ABSTRACT
Soils make up the upper part of the unsaturated zone, where water flow occurs mainly under unsaturated conditions. The unsaturated zone consists of a complex arrangement of mostly connected solid, liquid, and gaseous phases, with the spatial distribution and geometrical arrangement of each phase, and the partitioning of solutes between phases, controlled by physical, chemical, and biological processes. The unsaturated zone is bounded by the soil surface and merges with the groundwater in the capillary fringe (see the article Soil Water: Flow under Saturated Conditions). The distinction between groundwater and water in the unsaturated zone is determined by the degree of water saturation (see the article Soil Water: Capillary Rise ). For groundwater, it is generally assumed that the pore space within the solid matrix is saturated with water, and that the hydrostatic pressure in the water is larger than atmospheric pressure. In contrast, in the unsaturated zone, the pore space is only partly filled with water, while the remaining space is occupied by the gas phase. Water is held in the soil matrix of the unsaturated zone by capillary and adsorptive forces. The unsaturated zone is usually considered to be the region for water flow and its concomitant transport of chemicals between the atmosphere and groundwater. Although, the importance of water as carrier of chemicals is paramount, it is becoming increasingly clear that chemical and biological phenomena in the unsaturated zone play a profound role on chemical fate. It is therefore that vadose zone notation is preferred, emphasizing the multidisciplinary approach in subsurface characterization.
