What type of material is required to surround the bottom of a drainfield for pollutant absorption?

The correct choice is gravel because it plays a crucial role in facilitating both drainage and aeration in the drainfield. Gravel is highly permeable, allowing wastewater to flow through it easily while providing the necessary space for air circulation, which is essential for the microbial processes that help break down pollutants. This permeability enables effluent to disperse evenly throughout the drainfield, promoting effective absorption and treatment as it moves into the surrounding soil.

While sand can also be used in certain contexts for drainfields due to its drainage properties, it does not provide the same level of structural stability and airflow as gravel. Clay, on the other hand, is not suitable as it tends to retain water, creating saturated conditions that inhibit the proper functioning of a drainfield. Bentonite, a type of clay used for sealing applications, does not offer the required permeability for effective pollutant absorption in a drainfield context. This makes gravel the most appropriate and commonly used material for surrounding the bottom of a drainfield.