Assessment of water quality, heavy metal pollution and human health risks in the Canal system of Ho Chi Minh City, Vietnam

This study was conducted to determine the surface water quality and health risks in Ho Chi Minh City (HCMC) canals. 180 water samples and 180 sediment samples were collected from 15 canal locations in HCMC in 2021 and 2022. The Water Quality Index (WQI) assessment results indicated that the water quality in 2021 ranged from unsuitable to good, with a trend towards improvement in 2022, where good quality water was predominant. TMs PCA/FA identified domestic and agricultural wastewater from HCMC residents as influencing the water quality. Most TMs detected in surface water were within the limits the Vietnamese Ministry of Natural Resources and Environment allowed, except for Pb in 2022 (> 0.02 mg l−1). For sediments, TMs concentrations were higher than in water samples but showed a decreasing trend over the survey period in the order of Hg < Cd < As < Pb < Cu. The findings show that sediments are more strongly affected by TMs than surface water, with the main sources being industrial and agricultural human activities. The non-cancer risk assessment showed that children are more exposed to TMs than adults, mainly through ingestion. Additionally, the cancer risk assessment (CR) identified As in sediments as posing an unacceptable cancer risk (TCR > 1 × 10−4). Therefore, it is necessary to establish high-frequency monitoring policies to analyze and reduce TMs concentrations in water and sediments of the canals to protect human health.


Introduction
Surface water is essential for human activities and the ecosystem of flora and fauna on Earth.However, surface water quality globally is being severely impacted by pollutants from human activities such as domestic life, industrial and service developmentK.These pollutants in the water environment can settle and accumulate in sediments in canals and rivers.Sediments comprise various components, including minerals, organic debris, and other forms.Trace metals (heavy metals) (TMs) are natural components of minerals that can attach to suspended particles, forming complex bonds with other compounds, leading to their deposition and absorption in sediments.In addition to being poisonous and non-biodegradable, TMs can build up in the food chain (Varol and Tokatlı 2023).Consequently, the sediments of aquatic ecosystems can accumulate significant amounts of pollutant TMs, posing health risks to humans through eating fish, drinking water, coming into contact with water on the skin, and consuming agricultural products irrigated with contaminants (Usman et al 2021, Varol and Tokatlı 2023).Studies on TMs in water and sediments have been conducted in rivers worldwide, like Bangladesh (Ali et al 2021), Iran (Aghadadashi et al 2019), and Turkey (Omwene et al 2018, Varol et al 2021, Varol and Tokatlı 2023).These studies reported the detection of heavy metals in water and river sediments but were limited to determining metal concentrations and assessing ecological risks.Previous studies did not address the correlation between different TMs or evaluate human health risks.Therefore, the current study has undertaken this entire domain.Nowadays, methods like correlation analysis and principal component analysis (PCA)/factor analysis (FA) are commonly used to identify pollution sources in the environment (Dey et al 2021).These methods facilitate quantifying and identifying pollution sources, helping managers to implement appropriate remedial measures timely.Besides identifying TMs pollution sources in the environment, conducting health risk assessments related to TMs due to potential environmental impacts is crucial (Islam et al 2020, Setia et al 2020).Risk assessment is a methodological approach for identifying, characterizing, and analyzing hazardous elements to qualitatively assess risks based on adverse impacts and quantitatively measure risk levels (USEPA 1989).Commonly used indices in human health risk assessment due to TM pollution include Consumption Daily Intake (CDI), Hazard Quotient (HQ), Hazard Index (HI), and Cancer Risk (CR), as observed in studies on rivers like Pardo in Brazil (Alves et al 2014), Ganges in India (Mitra et al 2018), Ajay in India (Singh and Kumar 2017), Sutlej in India (Setia et al 2020), and rivers in Bangladesh (Islam et al 2020).
Currently, the quality of surface water in Vietnam is strongly affected by urbanization and industrialization.In particular, Ho Chi Minh City (HCMC) has a relatively dense canal network that receives most of the waste from local residents' daily activities, agriculture, and industry.There have been some studies on assessing river water quality in HCMC (Strady et al 2017, Nguyen et al 2019a, Vu et al 2022), but the water quality in the canal system still needs to evaluated.Khoi et al (2022) used 10 parameters to calculate the Water Quality Index (WQI).However, their study was limited to only four locations along the La Buong River, a tributary of the Dong Nai River.
Meanwhile, the canals in HCMC, which directly receive human waste from the districts before flowing into rivers, have yet to be studied for WQI assessment in HCMC.Additionally, until now, the pollution of trace metals (TMs) and the potential health risks associated with water and sediments in the canals of HCMC, Vietnam, have yet to be researched.Therefore, (1) assessing water quality and sources of pollution; (2) determining the concentrations and sources of TMs pollution in water and sediments; and (3) evaluating the human health risks associated with exposure to TMs in water and sediments of the canals in HCMC are urgent.The results of this research will provide a database to help managers propose pollution mitigation measures suitable for the environmental status of HCMC's canals and ensure the health of residents.

