Which of the following is a primary concern of non-point source pollution?

Non-point source pollution is characterized by its diffuse nature, meaning that it can originate from a variety of sources rather than a single, identifiable source. This is a significant concern because it complicates both assessment and management. For instance, pollutants can come from agricultural runoff, urban areas, and even atmospheric deposition, all of which mix and contribute to water quality degradation. Identifying specific sources of pollution is challenging as they can vary based on land use, weather conditions, and hydrologic factors.

The other options present characteristics that do not accurately reflect non-point source pollution. The nature of non-point source pollution makes it inherently difficult to regulate, as there isn't a central point of emission to target. While rain events can exacerbate non-point source pollution by mobilizing pollutants, it is not limited to these conditions and can occur during dry weather as well. Lastly, non-point source pollution is not primarily industrial; it is often associated more with agricultural practices, urban runoff, and general land use, making the focus of the concern broader than just industrial activities.