2. 1. Introduction

Due to its potential harm to aquatic ecosystems and human health, MP contamination has recently become a much more concerning issue on a global scale (Wanner, 2021). Microplastics (MPs) are defined as plastic particles with a diameter of less than 5.0 mm, comprising primary MPs discharged during industrial production and secondary MPs produced during the breakdown of more substantial plastic products (de Falco et al., 2019). Because of the ability to absorb toxic chemicals or pathogens that can then be transferred to living organisms through the food chain, MPs are considered a contaminant (Baldwin et al., 2020; Dai et al., 2022; Sulistyowati et al., 2022). (F. Wang et al., 2018) examined the chemical distribution and adsorption behaviour of MPs. In China, Greece, and Portugal, polychlorinated biphenyl (PCB) congeners, polycyclic aromatic hydrocarbon isomers, hexachlorocyclohexane isomers, and dichloro-diphenyl-trichloroethane metabolites were discovered in plastic pellets. Chemical PCB congeners concentrate more on black or aged MPs than on coloured or white MPs (Beckingham & Ghosh, 2017). It has been reported that MPs can travel long distances in the water flow because of their small size and ability to remain in the environment for long periods due to their resistance to degradation. Microplastics have recently been discovered and traced in a variety of environments, including drinking water (Schymanski et al., 2018), the atmosphere (Liu et al., 2019), soil (Nizzetto et al., 2016), food (Vitali et al., 2022) and even human bodies (Wright & Kelly, 2017). Therefore, it is critical to understand the occurrence, abundance, behaviour, and fate of MPs in the natural environment.

Vietnam is one of the top 20 countries in terms of plastic waste input into the ocean, with approximately 1.8 million tons of plastic waste discharged each year, which is 10% more than the global average (Jambeck et al., 2015). The abundance of MPs aquatic system in Vietnam has been reported in previous studies with the MP size ranging from 0.35 to 2522 items/m³ (Chau et al., 2020; Tran-Nguyen et al., 2022). In the Red River, the concentration of MP was 2.3 items/m³ reaching the lowest level among the investigated areas, while the highest concentrations were measured in urban and smaller rivers: 93.7 items/m³ in the Nhue River and 2522 items/m³ in the To Lich River (Strady et al., 2021). In Danang city, the Phu Loc channel was reported as an MPs pollution hotspot with an MPs concentration of 1482.0 ± 1060.4 items/m³ in water (Tran-Nguyen et al., 2022). In the south of Vietnam, fibers and fragments are highly concentrated in the Saigon River basin, respectively 172,000 to 519,000 items/m3 and 10 to 223 items/m3 (Lahens et al., 2018).

The Nhue and Day rivers flow through Hanoi and four other provinces (Hoa Binh, Ha Nam, Ninh Binh, and Nam Dinh) before joining Phu Ly city in Ha Nam province. This river basin, which has a length of 236 km and an area of around 7655 km², used to be a source of critical biodiversity and support for the economic growth of about 12 million people. The effluents from large residential areas like Hanoi, which has over 4000 industrial facilities, about 500 traditional artisan villages, and roughly 1400 hospitals and healthcare facilities, hurt the water quality of these rivers (Le et al., 2022). This river system provides most of the region's freshwater for agriculture and aquaculture. The presence of MPs in this river system has been reported by Strady (Strady et al., 2021). However, information on MPs pollution in these systems is still limited to exploring the source of MPs in the investigated area. Therefore, investigating MP pollution in this river basin is very important for a comprehensive understanding of MPs content situation to provide information to trace the sources and pathways of MPs in the aquatic environment. In this study, the MPs in surface water along the Nhue- Day River basin were investigated to explore the effects of seasonal variation and anthropogenic activities on MP contamination and characteristics in the Nhue- Day River basin. Understanding the presence of MPs in river water will help to develop workable solutions and strategies to reduce the conflicting effects of MP contamination on the ecosystem and public health. The findings from this study can be used as trustworthy support for future research on the impact of MP pollution on human health in Northern Vietnam.

3. 2. Materials and Methods

River water samples were collected using a plankton net (mesh size 300 µm, mouth diameter 30 cm) fitted with a General Oceanic flowmeter (ensure the mouth of the net is completely under the surface of the water) (Dris et al., 2018; Kataoka et al. al., 2019) for 120-240 seconds. Depending on the conditions at the sampling location, the two below methods were applied:

- At monitoring sites with large currents and wide riverbeds, samples were collected from a boat in the middle of the stream. Sampling time and boat speed are calculated to ensure that the sample volume in the net is at least 1m³.

- For the site with a small flow and narrow riverbed area that cannot be netted, a 100-200L sample was collected into a bucket and slowly poured into the sampling device or through a sieve with 0.3mm mesh size.