Study area
Ho Chi Minh City (HCMC), located at 10°10'-10°38' North and 106°22'-106°54' East, lies in the transitional region between the Southeast and Southwest of Vietnam, covering a total area of 2095 km 2 (ESCAP 2018).It is the largest city in Vietnam in terms of population and urbanization scale.In 2023, HCMC's population was 9.3 million, with an estimated increase to 12.2 million by 2035 (World Population Review).HCMC is an industrial center, contributing one-third of the national GDP from this urban area.However, canal water pollution in HCMC is a major issue due to the lack of waste treatment services and outdated wastewater treatment systems, with most domestic wastewater being discharged directly into the canals.According to the report from Ho Chi Minh City Environmental Protection Agency (HEPA, http://www.hepa.gov.vn), about 150,000 m 3 of industrial wastewater, 17,000 m 3 of hospital wastewater, 500,000 m 3 of domestic wastewater, 400-500 tons of solid waste, and 300 tons of human waste are discharged directly into HCMC's canals daily (Thi Van Ha et al 2008).Therefore, the study focuses on the canal system in HCMC (figure 1).

Sample collection
Water and sediment samples were collected from 15 canal locations in Ho Chi Minh City (HCMC) as depicted in figure 1, including Thi Nghe 2 (C01), Dien Bien Phu (C02), Hai Duc (C03), Le Van Sy (C04), Cau So 1 (C05), An Loc (C06), Tham Luong (C07), Hoa Binh (C08), Ong Buong (C09), Cau Mong (C10), Cau Chu Y (C11), Cha Va (C12), Rach Ngua (C13), Phu Dinh (C14), and Nhi Thien Duong (C15).15 canals locations were chosen because they cover most districts of HCMC, representing the environmental characteristics of the entire area.Surface water samples were collected from 0-50 cm depth of the canals using a Van Dorn sampler (Nguyen et al 2019b).Two liters of water from each location were collected into high-density polyethylene (HDPE) plastic bottles.Before sampling, the bottles were sterilized, rinsed with dilute nitric acid (HNO 3 ), and twice with distilled water.Moreover, these plastic bottles were rinsed with canal water for sampling.The lids were wrapped tightly with Parafilm to prevent air contact with the sample.15 canal locations were sampled over two years, 2021 and 2022, with six sampling rounds each year, including in the months of February, April, June, August, October, and December (a total of 180 samples).The water samples were collected and transported to the laboratory in iceboxes, maintained at 4°C until analysis.The methods used in this study for sampling, storage, transportation, and testing of water samples were conducted as mentioned in the American Public Health Association (APHA) (APHA 2005).
Sediment samples from the canals (top 5 cm) were collected using a Petersen grab sampler (a total of 180 samples).The sediment sample at each location was placed in a plastic tray, mixed well, and then placed in a plastic bag (about 3 kg of sediment per bag).The plastic bags containing the samples were stored in a styrofoam box with ice (at approximately 4 °C) and transported to the analysis laboratory on the same day.

