*3.2. Water Quality Index (WQI)*

There have been many water quality assessment methods proposed by international scholars, such as set pair analysis [46–48], rough set and TOPSIS [49–52], entropy water quality index [53–55]. However, water quality index (WQI) is the most popular and widely adopted methods for overall water quality assessment [56,57]. In this study, the water quality index (WQI) was constructed using the weighted arithmetic average method as shown below [58].

$$\text{Calculation for water quality rating} \left(\text{Q}\_n\right) = 100 \times \frac{\left(V\_n - V\_0\right)}{\left(S\_n - V\_0\right)}\tag{1}$$

*Qn*: Water quality rating for the *n*th parameter, *Vn*: Observed value of the *n*th parameter, *V*0: Ideal value, *Sn*: Standard permissible value of *n*th parameter.

The unit weight of the corresponding parameter was an inverse proportional value to the recommended standard value of *S<sup>n</sup>*

$$\text{Calculation of unit weight } (W\_n) \; = \, \frac{\text{K}}{\text{S}\_n} \tag{2}$$

*Wn*: unit weight for the *n*th parameter, *Sn*: standard value of the *n* th parameter, *K* is the constant for proportionality: *K =* <sup>1</sup> Σ 1

*Sn* The total water quality index was calculated linearly by adding the quality rating to the unit weight:

$$\text{WQI} = \sum Q\_n \mathcal{W}\_n / \sum \mathcal{W}\_n \tag{3}$$
