What is the allowed range of percolation time for a shallow buried trench system?

• A. Between 0 and 0.5 min/cm
• B. Between 1 and 125 min/cm
• C. Between 10 and 50 min/cm
• D. Between 125 and 200 min/cm

Answer: (B)

The test is checking how soils in shallow trench systems must behave in terms of water movement to ensure proper treatment and drainage. Percolation time measures how long it takes for water to move through a given depth of soil; a range is set to balance effective treatment with reliable drainage.

If the percolation time is too short (soil is too permeable), wastewater can pass through the soil too quickly and receive inadequate treatment before reaching groundwater. If it’s too long (soil is very restrictive), the effluent may not infiltrate efficiently, causing surface pooling, odors, or system failure. The allowed window—between 1 and 125 minutes per centimeter—covers a broad spectrum of soils that are suitable for shallow trench designs, making sure there’s enough interaction with the soil for treatment while still allowing timely drainage.

Options that are much faster (near-instant) or much slower (very slow infiltration) fall outside practical design ranges for typical shallow trench systems, so they aren’t appropriate.