Generally, the risk of surface or groundwater contamination is greatest when applications are followed immediately by heavy rain.

• A. A Sudden Drop In Barometric Pressure
• B. A Cold Front
• C. Heavy Rain
• D. Prolonged Drought

Answer: (C)

The risk is driven by how weather after application can transport pesticides. When heavy rain comes right after you spray, pesticides on leaves or in the soil don’t have time to bind tightly or begin degrading. The rain physically washes them off surfaces and carries them into surface water as runoff, or pushes them downward through the soil toward groundwater (leaching). This transport happens more with high rainfall intensity and short time between application and rain, especially for pesticides that are more soluble in water or less strongly adsorbed to soil particles.

In contrast, a sudden drop in barometric pressure doesn’t create a direct movement path for pesticides. Prolonged drought minimizes water movement, reducing runoff and leaching. A cold front may bring rain, but the crucial factor is the heavy rain itself and how soon it follows the application.