Surface water in Hanoi city
Hanoi city
Surface water quality in Hanoi is also an environmental issue of concern (Le et al., 2023; 2024).
Physico-chemical variables and microbial variables
Several studies have been conducted in Hanoi to evaluate the water quality of rivers in urban areas. For instance, a study conducted by Le et al. (2023) reported the mean concentrations of some water quality variables (eg., water temperature, pH, conductivity, TDS, DO, NH4+, PO43-, TSS and POC for four rivers in Hanoi city were as followings: pH values fluctuated from 5.6 to 9.9 averaging 7.4 ± 0.9; water temperature: 20.1 - 30.8, (26.6 ± 3.0) oC; conductivity: 0.5 - 1.2 (0.9 ± 0.2) mS.cm-1; TDS: 0.3 - 0.8, (0.6 ± 0.1) g.L-1; DO values: 0.6 - 6.7, (2.7 ± 1.8) mg.L-1; NH4+ concentrations: 4.2 - 19.5, (10.3 ± 3.9) mgN.L-1; PO43- concentrations: 0.2 - 4.3, (1.5 ± 0.7) mgP.L-1, TSS concentrations: 14.3 - 138.4, (35.0 ± 24.0) mg.L-1, and POC concentrations: 1.2 - 6.0, (3.4 ± 1.0) mgC.L-1. Comparing with values proposed by the Vietnamese technical regulation QCVN 08-MT:2015/BTNMT, pH was within the permissible value whereas three variables (DO, NH4+, and PO43-) exceeded the allowed ones.
These authors also demonstrated that total coliforms (TC) density in water of the four rivers varied from 16x104 to 312x104 CFU.100mL-1, averaging 89.5x104 CFU.100mL-1 which far exceeded the allowed value of the Vietnamese technical regulation for surface water quality QCVN 08-MT:2015/BTNMT column B1 (7.500 CFU.100mL-1) from 21 to 416 times. Similarly, Escherichia coli (E. coli or EC) density highly fluctuated from 4x104 to 240x104 CFU.100mL-1, averaging 58.9x104 CFU.100mL-1, much higher than the allowed value from 400 to 24,000 times. The author emphasized that microbial pollution of river water in inner Hanoi was caused by domestic wastewater, especially in the dry season Le et al. (2023).
Fig: The To Lich river
Antibiotics
One of the emerging contaminants is antibiotic residues; however, research on antibiotic residues has not been very extensive or diverse in Vietnam in general or in big cities in particular. Using antibiotics improperly and leaving antibiotic residues that have not been thoroughly treated can have an impact on the environment, including the growth and development of aquatic species. The emergence of antibiotics in surface water can be from different pollution sources such as wastewater leakage, illegal discharge of raw wastewater into the environment, sewer overflow, or urban storm water runoff (Tran et al., 2019). Recently, the results of surveying the concentration of eight antibiotics in urban river water in Hanoi showed that the concentration of antibiotics in the samples ranged from 3,050 to 16,700 ngL-1 (average 7,800 ngL-1), higher than the concentration in water in some rivers around the world (Le ND et al., 2021). The most frequently detected and predominant compounds (100% detection rate) were amoxicillin, oxfendazole, and lincomycin, accounting for 76±14% of total concentrations. Sulfacetamide and sulfamethoxazole were detected at moderate concentrations in more than two-thirds of the samples analyzed. Other antibiotics (azithromycin, ciprofloxacin, and ofloxacin) were found at lower frequency and detectable concentrations (Le ND et al., 2021). The survey results on the concentration of antibiotics in some inner-city rivers in Hanoi are much higher than those in the Cau River, with the samples taken in less urbanized areas in the North of Vietnam (Ngo et al., 2020).
The study of antibiotic residues by Hoang VC et al. (2021) showed a large variation of antibiotic residues in different urban rivers in the Hanoi inner city. The study presented the antibiotics residues in the surface water of the To Lich River, Nhue River, and the Red River. The main cause of pollution was urban domestic wastewater, which was discharged directly into rivers without treatment. The results showed that the total antibiotic concentration in the To Lich River water was the highest at 12.09 µgL-1, followed by the Nhue River (5.09 µgL-1), and the lowest was in the Red River (1.69 µgL-1). Among the 8 antibiotics monitored (AMO, AZI, CIP, OFL, OXF, LIN, SCE, and SME), two antibiotics, AMO and OXF, were detected in all water samples from all 3 rivers (the Red, the Nhue, and the To Lich). It was found that relatively high levels of some antibiotics were found in some water samples, may reflect the risk of antibiotic effects on aquatic ecosystem in the Hanoi rivers. High antibiotic concentrations in surface water in the urban rivers also revealved the impact of untreated wastewater from Hanoi city.
