For effective wastewater treatment, soil must be relatively permeable and should remain unsaturated for several feet below the system depth.
Soil absorption is the ability of soil to combine with, and hold water and other liquids. It is the process of soil absorption that allows a septic system drain field to process effluent from the septic tank. When soil absorption stops it causes flooding of the field resulting in effluent rising to the surface of the soil. Flooding also deprives the soil of oxygen, which in turn kills off beneficial bacteria needing oxygen to survive.
The size of a soil absorption system (drain field) is based on the size of the house and the soil characteristics. Soil is generally evaluated using a "percolation rate" to determine the suitability of the site for septic system placement.