The depth where restrictive horizons are located is important when determining the suitability of a site. Because restrictive horizons retard or stop water or wastewater flow, the presence of these horizons, if they are too close to the soil surface, can disqualify a site for on-site system installation.

As defined int he rules, restrictive horizon means “a soil horizon that is capable of perching ground water or sewage effluent and that is brittle and strongly compacted or strongly cemented with iron, aluminum, silica, organic matter, or other compounds. Restrictive horizons may occur as frangipanis, iron pans, or organic pans, and are recognized by their resistance in excavation or in using a soil auger.”

For purposes of site evaluation, restrictive horizons are defined as layers of material that are at least three inches thick that effectively prevent water from flowing through. See Figure 4.5.6, which shows the relationship between restrictive horizon depth and suitability for on-site system installation.
The only exception to the restrictive horizon depth requirement is for sites where the restrictive horizon is less than 36 inches from the surface and where the site evaluation has determined that a modified or alternative system can be installed. See 15A NCAC 18A.1956 or .1957 for the rules governing installation of modified or alternative septic systems.

From the North Carolina Onsite Guidance Manual