Hydrogeochemical characteristic and water quality index of groundwater and streamwater at Nam Mu River basin, Lai Chau province in northwest Vietnam

A total of 22 groundwater samples and 32 streamwater samples were systematically collected and preserved during the dry season between 2021 and 2022 (figure 1). Groundwater samples were sourced from various locations, including drilled wells, dug wells, and surface water bodies, to encompass a representative sample of the study area. Streamwater samples were extracted from upstream segments within the Nam Mu River basin, serving as a comparative reference to the groundwater samples within the study area. Water samples were stored in 500 ml polyethylene bottles. Each bottle underwent a thorough rinsing with sample water before collection and was clearly labeled. Each sampling location yielded three bottles. Among those, one bottle was subjected to filtration through filter paper with a size of 10–15 μm and subsequently acidified with 0.1 M HNO₃. This acidified sample was reserved for the analysis of cations and trace elements, which was carried out using the ICP-MS method on an Agilent 7900 device at the Institute of Geography, VAST (comprehensive analysis methods, in Lim et al 2022). Non-acidified samples were designated for the analysis of ions, including NO₃ ⁻, NH₄ ⁺, and PO₄ ³⁻, which were assessed via the photometric method. Additionally, Cl⁻, HCO₃ ⁻, and SO₄ ²⁻ were analyzed using the titration and turbidimetry methods. All collected samples were promptly refrigerated at 4 °C and remained so until they underwent laboratory analysis. In addition to laboratory analysis, certain parameters were measured in the field using a HANNA HI9829-01042 handheld device. These parameters encompassed temperature (T), pH, electrical conductivity (EC), and total dissolved solids (TDS).

Quality control for the analytical results in this study involved calculating the ion balance error, which is based on the relationship between total cations and anions for each water sample. As per the guidelines outlined in the American Public Health Association (APHA 2005), the calculated ion balance errors were found to fall within the acceptable range of ±5%. This validation confirms the reliability of the analysis results obtained in the study.

Groundwater and streamwater hydrogeochemical facies classification and main processes governing the overall chemistry using Piper (1944), Gibbs (1970), and bivariate diagrams were discussed. The Schoeller indexes (Schoeller 1977) such as CAI-I and CAI-II were calculated to analyse the base ion exchange reactions, as follows:

$$\begin{eqnarray}&&{CAI}-I=\frac{{{Cl}}^{-}-({{Na}}^{+}+{K}^{+})}{{{Cl}}^{-}}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{CAI}-{II}=\frac{{{Cl}}^{-}-({{Na}}^{+}+{K}^{+})}{{{SO}}_{4}^{2-}+{{HCO}}_{3}^{-}+{{NO}}_{3}^{-}}\end{eqnarray} \tag{ 2 }$$

The saturation index (SI) is determined using the following formula:

$$\begin{eqnarray}&&{SI}=\left[\mathrm{log}\left(Q\right)\right]/[\mathrm{log}\left({Ksp}\right)]\end{eqnarray} \tag{ 3 }$$

In which, Q represents the ion activity, while Ksp signifies the mineral dissolution constant. A positive SI value (SI > 0) indicates supersaturation, signifying the propensity of minerals to precipitate from groundwater. Conversely, negative SI values (SI < 0) indicate subsaturation, reflecting the inclination of minerals to dissolve into groundwater and surface water. The equilibrium SI value (SI = 0) signifies that the mineral phase is in a state of equilibrium (Srinivasamoorthy et al 2008, and Pant et al 2018). In this study, the SI index was calculated for 22 groundwater samples and 32 streamwater samples within the study area using the thermodynamic software PHREEQC version 2 (Parkhurst and Appelo 1999). PHREEQC (pH-Redox-Equilibrium in the C programming language) was developed by United States Geological Survey. It is a computer program that can simulate chemical reactions and transport processes in natural or polluted. The program uses an ion-association aqueous model and can perform speciation and SI calculations (Parkhurst and Appelo 1999).

The WQI is a crucial technical parameter employed to assess water quality by considering various physicochemical parameters individually and in aggregate. It serves as a vital tool for categorizing the quality of both surface water and groundwater. In recent years, the WQI has gained widespread usage in the field of environmental science and monitoring to track developments in water quality (Horton 1965, Bordalo et al 2001, Rubio-Arias et al 2012, Bartsch et al 2015, Das Kangabam et al 2017). Sahu and Sikdar 2008, Shinde and Ningwal 2013 have employed the WQI as a valuable tool for assessing the quality of both surface water and groundwater within river basins. These studies have utilized the relationship between various water quality parameters and aquifer characteristics. As a result, the authors have proposed basin-specific water quality parameters, aiming to simplify complex water quality data into easily interpretable indicators. This approach facilitates more effective monitoring and management of water quality within these river basins.

