How is a pollutant best defined?

A pollutant is best defined as a substance that negatively impacts water quality, regardless of whether it is toxic or nontoxic. This definition encompasses a wide range of materials, which can include sediments, nutrients, and various other compounds that may alter the physical, chemical, and biological characteristics of water.

In the context of water quality, pollutants can disrupt ecosystems, harm aquatic life, and affect human health and recreational activities. Thus, recognizing that pollutants can be nontoxic yet detrimental to water quality allows for a broader understanding of water management and environmental protection efforts.

Other options misinterpret the concept of a pollutant. A toxic substance causing health risks might be a pollutant, but not all pollutants are necessarily toxic. Similarly, a chemical used for filtration may improve water quality but does not fit the definition of a pollutant. Finally, a natural component of water is typically not classified as a pollutant unless it is present in such quantities that it causes adverse effects.