Surface water quality assessment in the Bach Dang river basin, Vietnam: using water quality index and geographical information system methods

The study evaluates the surface water quality of the Bach Dang river basin. During the rainy season, the temperature, pH, EC, BOD5, COD, coliforms, ammonium and phosphate values were 27 °C–37 °C, 6.6–8.9, 300–2200 μS cm−1, 5–25 mg l−1, 5–26 mg l−1, 0–3000 MPN/100 mL, below 0.01 mg l−1, and 0.02 mg l−1, respective. During the rainy and dry seasons, Zn, Pb, Cr, As, Fe and Mn levels were below drinking water limits. In the rainy season, the water quality index varied from 76 to 91, signifying medium to excellent water quality. The most outstanding water quality sites were the QN12, QN18, and QN22. During the dry season, most areas had moderate to excellent (WQI values between 61 and 93), except for QN01, QN03, QN04, QN05, QN06, QN07, QN11, QN16, QN17, QN19, and QN23, where it dropped to moderate (61–73). The temperatures were greater and higher EC values in the southeast. The DO concentrations were higher in the northwest and southeast in both seasons. The southern and southeastern regions exhibited more excellent COD and BOD5 values than the western, southwest, north, and northeast regions. The ammonium concentrations were higher in the southeast. During the dry season, the phosphate levels exceeded the permissible limit in the west, southeast, east, and northeast. Coliforms were below acceptable in both seasons, with greater levels in the northwest and northeast during the dry season. In addition, the Zn and Pb are favorably associated with the As, Mn, ammonium and phosphate in the rainy season. The COD positively correlated with the coliforms and BOD5. In the dry season, the pH inversely correlated with the Pb and Fe positively. The ammonium correlated negatively with the DO and positively with the phosphates. The pH negatively connected with the alkalinity, while the As positively correlated with the BOD5 and COD.


Introduction
Water is essential to both life and globalization. Despite this, the global water supply has deteriorated over time for natural and anthropogenic causes. The Water quality assessment aims to identify the sources of water 2. Materials and methods 2.1. Study area and sample collection Study area: Bach Dang river is part of the Thai Binh river system; it begins at the confluence of the Da Bac and Gia rivers (near Ben Rung) and runs approximately 43 kilometers south to Vu Yen island (figure 1). This is the boundary between the Vietnamese provinces of Hai Phong and Quang Ninh. The northern bank includes the districts of Dong Trieu, Uong Bi, and Quang Yen (Quang Ninh province), while the southern bank includes the districts of Hai An, Thuy Nguyen of Hai Phong province, and Kinh Mon of Hai Duong province. The Bach Dang river basin is evaluated as a complicated hydrological system with multiple rivers and estuaries draining into the Gulf of Tonkin. The Da Bac river is the primary stream of the river basin in Quang Ninh province; tributaries include the Uong river from Uong Bi district, the Cam river from Dong Trieu district, and the Khoai river tributary; additional the Chanh and Rut rivers help drain water into the Gulf of Tonkin. The Bach Dang estuary is both a confluence of tributaries and a source of rivers that go into the ocean. The region features a dense network of rivers and streams with an average density of 1.0-1.9 km km −2 and a maximum density of 2.4 km km −2 in certain locations. The width of the riverbed in the Da Vang portion is between 160 and 270 meters, while the width of the Da Bac section averages between 600 and 700 meters. At high tide, the Da Bac River is deep enough to accommodate ships of up to 5,000 tons and barges of between 400 and 500 tons.

Sampling and analysis
We collected 23 water samples at the Bach Dang river basin points in Quang Ninh province, Vietnam for the dry and rainy seasons (table 1). The samples were taken at a depth of 40-50 cm using polypropylene bottles for physical and chemical examination and sterilized glass bottles for bacteriological investigation, all in accordance with established standard methods [13].
Electrical Conductivity (EC), Potential of Hydrogen (pH), temperature, alkalinity, ammonium (NH 4 + ), phosphates (PO 4 3− ), Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD 5 ) Chemical Oxygen Demand (COD), coliform, and heavy metals: Zn, As, Pb, Cr, Fe, and Mn are among the sixteen parameters analyzed for each sample in table 2. The EC, pH and temperature of water samples were detemined by using High Range pH/ EC/TDS/Temperature Waterproof Meter HI991301, Hanna, Italy. Using a sulfuric acid, a digital titrator is used to assess alkalinity [14]. The DO was determined by an electrochemical method [15]. Microorganisms in the water sample measure the BOD and COD after five days of incubation and the amount of oxygen necessary to oxidize the organic content in a water sample completely [13], respectively. The amounts of ammonium and where a WQI a represents the chemical parameters (the DO, BOD 5 , COD, ammonium, and phosphates, Zn, Pb, Cr, As), the WQI b represents the physical parameters (the alkanility, temperature, EC), a WQI c represents the biological parameter of coliform, and a WQIpH represents the pH. On the basis of the determined the WQI score, the quality of the water is categorized into five categories: Level 1: A WQI score between 0 and 25 indicates very dirty water that must be treated immediately before reuse.
Level 2: A WQI score between 26 and 50 shows that the water is suitable for transport and comparable uses. Level 3: A WQI score between 51 and 75 indicates appropriate water quality for irrigation and related uses. Level 4: A WQI score between 76 and 90 indicates domestically usable water quality. Level 5: A WQI score between 91 and 100 indicates home water supply quality.
The resulting water quality evaluation can be displayed on a map to indicate the quality of the water.

Geographical information system (GIS) technique
Using a geographic information system (GIS), spatial distribution maps were made, specifically for groundwater quality parameters [17], using the inverse distance weighted interpolation technique. This efficient method made the estimation of average values in unsampled areas possible, which generated weights based on the proximity of neighboring places and distributed them to various characteristics. The weights were proportionally determined by the inverse distance weighted power coefficient, which took into account the closeness of sampled locations to unsampled places. This GIS interpolation method is useful for creating maps of the surfae water quality, identifying potential surfae water areas, and supporting the creation of new surfae water schemes. The GIS approach is used to generate maps displaying the spatial distribution of water quality metrics, including the Zn, Pb, Cr, and As, as well as parameters such as the DO, BOD 5 , COD, ammonium, phosphates,  coliforms, pH, temperature, EC and WQI. This instrument is used to depict the distribution of water quality characteristics, which serves as the basis for evaluation.

