The unsaturated zone is the portion of the subsurface above the groundwater table. The soil and rock in this zone contains air as well as water in its pores. In some places the unsaturated zone is absent, as is common where there are lakes and marshes, and in some places it is hundreds of meters thick, as is common in arid regions. Hydrologically, the unsaturated zone is often the main factor controlling water movement from the land surface to the aquifer. Flow rates and chemical reactions in the unsaturated zone control whether, where, and how fast contaminants enter groundwater supplies.