A mound or soil substitution drainfield is often used?

A mound or soil substitution drainfield is specifically designed to address situations where the existing soil conditions are not suitable for effective sewage treatment. When soils do not have the appropriate depth or texture to adequately treat effluent—meaning they may be too shallow, have poor permeability, or might not provide sufficient filtration capabilities—alternative systems like mound systems are employed.

These mounds are constructed by artificially creating a raised bed of sand or other suitable material that allows for better drainage and treatment of wastewater. By elevating the drainfield, it enables the effluent to be treated more effectively before it reaches the underlying soil layer or groundwater, which helps mitigate potential contamination and ensures compliance with health standards.

This is crucial in areas with high water tables or dense clay soils, where traditional drain fields would fail. The other options do not accurately capture the primary purpose of using a mound drainfield, as they either imply situations that do not require the specialized design of a mound system or suggest improper sewage treatment practices.