The methodology limitation
The WQI has some restrictions even though it thoroughly evaluates water quality by taking into account a number of characteristics. The WQI calculation uses preset parameters, which could not always match the unique characteristics of the Bach Dang River basin exactly. Additionally, a predetermined set of parameters, including pH, dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, ammonium, phosphates, and specific heavy metals, are used in this study's WQI calculation. Even though these metrics are crucial markers of water quality, they might not fully account for all pollutants or toxins in the Bach Dang River basin. The GIS method used in the study is another area of constraint. Although GIS offers useful spatial analysis and mapping capabilities, it depends on precise and current data for insightful interpretation. The GIS approach may not sufficiently capture temporal variations because it primarily concentrates on the spatial representation of water quality measures.

Surface quality parameters
The parameters observed during the rainy season can be used to evaluate the water quality in the Bach Dang river basin, as shown in table S1. The table lists the concentrations of various pollutants, including the Zn, Pb, Cr, and As, as well as parameters such as the BOD 5 , COD, ammonium, coliforms, and pH. In addition to the DO, temperature, alkalinity, EC and WQI, additional parameters are provided. Rainy season: Physical parameters: The water temperatures of all rainy season samples range from 27 to 37°C, which is within the normal range for freshwater temperatures. The EC values vary between 300 μS cm −1 and 2200 μS cm −1 . When the numbers are greater, this implies that the water contains more dissolved salts. It is a vital component in the process of determining the overall quality of river water and is used to investigate the numerous pollution causes. All samples showed the pH values indexss between 6.6 and 8.9, indicating that the water of the study area is either mildly acidic or slightly alkaline. The concentration of hydrogen ions, or the pH, in water is important because it affects its biological, chemical, and physical properties. The alkalinity ranges between 0.0846 mgCaCO 3 /L and 0.5636 mgCaCO 3 /L. It is water's ability to inhibit an acid from doing its function. This criterion plays a crucial role in the process of determining whether the water is suitable for agricultural, industrial, or domestic usage. The DO measurements range from 7.4 mg l −1 to 8.4 mg l −1 for all samples. The DO is essential since it influences the amount of oxygen aquatic organisms can utilise. Higher values of the DO indicate that the water has a higher quality.
Chemacal and biological parameters: The BOD 5 and COD values range from 5 mg l −1 to 25 mg l −1 and 5 mg l −1 to 26 mg l −1 , respectively. When considering organic pollution in water, the BOD and COD values are significant factors. The coliform levels range from 0 to 3000 MPN/100 mL for each sample. At the same time, the ammonium levels in all sampling points are below 0.01 mg l −1 , indicating that the ammonium index in water sources in the Bach Dang basins is low and within acceptable limits, and the environment is not polluted. The phosphates are also an indicator of the nutritional quality of water. The concentration of phosphates in 23 water samples from the Bach Dang basins was less than 0.02 mg l −1 . Consequently, no phosphate-contaminated water samples have been found in the region. The presence of coliform bacteria in water is a common indicator that the water contains faeces. The coliforms count that are above average suggest an increase in faeces contamination.
Heavy Metals: The values of Zn, Pb, Cr, As, and Fe, as well as Mn, fall below the World Health Organization (WHO) permissible ranges for consumption in the drinking water. On the other side, their presence in water has the potential to result in long-term health problems. The concentrations of Zn, Pb, Cr, As, Fe, and Mn in the surface water of the Bach Dang river basin during the rainy season are all below the WHO's allowable levels for consumption in the drinking water, according to the data provided. In terms of heavy metal contamination, this is good news and indicates that the water is safe for drinking. Despite the fact that the amounts of these heavy metals are below the allowable levels, their presence in water still has the potential to cause long-term health concerns. Thus, continuous monitoring of heavy metal concentrations in the surface water of the Bach Dang river basin is required to guarantee the safety of the population's water supply.
Dry season: Physical parameters: According to the data, the pH ranges from 6.5 to 9.1, with an average of 8.0, indicating that the water is slightly alkaline. The pH levels suggest that the water is slightly alkaline, which is typically seen as being beneficial for aquatic life. The alkalinity ranges from below 5 mg l −1 to above 240 mg l −1 . Some samples have extremely low alkalinity levels (less than 5 mg l −1 ), whereas others have extraordinarily high alkalinity levels (240 mgCaCO 3 /L). Notably, the alkalinity levels can affect the ability of water to support aquatic life, as well as its suitability for human consumption and industrial usage. Some samples have exceptionally low alkalinity values, whilst others have excessively high alkalinity levels. This may affect the water's appropriateness for various uses, such as drinking water and industrial use. The DO levels vary from 4.5 to 9.1 mg l −1 , with an average of 7.7 mg l −1 , indicating moderate to good water quality. The temperatures ranged from 19 to 93°C, with an average of 57.5°C, indicating high water temperatures. The EC index ranged from 270 to 2040 μS cm −1 , with a mean of 1352 μS cm −1 . Observed high water temperatures may impact the DO concentrations and promote the growth of hazardous algae and other organisms. The measurements for the EC show that the water has moderate to high amounts of dissolved salts, which may impact its usefulness for irrigation and other purposes. In addition, the broad range of EC values shows that different river basin regions may have distinct water quality features.
Chemical and biological parameters: The BOD 5 and COD indexes range between 5 and 27 mg l −1 and 8 and 30 mg l −1 , respectively, which indicates a moderate to high amount of organic contamination. The ammonium values range from 0.1 to 0.8 mg l −1 , with an average of 0.4 mg l −1 , indicating low pollution levels. The phosphate concentrations range from 0.2 to 1.9 mg l −1 , with an average of 1.0 mg l −1 , indicating moderate nutrient contamination. The coliforms values range from 0 to 3200 MPN/100 mL on an average, indicating moderate to high faecal contamination. Although the ammonium concentrations indicate that pollution levels are relatively modest, they may lead to eutrophication and other water quality issues. Similarly, the phosphate amounts indicate significant nutrient contamination, which may also result in problems such as algal blooms and decreasing oxygen levels. The coliform levels indicate that the water is contaminated with moderate to high levels of faeces, which could threaten human health. This may be caused by various factors, such as insufficient wastewater treatment, agricultural runoff, or other human activities.
Heavy Metals: The heavy metal concentrations in the river are relatively low, with the highest concentrations observed for Zn (0.1759 mg l −1 ), followed by Pb (0.0083 mg l −1 ), and Cr (0.0163 mg l −1 ). Furthermore, the As contents were minimal, averaging 0.0023 mg l −1 . The concentrations of Fe and Mn in the river were generally low, with the Fe having the highest concentration (1.3361 mg l −1 ). The Zn was found to have the highest quantities, followed by the Pb and Cr, however all values are under the regulatory thresholds. The As levels were low, which is a positive indicator given that the high As levels pose substantial health hazards. The Fe and Mn concentrations were also usually low, which were good because elevated levels of these metals can create water quality issues such as discolouration, foul taste, and odour.
