(2) Shukalev classification

Shukalev classification is a common method of classifying hydrochemical facies and mainly based on the mole fractions of the main ions (Ca2+, Mg2+, Na<sup>+</sup> + K<sup>+</sup> , HCO<sup>3</sup> −, SO<sup>4</sup> 2− and Cl−). Ions with a mole fraction greater than 25% should participate in the classification of the hydrochemical facies. Based on this, there are 49 hydrochemical facies [30].

Noticeably, the NO<sup>3</sup> − ion is not considered according to the Shukalev classification. This is mainly because the NO<sup>3</sup> −content is lower than 25% in the natural groundwater environment. However, due to the disturbance of human activities, the content of NO<sup>3</sup> − may increase and influence the hydrochemical evolution. Kpa o et al. [39] pointed out that the new hydrochemical types (NO3–Ca and NO3–Na type) were widely found in many agricultural areas of the former Soviet Union and the United States. Huang et al. [40] improved the Shukalev classification and took NO<sup>3</sup> − into consideration when determining the hydrochemical facies. Due to this, the NO<sup>3</sup> − ion was taken into consideration in this study to indicate nitrate contamination.
