A relative's home is having an issue with sewer gas smell. Started many months ago and then went away for several months but is back again. They live in a home outside of the city limits and have an old septic tank system with leach lines. Actually they have 2 systems on each end of the house but the smell is coming from the bathroom area on one end of the house. They noticed the smell was worst when a lot of family members were staying over and when it was a wet part of the season. It appeared to go away during the dry summer but has come back after the recent rains. If they put a cup over the bathroom vent pipe, the smell goes away but today, they removed the cup and the smell is really bad.

Any ideas as to what is causing this? TIA

I recently did a reno where we kept getting sewer gas smells anytime someone flushed a toilet.

Turns out, some dumbass plumber placed the vents almost directly in front of the gabel vents and the sewer gas was venting into the attic.

Took a while to connect the flushing of a toilet and associated sewer gas being pulled into the house