Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .386 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .386 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .388

Chemicals become associated with surface soils as a result of various human activities and other natural processes. These activities include the direct placement of waste materials onto the soil for storage or treatment and the application of pesticides for the control of agricultural pests. In addition, some chemicals arrive at the soil surface by the process of deposition from the air phase. These substances may originate from nearby or remote sources including power plant stack emissions and volcanoes in either vapor or particulate form and are transported via the atmospheric pathway prior to deposition. Once in place on the soil surface mixing and transport processes occur within the upper layers to move this material deeper into the soil column. Once contaminated with chemical substances, it is not uncommon for the soil surface layers to be the source of chemical transport back to the atmosphere. Evaporation and particle resuspension contribute to atmospheric transport but mixing in the upper soil column actively moves contaminants upwards and maintains an elevated concentration level at the surface. Consequently soilmixing in the upper layers is important to quantifying the transport chemodynamics in either direction. This chapter examines soil mixing processes in more detail.