Which of the following are examples of non-point source discharges?

Non-point source discharges are characterized by the lack of a single identifiable source, often arising over a wide area and collecting pollutants as they flow to water bodies. Among the provided options, highway runoff exemplifies a non-point source discharge because it consists of rainfall or melted snow that collects pollutants such as oil, heavy metals, and sediments from the road surface as it drains into nearby streams or rivers. This runoff is distributed across a large area and does not originate from a single point, making it a classic example of non-point source pollution.

In contrast, wastewater treatment plant discharges are classified as point source discharges because they originate from a specific, identifiable facility. Septic tank leachate, while it can contribute to groundwater contamination, typically leads to issues in localized areas rather than broadly affecting surface waters as a non-point source would.

Thus, the option highlighting highway runoff is supported by its nature as a diffuse source of pollutants, distinguishing it as a clear example of non-point source pollution. The combination of septic tank leachate and highway runoff reflects a broader understanding of non-point sources, but since the question seeks an example, the focus solely on highway runoff aligns with the defining characteristics of this type of pollution.