The WQI reflects the influence of each different water quality parameter, depending heavily on the characteristics of the study area and the intended use. Water quality is calculated using the WQI formula and compared with Vietnam's National Technical Regulations on clean water for domestic use (QCVN 01-1: 2018/BYT 2018). The quality scale (Qi) for each water quality parameter is calculated according to the following equation (4):

$$\begin{eqnarray}&&{Q}_{i}=\displaystyle \sum _{i=1}^{n}\left[\left\{{M}_{i}\left(-\right){I}_{i}/({S}_{i}-{I}_{i}\right\}\right]\times 100\end{eqnarray} \tag{ 4 }$$

Where Mi is the concentration of the ith parameter, Ii is the ideal value of each ith parameter, Si is the allowable limit value of the ith parameter as recommended by national standards or the World Health Organization. In this study, the Si value is used according to the recommendations of QCVN 01-1: 2018/BYT 2018. The S_i value is taken as an absolute value.

Wi is calculated according to equation (5) as follows:

$$\begin{eqnarray}&&{W}_{i}=1/{S}_{i}\end{eqnarray} \tag{ 5 }$$

The water quality index is calculated according to equation (6) as follows:

$$\begin{eqnarray}&&{WQI}=\left\{\left(\displaystyle \sum _{i-1}^{n}{Q}_{i}{W}_{i}\right)/\left(\displaystyle \sum _{i-1}^{n}{W}_{i}\right)\right\}\end{eqnarray} \tag{ 6 }$$

Water quality index is used to discuss the intended use of water. In this study, water quality will be discussed for drinking and domestic purposes. Therefore, the water quality index is calculated and classified into five levels, from very good water quality to very poor water quality, see table 1 for details.

Water quality assessment using physio-chemical parameters and WQI. The descriptive statistics for analyzed parameters are summarized and presented in section 3.1, and tables 2, 4.

Hydrogeochemical characteristics of groundwater and streamwater can be influenced by many different factors including rock-mineral types, residence time in bedrock, water characteristics, flow through rock, and the location of initial groundwater formation (Toth 1999). Ion concentration in water reflects the basic hydrogeochemical characteristics of groundwater (Li et al 2018). In this study, the results of physicochemical analysis of groundwater and streamwater samples in the study area are summarized in tables 2 and 3, thereby serving as a basis for identifying and analyzing hydrogeological characteristics, and chemistry of streamwater and groundwater here as well as evaluating the controlling factors. Guidelines for clean water used for living and drinking purposes recommended according to Vietnamese guidelines (QCVN 01-1:2018/BYT 2018) and the World Health Organization (WHO 2022) are also included for comparison.

The analysis results presented in table 2 indicate that the pH values of groundwater samples span from 5.9 to 8.5, with an average pH of 7.4. In contrast, the pH values of streamwater samples range from 6.9 to 9.3, with an average pH of 7.8. These findings reveal that the groundwater within the study area tends to be slightly acidic to alkaline in nature, whereas the streamwater exhibits a neutral to alkaline pH range. The EC of groundwater samples falls within the range of 10 to 250 μS/cm, with an average value of 84.5 μS cm⁻¹. In comparison, the EC of streamwater samples spans from 10 to 440 μS cm⁻¹, with an average EC of 90.9 μS/cm. Furthermore, the TDS in groundwater samples exhibit a range of 24.9 to 199.9 mg l⁻¹, with an average TDS of 89.9 mg/l. On the other hand, the TDS values in streamwater samples vary between 17.9 and 227.3 mg l⁻¹, with an average TDS of 83.6 mg l⁻¹.

The average cation concentration in groundwater within the study area exhibits a distinct dominance of Ca²⁺ ions over other cations. These cations are observed in the following order of concentration Ca²⁺ > Mg²⁺ > Na⁺ > K⁺, with respective concentrations of 13.9 mg l⁻¹, 3.7 mg l⁻¹, 2.4 mg l⁻¹, and 1.8 mg l⁻¹. Similarly, in streamwater samples, the average cation concentration follows a similar order of predominance Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ with corresponding concentrations of 13.1 mg l⁻¹, 3.5 mg l⁻¹, 2.2 mg l⁻¹, and 1.4 mg/l (table 2).

The average anion concentration in groundwater within the study area follows the order of HCO₃ ⁻ > SO₄ ²⁻ > Cl⁻ > NO₃ ⁻, with respective concentrations of 58.7 m g /l , 4.5 m g /l , 3.1 mg l⁻¹, and 1.5 mg/l. Notably, HCO₃ ⁻ ions are significantly more abundant in the water compared to the other anions, highlighting their clear predominance. Similarly, the order and values of these anions are similar and closely aligned with those found in the upstream water of the study area, where the anion concentration is also in the order of HCO₃ ⁻ > SO₄ ²⁻ > Cl⁻ > NO₃ ⁻, with corresponding concentrations of 52.4 mg l⁻¹, 5.9 mg l⁻¹, 3.6 mg l⁻¹, and 1.2 mg l⁻¹ (table 2).