Analytical methods
Surface water samples were analyzed for concentrations of 11 physicochemical parameters including temperature, pH, Total Suspended Solids (TSS), turbidity, NH4 + _N, PO 4 3− _P, Dissolved Oxygen (DO), Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD 5 ), Coliform, and E. coli.Parameters measured in the field include temperature, pH using handheld device model Hanna HI8424 (Hana Instruments) and DO measured using handheld device model Hanna HI9142 (Hana Instruments).Turbidity was obtained using a handheld turbidity meter model Hach 2100AN (Hach Company, Loveland, CO).Total suspended solids (TSS) was determined by using the gravimetric method as stipulated by the APHA (1999) guidelines.NH 4 + _N and PO 4 3− _P concentrations were measured using spectrophotometry (Rice et al 2012).BOD 5 was determined after 5 days of incubation in the dark at 20 °C and COD was analyzed according to standard methods (Rice et al 2012).Coliform and E. coli was determined by membrane filtration method (Dean 1990).
For the analysis of Trace Metals (TMs) in water: Water samples, upon collection, were filtered through Whatman filter paper with a pore size of 0.45 μm and acidified with HNO 3 (to a pH < 2) before analysis (Jain et al 2005).Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Spectro Analytical Instrument GmbH, 47533 Kleve Germany) was used for the analysis of TMs.
For the analysis of TMs in sediments: Sediment samples were air-dried in the laboratory, avoiding direct sunlight.The dried sediments were then ground, sieved through a 2 mm mesh, and stored for TM analysis.0.2 g of sediment sample was directly digested in a solution of HNO 3 , HCl, and HF (5:4:1 v/v) at a temperature of 140 °C for 6 h.The digest was then filtered and diluted with 25 ml of distilled water.The diluted solution was analyzed for TMs concentrations similarly to water samples using ICP-OES (Carter and Gregorich 2008).
2.4.Health risk assessment 2.4.1.Non-cancer risk assessment Exposure assessment helps to categorize and determine the level of exposure to potentially harmful chemicals at a location.It considers the sources of emissions and the pathways of chemical transport into the environment, particularly the impact on humans.Exposure assessment is expressed as Consumption Daily Intake (CDI) through two main exposure routes: ingestion (Formula (1)) and dermal contact (Formula (2)), based on standards from the United States Environmental Protection Agency (USEPA) (USEPA 2021). Where: • CW is the concentration of each TMs (μg l −1 or μg kg −1 ), • IR is the ingestion rate (mg/day), • EF is exposure frequency (days/year), • ED is exposure duration (years), • BW is body weight (kg), • AT is average time (70 years × 365 days/year), • SA is skin area (cm 2 ), • ET is exposure time (hours/day), • CF is the unit conversion factor (1 × 10 −6 ), • and Kp is the dermal absorption factor of TMs (cm h −1 ) (USEPA 2004).
Human health risk assessment is represented as Hazard Quotient (HQ) by dividing CDI by the reference dose (RfD) (Eziz et al 2018).Thus, different risk assessment models vary depending on the exposure route, such as ingestion or dermal.HQ representing non-cancer risks is calculated as follows Formula (3) (Eziz et al 2018): Where: RfD ingestion and RfD dermal are the chronic reference doses for ingestion and dermal exposure routes, respectively (mg/kg/day) (Eziz et al 2018, IRIS 2019).
The Total Hazard Index (THI) is the sum of individual HIs for each environment (Formulas ( 4) and ( 5 If THI < 1: no significant impact on human health.THI > 1: potential for adverse health effects.

Cancer risk assessment
Cancer Risk (CR) for humans caused by different pathways (Formula ( 6)) (Setia et al 2020).The Total Cancer Risk (TCR) is obtained by summing the CR of two different exposure pathways (Formula ( 7)) (USEPA 2021).
2.5.Statistical analyses 2.5.1.Principal component analysis (PCA)/factor analysis (FA) PCA extracts eigenvalues greater than 1 and eigenvalues are a list of loadings.Factor analysis (FA) on PCA further reduces less significant variables by applying VARIMAX rotation and extracts a new group of variables, termed factors, from the covariance matrix of the original dataset.To clarify changes in surface water and sediment quality, separate PCA was performed for 2021 and 2022 to identify differences between the two study years.PCA was conducted for all 11 physicochemical water parameters, TMs in water and sediments from the 15 study locations using JMP Pro.16 software.Data assumption for multivariate normality was checked using Bartlett's test, which was performed to test the suitability of the data for structure detection.

Correlation coefficient
The Pearson correlation coefficient was used to calculate the relationships between water quality variables and TMs components, with a significance level of p < 0.01 considered.Statistical calculations were performed for all physicochemical parameters and TMs in water and sediments from the 15 study locations using JMP Pro 16 software.All graphics were created using Sigmaplot 16.0 software.

Water quality index (WQI)
The Water Quality Index was assessed using the equation defined by Cude (2001): Where: • V actual is the actual value of the water quality parameter obtained from laboratory analysis, • V ideal is the ideal value of this parameter in pure water (for most parameters V ideal = 0, except for pH = 7 and Dissolved Oxygen (DO) = 14.6 mg l −1 ) • V standard is the recommended standard value by the World Health Organization (WHO) for water quality parameters.
• Q i is the quality rating of the ith parameter among the total number of water quality parameters analyzed.
• Si is the standard permissible value for the ith parameter.

