In what scenario would surface water typically require treatment?

Surface water typically necessitates treatment primarily because it is exposed to the atmosphere and subject to runoff. This exposure means that surface water can easily become contaminated by various pollutants. Runoff can introduce fertilizers, pesticides, sediments, nutrients, and pathogens into the water from surrounding lands. These contaminants can pose significant health risks if the water is consumed without proper treatment.

In contrast, water derived from natural springs is generally cleaner due to its natural filtration through soil and rock layers. Rainfall collected in isolation, without any interaction with the ground or contaminants, may also be quite pure, though it can still require testing for safety. Lastly, water stored in household tanks can vary in quality based on its source and the cleanliness of the tank, but the need for treatment would depend more on those factors than on being classified as surface water. Hence, the exposure of surface water to various potential contaminants is why it typically requires treatment.