The concentrations of ion F⁻ in groundwater samples vary within the range of 0 to 0.17 mg l⁻¹, while ion NH₄ ⁺ range from less than 0.1 mg l⁻¹ to 0.25 mg l⁻¹. Similarly, in streamwater samples, the concentrations fluctuate within the corresponding range of 0.01 mg l⁻¹ to 0.18 mg l⁻¹ for ion F⁻ and from 0.1 mg l⁻¹ to 0.6 mg l⁻¹ for ion NH₄ ⁺.

Among the sampled water set, one particular sample, labeled as V12U532/2 and sourced from Trung Dong commune, Tan Uyen district, stands out with distinctive analytical results. This water sample exhibits a notably high water temperature of 39 °C, a Total Dissolved Solids (TDS) value of 1,408 mg l⁻¹, and an elevated fluoride ion concentration of 1,626 mg l⁻¹. The water is categorized as having a Ca²⁺ - SO₄ ²⁻ composition (figure 1, table 2). These exceptional physicochemical characteristics distinguish this water sample from the rest, classifying it as mineral water with a high temperature and an unusually high fluoride concentration. Such characteristics render it a valuable source of hot mineral water, warranting further investment and in-depth research for potential utilization and development.

In nature, surface water and groundwater are mostly composed of the main cations Mg²⁺, Ca²⁺, Na⁺, K⁺ and the main anions SO₄ ²⁻, Cl⁻ and HCO₃ ⁻. Recognizing the significance of these key components, Piper introduced the Piper triangle diagram in 1944. This diagram serves as a valuable tool for classifying various types of water based on their origins (Piper 1944). Beyond classification, the Piper triangle diagram also aids in understanding the distribution patterns of different water types and provide information about flow and water quality (Back 1960, Freeze and Cherry 1979, Ophori and Toth 1989, Sikdar et al 1993, Sahu and Sikdar 2008, Sunitha and Reddy 2022, Yang et al 2022). In the context of the study area, the Piper chart was generated, revealing notable patterns. The majority of samples, encompassing both streamwater and groundwater originating from Triassic sedimentary formations, are concentrated within zones A and G on the diagram. This concentration signifies that Ca²⁺ and HCO₃ ⁻ are the predominant cation and anion within these samples (figure 2). In contrast, within the Permian felsic volcanic formations, the Piper diagram illustrates distinct patterns. The majority of both streamwater and groundwater samples are concentrated in zones D, C, and G. Among these, streamwater is predominantly clustered in zone D, showing no pronounced dominance between cations and anions. However, groundwater samples within this geological context exhibit some concentrations in zone C, providing evidence of Na⁺ as a prevalent cation in these specific samples. This distribution highlights the variability in cation and anion compositions within the study area's water sources (figure 2). On the Piper diagram, most streamwater and groundwater samples in Triassic sedimentary formations are distributed in zone I, belonging to the Ca²⁺-Mg²⁺-HCO₃ ⁻ water type. Groundwater and streamwater samples in Permian felsic volcanic formations have quite diverse water types, belonging to zone I corresponding to Ca²⁺-Mg²⁺-HCO₃ ⁻ water type, zone III with mixed Ca²⁺- Mg²⁺- Cl⁻ water type, and zone IV belongs to mixed Ca²⁺-Na⁺-HCO₃ ⁻ water type (figure 2). Of which, zone III is mainly streamwater samples and zone IV is mainly groundwater samples. The similarity in water types of streamwater and groundwater in the dry season in the study area shows the similarity in recharge sources, possibly mainly from rainwater. In mountainous areas, rainwater is commonly composed of Ca²⁺ and HCO₃ ⁻ ions and the penetration of rainwater into groundwater has increased the dissolution rate of carbonate minerals rich in Ca²⁺ and HCO₃ ⁻, thus creating a type of Ca²⁺-Mg²⁺-HCO₃ ⁻ water (Gao et al 2020, Wang et al 2022). The formation of mixed water Ca²⁺-Mg²⁺- Cl⁻ and Ca²⁺-Na⁺-HCO₃ ⁻ may be due to evaporation being much larger than precipitation, increasing the Cl⁻ and Na⁺ concentration thereby creating the water type Ca²⁺-Mg²⁺- Cl⁻ (Gao et al 2020, Wang et al 2022). These are representative of evolved water patterns, the meteorological signature of which disappears due to rock-water interactions and ion exchange reactions.