After being taken from the river, the sampling device will be washed with clean water from the outside to wash away the dirt and the MPs (if any) on the mesh wall and move to the cup at the bottom of the device. This process is repeated several times until the sampler is clean of MPs. After that, water samples were transferred to 100ml glass vials and kept refrigerated and dark before being transported to the laboratory. Then, the water sample was refrigerated at 4oC in the field for immediate analysis or stored in the refrigerator at 20oC for further analysis (stored within 15-20 days).

Besides that, the study also surveyed and sampled surface water at nine sampling sites according to ISO 5667-6:2014 procedure for measuring environmental parameters.

A volume of 100 L of water was pumped through a stainless-steel sieve with a mesh size of 0.3mm - 5mm. Then, samples were dried at 400C for 24 to 48 hours. After that, samples were transferred to a pre-cleaned glass beaker (to prevent sample contamination), and 30% H₂O₂ and FeSO₄ 0.05 M were added to dissolve the organic matter. The process lasted for 24 hours until the appearance of the solution was clear. The samples were then filtered through 0.45 μm filter paper and air-dried.

Carbonate compound on samplers was eliminated by adding 10-20ml of HCl 1M solution to the sampler bottles after drying and shaking well for complete reactions (no bubbling). The response time is from 2 to 4 hours before letting it dry at 400C for 12 hours and taken away for ratio separation. After the drying step, samples were slowly added to 30ml of ZnCl₂ solution d=1.6g/mL and then put into 50ml PE centrifuge plastic tubes. The mixture in the canister was centrifuged at a rotational speed of 3000 Relative Centrifugal Force (RCF) per min to separate MPs and other solids remaining in the water. Microplastics with light density float on the surface of the ZnCl₂ solution. The solution at the top of the test tube will be filtered to separate the MP particles from the remaining solid layer. The ZnCl₂ solution containing the supernatant MPs was collected by the overflow method and placed in a 500 ml Nalgene filter cup. And then filtered through grid paper with a 47mm diameter 0.45µm filter pore size; each piece of paper has 100 cells, with the size of each cell being 3.1x3.1mm. With the smallest MP particle size of 0.35mm, filter paper with the above pore size can be used. The filter paper was gently removed and stored in aluminium foil bags and dried at 45oC for about 6 hours before determining the quantity and composition of MPs.

Dried samples containing MPs were carried out to determine the number of MPs using a Euromex stereo microscope, with a maximum focal length of 40x, and a DC5.0 Stereo Camera with ImageForcus 4 English Version software with cam to determine the composition and quantity of MPs. Environmental parameters and microplastic abundance were analysed by correlation value and PCA (Principal Components Analysis) using SPSS v20.

4. Results and discussion

The results shown in Fig. 2 and 3 indicated that the distribution and abundance of MP in Nhue- Day river system was significantly affected by season and geographical location.  In the dry season, the concentration of MPs in surface water ranged from 49 to 2,827 items/m³, with an average abundance of 754 items/m³. The sampling area with the highest abundance of MPs was the ND3. The ND3 site is at the Day River's confluence, the Hoa Vien bridge section, and near ND3 market, Ung Hoa district, Hanoi. This is a densely populated area. Moreover, in recent years, various kinds of trading activities along the river in ND3 site have produced untreated wastes, and the survey area pollution has been quite severe. In addition, various kinds of small, scattered enterprises and industrial activities such as wool and fabric factories, garment factories, and some manufacturers and traders of plastic products have been recorded as sources of MP discharge in the area. Besides, site ND6 was also reported to have high levels of MPs in surface water (1,423 items/m³). Previous studies connected the presence of MPs in the aquatic environment to the effects of human activity (W. Zhao et al., 2020). It explained that the highest MP concentration at ND6 runs through a densely populated core area and industrial area related to engineering, electronics, assembly, food production, and some light industry. Additionally, the close distance between ND6 and rubbish dumping indicated a high MP concentration at this site. The fact that intense economic activities, such as industrial discharge and land use, can cause poor water quality to some extent has been alarming. The idea that MP abundance is driven by economic activity is indirectly supported by the potential positive association between poor water quality and high MP content.

In the rainy season, the abundance of MP in Nhue- Day River ranged from 400 to 1,133 items/m³, with an average abundance of 706,67 lower than that in the dry season. The MPs are distributed more evenly along the river basin due to the influence of the river flow in the rainy season instead of concentrating at some points in the dry season. This phenomenon indicates that the abundance of MPs was gradually diluted when the water volume increased. The two survey sites with the largest concentration of MPs were recorded at ND2 and ND6 with the corresponding value of 1,133 and 1,066 items/m³, respectively. This difference in the distribution pattern of MPs may be due to hydraulic and flow conditions significantly affected by seasonal factors. In the rainy season, tributaries, and overflows in residential areas on both sides of the river are formed. So, it may partly explain why the number of MPs at the lower limit increases and the recorded upper limit decreases in the rainy season. The increase in hydrology is also a factor governing the distribution of these plastics.

