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. These factors are summarized in Table I . Similar factors influence pesticide movement in surface runoff, except that pesticides with low water solubility may move with surface runoff if they are strongly adsorbed to soil particles and have some degree of persistence. As shown in Table I , soil characteristics are important to pesticide movement. For example, clay soils have a high capacity to adsorb many chemicals including pesticides and soil nutrients.
Sandy soils have a much lower capacity to adsorb pesticides.
Organic matter in the soil also can adsorb pesticides. Soil structure influences the movement of water and pesticides. Coarse textured sandy soils with large macropores allow more rapid movement of water than fine textured or compacted soils with fewer macropores. Other characteristics of the site, such as depth to groundwater, or distance to surface water, are important. Finally, the pattern of water falling on the soil through irrigation or rainfall is significant.