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The hydrogeochemical characteristics of water are affected and changed by various natural processes such as evaporation, precipitation, bedrock weathering or a combination of the above factors (Gaur et al 2022). These processes are clearly shown through the Gibbs diagram, based on the correlation between the ratios of cation Na⁺/(Na⁺ + Ca²⁺) to TDS and anion Cl⁻ /(Cl⁻ + HCO₃ ⁻) to TDS (figure 3). The Gibbs diagram can determine the geochemical reaction mechanism in aquifers, stream water and lithological factors, and explain the origin of water as well as the mechanism that affects the chemical composition of water,. natural water (Gibbs 1970, Shamsuddin Mohd et al 2019). Groundwater and streamwater samples in Tan Uyen and Than Uyen districts are shown on the Gibbs diagram (figure 3), showing that the majority of samples are located in the bedrock dissolution area and rainwater influence area. This proves that the two main factors affecting the chemical composition of groundwater and streamwater in the study area are the weathering process of rock-forming minerals and the recharge of rainwater into the aquifers. In particular, groundwater and streamwater samples in Permian felsic volcanic formations are influenced by the recharge process of rainwater, while groundwater and streamwater samples in Triassic terrigenous sediments are mainly affected by the weathering of rock-forming minerals (figure 3). This is also evidenced by the Schoeller indexes (Schoeller 1977) such as CAI-I and CAI-II.

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The CAI-I and CAI-II indexes can be positive or negative because they depend on the exchange of Na⁺ and K⁺ ions in water with Ca²⁺ and Mg²⁺ in rock or soil and vice versa. If these values are positive, it means the exchange of Na⁺ and K⁺ in water with Ca²⁺ and Mg²⁺ occurs. In contrast, if these values are negative, it shows that ion exchange occurs between Ca²⁺ and Mg²⁺ ions of surface water and Na⁺ and K⁺ ions in rocks or soil (Schoeller 1977, Kant et al 2018, Gaur et al 2022). This process occurs with bedrock formations or soils that serve as the main source of dissolved solids in water. The samples in figure 4(a) mostly show negative values, showing that the reverse ion exchange process controls the chemical composition of surface water as well as the interaction between rock and water. The patterns shown in the Na⁺ and Cl⁻ correlation plot (figure 4(b)) show that Na⁺ mainly originates from the dissolution of halite (Wu et al 2018, Wang et al 2022). However, some samples located above the 1:1 line show the predominance of Cl⁻, indicating that Cl⁻ can come from anthropogenic sources. Concomitantly, ion exchange between Na⁺, K⁺ with Ca²⁺ and Mg²⁺ from surrounding formations can also reduce Na⁺ concentration, causing Cl⁻ concentration to increase. Some samples are below the 1:1 line, possibly due to the weathering process of silicate rocks, increasing the Na⁺ concentration in water samples and accompanying it with increased concentrations of HCO₃ ⁻. Figure 4(c) shows that samples are quite close to 0.5*Total cations, especially groundwater and streamwater samples in Permian felsic volcanic formations. This proves that silicate weathering plays a dominant role in Na⁺ concentration in water samples of the study area, especially for groundwater and streamwater in Permian felsic volcanic formations. The correlation chart of Ca²⁺ with Mg²⁺ with most samples lying on the 1:1 line further confirms this role (figure 4(d)).

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On the correlation chart between (Ca²⁺ + Mg²⁺) and (HCO₃ ⁻ + SO₄ ²⁻) (figure 4(e)), the research samples lie on the 1:1 line, proving that in different formations of the study area, reaction is mainly dissolution of calcite, dolomite, anhydrite, gypsum. In addition, previous studies have shown that, if sulfate minerals (e.g., gypsum) are the main source of Ca²⁺ in water, the ratio of Ca²⁺ to SO₄ ²⁻ is 1:1 (Bahir et al 2018, Wang et al 2022). Samples of the study area were mostly below the 1:1 line (figure 4(f)), demonstrating that Ca²⁺ and SO₄ ²⁻ did not completely originate from gypsum dissolution.

