Which contaminants affect mainly the aesthetic qualities of water?

Secondary contaminants primarily affect the aesthetic qualities of water, such as taste, odor, and appearance. These substances do not typically pose direct health risks but can make the water less appealing for consumption and use. Common secondary contaminants include substances like chlorine, which can give a distinct smell and taste, as well as iron and manganese, which might cause staining or color issues in water.

In contrast, while microbial contaminants can cause serious health risks and heavy metals can be toxic, they don't usually impact the aesthetic qualities of water in the same way. Primary contaminants also focus more on health risks rather than aesthetics. Therefore, secondary contaminants are the correct choice when considering substances that mainly affect how water looks, tastes, or smells.