**4. Materials and methods**

#### **4.1 Study area**

Gaborone is Botswana's capital and largest city with a population of 231,626 based on the 2011 census, which is about 10% of the total population of Botswana. The city is supplied by raw water from Gaborone, Bokaa, Letsibogo, and Dikgatlhong dams. Raw water from the northern part of the country (Letsibogo and Dikgatlhong dams) is transported *via* the north-south carrier pipeline and undergoes treatment at different water treatment plants before it is eventually distributed to the existing distribution network.

#### **4.2 Sampling**

**Figure 1** shows the Gaborone drinking water distribution system map and sampling sites. Sample collection and analysis procedures were performed in accordance with quality assurance measures prescribed by the Council for Scientific and Industrial Research (CSIR) Environmental Laboratory (Method No. CMP33).

The method was validated using conventional analytical parameters, such as linearity, limit of detections (LODs), and spike and recovery experimentation. In all instances, field and method blanks were assayed to assess potential cross-contamination.
