How does groundwater become contaminated with pesticide?

• A. When irrigation runoff carries pesticides into streams
• B. When recharge water carries dissolved chemicals through the soil profile into the water table
• C. Through direct aerial deposition onto the water table
• D. By evaporation from the soil surface

Answer: (B)

Groundwater contamination from pesticides mainly happens when water moves downward through the soil and carries dissolved pesticides with it to the water table. This infiltrating recharge water from rain or irrigation percolates through the soil profile, dissolving pesticides and transporting them downward until they reach groundwater. That downward movement of dissolved chemicals into the water table is the best description of how groundwater becomes contaminated.

Other options describe surface processes or non-downward transport. Irrigation runoff into streams affects surface water rather than groundwater directly. Direct aerial deposition onto the water table isn’t a realistic pathway for groundwater contamination, since pesticides deposited on the land or in the air don’t immediately enter groundwater. Evaporation from the soil surface reduces moisture, and while pesticides may volatilize, evaporation itself doesn’t move dissolved pesticides down into groundwater.