Most metrics for both the rainy and dry seasons for the Bach Dang basin are within the QCVN 08-MT:2015/ BTNMT allowed limits, as shown by the data in the table S1. Nonetheless, sample spots and characteristics still exceed permitted limits in both seasons. Particlely, the sampling point QN19 and QN23 have the Mn concentration that exceeds the permissible limit, the sampling point QN06 and QN17 has the BOD 5 concentration that exceeds the permissible limit during both the rainy and dry season, and the sampling point QN05 has a slightly higher BOD 5 concentration than the permissible limit during the dry season. The COD concentration from QN17 has also slightly higher than the permissible limit in dry season. Valuabels of pH at the QN16, QN19 and QN23 are also out range of limitation accoding to the QCVN 08-MT:2015/BTNMT. In addition, it has been noted that certain parameters, such as the Fe, BOD 5 , and COD, ammonium, phosphates and coliforms tend to be higher during the dry season, whilst the alkanility, temperature and EC concentration tend to decrease. This can be explained by the impact of climatic elements such as precipitation, temperature, visibility, human activities, and wastewater sources. Per the QCVN 08-MT:2015/BTNMT, most parameters in the Bach Dang River basin are within the allowable limits. Yet, sample spots and characteristics still exceed the permissible limits, particularly during the rainy and dry season. This suggests that the management and monitoring of water quality in this river basin must be addressed to ensure the availability of clean, safe water for human and animal consumption. According to a research on surface water quality in Southwestern Vietnam, during the dry season, the levels of Fe, BOD 5 , and COD, ammonium, phosphates, and coliforms tend to be greater, whereas the alkanility, temperature, and EC concentrations tend to drop, [18]. The water temperature also affects the amount of DO and BOD 5 , with cold water holding more oxygen than warm water [19]. Except for the BOD 5 , all chemical parameters measured during the dry season of an Oluwa river research were significantly different from those measured during the rainy season [20].
The main finding of the study reveals that while the majority of parameters in the Bach Dang river basin were within acceptable ranges, there were some specific areas and features where certain contaminants, particularly the Mn, BOD 5 , COD and pH, exceeded the limits. This finding is consistent with studies by Dang and Liu (2022) [21], which emphasize the importance of thorough monitoring and analysis in order to accurately reflect water quality conditions and make wise judgments regarding water management. Furthermore, our results concur with those of Mosiej and Bus (2015) [22], who emphasize that water reservoirs might act as the causes of water contamination, particularly if they get insufficient or subpar water supplies. The Bach Dang river basin's elevated pollutant levels point to potential contamination from a variety of sources, including industrial and agricultural operations. The study also found that during the dry season, alkalinity, temperature, and electrical conductivity (EC) concentrations decreased while markers of nutrient and organic contamination increased. These results are in the line with the study of Stets et al (2020) [23], which analyzes the degrading effects of increased nutrient loading and changes in the main ion composition brought on by human activities on freshwater resources. The Bach Dang river basin's increased the organic and nutrient contamination during the dry season emphasizes the need for the efficient management and monitoring of water quality to guarantee the supply of clean and safe drinking water. It's crucial to recognize the limits of this study, though. The investigation only included a small number of factors and did not consider how poor water quality can affect people's health. This restriction is in the line with the conclusions of Levy et al (2012) [24], who underline the necessity for further consideration of research design elements, such as the sample size and measurement variability, when using indicator species to assess the quality of water.
According to the statistics supplied previously, the quality of the water in the Bach Dang basins during the rainy and dry season can be regarded as somewhat contaminated. After undergoing the requisite purification processes, the water is suitable for using in an agriculture, industry, and a number of other non-drinking purposes. Nevertheless, an untreated water is not safe for human consumption and should not be consumed.

The water quality index (WQI)
The supplied table S1 shows the WQI values for 23 sampling locations in the Bach Dang river basins throughout the rainy and dry seasons. The water quality for both the rainy and dry seasons is generally moderate to very good, with a few stations having average quality. A greater percentage of stations have a very good quality rating during the rainy season, which results in somewhat better water quality than during the dry season. According to the table S1, the WQI for the Bach Dang river basins ranges from 76 to 91 in the rainy season, suggesting that the water quality is generally good to very good. The top three stations with very good water quality ratings (I) in the rainy season are the QN12, QN18, and QN22, suggesting that their water quality is continuously high all year long. The WQI values of QN15 also ratings I in the dry season. In terms of seasonality, the WQI tends to be higher during the rainy season than the dry season. This may be the result of increasing runoff from agricultural and urban activities, which can raise nutrient concentrations in the water. However, the WQI value ranges from 61 to 93 and indicates that the water quality is from moderate to very good during dry season. Only sampling point QN15 reaches the level I of the WQI values during the dry season. The water quality decreased to moderate at numerous locations throughout the dry season (the WQI values between 61 and 73), including sample locations QN01, QN03, QN04, QN05, QN06, QN07, QN11, QN16, QN17, QN19, and QN23. This might be as a result of the dry season's lower rainfall and water flow in the dry season, which results in higher levels of contaminants and lower dissolved oxygen levels in the water. The remaining samples' WQI values demonstrate that during the dry season, the water quality is good.
Based on the analysis of the WQI, the main finding of this study is that the Bach Dang river basins exhibit high water quality during both the rainy and dry seasons. Most sampling locations have moderate to excellent water quality, while a few stations have ordinary water quality. Comparatively, the water quality during the rainy season is marginally superior, with a greater proportion of stations receiving an excellent rating. This assessment indicates that the Bach Dang river basins maintain high water quality, although specific areas may require management to maintain this standard [25]. This result is consistent with previous research conducted in various regions, indicating uniformity in water quality evaluation. For instance, a global study evaluating fecal contamination in drinking water sources found that 1.8 billion people worldwide rely on feces-contaminated water sources, indicating inadequate water quality [26]. In contrast, the study of the Bach Dang river basins generally reveals excellent water quality, indicating a positive state. Nonetheless, it is essential to recognize the potential differences in water quality between locations and seasons, as observed in other studies. Studying the trophic state and density of toxic cyanobacteria in multi-reservoir water systems, Stefanoski et al (2013) [27] discovered that water quality can be affected by numerous factors, including algal blooms. In addition, land use and management practices can have an effect on water quality. Boix et al (2005) [28] highlight the significance of soil management in assuring soil conservation and agricultural sustainability by highlighting the effects of land use on soil physical quality.
