In soils with very rapid percolation (less than 50 min/cm), a 900 mm clearance distance would be unnecessary.

• A. True
• B. False
• C. Not specified
• D. Only under certain conditions

Answer: (A)

The idea being tested is how soil drainage rate affects the need for vertical setbacks in an on-site system. Clearance distances protect groundwater and surface water by giving the effluent time to be filtered and diluted in the soil. When the soil percolates very quickly, the water moves through the upper layer and into deeper soils rather than lingering near the surface. That rapid movement means there’s less opportunity for effluent near the surface to migrate upward or contaminate shallow groundwater, so the extra vertical buffer (the 900 mm clearance) isn’t necessary. In slower-percolating soils, the effluent stays in the upper zone longer and a larger clearance helps protect groundwater. Therefore, for soils with very rapid percolation, the 900 mm distance would not be required.