Results
3.1.Assessment of surface water quality in Ho Chi Minh city canals 3.1.1.Physicochemical and microbiological characteristics The water quality parameters from 15 canal locations in Ho Chi Minh City (HCMC) for the years 2021 and 2022 are presented in figure 2. The study results show that the average temperature of canal water in the HCMC area in 2021 was 30.57± 0.98 °C, not significantly different from 2022, which was 30.26 ± 1.00 °C across all 15 observation locations (figure 2).The canal water temperature is affected by point sources where wastewater with high temperatures mixes with river water.The water samples collected in 2021 and 2022 were slightly acidic, with pH values of 6.64 ± 0.37 and 6.84 ± 0.43, respectively.The Total Suspended Solids (TSS) concentration increased from 37.96 ± 25.7 mg l −1 (in 2021) to 39.39 ± 32.26 mg l −1 (in 2022).Notably, turbidity showed a significant increase from 16.33 ± 11.11 NTU (in 2021) to 52.07 ± 52.78 NTU (in 2022).Similarly, the average concentrations of NH 4 + _N, PO 4 3 -_P, and E. coli in 2021 were lower than in 2022, indicating an increasing trend in concentration.Conversely, the water samples in 2021 showed a decreasing trend in average concentrations of DO, COD, BOD 5 , and Coliform compared to 2022 (figure 2).

Sources influencing physicochemical and microbiological parameters in surface water
After obtaining the dataset on physicochemical and biological parameters, PCA/FA was applied to identify the links between the parameters and the pollution sources present in the canals of HCMC.PCA/FA provides information on the most significant parameters describing the entire dataset with the least primary information change.With eigenvalues > 1, PCA applied to two different datasets extracted three main factors for 2021 and two factors in 2022, explaining 92.78% and 85.90% of the accumulated variance of the entire dataset, respectively (table 1).Based on factor loading classification, water samples collected in 2021 included factor 1 (F1) explaining 68.32% of the total variance, indicating strong loading with temperature, turbidity, NH 4 + _N, PO 4 3− _P, DO, COD, and BOD 5 ; weak loading for the remaining parameters.The main pollution sources are from domestic wastewater and runoff from landfills.F2 explained 13.17%, strongly loaded with Coliform and E. coli, with medium and weak loadings for the other parameters, representing pollution sources like urban wastewater and agricultural activities such as livestock waste.F3, explaining 11.29% of the variance, described pollution from industrial activities with strong loading with pH and TSS, medium loading with turbidity, and weak loading with the remaining parameters.For water samples collected in 2022, F1 explained 65.00% of the variance and was strongly loaded with NH 4 + _N, PO 4 3− _P, DO, COD, BOD 5 , Coliform, and E. coli, medium loading with DO, and weak loading with temperature, pH, TSS, and turbidity.Pollution could be due to agricultural activities such as livestock waste and atmospheric deposition in the area.F2 explained 20.90% of the variance, strongly loaded with pH, TSS, turbidity, medium loading with temperature, DO, and weak loading with the remaining physicochemical parameters, indicating pollution from industrial activities.The PCA results indicate that physicochemical parameters impacting the quality of canal water in HCMC are due to human waste sources.

Correlation among water quality parameters
Pearson correlation is a commonly used statistical test method to determine the degree of association or correlation between variables.The statistical results of the physicochemical parameters of canal water in HCMC demonstrate a strong positive correlation of NH 4 + _N with PO 4 3− _P (r = 0.95), COD (r = 0.90), and BOD 5 (r = 0.92) (p < 0.01).PO 4 3− _P is strongly positively correlated with COD (r = 0.94) and BOD 5 (r = 0.95) (p < 0.01).Notably, COD parameter is found to have a strong positive correlation with BOD 5 (r = 1.0) (p < 0.001) (figure 3).The strong correlation among parameters is due to human discharge activities in the surrounding areas of the sampling locations.