Despite the generally excellent water quality in the Bach and Dang river basins, it is crucial to identify potential limitations and problem areas. Continuous monitoring and evaluation of water quality are indispensable for its preservation. Long-term observations are essential in hydrology to understand the dynamics of biological and physicochemical processes that water flux affects [29]. In addition, according to a study of unregulated domestic wells in the United States, contaminant levels can fluctuate between wet and arid seasons, highlighting the need for routine monitoring and management of water sources [30].
3.3. Spatial distribution pattern 3.3.1. pH, EC, alkanility and temperature Figure 2 illustrates the spatial interpolation map of the pH, EC, alkanility and temperature in the Bach Dang river basin during the rainy and dry seasons. The majority of water regions in the basin are slightly acidic to slightly alkaline during both the rainy and dry seasons, as indicated by the map. Typically, the alkalinity levels are higher in the southeast in the dry season. Due to the influence of seawater, the southeast regions adjacent to the ocean have higher pH levels than the upstream regions of the basin during both the rainy and dry seasons. The majority of pH levels fall within the WHO recommended range for the drinking water (6.5-8.5). While the majority of pH values in the region are within the permissible range for surface water according to the QCVN 08-MT:2015/ BTNMT (5.5-9.0), certain monitoring locations, including the QN16, QN19, and QN23, have the pH levels lower than 5.5. This tendency is linked to the influence of Quang Ninh coal mine wastewater on the affected streams, resulting in low pH values. The effluent discharged from the coal mines in Quang Ninh province, Vietnam, contributes to the acidic nature of the water, resulting in the low pH levels. The coal mining generates acid mine drainage, which is created by chemical and biological processes in ore-exposed surface water in the presence of oxygen. This wastewater has severe negative environmental effects, including soil and water pollution, and can lower the pH of water sources [31,32].
These findings shed light on the characteristics and water quality of the Bach Dang river basin. The pH levels range from slightly acidic to slightly alkaline, indicating a relatively acceptable pH range for aquatic ecosystems. The elevated alkalinity levels in the south-east and during the dry season may be influenced by geological composition and the intrusion of seawater. The results of this study regarding pH and alkalinity in the Bach Dang river basin are consistent with previous research on the effect of seawater acidification on pH levels in reef corals [33]. These studies indicate that corals can modulate pH levels at the interface of their tissue and skeleton to mitigate the effects of seawater acidification. In agreement with research on the treatment of polluted water from coal mining [34], the findings also emphasized the potential impact of coal mine wastewater on pH levels in the river basin.
Similar to the pH index, the regional variability in water temperature values in the Bach Dang river basin exhibit an upward tendency toward the southeast (figure 2). The water temperature fluctuates from 28.01 to 37°C, with greater temperatures during the rainy season. During the rainy season, the region with the greatest temperature is the QN07 (37°C), while the zone with the lowest temperature is the QN22 (16°C). Closer to the coast, the tendency toward greater temperatures is observed. While warm water occupies more space than cold water, ocean warming also contributes to sea level rise. Coastal places are especially susceptible to rising temperatures due to the seas' warming. As greenhouse gases absorb more solar energy, the oceans absorb more heat, resulting in an increase in sea surface temperature.
These findings suggest that the water temperature in the Bach Dang river basin is influenced by a variety of factors, including regional variations, seasonal variations, and coastal influences. It is possible that geological composition, land use patterns, and climate patterns are responsible for the southeastward trend. Increased solar radiation and decreased water flow may account for the higher temperatures during the monsoon season. The coastal influence may be caused by ocean currents, atmospheric circulation, and emissions of glasshouse gases. Temperatures range between 28.01 and 37°C, with higher temperatures during the rainy season. Coastal regions experience greater temperature increases as a result of ocean warming induced by the absorption of glasshouse gases [35]. This result is consistent with other studies that have examined river basin temperature patterns. a study conducted in England and Wales found spatial patterns and inter-annual variability in river temperature regimes [36]. Another study in Ethiopia investigated temperature trends in the Abbay River Basin and observed a slightly increasing trend in annual temperature [37]. These studies emphasized the significance of comprehending the temperature dynamics in river basins and the potential consequences of climate change. Nonetheless, a study in the Mediterranean Thau Lagoon found that rising water temperature was the main cause of phytoplankton blooms, indicating a distinct relationship between temperature and ecological processes [38]. In addition, a study in the Contiguous United States evaluated water security based on water footprint concepts and found that spatial and temporal variations in temperature impacted river basin water supply [39]. It could be due to distinctions in geographical location, regional characteristics, and ecosystems between the investigated area and other study areas.
In figure 2, the spatial variations of EC values reveal that the regions of QN01, QN02, QN03, QN04, QN8, QN9, QN10, QN19, and QN20 have greater values than other regions. These locations correlate to the most significant river and stream networks within the Bach Dang river basin. The high concentrations of chloride and dissolved solids in the river and lakes during low-flow conditions are responsible for the elevated EC levels. In addition, the geology of the basin can influence the chemical composition of soil and water, resulting to the elevated EC concentrations [40]. In addition to the pH levels, agricultural runoff, and industrial fossil fuel emissions, other factors such as these can contribute to the deterioration of river water quality [41].
