What is meant by backflow in pesticide application?

Backflow in the context of pesticide application refers specifically to the situation where pesticides are inadvertently siphoned or drawn back into the water supply or source due to a drop in water pressure. This can happen if there is a significant change in the pressure of the water system, such as when water is being drawn from the system faster than it can be replenished.

When this occurs, the negative pressure can create a vacuum that pulls water—and any pesticides mixed in it—back towards the source. This is a critical concern in pesticide application because it can lead to contamination of the drinking water supply or other water sources, posing environmental and health risks.

The other options do not accurately capture the essence of backflow: water pressure engaging the pesticide equipment pertains to operational aspects but does not define backflow; the water source running dry is unrelated to the dynamics of pressure causing backflow; and introducing clean water into the pesticide mixture does not reflect the reversal of flow characteristic of backflow situations. Thus, the definition provided in the correct choice effectively highlights the potential hazards associated with backflow during pesticide application.