*2.3. Water Quality Index*

The WQI approach can not only comprehensively express the water quality information of groundwater but also quantitatively evaluate and compare the pollution degree of different water quality parameters [57]. This index is a mathematical instrument used to transform large quantities of water characterization data into a single number, representing the water quality level [32]. Firstly, each chemical parameter was assigned a weight (w<sup>i</sup> ) according to its impact on human health and groundwater quality. In this study, the highest weight of 5 was assigned to the parameters like TH, TDS, SO<sup>4</sup> <sup>2</sup>−, F−, Fe, Mn, Cd, Cr6+, and Pb due to their major importance in water quality assessment. These parameters are characterized by serious health effects and, when above critical concentration limits, may limit the usability of groundwater for domestic and drinking purposes [37,58]. Other parameters were assigned different weights ranging from 2 to 4. The relative weight is computed using the following formula:

$$\mathbf{W\_i} = \frac{\mathbf{w\_i}}{\sum\_{i=1}^{n} \mathbf{w\_i}} \tag{1}$$

where W<sup>i</sup> is the relative weight, w<sup>i</sup> is the weight of each parameter, and n is the number of parameters.

Then, the quality rating for each parameter is assigned by dividing its concentration in each water sample by its limit defined by the Chinese national standards [59] and multiplying the result by 100:

$$\mathbf{q}\_{\mathrm{i}} = \frac{\mathbf{C}\_{\mathrm{i}}}{\mathbf{S}\_{\mathrm{i}}} \times 100 \tag{2}$$

where q<sup>i</sup> is the quality rating, and C<sup>i</sup> is the concentration of each parameter in each water sample. S<sup>i</sup> is the drinking water standard for each parameter set by the Chinese national standard [59].

To calculate the WQI, the SI<sup>i</sup> has to be determined firstly:

$$\mathbf{SI}\_{\mathbf{i}} = \mathbf{W}\_{\mathbf{i}} \times \mathbf{q}\_{\mathbf{i}} \tag{3}$$

$$\text{WQI} = \sum\_{\mathbf{i}=1}^{\mathbf{n}} \text{SI}\_{\mathbf{i}} \tag{4}$$

where SI<sup>i</sup> is the subindex of the ith parameter. The WQI values are classified into five categories: excellent water (<50), good water (50–100), poor water (100–200), very poor water (200–300), and unsuitable water (>300) [57].