The abundance of MPs in the surface water from urban areas in other parts of Vietnam and over the world is displayed in Table 2. The results in this study were compared with those in other rivers in Vietnam, such as the Red River, Han River (which runs through Da Nang), and Dong Nai River. It is clear that except for the Saigon River (22,000–251,000 items/ m³; Strady et al., 2020), the MPs concentration in the surface waters of the Nhue- Day River is at high levels. In particular, the average value of microplastics recorded in an m³ of surface water in the study area is 327 times greater than the number of microplastics recorded in the Red River, nearly 280 times greater than the number of microplastics received from Han River water, and more than 193 times greater than the number of microplastics received from Dong Nai River. Especially, compared to the Nhue-Day River in 2020, there is a significant increase in the microplastic abundance in 2021 (sample collected time of this study), alarming the pollution issue of this area.

The Nhue-Day river basin has a greater MP concentration in waters than rivers in Asian countries, Europe, and the United States, including the Surabaya River in Indonesia, where MP concentrations were barely a few items per m³ (Lestari et al., 2020; Sembiring et al., 2020), Han River in South Korea (Park et al., 2020). In contrast, there are many other Asian rivers containing a higher MP concentration than Nhue- Day River such as rivers in China, for example, the Maozhou River (4000 ± 1000 to 25,500 ± 3500 items m⁻³; Wu et al., 2020); Wei River (3670–10,700 items m⁻³; Ding et al., 2019) (Table 2).

Research results show that microplastics in surface water are mainly secondary microplastics detailed in  Fig. 4 and 5. In both seasons, fibers identified as the most dominant type of MPs collected from the sampling sites were similar to those of other studies on freshwater MPs (Koelmans et al., 2019). Fibers come from various sources, such as washing clothes, using and wearing plastic objects, and the waste plastic produced during industrial production (Browne et al., 2011). Fibers were 93.26 % of all MP types, while fragments were 6.47% in dry season. In the rainy season, there was a light increase in the number of fragmented MP. Fragment MPs have a higher density than fibers; some types of density can increase to 1.2 g/L. In the rainy season, the river water discharge increases, causing a high-water velocity which facilitates the transportation of fragments in the flow. Therefore, the concentration of MP in fragment shape in dry season was higher than in rainy season.

Microplastics with different colours are presented in Fig.5. In dry season, black was seen to be the dominant (33.6%), followed by blue (31.6%) and red (18.5%). Other pigmented particles, such as green (4.46%), yellow (1.53%), and grey (0.03%), occurred only at low proportions. The coloured particles are more likely to be mistaken for food by aquatic biota because they resemble low-trophic-level organisms (Browne et al., 2008). Furthermore, green MPs with irregular forms may be formed because of the breakdown of single-use plastic products containing colorants (W. Wang et al., 2018a). In contrast, in rainy season, white was the most common MP colour, with a percentage reaching 49.6%, followed by blue (31.6%). In both seasons, grey is the least common one. This result agreed with several previous studies in which a high proportion of colourless MPs (transparent and white) was found (Lestari et al., 2020; Phuong et al., 2018; Wu et al., 2020). In addition, it was reported that the reason might be caused by the overflow of plastic manufacture from nearby companies and fading during significant weathering processes such as wave action by tidal currents.

Fig. 6 shows the distribution of MPs in the dry season with the minimum size value of 0.301mm and the largest size of 4.71mm. Microplastics tend to be unevenly concentrated in sizes while MPs with the size 0.5- 2.0 mm took up most of the samples. In the rainy season, MPs are smaller in size than in the dry season. This can be explained by the fact that MPs in the dry season are subjected to adverse weather conditions with prolonged high temperatures, as well as the influence of UV, causing these MPs to break into smaller pieces. In the rainy season, these particles are transported everywhere by the water flow, so encountering small-sized pieces of MPs is more frequent.

