Mounding of the groundwater table beneath a leaching bed may cause

• A. flooding of the soil/ effluent interface, thus removing oxygen from the soil and inhibiting natural treatment
• B. drying of the soil
• C. increased biodegradation
• D. improved drainage

Answer: (A)

When groundwater mounds beneath a leaching bed, the infiltration zone becomes waterlogged. That flooding of the soil/effluent interface reduces the diffusion of oxygen from the air into the root zone and treatment zone, creating anaerobic conditions. Oxygen is essential for the aerobic microbes that do most of the natural wastewater treatment, so with oxygen limited, treatment efficiency drops and natural degradation processes are inhibited. Drying the soil wouldn’t occur under mounding; increased biodegradation would rely on oxygen and thus is unlikely in flooded conditions; and improved drainage is the opposite of what mound indicates.