Certain regions of the Bach Dang river basin have higher EC levels than others, based on the spatial variations in EC values. These locations correspond to significant networks of rivers and streams within the basin. The elevated EC levels are attributable to high concentrations of chloride and dissolved solids during lowflow conditions and the effect of basin geology on the chemical composition of sediment and water [42]. This finding is consistent with other studies that have examined the impact of anthropogenic sources and geological factors on electrical conductivity in water bodies. For instance, according to a study conducted in the United States, salinization and alkalinization, which are linked to elevated EC levels, are caused by anthropogenic inputs of ions containing strong bases and carbonates and accelerated weathering assessed the physicochemical parameters of lake water in Bangladesh and found that EC levels were within the acceptable range established by the Bangladesh Department of Environment [43]. However, a Nigerian study of the physicochemical parameters of well water found that EC levels were within the World Health Organization's (WHO) allowable limits, with the exception of certain locations [44]. In addition, studies on zooplankton diversity and geoelectrical monitoring focused on distinct aspects of EC and did not explicitly address spatial variations in EC concentrations in river basins [45]. Limitations of the study include its focus on a specific river basin, which may restrict the applicability of its findings to other regions. Figure 3 displays the spatial distribution of the DO, BOD 5 and COD in the area under investigation. In accordance with the QCVN 08-MT:2015/BTNMT guideline values, the DO concentrations must be greater than 4 mg l −1 . During the rainy and dry seasons, the water samples in the Bach Dang river basin had the values ranging from 7.3 to 8.6 mg l −1 and 6.9 to 8.4 mg l −1 , respectively, which are within the acceptable range. Throughout the rainy season, the DO concentrations were the greatest in the northwest and southeast, and lowest in the certain regions. During the rainy season, the locations with the highest values were the QN01, QN15, QN18, and QN19; during the dry season, these areas were the QN01, QN02, QN12, QN15, QN18, QN19, and QN20. These regions are impacted by an estuary (e.g., QN01) that result in the mixing of fresh and saltwater, causing turbulence and oxygen diffusion into the water from the atmosphere. Certain regions are additionally affected by streams (e.g., QN18, QN19) and overflow dams (e.g., QN20). The QN03 is located close to the confluence of the Rut and Bach Dang rivers, and the QN12 is the point at which wastewater enters the lake, which runs into the river. In addition, the region near a sand mining facility (QN15) had greater DO concentrations than other regions. Many reasons cause the DO value of lake and river water to be higher during the rainy season and lower during the dry season. There is more precipitation during the rainy season, which increases the water level and flow rate of rivers and lakes. This increased flow velocity aids in water mixing and increases the DO concentration. In addition, the colder temperatures of the rainy season might boost the DO value [46]. In contrast, there is less precipitation during the dry season, which might result in decreased water levels and flow rates. The findings are consistent with those of a study conducted under tropical circumstances [47]. According to that study, the increased temperature seen in this investigation contributed to the decrease in dissolved oxygen levels in the water samples throughout the dry season. This study's findings are congruent with those of another study. Lower quantities of the DO in river water samples collected during the dry season may be related to nitrification activity that happens all along the river's path [48].

DO, BOD 5 and COD
On the map indicated in figure 3, the regional distribution of the BOD 5 and COD concentrations are depicted. Clearly, a portion of the samples exceed the concentration threshold defined by the QCVN 08-MT:2015/BTNMT. Particularly, the BOD 5 levels of samples obtained at sites QN06 and QN17 during both the rainy and dry seasons, as well as at the point QN05 during the dry season, were significantly higher. Only one of the samples, the QN17, has the COD level that exceeds the allowable limit. The BOD 5 and COD concentrations of the remaining samples are all below the acceptable limits of 15 mg l −1 and 30 mg l −1 , respectively. The southern and southeastern areas of the Bach Dang river basin include the highest COD and BOD 5 values, whereas the western, southwest, north and northeast contain the lowest quantities. The concentrations of the COD and BOD 5 are greater during the dry season than the rainy season. These findings are comparable to those of previously published studies, the COD and BOD 5 concentrations are higher during the dry season than the rainy season [20,49]. In a while, there are variances in the results based on the region and water source under investigation. According to one study, the BOD 5 was higher during the dry season, whereas the COD and DO were higher during the rainy season [31]. Another study discovered that the concentration of the COD was greater during the rainy season than the dry season [50].
Greater COD and BOD 5 concentrations in the southern and southeastern portions of the Bach Dang river basin can be attributed to a variety of factors. First, these places have a higher population density and concentration of industrial and commercial activity, leading to an increase in the anthropogenic pollution inputs into the river system. The use of fertilizers and other agricultural chemicals contributes to higher COD and BOD 5 concentrations in the southern and southeastern regions of the basin, where agricultural operations are more prevalent. In addition, the QN05 sampling site is situated in a region that receives both home wastewater and slaughterhouse effluent. The river estuary location of the BN06 puts it vulnerable to the effects of coal port operations and shipping traffic. The QN17 is situated in a lake region that collects both domestic and agricultural wastewater. Consequently, the COD and BOD 5 concentrations in these places surpass the allowable limit. Surprisingly, the observed increase lacks statistical significance.
The study's main result is that the DO concentrations in the Bach Dang river basin were within the permissible range (>4 mg l −1 ) durng both the rainy and dry seasons. Nonetheless, DO concentrations varied spatially, with the maximum levels observed during the rainy season in the northwest and southeast. Certain regions had reduced the DO concentrations, influenced by estuaries, streams, overflow dams, and a sand mine [51]. Additionally, the study found that the rainy season had higher DO levels than the dry season. This is a result of the rainy season's increased water levels, flow rates, water mixing, and reduced temperatures. In contrast, dry season DO levels were lower due to decreased water levels, flow rates, and increased nitrification activity [51]. In addition, the DO concentrations, the BOD 5 and COD values in the Bach Dang river basin were also investigated. The southern and southeastern regions of the basin had higher levels of the BOD 5 and COD than the western, southwest, north, and northeast regions. Consistent with previous research, the BOD 5 and COD concentrations were higher during the dry season compared to the rainy season. In certain regions, elevated BOD 5 and COD levels were caused by factors including a greater population density, industrial and commercial activities, agricultural operations, and pollutant inputs. Specific sites were impacted by domestic wastewater, slaughterhouse effluent, coal port operations, and shipping traffic, resulting in increased BOD 5 and COD concentrations [51]. This study's findings are consistent with other published research in the field. For instance, a study on sedimentation and water quality deterioration in the Terengganu River Basin in Malaysia discovered similar seasonal patterns of higher sediment and turbidity levels during the dry season compared to the rainy season [52]. Wahab et al (2019) [52] ascribed these results to human activities and unsustainable development management in the river basin. Using the SWAT model, a second study on the effect of hydrological conditions on phosphorus loads in an agricultural river basin found that the quantity and temporal distribution of precipitation influence nutrient pollution processes. From a rainy year to a dry year, the total phosphorus concentration in the examined river decreased, which is consistent with the findings of the Bach Dang river basin study. Cui et al (2017) [53] emphasized the significance of considering climate and geographic conditions when modeling nutrient loading in river basins. In addition, a study on nutrient budgets for the Red River basin in Vietnam and China revealed that the upstream Seine river basins and the Mississippi river basin are examples of autotrophic systems with moderate population densities that export substantial agricultural products. This is pertinent to the Bach Dang river basin study because agricultural operations were also identified as contributing to elevated BOD 5 and COD levels in certain regions [54]. Figure 4 demonstrates the geographical variation of ammonium, phosphate and coliform concentrations in the surface water of the Bach Dang river basin. According to the QCVN 08-MT:2015/BTNMT, the guideline limits for ammonium and phosphate are 0.9 and 0.3 mg l−1, respectively. The quantities of ammonium in the surface water of the Bach Dang river basin were continuously lower than the allowable limits. Ammonium concentrations were higher during the dry season than the rainy season. Moreover, the north, northwest, west and southwest which had low ammonium concentrations, the southeast portion of the research area had high ammonium concentrations. During the rainy season, the phosphate concentrations were typically below the allowed the level of 0.3 mg l−1 in all regions, however during the dry season, certain sampling locations surpassed the limit and higher in the west (QN14, QN14), southeast (QN03, AN05), east (QN07) and northeast (QN09). Specifically, during the dry season, sampling locations QN01, QN03, QN04, QN06, QN07, QN09, QN11-QN14, QN16, QN17, QN22, and QN23 had higher phosphate concentrations than allowed. In the estuary of the Bach Dang river, where it meets the sea, and at the junction of the Da Bach river, the phosphate concentrations were high (QN04). These regions are utilised for aquaculture (many households in the QN03 raise animals) or are impacted by coal mining and domestic wastewater operations (QN06, QN07, and QN09). In addition, the locations with heavy boat traffic, coal ports (QN01, QN11-QN14, QN16, and QN23), and tourism activities (QN22) were found to have the high phosphate concentrations. The coliform index remained below the permissible level during both seasons, with higher levels observed during the dry season. The coliform counts were higher in the northwest and northeast regions, while they were lower in the central, northern, and partially eastern regions.