The relation between environmental parameters and microplastic abundance

The environmental parameter values of Nhue Day river water in dry and rainy seasons include pH, Turbidity, Conductivity, TDS, DO, TSS, COD, BOD₅, NH₄⁺, NO₃^-, and NO₂. Research results show that the pH value is relatively stable in the whole study area and between the two seasons. The average pH values ​​corresponding to the dry and rainy seasons are 7.76 ± 0.21 and 7.55 ± 0.24, respectively, which is typical for a weakly alkaline environment. The values ​​of Turbidity, conductivity, and DO are also relatively stable between the two seasons, with the average value in the dry season being 32.42±16.13; 410.22±188 µS/cm; 3.82±1.92 mg/l, and in the rainy season, is 29.51±7.64, 435.11± 91.26 µS/cm, 4.14± 1.68 mg/l. These parameters in two seasons are within the allowable threshold according to national standard QCVN 08 – MT/2015 for surface water of class B1. Except for some sampling sites such as ND-1, and ND-2, the DO value exceeds the allowable threshold.

The values of TSS, COD, BOD₅, and NH₄⁺ ​​in the study area tend to increase in the dry season due to increased temperature and low river water discharge. Average values ​​of TSS, COD, BOD₅ and NH₄⁺ in the dry season respectively 41.29 ± 17.30 mg/l; 51.11 ± 30.11 mg/l; 34.30 ± 20.96 mg/l; 10.51 ± 13.72 mg/l and in the rainy season is 26.31 ± 15.67 mg/l; 34.14 ± 31.53 mg/l; 20.38 ± 18.95 mg/l; 5.67 ± 7.73 mg/l. During the rainy season, the river level rises, and the flow is stronger, dissolving the wastes and carrying the river flow, causing the concentration to decrease in the rainy season. At the same time, the results also show that the surface water of the study area is contaminated with organic matter, especially at the survey points ND-1, ND-2, ND-3, and ND-4. These points all have TSS, COD, BOD₅, and NH₄⁺ values ​​exceeding many times the allowable threshold according to national standard QCVN 08 - MT/2015 for surface water of class B1. Therefore, it is necessary to take timely measures to prevent pollution from becoming more serious, affecting the quality of the environment and the lives of people in the area.

The results showed that no close relation was found between the environmental parameters and microplastics abundance in the study area (Fig.8; Tab.3). The behaviour of microplastics in the environment may be a separate process influenced by many environmental influences such as temperature, UV rays, terrain, flow rate, and especially the sources of plastic waste... Therefore, more detailed studies are needed on the impact of environmental factors on the behaviour of microplastics. The study also shows a relation between the values of TDS, NH₄⁺, COD, and BOD. May be these indicators have the same pollutant source. More detailed studies are needed to clarify this issue.

5. Conclusions

The widespread pollution of the environment by MPs has received much interest from the public and scientists. All the surface water from Nhue- Day river contained MPs, varying abundance in different seasons. In the rainy season, the MPs are distributed more evenly along the river basin due to the influence of the river flow instead of concentrating at some points, as in the dry season. Fibers were dominant in both seasons.

Most of the MPs collected for this investigation were small particles, with a sizable portion falling within the 0.5–2 mm size range. In the rainy season, MPs are smaller than in the dry season. The research did not find a close correlation between the environmental parameter and the number of microplastics. It is of practical significance to understand the sources and sinks of MPs in inland freshwater environment.

Comparison with other rivers allows us to conclude that the Nhue- Day river is highly polluted with MPs compared to other rivers in Vietnam as well as in the world. Besides, this river is also facing a high risk of organic pollution. So It is necessary to take timely remedial measures to limit pollution in the area.

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Author Contributions: Conceptualization: Huu Thang Nguyen; Methodology: Huu Thang Nguyen, Thi Ha Nguyen, Dung Luu Viet; Formal analysis and investigation: Huu Thang Nguyen, Thi Hien To, Dung Luu Viet, Minh Trang Hoang; Writing - original draft preparation: Huu Thang Nguyen; Writing - review and editing: Thi Ha Nguyen, Minh Trang Hoang, Dung Luu Viet; Supervision: Thi Ha Nguyen

Huu Thang Nguyen^1,2*, Thi Ha Nguyen², Thi Hien To³, Minh Trang Hoang², Dung Luu Viet⁴ Truong Huu Duc⁴, Duong Thanh Nghi⁵

¹Centre for Environmental Monitoring, Pollution Control Department (PCD), MONRE, Ha Noi, 10 Ton That Thuyet Road, Ha Noi, Viet Nam.

²Faculty of Environmental Sciences, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam

³Faculty of Environment, University of Science, Vietnam National University Ho Chi Minh City

⁴Key Laboratory of Geoenvironment and Climate Change Response, VNU University of Science, Vietnam National University, Hanoi, Vietnam

⁵Institute of Marine Environment and Resources, Vietnam Academy of Science and Technology, 246 Da Nang street, Ngo Quyen district, Hai Phong city, Vietnam

^*Corresponding author: Huu Thang Nguyen

Email address: thangcemdi@gmail.com