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

from indoor taps could be due to the reasons alluded to above, as well as impurities in the zinc of galvanized pipes and some metal fittings [73]. In comparison to other countries, such as Canada, concentrations in our study were lower compared to the mean of 0.044 μg/L [74]. Similarly, a survey carried out in Malaysia found Cd concentrations from household tap water was high, the contamination of which was associated with indoor plumbing material. This could also be a possible explanation in our study, where the concentrations of Cd and Pb are higher in indoor tap water than in raw water [75].

Whilst Cd levels in water were within acceptable water quality standards, recent environmental investigations on Cd have demonstrated that prolonged and low-level exposure to Cd is a risk factor for adverse effects to many organs and systems such as kidneys, liver failure, skeletal system, certain cancers, and the cardiovascular system including hearing loss [10–12, 76].

The levels of arsenic in raw water fell within the expected levels in natural waters at concentrations of less than 1–2 μg/L [3]. Arsenic concentrations were highest in raw water (0.28 ± SD:0.14) μg/L, with a reduction after treatment (0.18 ± SD:0.03 μg/L), during distribution (0.20 ± SD:0.06 μg/L) and indoor household taps (0.23 ± SD:0.05) μg/L. These levels fell within the current recommended limit of 10 μg/L in drinking water [3]. Contamination of drinking water from reservoirs by As might be due to paint, fertilizers, pesticide and pharmaceutical industries, and agricultural activities [77]. However, the indoor As concentration was still higher than the distribution line concentration at 0.23 ± 0.05 μg/L. A metalloid widely spread in rocks, soil, water, and air is usually ubiquitous in the environment, leading to high human exposure [78]. The contamination of drinking water from taps might be due to leaching from water distribution pipes and corrosion of plumbing systems 79).

**Table 2** shows that Pb concentrations increased by 612.5% between raw water (0.08 ± [SD:0.02]) and after treatment (0.57 ± [SD:0.47]). This is indicative of contamination of water from the treatment plant. A 93% increase in lead concentration was detected between after-treatment and outdoor household tap water (distribution line) and an 8.2% increase between the outdoor and the indoor fixture. Among the heavy metals studied in New South Wales, Australia, almost 56% of 212 samples contained Pb, which exceeded the Australian Drinking Water Guidelines (ADWG) by 8% [79].

Our study showed that the mean concentration of Pb exceeded the set standards by U.S-Environmental Protection Agency of 0 μg/L. **Table 3** shows that the highest mean Pb concentrations were in Mogoditshane village (1.938 ± 1.587 μg/L) followed by Phase IV (1.874 ± 2.189 μg/L) and Tlokweng (1.802 ± 1.413 μg/L) with the lowest at Bontleng 0.445 ± 0.379 μg/L. The overall Pb mean concentration for indoor and outdoor water in all locations was 1.185 ± 1.254 µg/L and 1.096 ± 1.1451 μg/L, respectively.

The variability of lead concentrations from the different locations (**Table 2**) could be attributed to several factors, including the type of material used for plumbing, the age of the plumbing system, water flow rate, and standing time of the water at the time of sample collection [5, 80]. Investigations have shown that the newer the home, the greater the risk of lead contamination [81]. New homes fitted with soldered copper piping connections are estimated to release as much 210–390 μg/L of Pb that could cause intoxication, particularly in children [82]. Even though Pb can be leached from piping systems indefinitely, the leaching from soldered joints and brass taps may decrease with time. As buildings age, mineral deposits form a coating on the pipes' inside, reducing the leaching of Pb from soldered joints. These factors could have caused the variability on lead and other heavy metal concentrations from different locations. **Table 4** shows the different plumbing materials that households


*Poisoning – Prevention, Diagnosis, Treatment and Poison Repurposing*

**Table 3.** *Mean ± standard deviation of heavy metals by location.*


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

#### **Table 4.**

*Mean ± standard deviation of heavy metal concentrations by type of plumbing materials used in households.*

use. Copper and copper-based alloys had the highest mean Pb concentrations (1.40 ± 1.62 μg/L) followed by and galvanized steel fittings (1.18 ± 1.37 μg/L).

Pb is a harmful metal that can affect the nervous system, weaken the fingers, wrists, and ankles and may lead to high blood pressure and mental illness due to brain damage. According to Mohod and Dhote (2013), Pb can lead to serious biochemical effects and interference with heme synthesis, leading to hemoglobin disorder [64]. According to WHO (2022), even though the set standard is 10 μg/L, this is no longer a health-based guideline value because Pb has no threshold for critical health effects [2]. WHO further emphasizes that the Pb concentrations should be maintained as low as reasonably practical since lead is highly toxic to the human body even at a deficient concentration.

Triantafyllidou and Edwards (2012) concluded that the alloys that contain lead and are used for plumbing, fittings, and soldering are a source of contamination in household taps [83]. This is corroborated by WHO (2022), emphasizing that Pb corrodes more rapidly when it is coupled to copper (Cu) and that the rate of galvanic corrosion is faster than that of simple oxidative corrosion [73]. The Cu lining may result in general corrosion, impingement attack, and pipping corrosion. Galvanized pipes are the significant sources of zinc that can leach Cd and Pb in drinking water. This corrosion typically occurs when galvanized steel or iron piping is connected to inappropriate materials, such as brass in taps and fittings [73].

In this study, only mercury was highest (0.24 ± 0.9 μg/L) in raw water compared to indoor household tap water (0.18 ± 0.14 μg/L). Mercury concentrations from the indoor households' taps had lower mean concentration as compared to the outdoor/distribution water line: 0.181 ± 0.138 and 0.192 ± 0.172 μg/L, respectively but lower than the permissible limit by US-EPA, WHO, and BOBS. Mogoditshane (0.263 ± 0.302 μg/L) had the highest mean concentration of Hg of any other location, with the lowest at Maruapula (0.129 ± 0.043 μg/L). A study evaluating the concentration of mercury, zinc, arsenic, lead, and cobalt in the Ilam city water supply network showed mercury levels ranging from 0.605 ± 0.1938 μg/L to 2.15 ± 0.0233 μg/L [84]. Global occurrence of potable water HM contamination has variably affected developed and developing countries [85].

None of the bottled water contained Pb and Cd. However, Hg (0.24 ± 0.06 μg/L) and As (0.15 ± 0.020 μg/L) were detected. These levels were less than the permissible limit by the US-EPA, WHO, and BOBS. Compared with research from elsewhere, the results obtained in Nigeria showed that in all the bottled water, Pb was found to be lower than the permissible limits by WHO and BOBs (10 μg/L) [86]. Whilst Pb was not detected in bottled water (**Table 2**) in our study, Hg was detected at slightly higher concentrations in bottled water than in tap water. Additionally, bottled water had slightly lower AS concentrations than indoor tap water. These findings are similar to other studies, which showed higher heavy metal concentrations in tap water compared to bottled water [87, 88]. A similar study carried out in Croatia showed that tap water contained higher levels of Cd, Cu, and Pb among others than bottled water [89].

The authors concluded that partial solubilization of the materials involved in the treatment, such as metal supply systems, tanks, pipes, valves, and pumps, were possible origins of the heavy metals. The low levels of heavy metals in bottled water could be due to filtration through multi-barrier filtration systems, reverse osmosis, and microfiltration. Other treatments may include exposure to ultraviolet light or ozonation. It is estimated that these methods remove up to 80% of H.M. [90]. This could explain bottled water's non-detection or low-level heavy metal content. Finally, the slight elevation of Hg in our bottled water results resembles those of other studies [87].
