2.4.1. Water Quality Index (WQI)

To evaluate groundwater quality status in the study area, method of water quality index (WQI) was used to integrate comprehensive information through the analysis of physicochemical parameters [31,48–51]. In other words, WQI is a single numerical value obtained by combining a large water quality data [52,53]. First, each chemical parameter is assigned with a weight (*w<sup>i</sup>* ), which is determined by affecting the degree of the parameters to groundwater quality. The relative weight (*W<sup>i</sup>* ) is computed as:

$$\mathcal{W}\_{i} = \frac{w\_{i}}{\sum\_{i=1}^{n} w\_{i}} \tag{3}$$

where, *W<sup>i</sup>* is the relative weight, *w<sup>i</sup>* is the assigned weight of each parameter, *n* is the number of parameters. The value of *w<sup>i</sup>* ranges from 1 to 5 according to the impact of the contaminant on human health.

Then, the quality rating scale (*q<sup>i</sup>* ) can be computed by:

$$q\_i = \frac{\mathcal{C}\_i}{\mathcal{S}\_i} \times 100\tag{4}$$

where, *q<sup>i</sup>* is the quality rating scale, *C<sup>i</sup>* is the concentration of each chemical parameter in each water sample in mg/L, and *S<sup>i</sup>* is the standard for each chemical parameter.

To calculate the WQI, *SI<sup>i</sup>* has to be determined with the following equations:

$$SI\_i = \mathcal{W}\_i \times q\_i \tag{5}$$

$$\text{WQI} = \sum SI\_{\bar{l}} \tag{6}$$

where, *SI<sup>i</sup>* is the sub-index of the *i*th parameter and WQI is the water quality index.

The computed WQI values are classified into five categories [15,31,48,54]: excellent water (<50), good water (50–100), poor water (100–200), very poor water (200–300), and unsuitable water (>300).