Water quality index (WQI)
The Water Quality Index (WQI) in the study area varied depending on the location and year of sampling (table 2).The WQI values of samples in 2021 were higher than in 2022, indicating improving water quality.In 2021, poor water quality (locations C01, C02, C03, C04, and C10) and unsuitability (locations C09, C13, and C14) were more frequently detected than locations with good water quality.By 2022, good quality was detected in most sampling locations, except for locations C02 and C03, which still had poor quality.The survey results show that the water in this area is safe for processing for domestic purposes, irrigation, and industrial use.Although the water quality of canals in HCMC improved over the two years of study, due to the impact of industrialization and modernization processes, canal water in HCMC still poses a risk of pollution, affecting the quality of life of humans and wildlife.Therefore, waste management measures from domestic activities and economic development activities in the study area are needed.4 shows the TMs values in canal water in HCMC in 2021 and 2022.The concentration of Lead (Pb) (mean ± SE) was detected to increase significantly from 0.001 ± 0.003 mg l −1 (in 2021) to 0.041 ± 0.103 mg l −1 (in 2022).However, the concentration of Cadmium (Cd) in 2021 was 0.001 ± 0.001 mg l −1 , and no pollution of this TM was detected in 2022.Similarly, Chromium (Cr) and Copper (Cu) also showed a decreasing trend over two years, from 0.005 ± 0.002 mg l −1 to 0.003 ± 0.003 mg l −1 (for Cr) and from 0.006 ± 0.002 mg l −1 to 0.003 ± 0.003 mg l −1 (for Cu).
Changes in TMs concentrations in canal sediments over two years of survey (2021 and 2022) are presented in figure 5.The analysis results show that most TMs detected in sediments tended to decrease over two years of sampling, except for Cd.Specifically, the average concentration of Pb decreased from 31.447 ± 17.156 mg kg −1 to 25.103 ± 18.738 mg kg −1 , Cd −1 increased from 0.069 ± 0.07 mg kg −1 to 0.328 ± 0.298 mg kg −1 , C −1 u −1 decreased from 59.257 ± 31.872 mg kg −1 to 56.575 ± 80.524 mg kg −1 , Mercury (Hg) decreased from 0.05 ± 0.049 mg kg −1 to 0.011 ± 0.009 mg kg −1 , and Arsenic (As) decreased from 10.78 ± 7.148 mg kg −1 to 6.513 ± 8.122 mg kg −1 (figure 5).These TMs enter the canal system, leading to the risk of water and sediment pollution in the future.

Sources of TMs pollution in water and sediments
Table 3 presents the eigenvalues, % variance, and % cumulative of the first principal components (PCs) of TMs in water and sediments over two survey years (2021 and 2022).PCA/FA analysis shows that TMs have common pollution sources.For water samples collected in 2021, factor F1 explains 15.23% of the total variance, showing medium loading with Pb and Cd, and weak loading with Cr and Cu.F2 explains 2.24%, with weak loading of the detected TMs in water.In 2022, factor F1 explains 15.89% of the variance and shows medium loading with Pb, weak loading with the other TMs.F2 explains 2.45% of the variance and weak loading with all TMs (table 3).PCA/FA analysis indicates that Pb is the primary metal impacting water quality in the area, necessitating strict monitoring of emission sources such as household waste burning and industrial wastewater from factories.
For sediment samples collected in 2021, F1 explains 25.87% of the total variance and shows strong loading with Pb and Cu, weak loading with Cd, Hg, and As, with no medium loading detected.F2 explains 7.56% of the variance, medium loading only with Hg, and weak loading with the other TMs.In 2022, factor F1 explains 49.31% of the variance, with strong loading for Pb, Cd, and Cu; medium loading for Hg, and weak loading for As.The pollution source is identified from industrial processes such as metal plating, painting activities, transport equipment, machinery, ceramic glazes, etc.Meanwhile, factor F2 explains 29.71% of the variance and shows strong loading with Hg and As; medium loading with Cd; and weak loading with Pb and Cu.The study finds that agricultural fertilizers are closely related to TMs pollution.Additionally, TMs tend to increase due to the growing accumulation in the sediment layers of canals over the two-year survey in the study area.Therefore, timely and appropriate waste management measures are needed.

Correlation between TMs in water and sediments
The relationships between different TMs in water and sediments were determined using the Pearson correlation coefficient (figure 6).The analysis results show a strong positive correlation between Cr in surface water and Cu in water (r = 0.84) and Hg in sediments (r = 0.82) (p < 0.01), indicating similar sources for Cr, Cu, and Hg.However, there is a strong negative correlation between Cr in water and Cd in sediments (r = -0.83)(p < 0.01).Additionally, a strong positive correlation of Cu in surface water with Hg in sediments (r = 0.81) was observed (p < 0.01) (figure 6).No strong correlation was detected between TMs in water or between TMs in sediments, indicating that the pollution status in the study area does not originate from natural sources (e.g., bottom sediments) but mainly from human activities releasing pollutants.

