What is the minimum depth below the ground surface required for absorption trenches?

• A. Sixty centimeters.
• B. Thirty centimeters.
• C. Ninety centimeters.
• D. One hundred twenty centimeters.

Answer: (A)

The key idea is that the soil above the absorption trench must provide a proper vertical uncompromised zone for treating effluent as it percolates downward. A minimum depth of 60 cm below the ground surface gives enough soil for filtration, adsorption, and microbial activity to occur before the liquid reaches any groundwater or surface water. This depth also helps protect the trench from disturbances like surface runoff, root intrusion, and vehicle or foot traffic, and it provides a buffer from shallow groundwater.

If the trench were placed shallower than this, there wouldn’t be enough soil beneath the surface for adequate treatment and protection, increasing the risk of surface or groundwater contamination. Placing it much deeper isn’t necessary for the minimum requirement and would add cost and potential issues without improving the essential function.