What is meant by the term "run-in" regarding pesticide application?

The term "run-in" in relation to pesticide application refers to the movement of pesticides from the soil surface directly into groundwater. This phenomenon is significant because it highlights the potential for pesticides to contaminate water supplies, particularly when they are applied before significant rain events or in conditions that favor leaching. Understanding this process is crucial for pesticide applicators, as it reflects the importance of applying pesticides responsibly and in accordance with recommended practices to minimize environmental impact.

The other options address different aspects of pesticide behavior but do not accurately capture the essence of "run-in." The absorption of pesticides by plants is related to their effectiveness in controlling pests but does not involve groundwater movement. Accumulation in the soil layer explores a different aspect of pesticide persistence and does not involve direct movement into groundwater. Pesticide degradation refers to the breakdown of pesticides over time, which is also distinct from the direct transport of pesticides to groundwater, making the understanding of "run-in" specific to groundwater contamination.