A raised leaching bed is considered when the high groundwater table is less than 900 mm from the ground surface.

A raised leaching bed is considered when the high groundwater table is less than 900 mm from the ground surface.

• A. True
• B. False
• C. Only if the percolation rate is fast
• D. Only if the surface is sloped

Answer: (A)

When groundwater is close to the surface, there isn’t enough unsaturated soil above the leach field to properly treat and disperse effluent. A raised leaching bed is considered to create additional vertical separation, keeping the infiltrative surface in unsaturated conditions and reducing the risk of contaminating groundwater. Using a raised bed effectively places the leach field higher than the natural grade, often with fill and proper drainage, so the system can function reliably even with a high water table. The 0.9-meter (900 mm) depth is the guideline used here. Percolation rate or slope influence performance and design in other ways, but they don’t determine the need for a raised bed—that decision hinges on groundwater depth.

When groundwater is close to the surface, there isn’t enough unsaturated soil above the leach field to properly treat and disperse effluent. A raised leaching bed is considered to create additional vertical separation, keeping the infiltrative surface in unsaturated conditions and reducing the risk of contaminating groundwater. Using a raised bed effectively places the leach field higher than the natural grade, often with fill and proper drainage, so the system can function reliably even with a high water table. The 0.9-meter (900 mm) depth is the guideline used here. Percolation rate or slope influence performance and design in other ways, but they don’t determine the need for a raised bed—that decision hinges on groundwater depth.