Ammonium, phosphates and coliforms
Our results indicate that the sources and variables influencing the distribution of the ammonium and phosphate concentrations in the surface water of the Bach Dang river basin vary regionally and are influenced by seasonality. This is because, during the dry season, the river's water flow is often reduced, which reduces the diluting effect of pollutants such as the ammonium and phosphate, resulting in higher concentrations. According to a previous report, the concentrations of ammonium and phosphate were higher during the dry season than the rainy season [55][56][57]. This is also due to a combination of causes, including initial flushing runoff from human and animal waste and spring fertiliser application [57]. Some studies [57,58] indicate that the concentrations of ammonium and phosphate are influenced by the rainy and dry seasons. Less precipitation occurs during the dry season, which might lead to a concentration of these ions in the water. In contrast, during the rainy season there is greater precipitation, which might dilute and wash away these ions. In addition, the usage of fertilisers and discharge from human and animal waste can lead to an increase in the ammonium and phosphate during the change from a dry to a rainy climate [57]. Figure 4 also exhibits the spatial distribution of coliforms in the research area's surface water. Throughout both the rainy and dry seasons, the coliforms index in the Bach Dang river basin was below the permitted requirements, as stipulated by the QCVN 08-MT:2015/BTNMT (7500 MPN/100 mL). Nonetheless, the coliforms index is still greater during the dry season than the rainy season. This indicates that the quality of the water in the Bach Dang river basin is within the permitted level for human consumption. Due to many causes, the coliforms index is still higher during the dry season than the rainy season. There is less precipitation during the dry season, and the temperature, Sunlight intensity, and wind speed are all high. The higher temperature offers numerous fecal coliforms with a warmer environment in which to thrive. During the dry season, there is also a large amount of particle discharge into rivers.
The study on the Bach Dang river basin found that the distribution of ammonium, phosphates, and coliforms varied by region and season. The ammonium levels were higher during the dry season compared to the rainy one, but they were constantly below the allowed limits. While the north, northwest, west, and southwest had lower quantities of ammonium, the southeast had higher concentrations. throughout the rainy period, phosphate concentrations were often within the permissible level; but, throughout the dry season, the limit was surpassed in some sample locations, notably in regions affected by varied anthropogenic activities. High phosphorus concentrations were found in the Bach Dang River estuary. Although it was greater during the dry season, the coliforms index remained below the allowed level during both seasons. Less precipitation, higher temperatures, more Sunlight, faster winds, and significant particle discharge into rivers were all blamed for the dry season's elevated coliforms index. The results of the investigation agree with earlier studies in the area. A study on the kinetics of iron oxidation and phosphate immobilization along the flow-path from groundwater to surface water was done by Grift et al (2014) [59]. They observed that groundwater exfiltration, whether shallow or deep, could be the source of dissolved-phosphate loading in surface waters. This suggests that the flow of groundwater and its interactions with the environment can have an impact on the presence of phosphates in surface water. Another study on the distribution of dissolved trace metals in the Southern Ocean and the Southeast Atlantic found nutrient-like profiles for cadmium (Cd), comparable to phosphate, and nutrient-like profiles for copper (Cu) and silver (Ag), similar to silicic acid. This suggests that other trace metals' behavior and associations with nutrient-like profiles may have an impact on the distribution of phosphates in surface water [60]. Additionally, the heavy metal pollution index analysis can be used to assess the characteristics of surface water quality, according to study on heavy metal pollution in surface and spring water close to a limestone mining area. This implies that the evaluation of pollution indexes might offer insightful information on the overall pollution brought on by different contaminants, such as phosphates, in water sources [61]. Figures 5 and 6 depicts the regional variation of heavy metals during the rainy and dry seasons. The interpolated spatial variation map of heavy metals reveals that the majority of heavy metals, including the Zn, Pb, Cr, As, Fe, and Mn, are below the permissible limit in accordance with the QCVN 08-MT:2015/BTNMT, with the exception of Mn in samples QN08, QN09, QN12, QN16, QN19, and QN23, which exceed the permissible level (0.5 mg l −1 ). The Zn index is high in the north and gradually falls towards the northwest, whereas the As index is low in the north and gradually increases towards the southeast during both the rainy and dry seasons. In contrast, during both seasons, the Fe, Pb, and Cr indices are all extremely low and within the acceptable range.

Heavy metals
During the dry season, the Pb, and Cr indices are low in the north, northeast, northwest, west and southwest sections of the Bach Dang river basin in Quang Ninh province, whereas they are elevated in the southeast, southeast and the part of the southwest regions. The changes in heavy metal concentrations can be attributed to the precipitation, soil properties, and human activity, among other variables. Heavy metals may be transported by the precipitation during the rainy seasons, which could explain for their low concentrations. During the dry season, however, a lack of precipitation and an increase in human activities may contribute to increased heavy metal concentrations in certain locations. Figure 6 depicts the distribution of the Fe and Mn in the basin of the Bach Dang river. The results indicate that the Fe concentration is higher during the dry season than the rainy season, with the highest concentration appearing during the dry season in the southeast. In the meantime, the Mn is primarily concentrated in the northwest (the dry season) and northeast, as well as in the north to a lesser extent (the rainy season).
