What is a possible way a groundwater contamination plume can be formed beneath a leaching bed?

• A. Mounding of the groundwater table into the leaching bed
• B. Rapid drainage through soil
• C. Evaporation of effluent
• D. High soil porosity

Answer: (A)

A groundwater contamination plume under a leaching bed forms when the injected wastewater creates a hydraulic pressure that pushes water and dissolved contaminants into the surrounding groundwater. If the groundwater table mounds up into the leaching bed, the local head rises and the groundwater flow becomes directed into and away from the bed. This downward and lateral movement carries contaminants with the moving groundwater, producing a plume that can extend downgradient from the source.

Rapid drainage would tend to remove water quickly rather than push contaminants into groundwater, evaporation acts at the surface and doesn’t introduce contaminants into the groundwater system, and while high soil porosity affects how easily water can move, it doesn’t by itself generate the driving force for plume formation without that upward hydraulic pressure from mounding.