The boundary separating the zone of aeration and the zone of saturation is the water table. As groundwater seeps through the earth, different concentrations are present at each level. At some point, the ground becomes completely saturated and creates the water table.
Some areas of soil are very porous, with many gaps in which water can accumulate. Other areas may be impermeable to water. The rate at which water flows through soil is called permeability. When water reaches an impermeable rock and begins to flow laterally, an aquifer is created. Low permeability near the surface of the soil greatly increases the potential for flooding.