The dissolution process of various materials gives rise to distinct ion combinations, and as a result, the geological formations within the study area exert a significant influence on the chemical composition of both streamwater and groundwater. Notably, the presence of ions such as Ca²⁺ and Mg²⁺ can be attributed to the weathering process of silicate, carbonate, and evaporite rocks. In addition, ions like Na⁺ and K⁺ find their origins in the weathering of silicate rocks and the dissolution of evaporite formations. This understanding underscores the pivotal role of geological factors in shaping the chemical makeup of the study area's water sources. Notably, SO₄ ²⁻ and Cl⁻ ions predominantly originate from the dissolution of evaporite rocks, while HCO₃ ⁻ ions mainly derive from the weathering of silicate and carbonate minerals (Mortatti and Probst 2003, Pant et al 2018, Wang et al 2022). According to Meybeck 1987, carbonate minerals have 12–40 times higher solubility than silicates and are more easily weathered under natural conditions. Analytical results of the study area show that the average value of the ratio [(Ca²⁺ + Mg²⁺)/Total cation] is quite high for groundwater and streamwater samples in Triassic terrigenous sedimentary formations, with values of 0.87 and 0.88, respectively, and lower values for groundwater and streamwater samples in the Permian felsic volcanic formation, with values of 0.48, 0.63, respectively. In contrast, the ratio (Na⁺ + K⁺/Total cation) is quite low for groundwater and streamwater samples in Triassic terrigenous sediments, with values of 0.10 and 0.11, respectively, and higher for groundwater and streamwater in the Permian felsic volcanic formation, with values of 0.33 and 0.51, respectively (table 2, figure 4(c)). This observation suggests that in the groundwater and streamwater samples from Triassic terrigenous sediments, the weathering of silicate and evaporate minerals appears to be less pronounced when compared to the weathering of carbonate minerals. In contrast, it is apparent that in groundwater and streamwater sourced from Permian felsic volcanic formations, the processes of silicate weathering and evaporation exhibit greater intensity when compared to the weathering of carbonate minerals. These findings are further substantiated by the positive correlation observed between Si and cations such as Na⁺, K⁺, Ca²⁺, and Mg²⁺ in groundwater and streamwater samples from Permian felsic extrusive formations. In contrast, there exists a negative correlation between Si and most of these cations in groundwater and streamwater samples derived from Triassic terrigenous sediments (table 3). This phenomenon is also distinctly evident in the correlation chart between Ca²⁺/Na⁺ with HCO₃ ⁻/Na⁺ and Mg²⁺/Na⁺ (figure 5), thus water samples from Permian felsic volcanic formations cluster near the silicate field, signifying a substantial silicate weathering process. Besides, water samples from Triassic terrigenous sediments gravitate towards the carbonate end member of the field, demonstrating the prevalence of carbonate weathering processes in this geological context. This chart provides a visual representation of the differing geochemical processes in these two distinct areas (figure 5).

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The process of mineral precipitation or dissolution significantly influences the chemical characteristics of both streamwater and groundwater, and this influence is quantified through the mineral saturation index (SI). The SI value plays a crucial role in assessing the equilibrium between groundwater and surface water, providing valuable insights into the degree of saturation within specific mineral phases.

The SI values, arranged in descending order, exhibit the following trend: dolomite > calcite > aragonite > gypsum > anhydrite > fluorite > sylvite > halite (figure 6). Notably, the SI values for dolomite, calcite, and gypsum fall within the oversaturated range, indicating their propensity to precipitate from the water. In contrast, the SI values for minerals such as aragonite, anhydrite, fluorite, and halite indicate subsaturation, suggesting their tendency to dissolve into the water (figure 6). These results confirm the substantial influence of carbonate minerals on the geochemistry of both groundwater and streamwater within the study area, aligning with the prevalence of the Ca²⁺-Mg²⁺-HCO₃ ⁻ water type. This is further corroborated by the saturation indices of calcite (SIC), which demonstrate oversaturation for most samples in terms of Ca²⁺ (figure 7(a)), and dolomite (SID), which similarly exhibits oversaturation for most samples, particularly in relation to Mg²⁺ (figure 7(b)). This study, conducted during the dry season characterized by low rainfall, may have contributed to the oversaturation of the minerals mentioned above. This phenomenon aligns with the observations of previous studies (e.g., Garrels and Mackenzie 1967, Elango and Ramachandran 1991) that saturation indices can vary in response to rainfall conditions. During dry conditions, groundwater often remains supersaturated with calcite and dolomite due to factors like evaporation, leading to the precipitation of these minerals. In contrast, in the rainy season, these minerals are diluted. The observed oversaturation suggests that these minerals have a sufficient residence time to reach a state of equilibrium (Singh et al 2017).