Health risk assessment of TMs
Beyond identifying pollution concentrations, risk assessment indices help to enhance the reliability of TMs risks for water and sediment environmental management in HCMC canals.Therefore, non-cancer risk assessments through CDI, HQ (table 4), and THI (figure 7) values were calculated for children and adults in the water and sediment environments of HCMC canals in 2021 and 2022.In 2021, CDI ingestion for children and adults for TMs in canal water were determined in the order of Cu > Cr > Pb > Cd and in sediments as Cu > Pb > As > Cd > Hg.For CDI dermal of TMs in water and sediments in 2022, the order was Cu > Cr > Cd > Pb and Cu > As > Pb > Cd > Hg.In 2022, CDI ingestion in water for TMs was in the order of Pb > Cr > Cu > Cd and in sediments similar to 2021.CDI dermal in water and sediments followed the same order as CDI ingestion calculated in 2021.Total CDI values for TMs in water and sediments ranged from 3.32 × 10 -4-4.47 × 10 −2 mg l −1 day −1 and 1.16 × 10 −2 -6.49 × 10 1 mg/kg/day for children, while for adults, these values ranged from 1.42 × 10 −4 -1.92 × 10 −2 mg l −1 day −1 and 4.99 × 10 −3 -2.78 × 10 1 mg kg −1 day −1 .Total CDI values indicate that children are more exposed to TMs than adults, and ingestion exposure is identified to be higher than dermal.Also, the HQ assessment for children and adults shows that HQ ingestion > HQ dermal (table 4).Particularly, HQ ingestion for TMs in children was determined to be the highest in both water and sediment environments over two survey years.Except for As in sediments collected in 2022, which had the highest ingestion HQ for adults (1.02 × 10 4 ), HQ ingestion values for TMs in sediments in children were greater than 1, indicating potential health risks.THI values for children and adults ranged from 1.98 × 10 −3 -3.94 × 10 4 and 8.51 × 10 −4 -1.69 × 10 4 , respectively, and were much lower in water compared to sediments (figure 7).
Although the THI values for TMs in water collected in 2021 were less than the level impacting human health (THI < 1), there is a trend of increasing pollution in 2022, notably for Pb in children (THI = 12.8) and adults (THI = 5.48) (p < 0.05) (figures 7(a), (c)).Almost all TMs detected in sediments over two years of sampling for   .Particularly, children are more susceptible to health impacts from toxic chemicals in the environment, with THI values 1.68 times higher than for adults.
In addition to non-cancer risk assessments, cancer risk was also calculated through the Total Cancer Risk (TCR) value with the main exposure route being ingestion.The TCR calculation results indicate that As in the sediment environment poses an unacceptable cancer risk (TCR > 1 × 10 −4 ).TCR values for children and adults were 17.72 and 7.59, respectively, in 2021; 10.70 and 4.59 in 2022, demonstrating a higher health risk for children, 2.33 times greater than adults.

Surface water quality
Temperature is a crucial parameter for water quality management due to its impact on the variation of other physicochemical indicators.Additionally, the rate of chemical reactions generally increases at higher temperatures (Alam et al 2007).pH also plays a vital role in monitoring and assessing water quality due to its significant influence on biological and chemical processes in water bodies (Ahmed et al 2011).The pH limit according to Monre (2015), for water is 6-8.5, indicating that the pH levels of canal water samples in HCMC are within the recommended range.Previous studies in Lake Bogoria and Lake Nakuru in Kenya showed higher values than those in HCMC canals, possibly due to the use of nitrogen-based fertilizers on agricultural land (Jirsa et al 2013).However, an indicator of potential pollution is the high concentration of Total Suspended Solids (TSS) in the canal water samples in HCMC in 2022 (39.39 mg l −1 ), a sign of increasing suspended materials.High TSS levels cause water turbidity and disrupt the photosynthetic ability of aquatic plants (Goel 2000).The study results on HCMC canals align with other reports on widespread water environment pollution (Amadi et al 2006, Adekunle et al 2007).A study by Aftab et al (2011), on the water of the Lahore branch canal used for irrigation around the city, found most physicochemical parameters within standard limits, except for turbidity.Turbid water hinders recreational and aesthetic uses and can affect sensitive skin.NH 4 + _N represents 80% of the Dissolved Inorganic Nitrogen (DIN), with the highest values typically found in freshwater environments (Martin et al 2008).The spatial and temporal variations in NH 4 + _N concentration are due to the process of oxidation to other forms or the reduction of nitrate to lower forms in water (Sankaranarayanan and Qasim 1969).Moreover, Dissolved Oxygen (DO) is an important component of water, indicating current water quality and the balance of organisms in the water body.Elevated DO levels can result from high photosynthesis rates of phytoplankton, constituting the primary source of DO (Sharma and Rathore 2000).However, all water samples from HCMC canals showed low DO levels, leading to insufficient oxygen supply in the water environment.Decreased DO in water can cause aquatic species to die when oxygen is depleted due to microbial metabolism, increasing Biological Oxygen Demand (BOD) compared to the standard values of Monre (2015).
Elevated BOD and Chemical Oxygen Demand (COD) values compared to Monre (2015), due to organic and inorganic pollutants and oxygen demand in the water source indicate pollution (Adekunle et al 2007, Singh et al 2008).These factors demonstrate the impact of agricultural activities such as fertilization and erosion in highland areas during the rainy season (Shrestha and Kazama 2007).Additionally, high levels of Coliform and E. coli increase canal pollution in HCMC, particularly through the discharge of domestic wastewater and livestock activities.Therefore, using the Water Quality Index (WQI) to assess the quality of a specific water area helps managers easily gather information on the pollution status of the area.There is a significant difference in water quality in HCMC canals between 2021 and 2022 (table 2).The WQI calculation results show a positive change over two years, with most locations achieving good water quality in 2022.Other studies also assessed surface water quality through WQI, such as in the Gorganrood River, where only one location was categorized as average, while others showed poor water quality (Karbassi et al 2011).Bora and Goswami (2016), applied WQI calculations to the Kolong River in India, revealing very poor to unsuitable water quality at most of the 7 sampling points.Additionally, the Aksu River was assessed to have good water quality with calculated WQI values ranging from 35.61 to 337.52 (Şener et al 2017).However, poor and very poor water quality was also detected in the northern and southern basins of the river (Şener et al 2017).Mohammed and Abdulrazzaq (2018), used the WQI method to assess water quality at 11 locations along the Euphrates River at the Iraqi border in 2013-2014.The results showed that the water quality of the Euphrates River ranged from good to poor, receiving pollution loads from various sources such as domestic and industrial wastewater (Mohammed and Abdulrazzaq 2018).Varol (2020) showed the Sürgü Stream's water quality from good to excellent using WQI with significant.Based on current research results and global studies, the issue of ensuring water quality is global, necessitating unified management efforts and coordination with local businesses and residents.

