Sometimes pesticide transfer is essential for pest control. For example, certain preemergence herbicides must move within the soil to reach germinating weed seeds. Volatilization occurs when a liquid or solid converts to a gas and moves away from the initial application site. Runoff occurs when water is added to a field faster than it can be absorbed into the soil. Leaching is the downward movement of chemical through the soil, eventually reaching the groundwater.
Absorption is the uptake of pesticides or other chemicals into the plant or animal.
After absorption, the pesticide residue may be broken down or remain in the plant or animal until harvest. Crop removal through harvest or grazing may move pesticide residue. Degradation is the process of pesticide breakdown after application by either microbial action, chemical action or photodegradation. This process may take hours, days, weeks or years, depending on environmental conditions and the chemical characteristics of the pesticide. The factors influencing whether pesticides will be leached into groundwater include characteristics of the soil and the pesticide, and their interaction with water from irrigation or rainfall.