Restrictive layers in septic systems can include all of the following EXCEPT:

In the context of septic systems, restrictive layers are materials that impede the movement of water and waste through the soil, impacting the drainage and effectiveness of the system.

Ledge, which refers to solid rock or a similar hard material, serves as a significant barrier that prevents wastewater from percolating downwards, thus constituting a restrictive layer. Groundwater can also be considered restrictive; if the groundwater table is high, it can saturate the soil and hinder the infiltration of effluent from the septic system, leading to potential system failure or groundwater contamination. Impervious soil, such as clay, is another restrictive layer because its density traps water above it, prohibiting proper filtration.

In contrast, sand is a permeable material, allowing water to pass through it easily. It is typically not a restrictive layer and is often sought after when installing septic systems because it promotes good drainage and effluent movement. This characteristic of sand makes it suitable for effective septic system operation, as it facilitates the absorption of wastewater into the surrounding soil. Thus, it does not qualify as a restrictive layer in septic systems.