TMs pollution and its sources
The TMs detected in the surface water of Ho Chi Minh City (HCMC) canals comply with Monre (2015), except for Pb in around 2023 (> 0.02 mg l −1 ).TMs significantly degrade water quality and have hazardous side effects on human and animal life.Based on the analysis of TMs concentrations in the water of HCMC canals (0-0.47 mg l −1 ), the levels are lower than those in the Sabarmati River and Kharicut Canal in Ahmedabad, Gujarat (below detection level-18.6mg l −1 ) (Kumar et al 2012), canals and the Reno River (0.3-5.7 mg l −1 ) (Ferronato et al 2013), but higher than the Oke-Afa Canal, Lagos Nigeria (0.029-0.069 mg l −1 ) (Aderinola et al 2012).Multivariate statistical methods such as PCA/FA are commonly applied in scientific studies to identify the origins of metal pollution in water and sediment environments (Hasan et al 2021, Ke et al 2017).The presence of TMs in HCMC canals is influenced by residential and small-scale commercial activities (gas stations and car exhaust) (Adekunle et al 2007).TMs in water settle into sediments, accumulating at high concentrations in the environment from various sources.TMs concentrations in HCMC canal sediments over two years of study comply with Monre (2017) for freshwater sediment quality.However, with economic development and increasing population, more TMs emission sources from human activities are expected.Cu (57,916 mg kg −1 ) was found in the highest concentration in sediment samples from HCMC canals, primarily from textile industries (Environmental Information Office 2001) and car lubricants (Fu et al 2014).Pb (28,275 mg kg −1 ) in canal sediments originates from leaded gasoline in transportation and manufacturing processes (Fu et al 2014).Similarly, TMs in Ismailia Canal sediments in Egypt showed decreasing concentrations in the order: Zn > Pb > Cu > Ni > Cr, with pollution sources from solid waste and untreated industrial wastewater (Ibrahim et al 2009).The Begej Canal contained high amounts of Cd, Cu, Zn, and Cr-contaminated sediments, but no increase in concentrations was found in reference samples, proving human-caused pollution (Dalmacija et al 2006).The current study shows higher TMs pollution in HCMC canal sediments compared to some canals in China (about 4 times) (Yu et al 2011), the Venice canal system, Italy (about 3 times) (Zonta et al 2020), Kharicut Canal in Gujarat (about 3 times) (Kumar et al 2012), and Tongi Canal, Bangladesh (about 5 times) (Zakir Hossen et al 2017).Overall, the presence of TMs in canal sediments is due to the cumulative impact of urban, agricultural, and industrial activities along the canals.

