Air in the discharge line of a recovery well system could be caused by which of the following?

• A. Cascading water
• B. Vent valve stuck open
• C. Hole in drop pipe
• D. Plugged screen and cavitation

Answer: (C)

Air in the discharge line is most often caused by a breach in the drop pipe that carries groundwater to the surface. When the pump creates suction, a hole in the drop pipe allows ambient air from the surrounding annulus or atmosphere to be drawn into the water column. That air then travels up the discharge line, forming air pockets and bubbles in the discharge stream. The drop pipe is the path for the water column, so any hole along that path directly introduces air into the system.

Other scenarios don’t fit as well. A vent valve stuck open would usually vent air out of the system rather than pull air into the discharge line. Plugged screen and cavitation affect flow and pressure but don’t inherently cause persistent air to enter the discharge line. Cascading water could entrain air at a surface drop, but it’s not the typical mechanism for air in the discharge line of a recovery well.