*3.1. Physicochemical Parameters*

Groundwater quality data were first checked for reliability and accuracy by calculating the correlation between EC and the sum of cations on one hand and with the sum of anions on the other hand. The results show a good correlation with R<sup>2</sup> > 0.8 (Figure 2a,b. The reliability of groundwater quality data was also checked by the ion charge balance between cations and anions as follows:

$$E(\%) = \frac{N\_c - N\_a}{N\_c + N\_a} \times 100\tag{16}$$

where, *N<sup>c</sup>* and *N<sup>a</sup>* denote total concentrations of cations and anions of a sample in meq/L, respectively. The biggest value of *E* was 3.14%, which indicated that the samples were reliable, as the *E* value was between −5% and +5%.

The physicochemical indices of groundwater samples were statistically analyzed, and the results are listed in Table 1. The pH values in this study ranged from 7.05 to 8.39, which were within the guidelines set by the WHO [42] for drinking water (6.5 to 8.5). Hem [56] concluded that the pH of groundwater was controlled by the equilibrium of CO<sup>3</sup> <sup>2</sup>−, CO<sup>2</sup> and HCO<sup>3</sup> −, and interpreted the chemical characteristics of natural water. The mean pH value of groundwater samples was 7.77, which was suitable for drinking purpose. Mechenich and Andrews [57] considered the range of pH values from 7.5–8.3 as an ideal values range for drinking water. Thus, it can be assumed that pH values for drinking water in Tongchuan City are good and ideal. However, 12 samples (25% of the total samples) showed slight alkalinity of the drinking water in the study area with pH ranging from 8 to 8.39. Alkalinity is not only associated with high pH values, but also with hardness and excessive TDS [33].

**Figure 2.** Ionic balance of groundwater data: (**a**) Σ of cations vs. EC/100; (**b**) Σ of anions vs. EC/100. **Figure 2.** Ionic balance of groundwater data: (**a**) Σ of cations vs. EC/100; (**b**) Σ of anions vs. EC/100.

The physicochemical indices of groundwater samples were statistically analyzed, and the results are listed in Table 1. The pH values in this study ranged from 7.05 to 8.39, which were within the guidelines set by the WHO [42] for drinking water (6.5 to 8.5). Hem [56] concluded that the pH of groundwater was controlled by the equilibrium of CO32−, CO2 and HCO3−, and interpreted the chemical characteristics of natural water. The mean pH value of groundwater samples was 7.77, which was suitable for drinking purpose. Mechenich and Andrews [57] considered the range of pH values from 7.5–8.3 as an ideal values range for drinking water. Thus, it can be assumed that pH values for drinking water in Tongchuan City are good and ideal. However, 12 samples (25% of the total samples) showed slight alkalinity of the drinking water in the study area with pH ranging from 8 to 8.39. Alkalinity is not only associated with high pH values, but also with hardness and excessive TDS [33]. According to the average pH value, the groundwater in the study area can be used as drinking water. However, when comparing the detected TDS and TH values with the drinking water standards, there were two samples (4.2%) with TDS exceeding 1000 mg/L, and five samples (10.4%) with TH exceeding 500 mg/L. At the same time, referring to the drinking water quality guidelines recommended by the Ministry of Health of the People's Republic of China, there were eight samples (17%) whose TH exceeded 450 mg/L. This would be considered as hard water [1]. However, this classification is far different from the drinking water classification early made by Freeze and Cherry [58] (Table 2) based on TH. The groundwater classification on the basis of TDS and TH [14,31,58,59] in Tongchuan are as follows (Table 2): 35.4% and 64.6% of samples were hard water or very hard water; 47.9% were desirable and permissible for drinking; 95.8% were fresh water and 4.2% were brackish.


as drinking water. However, when comparing the detected TDS and TH values with the **Table 2.** TDS and TH-based classification of groundwater for drinking purpose in Tongchuan.

According to the average pH value, the groundwater in the study area can be used

**Table 2.** TDS and TH-based classification of groundwater for drinking purpose in Tongchuan. **Parameters Range Water Type % of Samples**  TH <75 Soft 0 75–150 Moderately hard 0 150–300 Hard 35.4 >300 Very hard 64.6 TDS <500 Desirable for drinking 47.9 500–1000 Permissible for drinking 47.9 In addition, the TH values of water are the measures of the dissolved Ca2+ and Mg2+ content, which are expressed in CaCO<sup>3</sup> mg/L and can be associated EC, which is normally twice the hardness for uncontaminated water [23,57]. Otherwise, if it is higher than that proportion, it provides information on the presence of components such as Na<sup>+</sup> , Cl− or SO<sup>4</sup> <sup>2</sup><sup>−</sup> [57]. Through the analysis of the physical and chemical indicators of the samples in the study area, the average values of EC and TH were 869.75 µS/cm and 349.94 mg/L, respectively, and the conductivity was greater than two times of the TH, which indicated that slightly high concentrations of Na<sup>+</sup> , Cl−, and SO<sup>4</sup> <sup>2</sup><sup>−</sup> were in some groundwater samples.

