Which of the following pollutants is most associated with agricultural runoff?

Nitrates are most associated with agricultural runoff primarily because they are commonly used in fertilizers to enhance crop growth. When it rains or when irrigation occurs, excess nitrates can leach into nearby water bodies through surface runoff or through groundwater. This influx of nitrates can lead to a phenomenon known as eutrophication, where nutrient overload causes excessive growth of algae in water bodies, depleting oxygen levels and harming aquatic life.

In contrast, other pollutants listed, such as lead, PCBs, and arsenic, typically originate from different sources. Lead often comes from urban runoff, industrial processes, or deteriorating infrastructure rather than agricultural activities. PCBs are primarily associated with industrial discharges and were commonly used in electrical equipment and lubricants, while arsenic is often linked to mining operations or contamination of groundwater from certain geological formations.

Thus, the connection of nitrates to fertilizer use and their direct pathway into aquatic systems through agricultural practices makes them the most relevant pollutant in this context.