Which of the following is a method by which pesticides can move into groundwater?

• A. Runoff
• B. Vapor drift
• C. Leaching
• D. Erosion

Answer: (C)

Pesticides reach groundwater mainly through leaching, the downward movement of dissolved chemicals with infiltrating water as it percolates through the soil toward the water table. This process happens when rainfall or irrigation drives water through the soil profile, and pesticides that are soluble or weakly sorbed can be carried downward. Factors that influence leaching include the pesticide’s solubility, how strongly it binds to soil, soil texture (lighter, sandy soils transmit water more easily), organic matter content, and the depth to groundwater. Runoff moves pesticides across the surface into surface water, not directly into groundwater; vapor drift involves movement through the air, not into groundwater; erosion removes soil (and pesticides with it) but does not inherently transport pesticides down to the groundwater.