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4.4.1. Water quality assessment based on physicochemical parameters and indicators

4.4.2. Water quality assessment using Water Quality Index (WQI)

Groundwater and surface water are primary sources of water supply for domestic, agricultural, and industrial purposes (Shinde and Ningwal 2013, and Osta et al 2020). Consequently, assessing water quality is a vital parameter that demands continuous monitoring and evaluation. In this study, the quality of both groundwater and upstream streamwater was evaluated, particularly regarding domestic use, utilizing the WQI method. All water quality parameters monitored during the dry season in the study area are statistically displayed according to the minimum (Min.), maximum (Max.), and average (Mean) values of the data set (table 5). Major parameters including pH, TDS, TH, Cl⁻, SO₄ ²⁻, NO₃ ⁻, NH₄ ⁺, Fe, Mn and F⁻ in water samples are compared with clean domestic water according to National technical regulations on quality of clean water used for domestic purposes QCVN 01-1:2018/BYT 2018 (table 5). The WQI index is calculated according to the arithmetic ranking method depending on the weight of the above mentioned domestic parameters. The results of the WQI assessment provided in table 5 and figure 8 illustrates that the majority of water samples, including both groundwater and streamwater, received ratings of 'very good' and 'good' in terms of water quality. This indicates that the water is well-suited for drinking and domestic purposes. According to geological formations, water samples within Permian felsic volcanic eruption formations, both in streamwater and groundwater, are generally well-suited for domestic water use. Specifically, in the case of groundwater, all samples fall within the 'very good' category with a WQI of less than 25 (table 6). For streamwater, 54.55% of samples achieved a WQI score of less than 25, signifying very good water quality. In addition, 18.18% of samples fell within the 26–50 WQI range, indicating good water quality. However, 9.09% of samples had WQI values between 51 and 75, which reflects poor water quality, and the same percentage of samples had WQI values exceeding 100, signifying very poor water quality. The water quality is classified as very poor and unsuitable for daily use, primarily due to the elevated levels of Fe and Mn surpassing standards (tables 5 and 6).

Table 5. WQI at individual sampling site.