Similarly, a study by Yen et al. (2017) evaluated antibiotic residues in lake water in Hanoi such as the West, the Truc Bach, the Ngoc Khanh, the Thu Le, and the Yen So. Some antibiotics were found at the highest concentrations (in ngL-1) in different lakes: SMX: 1619.35, STZ: 13.78 in the Yen So; SMZ – 30.19 in the Thu Le; TRI: 118.0, CIP: 823.55, OFL: 430.11 in the Ngoc Khanh; ENR: 16.88, NOR: 79.0 in the West; SMR: 16.34 in the Truc Bach. The research showed that the CIP and OFL were detected at the highest values (823.55 and 430.11 ngL-1, respectively) and at high frequency from 55.6 to 100% of samples in all lakes observed. In constrast, NOR was relatively low, ranging from less than the detection limit to 79.0 ng/L, with a frequency < 57.1%. As known, NOR is largely used in agriculture. In addition, a previous study revealed that the NOR concentration in the wastewater before and after treatment of six major hospitals in Hanoi were in the range of 0.9–17 µgL-1. However, NOR is known to be easily metabolized by different processes such as adsorption and photochemical decomposition under the national conditions.
Fig: the Thu Le lake
Fig: the Truc Bach lake
Another study by Tran et al. (2019) investigated antibiotic residues in the To Lich and the Kim Nguu Rivers, and three freshwater lakes: the Hoan Kiem, the West, and the Yen So. The authors found that most of the antibiotic concentrations in the West and the Yen So were higher than in the Hoan Kiem Lake which is a small lake and only collecting rainwater runoff. Among the three inner-city lakes, the highest value, expressed as SMX concentrations as high as 3,508 ng/L, AMX concentrations up to 1,126 ngL-1, and ERY concentrations up to 741 ngL-1 were detected in the West, followed by the Yen So (SMX, AMX, and ERY concentrations of 160; 356; and 11 ngL-1, respectively), because the latter receives treated wastewater instead of raw wastewater. The lowest values were reported for the Hoan Kiem lake (concentrations of SMX, ERY, and AMX were 58.32 ngL-1 and below the LOD, respectively).
Recently, Anh et al. (2021) working on the occurrence and distribution of antibiotics in the West and Yen So lakes in Hanoi and found that the average concentration of 5 groups of antibiotics decreased in order: sulfonamid (117.9 ngL-1) > β-lactam (31.28 ngL-1) > quinolon (20.19 ngL-1) > macrolide (17.74 ngL-1) > trimethoprim (8.93 ngL-1). While the highest concentrations of SMX were detected at 806.5 ngL-1 in the surface water, its concentration was at only 1.35 ng g-1 in the sediment samples due to their high solubility in water. Quinolones as OFL were found at a maximum concentration of 158.7 ngL-1 in the water phase (Anh et al., 2021).
Microplastics
The determination of the microplastics abundance in rivers exhibit diverse fluctuations in density in both water and sediment ranging from 2.3 items m-3 (Red River, Hanoi city, Strady et al., 2021) to 2,522 items m-3 (To Lich River, Hanoi city), in which the fiber form dominated over the others (Duong et al., 2022). These authors emphasized that wastewater or untreated wastewater from Hanoi city may be responsible for the particularly high microplastics abundance in inner-city river. Furthermore, the microplastic densities in the To Lich River samples were unexpectedly high compared to the Nhue River samples due to that the To Lich River (with high population density in its basin) receives higher large untreated domestic wastewater. This may show that the microplastics abundance in surface water tends to decrease gradually when the flows from the high population density places to the low ones, which further confirms the microplastics abundance directly affected by plastic waste and wastewater discharged by humans in production and living. The amount of microplastics released during human activities is extremely large, especially in urban wastewater.
The Day River system which locates in the Red River Delta, Northern Vietnam is a typical example of a peri-urban river strongly affected by human activities. The Day River receives wastewater of Hanoi city via the Nhue river. The research by Doan et al., (2021) on microplastics in the downstream of the Day River showed that microplastic density in the rainy season was higher than in the dry season and increased at points near urban areas with high population density and economic development associated with various human activities. Fibrous microplastics accounted for over 92% of the total microplastic numbers with the small sizes of 300-1000 µm and 1000-2000 µm, accounting for 78.5-85.7% dominated. Purple was the most common color. PE and PP were the main polymers in surface water samples downstream of the Day River (Doan et al., 2021). Nguyen HT and Nguyen TH (2022) studied on the Nhue and Day rivers and found that microplastic densities, mainly in the form of fibers and fragments with various colors, were in the range from 49-2,827 items m-3 with the microplastic sizes ranging from 0.3-5mm. The authors demonstrated that domestic wastewater in which a microplastic density was of 933-1999 items m-3, was the main microplastic source of the Day – Nhue rivers (Nguyen HT and Nguyen TH, 2022).
