Where is a 900 mm clearance distance unnecessary?

• A. Soils with a percolation time less than 50 min/cm
• B. Soils with a percolation time greater than 100 min/cm
• C. Any soil type
• D. Only in sandy soil

Answer: (A)

The distance a leach field must be kept from potential sources of contamination depends on how quickly water moves through the soil. When percolation is very fast (less than 50 minutes per centimeter), the soil itself handles rapid drainage and dilutes any effluent quickly, so the extra vertical buffer of 900 mm isn’t necessary. In contrast, slower-percolating soils (longer percolation times) need that clearance to protect groundwater by giving more distance for treatment and dispersion.

So this 900 mm clearance isn’t required in soils with rapid percolation, since the risk the clearance is meant to mitigate is already reduced by the soil’s fast infiltration. It isn’t about soil type alone (not just sandy soils) and isn’t universal to all soils (it’s tied to the rate of percolation).