**6. Results and discussion**

#### **6.1 Water sample collection sources**

A total of 200 water samples were collected from different water sources ranging from raw untreated water to indoor household tap water (**Figure 3**). The majority of water samples were from outdoor household tap water (48%) followed by indoor household tap water (43%). In contrast, raw water was collected from three reservoir tanks, accounting for 2% of the total samples collected; similarly, commercial bottled water comprised 4% of the total samples.

#### **6.2 Evaluation of physicochemical parameters**

The measured values of pH, temperature, and total dissolved solids (TDS) were all within the recommended limits in this study. Our results showed an overall mean pH of 7.93 ± 0.29, against a pH = 8 or less as recommended by WHO (2020). Similarly, the

*Occurrences of Cadmium, Arsenic, Lead, and Mercury in Potable Water in Greater Gaborone… DOI: http://dx.doi.org/10.5772/intechopen.113716*

#### **Figure 3.**

*A bar chart depicting sample collection sources. The numbers within each bar represent the actual individual samples collected at each point.*

US EPA recommends a pH level of between 6.5 and 8.5 in drinking water for public consumption [3]. This is in agreement with the WHO [2]. A pH lower than 6.5 is considered acidic and likely to be corrosive. While pH generally has no direct impact on consumers health, it is among the most important water quality parameters for consideration at all stages of water treatment because low pH levels readily ionize cations and make them available and vice versa in that at high pH (basic conditions) cations are bound to organic complexes, and therefore unavailable. Therefore, pH is a necessary parameter during water treatment and plays a critical role in releasing heavy metals *via* oxidative processes such as corrosion [62].

A slight increase in pH was observed in samples collected in the afternoon (7.97 ± 0.17) compared to the morning (7.94 ± 0.20). This may be associated with the increase in water temperature from 27.0 (±2.2) in the morning to 29.5 (±2.1) in the afternoon. It has been shown that as water temperature increases and pH also increases [2, 68]. These values were within the permissible limits set by the US EPA, ranging from 6.5 to 10.5 [69]. Further, WHO (2022), advises close monitoring of the pH of water entering the distribution system to minimize the corrosion of water mains and pipes in household water distribution systems as nonadherence to such may result in the contamination of drinking water and the deterioration of taste, odor, and turbidity [2].

Similarly, TDS is among the most significant factors in giving water an acceptable taste, as well as in providing important elements such as calcium (Ca), magnesium (Mg), and potassium (K) [3]. The US EPA and WHO recommend a TDS of less than 500 and 600 mg/L, respectively. The overall mean TDS measured in this study complied with the threshold values above. For example, a mean TDS of 111.3 (±28.6) mg/L was measured for samples collected in the morning and 112.7 (±5.5) mg/L for samples collected in the afternoon. High TDS levels have been blamed for the accumulation of C.A. and Mg scales in water boilers and heaters.

Mittal et al. (2017) found that TDS in groundwater samples ranged from 535 to 2460 mg/L with a mean value of 1192.5 mg/L [70]. The study further showed that the lower the TDS, the lower the radioactivity value, and this was proven by positive correlation of TDS with heavy metals. There was no direct impact of TDS on the human body except that it leads to hard water, salty taste, and films on fixtures leaving deposits, eventually leading to corrosiveness, which, in turn, results in the leaching of heavy metals in drinking water [70, 71].
