What type of soils can be considered restrictive layers for septic system function?

Clay soil is considered a restrictive layer for septic system function primarily due to its density and low permeability. This type of soil can hinder water movement, which is essential for the proper treatment of wastewater in a septic system. Since clay has tiny particles that fit tightly together, it does not allow water to percolate through easily. This slow movement can lead to saturation and potentially cause system failure, as effluent cannot effectively disperse into the surrounding soil where natural filtration takes place.

In contrast, sandy soil and gravel allow for faster drainage and better absorption of water, making them conducive for septic system use. Peat, while being somewhat less favorable due to its organic content and potential for water retention, is not as definitively a restrictive layer as clay, which is the most well-known for its limitations in water drainage. Therefore, clay soil is the most significant restrictive layer impact on the functionality of a septic system.