Which material is unsuitable for the disposal of sewage by subsurface means?

When considering materials for the disposal of sewage by subsurface means, clay is deemed unsuitable due to its physical properties. Clay has very fine particles that create a dense and compact structure, which significantly limits its permeability. This low permeability inhibits the effective movement and infiltration of liquids, including sewage effluent, through the soil. Consequently, using clay for sewage disposal can lead to surface pooling, increasing the risk of contaminants reaching the surface and posing public health risks.

In contrast, materials like loam and gravel possess better drainage characteristics, allowing for efficient infiltration and treatment of sewage effluent underground. Loam, being a mixture of sand, silt, and clay, provides a balance of drainage and nutrient retention, making it suitable for subsurface disposal. Gravel has high permeability, facilitating rapid drainage, which is also beneficial for sewage dispersal. Silt, while more problematic than loam and gravel, can still be more effective than clay in certain circumstances due to its finer texture and ability to retain some moisture.

Thus, given these characteristics, clay is particularly unsuitable for subsurface sewage disposal.