Surface water quality in Ho Chi Minh city
Ho Chi Minh City is the largest city in Vietnam in terms of population and scale of urbanization. According to the General Statistics Office (2022), Ho Chi Minh City has an area of 2,095.39 km2, and the city's population will reach 9,166 million people, equaling the population density of 4,375 inhabitants.km-2 in 2021.
There are three main canal systems in HCMC, these being Tan Hoa-Lo Gom (TH-LG), Nhieu Loc-Thi Nghe (NL-TN), and Tau Hu-Ben Nghe (TH-BN). These three canals belong to the network of waterways and drainage system of HCMC. TH-LG, with a length of 7.84 km, passes through 4 urban districts. NL-TN is in the city center with a length of 9.47 km, flowing through 5 districts. TH-BN is a large tributary (25.4 km in length) of the Saigon River in the south of the city center, flowing through 5 urban districts. Along these canals, there are many residential areas and markets with different anthropogenic activities.
Physico-chemical variables
The Saigon River (which flows along the territory of HCMC, about 80 km) is downstream of these canals; it is also the main waterway for ships entering and leaving Saigon harbor and has an average flow of about 54 m3/s. The width of the Saigon River in the city varies from 225 to 370 m, and the depth is up to 20 m. After leaving the inner city, the Saigon River merges with the Dong Nai River and divides it into tributaries that flow into the Can Gio Sea. Le et al. (2022) studied water quality for the surface water along the Saigon River in Ho Chi Minh City and assessed for four groups of water samples collected at the agricultural, industrial, residential, and less impacted areas. The COD, PO4-P, NH4-N, NO2-N, and NO3-N values in all water samples ranged from 18.0 to 48.2 mg/L, 0 to 1.168 mg/L, 0 to 13.92 mg/L, 0 to 0.282 mg/L, and 0 to 54.545 mg/L, respectively (Le et al., 2022). The authors demonstrated that, overall, pH, turbidity, heavy metals, BOD5, COD, and nutrients in surface water samples for industrial and residential activities were found to be higher than those at the agricultural and less impacted sites. On the contrary, the DO was found to be highest in the less impacted areas and lowest in the industrial areas. This indicated high contamination of human and animal waste in the sampling sites of the Saigon River in the industrial and residential areas over the agricultural and less impacted areas (Le et al., 2022). On average, the turbidity, BOD5, COD, and nutrients in the river water in both seasons were higher than the Vietnam regulation for surface water class 2 (Ministry of Natural Resources and Environment 2015). Conversely, the presence of tested heavy metals in the surface water of the sampling sites in the Saigon River was found at lower levels than the limits according to the Vietnam regulation for surface water (Ministry of Natural Resources and Environment, 2015), which were found up to 75.03 μg/L (Cu), 15 μg/L (Pb), 47.41 μg/L (Zn), 2.6 μg/L (Cd), 2.4 μg/L (As), 7.06 μg/L (Cr), 29.31 μg/L (Ni), and 0.9 μg/L (Hg). In the canal water, Le Thy et al. (2024) identified that the average concentration of total coliforms, E. coli, at five primary canal systems in HCMC was from 6 to 400 times higher than that of the discharge limit standards of Viet Nam (QCVN 08:2015/BTNMT - Column B1). On the contrary, dissolved oxygen (DO) in the surface water of canals in HCMC were mainly lower than the discharge standard for surface water used for agricultural purposes. This study showed that rapid industrial development and social progress in Ho Chi Minh City have increased heavy metal concentrations and pathogens (e.g., E. coli and coliforms) in canal water bodies, affecting the aquatic ecological environment and the health of people living along the canal area (Le Thy et al., 2024).
Fig: The Saigon river
Antibiotics
In recent years, antibiotics have been known as emerging contaminants. Antibiotic residues in several areas have been investigated in Vietnam. A recent study revealed antibiotic residues in the water of the Saigon River in Ho Chi Minh City (Bao et al., 2022). The results showed that the highest concentration of sulfamethoxazole was 364.2 ng/L and 313.5 ng/L in the dry and rainy seasons, respectively. Total antibiotic concentration ranged from undetected to 3,048 ng/L in the rainy season and to 3,227 ng/L (in the dry season) at the Ong Co bridge sampling site (Bao et al., 2022). In another study on the Saigon River, 12 water samples were taken from along this river, starting from Ben Duoc in Cu Chi district and ending at the Truong Phuoc bridge, near the intersection of the Saigon and the Dong Nai rivers (Le et al. 2022). The concentrations of six antibiotics, including amoxicillin, amikacin, cefixime, ciprofloxacin, sulfamethoxazole, and trimethoprim were found. Overall, the concentrations of most selected antibiotics (amoxicillin, amikacin, cefixime, and ciprofloxacin) were low, with the exception of sulfamethoxazole and trimethoprim in industrial zones I2 (0.3 mg/L sulfamethoxazole and 0.01 mg/L trimethoprim) and I3 (0.11 mg/L sulfamethoxazole and 0.013 mg/L for trimethoprim). The presence of these antibiotics in water may play a role in the growth and spread of antibiotic-resistant bacteria in the aquatic environment.