No	Sample	Water and Rock type	pH	TDS mg/L	TH mg/L	Cl- mg/L	SO42- mg/L	NO3- mg/L	NH4+ mg/L	Fe mg/L	Mn mg/L	F mg/L	WQI	Water quality status
1	V12U516	Groundwater, Permian felsic volcanic rock	6.20	30	3	3.12	2.64	1.41	0.05	0.0040	0.0005	0.146	3.38	Very good water
2	V12U588	Groundwater, Permian felsic volcanic rock	6.90	38	5	2.50	3.00	1.04	0.05	0.0682	0.0036	0.005	8.73	Very good water
3	V12U509	Groundwater, Permian felsic volcanic rock	7.40	25	3	2.81	3.20	0.99	0.05	0.0118	0.0005	0.083	3.52	Very good water
4	V12U534	Groundwater, Permian felsic volcanic rock	5.90	24	3	3.12	3.58	2.01	0.05	0.0236	0.0027	0.005	5.80	Very good water
5	V12U581	Groundwater, Permian felsic volcanic rock	6.70	28	3	2.50	2.00	2.65	0.05	0.0208	0.0005	0.005	3.98	Very good water
6	V12W534	Streamwater, Permian felsic volcanic rock	7.40	23	3	2.80	3.10	1.80	0.05	0.0767	0.0119	0.03	14.54	Very good water
7	V12W609	Streamwater, Permian felsic volcanic rock	8.3	49	7	3.1	3.2	0.3	0.1	0.0460	0.0025	0.1	9.29	Very good water
8	V12W509	Streamwater, Permian felsic volcanic rock	7.4	24	5	2.51	3.02	2.08	0.10	0.0064	0.0005	0.031	5.06	Very good water
9	V12W516	Streamwater, Permian felsic volcanic rock	7.3	21	3	3.12	3.25	0.75	0.10	0.0930	0.0011	0.024	11.01	Very good water
10	V12W525	Streamwater, Permian felsic volcanic rock	9.3	18	3	3.01	2.85	1.52	0.050	0.0185	0.0005	0.010	4.69	Very good water
11	V12W546	Streamwater, Permian felsic volcanic rock	7.3	29	4	3.12	6.03	1.79	0.100	0.5988	0.1240	0.046	117.66	Very poor water
12	V12W556	Streamwater, Permian felsic volcanic rock	7.7	30	4	4.37	6.35	1.49	0.05	0.1607	0.1041	0.010	75.00	Medium water
13	V12W576	Streamwater, Permian felsic volcanic rock	6.9	25	3	3.74	3.17	1.17	0.20	0.0616	0.0366	0.017	33.86	Good water
14	V12W581	Streamwater, Permian felsic volcanic rock	7.0	80	13	7.80	5.06	9.49	0.25	0.4408	0.0712	0.010	82.04	Poor water
15	V12W582	Streamwater, Permian felsic volcanic rock	7.1	35	5	3.12	3.26	0.95	0.20	0.0481	0.0013	0.131	12.25	Very good water
16	V12W588	Streamwater, Permian felsic volcanic rock	7.1	37	4	3.12	3.01	0.88	0.20	0.2741	0.0294	0.070	43.72	Good water
17	V12U508	Groundwater, Triassic sedimentary rock	6.60	110.79	21.91	2.81	3.67	1.46	0.25	0.1074	1.8167	0.04	1,098.20	Very poor water
18	V12U513	Groundwater, Triassic sedimentary rock	8.00	38	8	2.81	3.20	0.83	0.05	0.0893	0.0057	0.004	11.80	Very good water
19	V12U522	Groundwater, Triassic sedimentary rock	7.70	30	5	2.81	2.83	0.44	0.05	0.0025	0.0005	0.005	2.80	Very good water
20	V12U532	Groundwater, Triassic sedimentary rock	7.90	49	10	3.12	3.00	0.70	0.05	0.0041	0.0005	0.005	3.02	Very good water
21	V12U541	Groundwater, Triassic sedimentary rock	7.90	52	9	3.43	3.58	0.18	0.05	0.0457	0.0005	0.045	5.83	Very good water
22	V12U543	Groundwater, Triassic sedimentary rock	8.40	137	29	3.12	5.00	0.29	0.05	0.0168	0.0094	0.033	9.45	Very good water
23	V12U551	Groundwater, Triassic sedimentary rock	8.10	77	15	2.81	5.66	0.44	0.05	0.4147	0.0448	0.023	56.62	Medium water
24	V12U561	Groundwater, Triassic sedimentary rock	8.50	162	34	2.81	6.22	0.81	0.05	0.0268	0.0024	0.051	6.07	Very good water
25	V12U565	Groundwater, Triassic sedimentary rock	8.00	102	20	3.12	6.2	1.12	0.05	0.0158	0.0056	0.013	6.93	Very good water
26	V12U567	Groundwater, Triassic sedimentary rock	6.80	79	16	3.74	7.06	5.33	0.05	0.4192	0.1246	0.060	104.44	Very poor water
27	V12U573	Groundwater, Triassic sedimentary rock	8.10	62	12	2.81	4.71	0.65	0.05	0.0611	0.0022	0.043	8.00	Very good water
28	V12U590	Groundwater, Triassic sedimentary rock	8.00	116	23	3.12	6.50	2.28	0.05	0.0123	0.0009	0.036	4.02	Very good water
29	V12U590/D	Groundwater, Triassic sedimentary rock	8.00	118	23	3.43	6.92	2.60	0.05	0.0169	0.0020	0.040	5.02	Very good water
30	V12U599	Groundwater, Triassic sedimentary rock	6.70	185	39	3.43	5.36	1.68	0.05	0.0177	0.0004	0.165	4.08	Very good water
31	V12U604	Groundwater, Triassic sedimentary rock	6.80	183	38	3.12	4.71	1.21	0.05	0.0145	0.0005	0.014	3.44	Very good water
32	V12U608	Groundwater, Triassic sedimentary rock	7.10	200	44	3.43	4.50	4.34	0.05	0.0166	0.0115	0.005	10.30	Very good water
33	V12U560	Groundwater, Triassic sedimentary rock	7.20	130	25	3.43	4.71	0.65	0.05	0.0525	0.0105	0.064	11.96	Very good water
34	V12W522	Streamwater, Triassic sedimentary rock	7.50	62	13	3.43	4.15	0.69	0.10	0.0113	0.0096	0.02	10.73	Very good water
35	V12W560	Streamwater, Triassic sedimentary rock	8.30	146	32	3.12	3.67	0.14	0.05	0.0909	0.0341	0.04	28.97	Very good water
36	V12W513	Streamwater, Triassic sedimentary rock	7.2	32	6	2.64	2.11	1.42	0.10	0.0771	0.0059	0.008	12.72	Very good water
37	V12W514	Streamwater, Triassic sedimentary rock	7.8	227	45	3.65	42.19	0.65	0.10	0.0005	0.0304	0.019	22.50	Very good water
38	V12W529	Streamwater, Triassic sedimentary rock	7.7	55	11	3.43	3.67	0.87	0.10	0.0742	0.0189	0.010	20.50	Very good water
39	V12W532	Streamwater, Triassic sedimentary rock	7.8	175	39	3.43	23.30	1.90	0.10	0.0329	0.0173	0.047	17.04	Very good water
40	V12W539	Streamwater, Triassic sedimentary rock	8.2	61	12	3.43	2.83	1.14	0.050	0.0364	0.0006	0.014	5.44	Very good water
41	V12W541	Streamwater, Triassic sedimentary rock	8.0	70	14	3.12	3.20	0.88	0.050	0.1002	0.0197	0.043	20.96	Very good water
42	V12W543	Streamwater, Triassic sedimentary rock	8.0	77	15	3.43	3.20	0.44	0.10	0.0194	0.0005	0.026	6.06	Very good water
43	V12W544	Streamwater, Triassic sedimentary rock	7.6	42	8	2.81	3.58	0.70	0.10	0.2699	0.0627	0.012	59.42	Medium water
44	V12W545	Streamwater, Triassic sedimentary rock	7.8	150	33	3.12	4.90	0.52	0.050	0.1037	0.0315	0.132	28.30	Very good water
45	V12W558	Streamwater, Triassic sedimentary rock	6.9	73	14	4.68	6.06	0.36	0.60	0.1268	0.0422	0.059	57.22	Medium water
46	V12W566	Streamwater, Triassic sedimentary rock	7.6	115	23	3.12	4.71	0.22	0.05	0.0321	0.0033	0.010	6.35	Very good water
47	V12W567	Streamwater, Triassic sedimentary rock	8.3	94	20	3.43	4.71	0.52	0.20	0.1237	0.0495	0.131	46.47	Good water
48	V12W590	Streamwater, Triassic sedimentary rock	8.7	93	19	3.74	5.07	0.77	0.20	0.0568	0.0089	0.048	17.86	Very good water
49	V12W590/2	Streamwater, Triassic sedimentary rock	8.4	38	7	4.37	3.57	0.65	0.20	0.0708	0.0055	0.025	16.55	Very good water
50	V12W596	Streamwater, Triassic sedimentary rock	8.8	75	14	3.43	4.80	0.50	0.20	0.0816	0.0432	0.036	39.88	Good water
51	V12W599	Streamwater, Triassic sedimentary rock	8.1	182	38	4.37	6.13	1.01	0.20	0.0293	0.0131	0.010	18.15	Very good water
52	V12W601	Streamwater, Triassic sedimentary rock	8.7	136	31	3.43	5.08	0.28	0.20	0.1302	0.1560	0.053	110.23	Very poor water
53	V12W604	Streamwater, Triassic sedimentary rock	7.0	178	37	3.74	2.92	2.22	0.05	0.1320	0.0318	0.183	30.27	Good water
54	V12W608	Streamwater, Triassic sedimentary rock	7.9	225	48	4.05	5.26	1.76	0.05	0.0040	0.0005	0.105	8.25	Very good water
	Mean		7.62	110.17	23.17	3.35	20.39	1.33	0.10	0.09	0.05	0.07
	QCVN01-1:2018 (Si)		8.50	1,000	300	250	250	8.86	0.39	0.3	0.1	1.5
	Ideal value (Ii)		7.00	0	0	0	0	0	0	0	0	0
	Unit weight (Wi)		0.118	0.001	0.003	0.004	0.004	0.500	3.333	3.333	10.000	0.667