Fig. The Day River
In addition, this lake is characterized by high urban and domestic pressure on its ecosystem, along with the direct discharge of untreated wastewater, thus the author concluded that the amount of biological wastewater and waste from human recreational activities can be the main sources of microplastic emissions in surface water (Nguyen et al., 2022). Similar results also indicate that a high density of microplastics was recorded in lakes located in densely populated urban areas, as studied by Strady et al. (2021), e.g. microplastics density was of 611 items.m-3 the West Lake (inner Hanoi city).
Fig: Microplastics under the microscope
Harmful algae and cyanotoxins
Harmful cyanobacterial blooms (HABs) in surface waters are associated with toxic secondary metabolites (cyanotoxins). Dang et al. (2006) conducted a study on the status of the nutrient quality of water bodies in Hanoi (the Thanh Cong and the Giang Vo lakes), The cyanobacteria in the above aquatic systems also fluctuated greatly, from 1.5% to 92.29%. In the lakes of Hanoi (the Hoan Kiem, the Giang Vo, the Thanh Cong, the Thien Quang, and the Bach Thao) at the time of survey (August 2004), cyanobacterial blooms were occurred with the dense scums of phytoplankton and Chl a content ranging from 30 μgL-1 (the Hoan Kiem Lake) to 412 μgL-1 (the Giang Vo Lake). The cyanobacteria community predominated, accounting for 96% of the biomass in the Thanh Cong Lake, 99% in the Giang Vo Lake, and 93.8% in the Thien Quang Lake. The ratio of non-toxic cyanobacteria, e.g., Spirulina, was not significant in the cyanobacteria population.
Fig. The Giang Vo lake
Along with the rapid development in Hanoi, the pollution of a series of water bodies in the inner city and surrounding areas has increased. The Hoan Kiem Lake, the West Lake is the largest natural lake in the city, regulating a large inner city, a famous landmark of the capital, was also polluted. The change in water quality lead to the regular and abnormal growth of cyanobacteria, which changed the species composition of the microalgae system in the lake. The decline of some endemic microalgae species, together with the increase in species composition and density of toxic cyanobacteria in the Hoan Kiem Lake has been warned and caused environmental concerns for the fauna plants living in the lake (Dang et al., 2006). The Hoan Kiem Lake was home to rare microalgal species that were endemic and diverse in species composition. Among the noxious cyanobacteria genera, Microcystis accounted for the highest densities (about 80% of all cyanobacteria causing cyanobacterial blooms). They produce the toxin microcystin, which has harmful effects on both animals and plants in the water body. The research on the microcystis toxic in the Hoan Kiem Lake investigated by the Duong et al. (2012), demonstrated that the Microcystis dominated in the cell community at the Hoan Kiem Lake. Microcystin-RR and LR were found in water samples with concentrations ranging from 2.08 to 46.00 µgL-1.
Fig: The Thu Le lake
POPs
The distribution of POPs such as hexachlorocyclohexane (HCHs), dichloro-diphenyl-trichloroethane (DDTs), hexachlorobenzene (HCBs), and polychlorinated biphenyls (PCBs) in the surface water of some selected individual rivers or lakes in Hanoi city in different periods since 1994. Iwata et al. (1994) conducted a study on the Red River, where elevated concentrations of DDTs, HCHs, CHLs, and PCBs were found at 0.68, 3.2, 0.045, and 0.84 ngL-1, respectively. Then, the study of Hung and Thiemann (2002) showed a large variation of POPs in different urban lakes (dry season: 0.21–4.8 ngL-1; rainy season: 0.65–120 ngL-1) or in the irrigation canal water (dry season: 1.6–130 ngL-1; rainy season: 1.2–110 ngL-1) in inner Hanoi city. Truong et al. (2023) found the total concentrations of six organophosphate triesters (tri-OPEs) and two di-OPEs were from 46 to 3,644 (average: 1,409) ngL-1 and from 2.6 to 6,138 (average 351) ngL-1, respectively.
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