Hospital wastewater contains a large number of pollutants that are being discharged directly into the environment with or without treatment. In a study on antibiotic residues of hospital wastewater in Ho Chi Minh City, Vo et al. (2016) found 7 common antibiotics present in all samples. The concentrations were: Sulfamethoxazole (2.5 ± 1.9 μg/L), norfloxacin (9.6 ± 9.8 μg/L), ciprofloxacin (5.3 ± 4.8 μg/L), ofloxacin (10.9 ± 8.1 μg/L), erythromycin (1.2 ± 1.2 μg/L), tetracycline (0.1 ± 0.0 μg/L) and trimethoprim (1.0 ± 0.9 μg/L). In wastewater from Trung Vuong hospital, a relatively high specific antibiotic residue was reported, which were from 0.036 to 43.610 μg/L. Ofloxacin (OFL) is a strong antibiotic, difficult to decompose, thus it is likely to accumulate in high concentrations in water (Pham et al., 2021). Although being treated in the wastewater treatment system of hospitals, the OFL content is still high in the water. The analysis results of OFL concentrations in the water samples of the 12 hospital wastewater treatment plants in Ho Chi Minh City were from 0.55 ± 0.21 (Tam Duc Heart Hospital) to 48.85 ± 2.76 μg/L (Saigon General Hospital) (Pham et al., 2021).
Microplastics
Microplastics were found in all surface water samples, with an average density of 0.80 ± 0.58 items/m3 (from 0.15 to 2.20 items/m3) in the Saigon River (Nguyen et al., 2023). According to this study, in the Saigon River, the density of microplastics at Binh Phuoc Bridge was the highest (2.20 items/m3) and the lowest at Binh Trieu Bridge (0.15 items/m3). The density of microplastics at locations on the Saigon River was uneven, indicating the influence of flow from inner-city canals as well as man-made activities on the river. According to the author, the density of microplastics in water at locations that receive flow from inner-city canals and streams was higher than at other locations (Nguyen et al., 2023). Lahens et al. (2018) also conducted a study on the density of fibrous and fragmentary microplastics in the Saigon River in 2015-2016 with surface water samples collected by buckets (300 mL for microplastic fiber analysis) and by plankton nets (300 μm mesh, 60 sampling time) for microplastic fragment analysis. This study resulted in a density of fibrous microplastics ranging from 172,000 items/m3 to 519,000 items/m3 at locations and microplastic fragments ranging from 10 to 223 items/m3 (Lahens et al., 2018).
According to the study by Nguyen et al. (2023), in the Saigon River, it can be seen that PE microplastics account for the largest proportion with about 35% (14/15 sampling locations have PE microplastics), followed by PP plastic (23.3%, 11/15 locations) and EVA plastic (16.2%, 15/15 locations). These are floating plastics (with a density of less than 0.95 g/cm3) and are common in the environment. Microplastics in the form of fragments account for the majority (82.2%), with white and transparent colors accounting for the highest percentage with 58.4% in the Saigon River. According to the author, the shapes found in the Saigon River mainly include fragments, fibers, and particles (Nguyen et al., 2023). Other studies show that MPs were detected at all sampling sites in canal systems of HCMC with 375 MPs pieces in total (Nguyen et al., 2022). In particular, the highest average abundance of MPs was in three canal systems (104.17 ± 162.44 items/m3), the abundance of MPs in NL-TN was the highest (165 ± 280.63 items/m3), followed by TH-BN (114.17 ± 46.45 items/m3) and TH-LG (33.33 ± 20.41 items/m3). In TH-BN, MPs abundance in most of the sampling sites was higher (100 items/m3). This is a large canal of HCMC with 20 km in length stretching through central districts to the Saigon River. The canal connects with other canals, such as TH-LG, and intersects with the Saigon River. The abundance of MPs was higher at two ends of the canal. At the beginning of TH-BN, there are still makeshift houses along the bank of the canal; meanwhile, at the downstream (central district of HCMC), there are many commercial activities such as tourist yachts, restaurants on the river, and water transportation. Therefore, the emission sources of MPs in canals could be from residential activities, waterway transport, tourism along the canal, wastewater, as well as road dust (Nguyen et al., 2022). The populations of urban districts are approximately 40,000 people/km2, showing the high potential for MP pollution. The reception of domestic wastewater from these places also led to higher abundances in these sampling sites (Nguyen et al., 2022).
Fig: Microplastics under the microscope
References
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