The rate of wastewater flow through the soil is critical to the ability of a soil to treat wastewater. If wastewater moves too rapidly through the soil, the chemical, physical, and biological reactions that must occur to retard, reduce, and transform the pollutants are impeded. Any soil condition that causes an increased rate of flow, such as a high water table, high hydraulic loadings, or shallow depth to seasonally high water tables, can potentially cause contamination of ground water because the wastewater has not been adequately treated.

Water moves though unsaturated soil more slowly than through saturated soil. The slower movement under unsaturated conditions provides more treatment and more protection of ground water than can be obtained in saturated soil.

In North Carolina, for soils in Groups II, III, and IV, 12 inches of unsaturated soil between the bottom of the trench and any soil wetness condition, ground water, or other unsuitable soil condition is required for proper treatment of wastewater.

For Group I soils (sandy soils), an 18-inch separation distance is required between the trench bottom and any soil wetness conditions or the ground water. Because the rate of wastewater movement through Group I soils is faster than Group II, III, or IV soils, a greater separation distance is needed to properly treat the wastewater before it enters the ground water.

Reference

15A NCAC 18A.1955(m)

From the North Carolina Onsite Guidance Manual