What is the minimum hydraulic conductivity allowed for covering material?

• A. 0.001 m/day
• B. 0.01 m/day
• C. 0.1 m/day
• D. 1 m/day

Answer: (B)

Water movement through the covering material is what matters here. Hydraulic conductivity tells us how easily water can pass through a soil layer. For the cover over an on-site system, you want enough permeability to drain rainfall and prevent surface water from pooling, but not so much that the layer loses its protective function or bypasses the treatment soil. If the cover is too slow to drain, water can sit at the surface or near the surface, leading to poor aeration, vegetation stress, and potential system issues. The smallest value that still allows adequate downward drainage is about 0.01 meters per day, so that a fairly fine-to-medium textured cover can shed water without creating perched water problems. Values lower than this would be too impermeable, causing drainage problems. Higher conductivities are acceptable because they simply indicate a more permeable cover, but they aren’t the minimum requirement.