For area-fill systems that meet the requirements of 15ANCAC 18A.1957(b)(l)(A), the lowest LTAR for the applicable soil group of the most hydraulically limiting soil horizon within 18 inches of the naturally occurring soil surface or to a depth of one foot below the trench bottom, whichever is deeper, must be used to design the system.
The LTAR values for Group I soil area-fill systems are reduced to spread the flow of the effluent over a larger area over these very permeable soils so that the effluent receives adequate treatment. There are two exceptions to the LT AR shown for Group I soils (Table 4.6.1) when using fill systems:
The LTAR cannot be greater than 1.0 gallon per day per square foot for gravity distribution, or
The LTAR cannot be greater than 0.5 gallons per day per square foot for low-pressure pipe systems installed on sites with at least 18 inches of Group I soils below the naturally occurring soil surface or to a depth of one foot below the trench bottom, whichever is deeper.
15A NCAC 18A.1957(b)(l)(D)
From the North Carolina Onsite Guidance Manual