Many environmental and climatic factors may influence the geographical fluctuation of heavy metals, such as the Zn and As, during the rainy and dry seasons. During the dry season, the east, southwest, south of the Bach Dang river basin may have the lower Zn index due to factors such as the decreased water flow and higher sedimentation, which can contribute to the accumulation and retention of heavy metals. In contrast, the higher Zn index in the northwest may be the result of increased human activity, such as mining and industrial activities, which can release heavy metals into the environment. During the rainy season, the higher As index in the southeast may be the result of enhanced leaching and erosion produced by heavy rainfall, which can mobilize and transport heavy metals downstream. The lower As index in the north and northeast may be attributable to dilution produced by greater water flow from upstream regions or changes in the local geology and soil composition. In addition, due to different hydrological and climatic conditions, the factors that determine the spatial distribution of heavy metals during the rainy season may differ from those during the dry season [62]. The Mn, Zn, Cu, Cr, Cd, and Pb are some of the common heavy metals found during the rainy season [63].
Concentrations of heavy metals such as the Hg, Cd, Zn, Pb, As, Cu and Cr have been measured in the surface water and sediment [64].
During the dry season, the elevated levels of the As, Pb, and Cr indexes in the south and southeast in the regions of the Bach Dang river basin could be attributed to anthropogenic activities such as industrial and agricultural practices, the discharge of untreated sewage and wastewater, and transportation activities in this region. These actions can lead to the release of heavy metals into the environment, which can accumulate over time in the water and soil, resulting in increased heavy metal concentrations [65][66][67]. On the other hand, the low levels of the As, Pb, and Cr indices in the noth of the Bach Dang river basin in Quang Ninh province, Vietnam during the dry season may be the result of numerous causes, including fewer human activities, less industrialisation, and fewer agricultural practices. The low levels of precipitation during the dry season may also contribute to the low levels of heavy metal concentrations, as a rainwater may not be present to flush the environment of heavy metals [68,69]. The heavy precipitation can result in the leaching of heavy metals, and the environmental impact of intensifying precipitation may be exacerbated by ongoing climatic change [68]. The summer's high warmth increases evaporation, which enhances the amount of metal ions settling from the water column into fluvial sediments and raises the concentration of metal [70].
As can be seen that the majority of heavy metals, including the Zn, Pb, Cr, As, Fe, and Mn, are below the permitted levels according to the QCVN 08-MT:2015/BTNMT recommendations, with the exception of Mn in certain samples that exceed the limit, according to the study's main results on the Bach Dang river basin. Regional differences can be seen in the distribution of heavy metals throughout both the rainy and dry seasons. Changes in heavy metal concentrations are influenced by elements like precipitation, the characteristics of the soil, and human activity. During the rainy season, heavy metals may be carried by precipitation, resulting in reduced concentrations. On the other hand, during the dry season, a lack of precipitation and increased human activity may cause higher concentrations in some areas. Seasonal fluctuations can be seen in the concentrations of Fe and Mn, with the Fe concentrations being higher during the dry season and the Mn concentrations being higher in the northwest during the rainy season. The geographical fluctuation of heavy metals is influenced by environmental and meteorological factors. In some areas during the dry season, high levels of heavy metals are caused by anthropogenic activities such as industrial and agricultural practices, untreated sewage and wastewater discharge, and transportation-related activities. These results are in agreement with those of other research studies in the area. For instance, a study on heavy metal contamination in sediments and marine species was carried out in Daya Bay, China [71]. The findings showed that anthropogenic activities had an impact on heavy metal concentrations and that these variations were regionally distributed [71]. This highlighted the ecological and health problems related to heavy metal pollution in the maritime environment. Similar research looked at heavy metal pollution in plants and water in Agodi Reservoir, Ibadan, Nigeria [72]. The results of the study, which revealed that certain cases surpassed the allowable limits for the concentrations of heavy metals in both water and plants, highlight the necessity of monitoring heavy metal pollution and its possible effects on ecological resilience and human health [72]. Additionally, a study looking at the levels of heavy metals and risk assessment in edible fish from the Persian Gulf found considerable regional variability in heavy metal concentrations [73]. Management procedures, wastewater discharge, and industrial activity are some of the causes of these variances. The study highlighted the ongoing requirement for constant monitoring and management of heavy metal pollution in aquatic ecosystems. Figure 7 illustrates the WQI spatial variation map. During both the rainy and dry seasons, the WQI of the Bach Dang river basin is noted to be the highest in the basin's centre, northeast, the part of the southeast and northwest regions and the lowest in its southeast and southern regions. The WQI values for all samples collected in the region varied from medium to good to excellent. Even though, the WQI values are greater during the rainy season and lower during the dry season. These results suggest that the area's water sources are still within acceptable limits and can be used for an irrigation or aquaculture without a risk. The autumn has the greatest WQI values, followed by the spring and summer, with the winter having the lowest [74].

Water quanlyty index
Many factors, including differences in land use, human activity, and terrain, are likely responsible for this pattern of the WQI fluctuation. Compared to the centre, northeast, part of the southeast and northwest and its southeast and southern sections of the basin, there may be less human activities and fewer sources of pollution in the basin's central region. In addition, the basin's geology may play a role, since the middle region may have greater natural filtration and the retention of water in the southeast and southern sections, which could lead to higher water quality. Many variables may explain why the WQI levels are higher during the rainy season and lower during the dry season. During the rainy season, there is an increase in surface runoff, which can dilute pollutants and improve the quality of the water overall. During the dry season, however, there is less surface runoff and the water becomes more sluggish, resulting in increased the concentrations of contaminants [75]. This can result in the greater WQI index during the rainy season than the dry season [74]. Moreover, during the dry season, water abstraction for agricultural and industrial reasons can further diminish the volume of available water, resulting in increased pollution levels [75]. The observed changes in the WQI values between the rainy and dry seasons in the Bach Dang river basin may be attributable to these variables.
