**4.3 Sample preparation and instrumental analysis procedures**

All reagents and chemicals used in this study were of analytical grade (Argon 99.999% purity, HNO3 Analytical Grade). Samples were filtered to remove suspended solids and improve turbidity using glass membrane filters (Whatman™ 0.45 μm pore size), digested using microwave digestion method, and subsequently assayed using the inductively-coupled plasma mass spectrometer (ICP-MS, Agilent™ 7900). Microwave digestion entailed adding 2.5 mL of concentrated HNO3 to 50 mL aliquots of samples and method blanks and digesting at 170°C for 30 min. The ICP-MS was optimized for the best sensitivity according to the standard operating procedure

**Figure 2.** *Drinking water sample collected from (a) indoor household tap and (b) outdoor tap.*

detailed in the CMP33 method [66]. Calibration standards in the range 1 to 500 μg/L were prepared daily by appropriate dilution of the 1000 mg/L standards containing target analytes, and this includes quality control samples. To ensure accuracy and eliminate human error, a calibrated Hamilton auto-diluter was used for all dilutions.

#### **4.4 Heavy metal pollution index**

To determine the overall water quality of drinking water in the distribution system, a heavy metal pollution index (HPI) was determined using Eqs. (1) and (2) [67]. In this study, metals considered in the HPI index calculation included Cd, As, Pb, and Hg.

$$HPI = \sum\_{i=1}^{n} W\_i Q\_i \tag{1}$$

Where Wi is the weighting factor of the ith metal and is a value ranging between zero and one, and Qi is the sub-index of ith parameter and is determined

$$\mathbf{Q}\_{i} = \sum\_{i=1}^{n} \frac{\mathbf{M}\_{i} - I\_{i}}{\mathbf{S}\_{i} - I\_{i}} \mathbf{n} \times \mathbf{100} \tag{2}$$

*Where;*

Mi is the analyzed result of the ith parameter in μg/L, Si is the permissible limit value given in the standard for the *i* th parameter, and Ii is the ideal value of the ith parameter. The n in both equations denotes the number of heavy metal parameters considered in the study, *n* = 4.
