How can nonpoint source pollution be defined?

Nonpoint source pollution is characterized by water-borne pollutants that are carried into bodies of water through runoff rather than from a single, identifiable source. This type of pollution typically arises when rainwater or melting snow flows over various surfaces like roads, fields, or urban areas, picking up numerous contaminants such as fertilizers, oils, sediments, and chemicals along the way. Because nonpoint source pollution does not originate from a discrete outlet, it poses significant challenges for management and regulation.

The other choices refer to more defined and controlled types of pollution. Air pollutants from industrial facilities are typically considered point sources since they emanate from specific locations. Municipal waste discharges involve controlled outputs from cities, which again can be traced back to specific points. Similarly, discharges from sewage treatment facilities are regulated point sources that can be monitored and treated before release. Thus, the essence of nonpoint source pollution lies in its diffuse nature and the multiple potential sources contributing to water quality degradation, encapsulated well by the description in the correct answer.