What type of soil is considered unsuitable for drainfield use?

Class 4 soil is considered unsuitable for drainfield use primarily due to its physical characteristics that impede the movement of water and can negatively affect the proper function of an on-site sewage facility. This classification indicates a high density or a significant presence of clayey materials, which restricts drainage and increases the potential for groundwater contamination.

In contrast, Class 1 soils have excellent drainage capabilities, while Class 2 and Class 3 soils have moderate drainage features that may allow for effective wastewater treatment under specific conditions. These other classes can generally support the required infiltration rates for drainfields. Therefore, Class 4 stands out as the least appropriate option for this purpose.