captain_spalding said:

Do we have any plumbers among our number?

Not qualified, but I have installed a dozen or so septic systems, and repaired a few more.

It sounds like a tree of some sort has got it’s roots into the system through a crack or the leach drains. If it’s connected to town sewage system, it’s probably a cracked pipe letting roots in. Try caustic soda, and follow the instructions. Don’t come crying to me if you melt your arm off.