A homeowner reports a persistent foul odour emanating from their basement floor drain, particularly after heavy rainfall. The building sewer appears to be functioning normally. What is the most likely cause of this odour?

Question: A homeowner reports a persistent foul odour emanating from their basement floor drain, particularly after heavy rainfall. The building sewer appears to be functioning normally. What is the most likely cause of this odour?

Answer options:

• A broken building sewer pipe allowing sewage to leak into the soil.
• A blocked main vent stack, causing siphonage of trap seals. ✅ A dry trap seal in the floor drain, allowing sewer gas to enter the building.
• Backpressure from the municipal storm sewer system.

Correct answer: A dry trap seal in the floor drain, allowing sewer gas to enter the building.

Explanation: A dry trap seal in a floor drain is a common cause of sewer gas odours, as the water barrier designed to prevent gases from entering the building has evaporated. Blocked vent stacks often lead to other fixture traps losing their seal, and a broken sewer pipe would likely manifest as sewage backup. Increased rainfall would not directly dry a trap unless contributing to system pressure changes that result in siphonage due to improper venting.