**1. Introduction**

Heavy metals are defined as naturally occurring cations having atomic number (Z) greater than 20 g/cm3 and an elemental density greater than 5 g/cm3 [1]. Their

unintended use in industrial, domestic, agricultural, medical, and technological applications has led to their wide distribution in the environment, and this has generated concerns over their detrimental effects on human health and the environment [2]. International organizations, such as the World Health Organization (WHO) and the United States Environmental Protection Agency (U.S-EPA), have set regulatory standards on permissible concentration levels of heavy metals in dietary constituents, such as drinking water [2, 3]. Beside existing in elemental forms, toxic metals, such as cadmium (Cd), arsenic (As), lead (Pb), and mercury (Hg), are also found as mixtures in various parts of the ecosystem [4]. Drinking water and other dietary constituents are considered as the main entry points of heavy metals into the human food chain [5, 6]. Other sources, although negligible, include pharmaceuticals and human care products and their metabolites.

Even at low doses, these metals accumulate in the body, causing detrimental health effects [7]. Cd, As, Pb, and Hg, referred to as CALM in this chapter, are closely linked to hypertension, neurological disorders, cognitive impairments, cerebral palsy, blindness, dysarthria, cancers, cirrhosis, and hyperkeratosis [8–12]. These metals, albeit in excessive concentrations and exposure, have no known health benefits in human physiology and are thus considered toxicants [13, 14]. Further, accumulation of CALM in the body may lead to a steady reduction in intelligence quotient (I.Q.) an acute reduction in mental concentration span, and have been linked to behavioral delinquency and violence [15, 16].

Due to limited technical and economic capacities, the bulk of developing countries, especially in sub-Saharan Africa, are incapable of monitoring human exposure to heavy metals in drinking water [5]. Botswana, a middle-income country, is not spared from these technical challenges. Until now, water authorities have placed emphasis on the microbiological quality of drinking water [17].

The aim of this study was to determine concentrations of the CALM cations (As, Cd, Pb, and Hg) in drinking water in the greater Gaborone area (**Figure 1**). In addition, the study evaluated the contribution of household plumbing and piping systems to the overall concentrations of CALM cations in drinking water.

**Figure 1.** *Gaborone drinking water distribution system map showing sampling sites.*

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