<1000 Fresh water 95.8 >1000 Brackish 4.2 The order of major cations in the groundwater samples from the study area was Ca2+ > Na<sup>+</sup> > Mg2+ > K<sup>+</sup> , with average values of 96.62, 51.81, 26.43, and 3.99 mg/L, respectively. The order of major anions of the samples was HCO<sup>3</sup> − > SO<sup>4</sup> <sup>2</sup><sup>−</sup> > Cl−, with average values of 389.29, 79.19, and 37.49 mg/L, respectively.

Indicated by the detected results of the samples, there was no HN<sup>4</sup> + contamination in the groundwater of the study area because the maximum HN<sup>4</sup> + concentration of the samples (0.13 mg/L) was in the range of natural levels of HN<sup>4</sup> + in groundwater (below 0.2 mg/L), according to WHO [42]. The concentration of HN<sup>4</sup> + in water is an indicator of possible bacterial, sewage, landfill, and animal waste pollution [30]. The concentration of Cl− was not excessive in the analyzed samples from drinking water as it ranged from 2 to 144 mg/L. The WHO [42] has not set a health-based guideline value for Cl−, but a concentration exceeding 250 mg/L can cause the water to be unsuitable for drinking as high Cl− waters have a laxative effect for some people [33,55].

Although there is no health-based guideline value for Na<sup>+</sup> in potable water according to WHO [42], if its concentration exceeds 200 mg/L, it may taste bad, and excessive intake may cause hypertension [18]. Na<sup>+</sup> concentrations of four samples (8.3% of the total samples) slightly exceeded that threshold for the present study. A value of K<sup>+</sup> exceeding 12 mg/L in drinking water gives it a bitter taste [31]. In this study, only two samples (4.2%) exceeded this permissible limit. SO<sup>4</sup> <sup>2</sup><sup>−</sup> was not excessive, except in one sample, where its concentration exceeded (572 mg/L) the SO<sup>4</sup> <sup>2</sup><sup>−</sup> concentration limit proposed by WHO [30] for potable water, which is 500 mg/L.

To check the simultaneous occurrence of NO<sup>3</sup> − and NO<sup>2</sup> − in drinking water, the sum of the ratios of the concentration of each over its guideline value (*GV*) should not exceed 1 [42]:

$$\frac{\mathbf{C}\_{\text{nitrate}}}{\mathbf{G}V\_{\text{nitrate}}} + \frac{\mathbf{C}\_{\text{nitrite}}}{\mathbf{G}V\_{\text{nitrite}}} \le 1\tag{17}$$

where *Cnitrate* is the concentration of NO<sup>3</sup> <sup>−</sup>, *Cnitrite* is the concentration of NO<sup>2</sup> −, and *GVnitrat*<sup>e</sup> and *GVnitrite* are the guideline values of NO<sup>3</sup> − and NO<sup>2</sup> −, respectively.

The application of this formula reveals that 16.6% of the groundwater samples were in the situation of simultaneous occurrence of NO<sup>3</sup> − and NO<sup>2</sup> − in drinking water. Furthermore, in the presence of microbial contamination, especially due to fecal contamination in drinking water, the health risk to infants is high [42].

In this study, 6.2% of the groundwater samples slightly exceeded the guideline value of permissible concentration in drinking water, which is 0.05 mg/L [43]. Fluoride is important for drinking water, with a concentration ranging from 0.7 to 1.2 mg/L, as it protects against dental cavities and strengthens the bones. When F− concentration exceeds 1.5 mg/L, it causes teeth mottling, fluorosis or discoloration [33,42,60,61] as well as other health problems such as nervous system harm and urinary tract disease [62,63]. Although there were two samples with F− concentration exceeding 1.5 mg/L, most of the samples (83.3%) were associated with low F− concentrations below 0.7 mg/L. Therefore, to ensure the good health of the population in Tongchuan City, F− should be added in drinking water to the majority of wells and be reduced in a few wells to avoid potential health hazards. In addition, 50 mg/L of the guideline value for NO<sup>3</sup> − was established by WHO [42] to protect the most sensitive populations. However, this population must be free of adverse health effects such as methemoglobinemia and thyroid effects at a concentration below 50 mg/L of NO<sup>3</sup> −. This health risk can seriously affect bottle-fed infants when mathemoglobinemia is complicated by the presence of microbial contamination and subsequent gastrointestinal infection that manifests as diarrhea.

Excessive boiling of water for microbiological safety purposes may increase the concentration of NO<sup>3</sup> −. Water for drinking should be heated until it reaches a rolling boil [42]. For NO<sup>3</sup> −, 45.8% of the samples exceeded the limits (20 mg/L) set by the Ministry of Health of the P.R. China [43].
