The quality and quantity of surface water varies from one place to another and over time, due to factors such as geology, climate and surrounding land use.
Why is surface water classed as an unimproved source?
Because rivers and lakes can be easily contaminated by run-off that washes pollutants into the water. Lack of effective sanitation and open defecation make contamination by microbiological pathogens much more likely.
The variable quality of surface water means it has to be treated to make it safe for domestic consumption. There are several different methods of water treatment at large and small scale that are described in later study sessions. The quantity of water in rivers and lakes obviously varies with rainfall and there can be wide fluctuations at different times of year. To ensure year-round supply, dams can be constructed to create reservoirs from which water can be extracted prior to treatment (Figure 3.12).