Health risks of TMs
Based on risk assessment guidelines (USEPA 2004), CDI, HQ, and HI values for TMs through ingestion and dermal routes for adults and children were calculated in HCMC canals, showing that ingestion is the primary exposure route.Children are more at risk due to higher exposure and sensitivity (Qu et al 2012).Beyond CDI and HQ, HI is an indicator of the non-cancer risk of TMs to health.The current study's THI assessment shows that TMs in sediments pose non-cancer health risks to children and adults (THI > 1), similar to previous findings in China (Eziz et al 2018), Turkey (Baltas et al 2020), and India (Nour et al 2022).
Cancer Risk (CR) is the likelihood of developing cancer over a lifetime due to exposure to carcinogens through ingestion and dermal routes (ATSDR 2012).The CR values of TMs in the current study (Pb, Cd, Cr, Cu, Hg, As) were evaluated for children and adults based on the acceptable cancer risk range from 1 × 10 −6 to 1 × 10 −4 (Rahman et al 2017).The TCR calculations show that exposure to TMs can cause serious cancer risks in children through ingestion, similar to sediments along the southern coast of China, accounting for 60.85% of TCR (Wang et al 2020).The study on HCMC canals shows an unacceptable cancer risk of As in sediments (TCR > 1 × 10 −4 ) due to aquaculture activities.Long-term exposure to As can cause cancer, skin damage, etc (He and Charlet 2013).Therefore, local residents, especially children, should be cautious of samples with high As content.Other TMs like Pb, Cd, Cr, Cu, Hg were found to pose no cancer risk to children and adults (TCR < 1 × 10 −6 ).Hence, there is a need for surveillance programs for As and other potential factors in the environment, combined with health education about As pollution in the future.

Conclusions
The findings demonstrate that using WQI to assess the quality of a specific water area helps managers quickly gather information on the pollution status of the study areas.Besides, the study results indicate positive changes in the quality of water in HCMC canals over two years of study (2021 and 2022).However, the values of water quality parameters such as TSS, NH 4 + _N, PO 4 3− _P, COD, BOD 5 , Coliform, and E. coli remain higher than MONRE standards, 2015, for surface water quality.Additionally, the current study's DO values are low, not in line with current standards.The main pollution sources are likely from residential and agricultural activities in the study area.The WQI calculations show that water quality in 2021 ranged from unsuitable to good, with a trend of improvement by 2022, but some locations still have poor water quality.For TMs in surface water samples, an increasing trend of Pb concentration was noted, while Cd, Cr, and Cu decreased over time in the order of Pb = Cd < Cr < Cu (in 2021), Cd < Cr = Cu < Pb (in 2022).For sediment samples, TMs concentrations were significantly higher than in water samples.However, TMs concentration tended to decrease over two years in the order of Hg < Cd < As < Pb < Cu.PCA/FA analysis shows that sediments are more strongly affected by TMs than surface water, with the main sources from industrial and agricultural human activities.Estimated non-cancer (CDI, HQ, HI) and cancer (CR) risks for exposure through ingestion and dermal routes indicate a higher potential health risk for children than adults.Additionally, the cancer risk assessment of As in sediments in the study area demonstrates a very high cancer risk for humans (TCR > 1 × 10 −4 ).Therefore, strict control of residential, industrial, and agricultural discharge is needed to improve environmental quality in the area and ensure community health.

Figure 1 .
Figure 1.Map of the study area of canals in Ho Chi Minh City.

Figure 2 .
Figure 2. Box plot of physicochemical parameters of Ho Chi Minh City canals for the years 2021 and 2022.

Figure 3 .
Figure 3. Correlation matrix of physicochemical parameters in water.

Figure 4 .
Figure 4. Box plot of TM values in the water of Ho Chi Minh City canals for the years 2021 and 2022.

Figure 5 .
Figure 5. Box plot of TM values in the sediments of Ho Chi Minh City canals for the years 2021 and 2022.

Figure 6 .
Figure 6.Correlation matrix between TM parameters in water (w) and sediment (s).

Figure 7 .
Figure 7. THI assessment at the water and sediment sampling sites in the canals for the years 2021 (a), (b) and 2022 (c), (d) by age group.

Table 1 .
Factor loadings for 11 water quality parameters collected in 2021 and 2022, analyzed using Principal Component Analysis/Factor Analysis (PCA/FA).Assessment of the current state of tms pollution in water and sediments of Ho Chi Minh city canals 3.2.1.TMs concentrations in canal environment Figure

Table 2 .
Water quality index (WQI) and status of canal water quality in 2021 and 2022.

Table 3 .
Factor loadings of TMs in water and sediment collected in 2021 and 2022 from PCA/FA analysis.

Table 4 .
Non-cancer risk to humans from TMs in water and sediments of HCMC Canals.