Septic Systems: Site Evaluation and Testing - Soil Wetness

Adequate treatment of wastewater can only occur in well-aerated soils. Because wet soils do not allow adequate treatment of wastewater, on-site systems must not be installed in wet soils.

Soil color is used to indicate soil wetness. Once the soil colors and the depth of these colors have been determined, the soil can be classified for suitability of on­site system installation based on wetness.

  • Chroma is the relative strength, purity, or saturation of the color of the soil. Chromas of2 or less on the Munsell color chart, either in mottles or as a solid soil mass, often indicate a wet soil. The wetness could be caused by a seasonal high­water table, perched water table, tidal water, soils that are saturated during the rainy season, or movement of ground water into and through the soil. The relationships between the depth to the soil with chroma 2 or less color and the site suitability for installation of an on-site system are presented in Table 4.5.6.

  • Sometimes soil color is an artifact of the original parent material and is not indicative of soil wetness. If the soil color is due to the original parent material, then color shall not be used to judge the suitability of this soil for the installation of an on-site system.

  • Other site characteristics can be used to indicate soil wetness. Vegetation and landscape position can both be used as an initial indicator of wet areas.

  • If the site has been drained, the soil must be evaluated for soil wetness by monitoring the site with monitoring wells from December through March to determine the water table depth.

  • In the Piedmont, interceptor drains may be used for lowering perched water tables. In the mountains, these drains can divert laterally moving water in colluvial soils. In the Coastal Plain, ground-water-lowering devices, such as subsurface tiles, ditches, or pumped drainage, are frequently used.

  • If the soil is UNSUITABLE because of wetness, the classification may be changed to PROVISIONALLY SUIT ABLE if an investigation determines that a modified or alternative septic system would function appropriately on this site. See 15A NCAC lBA.1956 or .1957 for the rules governing installation of modified or alternative septic systems.

From the North Carolina Onsite Guidance Manual