How can pesticides reach groundwater through soil?

Leaching is the process through which soluble substances, such as pesticides, are washed out from the upper layers of soil and permeate down into the groundwater. When pesticides are applied to the soil, especially if the soil is wet or saturated, they can dissolve in water and be carried downward through soil pores. The rate at which leaching occurs is influenced by various factors, including the chemical properties of the pesticide, soil type, and rainfall.

Other processes like runoff, evaporation, and surface drainage do not directly involve the movement of pesticides into groundwater. Runoff pertains to the flow of water over the soil surface, which can transport pesticides to surface water bodies rather than infiltrating through the soil to reach groundwater. Evaporation involves the transition of water from liquid to vapor but does not contribute to the downward movement of chemicals into the soil and groundwater. Surface drainage typically refers to the movement of excess water off the land surface, also not resulting in leaching of substances into the groundwater beneath the soil. Thus, leaching is the primary mechanism by which pesticides can migrate through soil layers and contaminate groundwater sources.