Which types of soil are considered unsuitable for On-Site Sewage Facilities (OSSF)?

Soil suitability is critical in determining the effectiveness of On-Site Sewage Facilities (OSSF) because it influences the treatment and dispersion of wastewater. The correct option identifies soil types that are generally poor at allowing water to infiltrate and percolate, which is essential for the proper functioning of these systems.

Class 1a soil is characterized by sand that has a high permeability, which is generally suitable for OSSF. However, when you incorporate sandy soil that contains more than 30% gravel, this composition can create issues with both retention of moisture and the biological treatment processes that depend on sufficient contact with the surrounding soil. Clay can also pose significant challenges since its high plasticity and low permeability impede drainage, leading to potential system failure due to ponding or backing up.

In contrast, the other choices include soil types that are generally more favorable for OSSF. Class 3, which includes silty loam and rocky soil, can have better drainage characteristics. Class 2, which consists of sandy clay and loam, generally provides a moderate balance of water retention and drainage. Class 4, which includes clay and organic soil, while they can have some limitations, typically do not encompass the problematic characteristics associated with excessive gravel content found in