(1) Mole fraction

The mole fraction is the ratio of the amount of substance in a solution to the sum of the amounts of substance in each component, and the equation is listed below [31]. The mole fraction can reflect the relative amount of a substance in a solution and is the basis of the classification of hydrochemical facies and ion proportional coefficient method.

isotopic data. The methodology flowchart (Figure 2) is shown below.

isotopes were conducted to analyze the source of groundwater. The relationship analysis among NO3−, SO42−, Cl−, Na+ and K+ and the 15N(NO3) and 18O(NO3) isotopes were com-

In this study, the hydrochemical characteristics of groundwater were analyzed from four aspects: the concentration characteristics of hydrochemical indices, spatial distribution, hydrochemical facies and correlation analysis among hydrochemical indices. Second, the hydrochemical evolution mechanisms were further studied by applying hydrochemical methods. Then, the main source of groundwater was recognized by using the D–18O isotope technique. Based on the evaluation of groundwater quality, the main sources of nitrate in groundwater were identified by integrating the hydrochemical and

bined to accurately identify nitrate contamination.

2.2.2. Methods

**Figure 2.** Methodology flowchart. At present, many methods are used to proceed the hydrochemcial research, such as Piper diagram, correlation analysis, Gibbs diagram, ion proportional coefficients, cluster–based methods [14,31–37]. According to previous studies and the study objectives, we used **Figure 2.** Methodology flowchart. At present, many methods are used to proceed the hydrochemcial research, such as Piper diagram, correlation analysis, Gibbs diagram, ion proportional coefficients, cluster–based methods [14,31–37]. According to previous studies and the study objectives, we used Excel to complete the statistical analysis. Statistics of the minimum, maximum, average and medium were used to reflect the concentration characteristics of the indices. The standard deviation (SD) was applied to reflect the variation degree between the average value and the actual value, and the coefficient of variation (*Cv*) was used to indicate the dispersion degree [38]. Based on the geostatistical methods, the spatial distributions of the main hydrochemical indices were obtained by using the Kriging interpolation with the help of ArcGIS 10.3. The Piper diagram drawn by Aquachem 4.0 and the iso–ionic–salinity (TIS) diagram were applied to classify the hydrochemical facies and the salinity distribution. Pearson correlation was undertaken to analyze the relationships among the hydrochemical indices via IBM SPSS Statistics 22. Gibbs diagram, ion proportional coefficients were conducted to reveal the hydrochemical evolution mechanism. Based on the binary phase diagram and chloro – alkaline indices, the cation exchange was judged. Integration of the δ <sup>15</sup>N(NO<sup>3</sup> ) – δ <sup>18</sup>O(NO<sup>3</sup> ) dual isotope technique and hydrochemical analyses was designed to trace the nitrate contamination. Origin 2020 was used to plot Gibbs diagram, TIS diagram, binary phase diagrams, and isotope distribution figures. The details of the methods are listed below.

$$\mathcal{C}\_{i} = \frac{\rho\_{i}/M\_{ri}}{\sum\_{i=1}^{n} \rho\_{i}/M\_{ri}} \times 100\% \tag{1}$$

where *C<sup>i</sup>* is the mole fraction of the *i* ion, %; *ρ<sup>i</sup>* is the mass concentration, mg/L; and *Mri* is the relative molecular mass, 1.
