this urban area of the estuary. The high concentration of CO 2 was contributed by the high concentration of organic carbon and mineralization process. GHGs fluxes at the Saigon River Estuary were comparable to other urbanized estuaries regardless of climatic condition. Control of eutrophication in urbanized estuaries through the implantation of efficient wastewater treatment facilities will be an effective solution in mitigating the global warming potential caused by estuarine emissions. [Display omitted] • Supersaturation of CO 2 , CH 4 , N 2 O are observed in the Saigon River, an urbanized estuary. • Concentrations of N 2 O and CH 4 have significant seasonal differences. • Eutrophication has linear relationships with CO 2 , CH 4 , N 2 O. • Supersaturation of CO 2 is dominant even under the extreme eutrophication state. [ABSTRACT FROM AUTHOR]

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. • Anthropogenic fibres in clams were 3.6 ± 2.1 fibres individual−1 or 2.7 ± 2.4 fibre g−1 ww. • Higher fibre accumulation was observed in the remaining tissues. • Fibre concentrations in all the clam tissues did not vary temporally. • Fibres may accumulate by adherence to remaining tissues. [ABSTRACT FROM AUTHOR]

The hydrological dynamics of the Saigon River is ruled by a complex combination of factors, which need to be disentangled to prevent and limit risks of flooding and salt intrusion. In particular, the Saigon water discharge is highly influenced by tidal cycles with a relatively low net discharge. This study proposes a low-cost technique to estimate river discharge at high frequency (every 10 min in this study). It is based on a stage-fall-discharge (SFD) rating curve adapted from the general Manning Strickler law, and calibrated thanks to two ADCP campaigns. Two pressure sensors were placed at different locations of the river in September 2016: one at the centre of Ho Chi Minh City and one in Phu Cuong, 40 km upstream approximately. The instantaneous water discharge data were used to evaluate the net residual discharge and to highlight seasonal and inter-annual trends. Both water level and water discharge show a seasonal behaviour. Rainfall, including during the Usagi typhoon that hit the megalopolis in November 2018, has no clear and direct impact on water level and water discharge due to the delta flat morphology and complex response between main channel and side channel network and ground water in this estuarine system under tidal influence. However, we found some evidences of interactions between precipitation, groundwater, the river network and possibly coastal waters. This paper can be seen as a proof of concept to (1) present a low-cost discharge method that can be applied to other tidal rivers, and (2) demonstrate how the high-frequency discharge data obtained with this method can be used to evaluate discharge dynamics in tidal river systems. Unlabelled Image • Development of a simple stage-fall-discharge (SFD) rating curve for tidal rivers • Evaluation of the net discharge in the Saigon River for a two-year period • Significant influence of the flood-plain exchanges and sea level on the net discharge • Flood risk of HCMC mostly impacted by local rainfall and sea water level [ABSTRACT FROM AUTHOR]

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. Image 1 • Anthropogenic fibers were monitored during contrasted seasons in a tropical river-estuary. • Anthropogenic fiber's concentrations ranged from 22 to 251 items L−1. • Fiber's concentrations are not influenced by change of TSS, pH, O 2 , conductivity. • No seasonal variation of fiber concentrations were observed. • Anthropogenic fiber's emission from river to sea were estimated 115–164 1012 items yr−1. Anthropogenic fibers in the Saigon River varied from 22 to 251 items L−1 over eighteen months and their variations were not related to rainfall, water discharge or abiotic factors. [ABSTRACT FROM AUTHOR]

The current study was conducted to (1) examine seasonal and spatial distribution of heavy metals and metalloid in sediment from the Saigon River and (2) apportion and quantify their pollution sources. Ninety-six sediment samples were taken in the rainy and dry season on 13 sampling sites, distributed over the lower reaches of the River, to analyze for exchangeable concentration of 11 heavy metals and metalloid (Al, B, Cd, Co, Fe, In, Mn, Ni, Pb, Sr, and Zn), pH, EC, organic carbon content, and particle-size distribution. Generally, the concentration of 11 elements was ranked in the order Mn > Al > Fe > Zn > Sr > In > B > Ni > Co > Pb > Cd. Hierarchical cluster analysis grouped 13 sampling sites into two parts based on the similar concentration of the 11 elements. Three-way analysis of variance showed that the total exchangeable concentration of 11 elements was significantly higher in the rainy season than in the dry season and in the upper part than in the lower part of the river. Principal component analysis/factor analysis and correlation analysis revealed that three pollution sources (PS) may contribute to enriching the 11 examined elements in the sediment. These sources included (PS1) from catchment through water erosion over natural areas, explaining 83%, (PS2) mixed sources from catchment through water erosion over agricultural fields and inside Ho Chi Minh City, accounting for 6%, and (PS3) mixed sources from lowland areas, explaining 7.8% of the total variance of the elements. In brief, the sediment concentration of 11 metals and metalloid varied with season and space and three major pollution sources from river catchment, inside Ho Chi Minh City, and lowland contributively enriched the elements in the sediment of the River. Image 1 • Sediment can serve as a sink/source of heavy metals (HM) for aquatic ecosystems. • 11-HM concentrations in sediment from the Saigon River varied with season and space. • Rainy season had significantly higher concentration of 11 HMs than the dry season. • River catchment, inside Ho Chi Minh City, and lowland were major sources of 11 HMs. [ABSTRACT FROM AUTHOR]