Which process involves the downward movement of pesticides through soil that may contaminate groundwater?

The correct answer is leaching. This process refers to the downward movement of water-soluble substances, such as pesticides, from the soil surface through the soil profile. When heavy rainfall or irrigation occurs, water can carry these dissolved substances down through the soil layers and potentially reach the groundwater aquifers. This poses a significant risk of groundwater contamination, which is a critical concern for environmental safety and public health.

In contrast, drift involves the movement of pesticide particles through the air, typically from the intended application area to non-target areas, which does not directly affect groundwater. Runoff occurs when water, often after rainfall, flows over the land surface and can carry pesticides into nearby rivers, lakes, or streams, but this process is surface-level and does not involve the downward movement necessary for groundwater contamination. Evaporation refers to the process of water turning into vapor and leaving a surface, which does not pertain to the movement of pesticides through soil. Understanding leaching is essential for herbicide applicators to implement practices that minimize the risk of groundwater contamination.