What is a method by which pesticides can enter groundwater?

Pesticides can enter groundwater primarily through the process of leaching, which involves the movement of water through the soil profile. When it rains or when there is irrigation, water percolates down through the soil layers. If pesticides are present in the upper layers of soil, they can dissolve in the water and be carried downward with it. This process allows pesticides to reach the groundwater, which can then contaminate drinking water supplies and harm ecosystems.

The other methods listed are less relevant to the issue of pesticide contamination in groundwater. For instance, evaporation pertains to the transformation of liquids into gas, which doesn't contribute to groundwater contamination. Surface runoff involves water flowing over the soil surface, which could lead to pesticide pollution in nearby surface waters but not directly to groundwater. Lastly, plant absorption refers to the uptake of pesticides by plants, which does not involve groundwater unless those plants eventually contribute to leaching through their root systems. Thus, leaching stands out as the most direct mechanism for pesticides to reach groundwater.