Breakdown is considerably slower in deeper soils and sediments. Some pesticides form intermediate substances during the breakdown process which may be more toxic than the original compound. The longer the half-life, the more persistent the chemical. Pesticides continue to degrade once they reach ground water, however, breakdown is generally much slower. Persistence and adsorption are the two most important characteristics of a pesticide affecting its potential to leach to ground water.
Adsorption describes how tightly a compound becomes attached to soil particles.
Some pesticides may be too tightly adsorbed to give proper pest control. It is another property that affects the behavior of a pesticide in the soil. As water percolates through soil, it carries water-soluble chemicals with it. This process is called leaching. The higher the water solubility value, the more soluble the chemical.