Which soil type is least suitable for the subsurface absorption of wastewater?

The soil type that is least suitable for the subsurface absorption of wastewater is clay soil. Clay has very fine particles that are tightly packed together, which limits the soil's porosity and permeability. This means that water cannot easily move through clay, causing wastewater to pool rather than being absorbed efficiently.

In contrast, sandy soil has larger particles and greater permeability, allowing wastewater to flow through more easily and be absorbed by the underlying layers. Mottled brown soil and black soil can have varying properties, but they often have better drainage capabilities compared to clay. Therefore, the slow absorption and drainage characteristics of clay soil make it the least suitable for subsurface absorption of wastewater.