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Table 6. The classification of water quality of the Nam Mu River basin according to WQI value.

WQI value	% of sample				Water quality status
	Permian felsic volcanic rock		Triassic sedimentary rocks
	Groundwater	Streamwater	Groundwater	Streamwater
0–25	100	54.55	83.33	71.43	Excellent
26–50	Nil	18.18	Nil	14.29	Good
51–75	Nil	9.09	5.56	9.52	Medium
76–100	Nil	9.09	Nil	Nil	Poor
>100	Nil	9.09	11.11	4.76	Very poor, unsuitable for drinking

Nil - Nothing

Water samples from Triassic sedimentary rocks are predominantly of good quality, with only a few exceptions that exhibit a water state unsuitable for domestic purposes (WQI > 100). Specifically, groundwater within Triassic sedimentary formations includes 83.33% of samples with a WQI < 25, indicating very good water quality, and 5.56% of samples with a WQI ranging from 51 to 75, signifying a state of poor water quality. However, 11.11% of samples, with WQI > 100, fall into the category of very poor water quality, rendering them unsuitable for domestic purposes, primarily due to elevated levels of NO₃ ⁻ and iron Fe that exceed established standards (tables 5 and 6). Streamwater in the Triassic sedimentary formation exhibits varying water quality levels. Specifically, 71.43% of samples are classified as having a 'very good' water quality status, 14.29% fall into the 'good' water quality category, and 9.52% are deemed to have 'average' water quality. However, 4.76% of the samples receive the designation of 'very poor' water quality, making them unsuitable for daily use. This unfavorable status is primarily attributed to elevated Mn levels exceeding established standards (tables 5 and 6).

In summary, the assessment of WQI indicators for the water sources in the study area reveals that groundwater and upstream streamwater from both Permian felsic volcanic formations and Triassic terrigenous sediments are generally of suitable quality for domestic purposes. However, some streamwater samples have higher concentrations of Fe, Mn, or NO₃ ⁻ exceeding established standards. Therefore, in cases where these contaminants are present, water treatment is necessary before domestic use to ensure safety and water quality.