what is the major difference between the saturated and unsaturated zones?
Unsaturated Zone. 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. Unlike the aquifers of the saturated zone below, the unsaturated zone is not a source of readily available water for human consumption.
The pores and fractures in soil and rocks are saturated with water. The zone of saturation is less corrosive than the unsaturated zone above the water table. The moisture content in the region is at one extreme while the other extreme is in the dry soil. Maximum corrosion occurs at the intermediate of the two extremes of soil-moisture content.