The main results of the study on the Bach Dang river basin's WQI point to seasonal and geographic fluctuations in the quality of the water. With higher values in the center, northeast, portion of the southeast, and northwest regions, and lower values in the southeast and southern regions, the WQI values vary across the different basin regions. Overall, the WQI values for all the samples that were taken in the area show that the water sources are risk-free, within acceptable bounds, and suitable for aquaculture and irrigation. In general, the WQI values are greater in the rainy season and lower in the dry season. The variations in the WQI are influenced by several variables, including land use, human activity, geography, and the basin's geology. Because there are fewer sources of pollution and better natural filtration in the central part of the basin, the water quality is better there. During the dry season, water volume is further reduced by water abstraction for industrial and agricultural uses, which raises pollution levels. These results are consistent with earlier research in the area. In this regard, the WQI technique was utilized in a study by Bytyçi et al (2018) [76] to evaluate the surface water quality in the Lepenc river basin. The study's findings showed that the WQI values varied within the basin indicating different water quality levels. The study showed the usefulness of using the WQI for categorizing water quality and the influence of anthropogenic pollution causes on surface water quality. The study by Gossweiler et al (2019) [77] examined spatial and temporal variations in water quality and land use in a semi-arid catchment in Bolivia. The WQI was used in the investigation to evaluate changes in water quality. The results demonstrated geographic diversity in the WQI values, determined by climate and land use practices. The study stressed the importance of implementing good management practices to protect the catchment area's water quality. In addition, Zhu et al (2022) [78] studied the characteristics of water quality changes in the middle and lower portions of the Lijiang River in China. The investigation used statistical techniques and data on water quality to examine the relationship between water quality indicators and environmental conditions. The results showed how environmental factors affect water quality and emphasized how crucial it is to understand these links to manage water resources effectively.

Correlation coefficient
Throughout the rainy and dry seasons, the correlation matrix demonstrates the interrelationships between physicochemical variables of surface water in the Bach Dang river basin (table 3). With values ranging from −1 to 1, the matrix gives information on the strength and direction of the relationships between variables. A number of −1 indicates a perfect negative correlation, whereas a value of 0 indicates no correlation, and a value of 1 indicates a perfect positive correlation.
During the rainy season, a number of variables exhibit robust associations. For example, the Zn has a moderately positive connection with the As and a large positive correlation with the Mn (0.84). (0.31). The Pb has a somewhat positive association with the ammonium (0.53) and phosphate and a high positive correlation with the Cr (1.0). (0.6). The COD has a somewhat positive association with the coliforms and a significant positive correlation with the BOD 5 (0.82) and (0.29). In addition, a significant negative correlation between the pH and alkalinity during the rainy season (−0.68) shows that these two variables have a strong inverse relationship. In other words, the water's alkalinity increases as its pH drops. This could be due to a number of variables, such as increased acid deposition during the rainy season or environmental changes that alter the chemical makeup of the water. A moderately positive connection between the DO and temperature (0.24) shows that these two variables are related. In this instance, it signifies that the concentration of dissolved oxygen in the water may increase to some degree when the temperature rises. However, other elements, such as a depth, flow velocity, and turbidity, may also influence the DO levels.
During the dry season, the results indicate that there are substantial correlations between numerous factors in the surface water of the Bach Dang river basin. The inverse relationship between the Pb and pH (−0.74) suggests that the content of lead in water increases as its pH falls. This may be related to the fact that lower pH levels can allow lead to leach from pipes and other items in contact with the water. The significant positive correlation between the Pb and Fe (0.9) shows that these two metals may have comparable origins. The somewhat positive connection between the As and BOD 5 (0.66) and COD (0.61) suggests that organic contamination may exist in the water. This may be the result of agricultural or industrial pollution in the nearby area. The weakly negative connection between the ammonium and DO shows that when the concentration of ammonium in water rises, the concentration of the DO falls. This may be because the ammonium serves as a fertiliser for algae and other microbes, which require an oxygen during respiration. The moderately positive connection between the ammonium and phosphates (0.27), and the weakly negative correlation between the ammonium and nitrate (−0.18), indicate that agricultural or sewage runoff may be present in the area, as these nutrients are typically connected with these sources. Lastly, the considerable negative correlation between the pH and alkalinity (−0.46) suggests that when the pH of water decreases, so does its alkalinity. This could be due to acidic compounds in the water or a lack of buffering capacity.
Using the correlation matrix, we can determine which variables are connected and which are not. This information can be utilized to comprehend the processes affecting the water quality in the Bach Dang river basin. The significant link between the Zn and Mn during the rainy season, for instance, suggests that these two metals may have a common source, such as mining or industrial operations. The correlation between the Zn and Mn during the rainy season suggests that these two metals may share a source, such as the mining or industrial processes [79,80]. The temperature, soil characteristics, and fertilisation can affect the concentration of these metals in sediments and soils [81,82]. Similarly, the significant link between the Pb and Fe during the dry season suggests that these two metals may have a source, such as vehicular or industrial activity. The high correlation between the Pb and Fe during the dry season indicates that these two metals may have a common source, such as a vehicle or industry. Seasonal variation and pollution can affect the concentration of these metals in sediments, water, soil, and crops activity [83][84][85]. In general, the correlation matrix can shed light on the causes and potential consequences of changes in water quality characteristics in the Bach Dang river basin.

Conclusion
Using physical, chemical, and biological parameters as well as heavy metals, the study evaluated the water quality of the Bach Dang river basin in Vietnam during the rainy and dry seasons. The study determined that the water quality measurements collected during the rainy season could be used to assess the water quality, and that the levels of heavy metals in the surface water were below the WHO and Vietnam goverment's allowable levels for use in drinking water during both seasons, indicating that the water is still safe for the consumption. However, continual monitoring is required to guarantee the safety of the population's water supply, as the presence of heavy metals in water may still pose long-term health hazards. In addition, the study revealed that agricultural runoff, industrial fossil fuel emissions, and the geology of the basin contributed to the decline in the river water quality. The results demonstrated the importance of continuous monitoring and management of water quality in the Bach Dang river basin in order to maintain the acceptable levels of contaminants. The study also examined the links between physicochemical variables of the surface water during the rainy and dry seasons, which could provide significant information regarding the origins of these variables and the potential repercussions of changes in water quality features. Further research could concentrate on the consequences of long-term exposure to heavy metals and the development of appropriate monitoring and management systems to assure the safety of the water supply for varied applications. This study's focus on surface water, which misses a thorough assessment of groundwater dynamics and long-term trends in contaminant concentrations, is one of its limitations. More research is required to evaluate the broader effects of pollution concentrations and investigate the linkages between surface water and groundwater systems.