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The day after moving into our recently purchased house in Oregon, we started noticing sewage smells on the back deck that were periodically overwhelming. We later began smelling it on the opposite side of the house as well. A new septic tank was installed prior to our arrival and the independent inspector thought it was off gassing and felt it would go away in a month. It didn't. A crack was noted in the old concrete tank at inspection pumping. It was removed and replaced. I watered all the P traps with no luck. Occasionally, the smell springs up in the basement bathroom as well.

As this got worse, we began noticing a sink drain on the main floor was draining slow. I pulled the trap and gooseneck and found a nasty plug of goo. The smell emanating from it was a musty, dank smell but didn't smell so much like sewage like we had noticed outside. It was still pretty nasty though. Cleaned and sanitized the fittings and replaced. The smell persisted so I flushed the drain with baking soda, vinegar, and about 3 gallons of boiling hot water. The smell then became much more intense and overpowered the bathrooms immediately above and below this one. It finally decreased in intensity after a week but still persists in the main floor bathroom where the original clog was and periodically involves the bathrooms directly above and below the main offender.

I'm not sure if these 2 issues are related. All drains are now flowing fine so didn't think it was a clogged main. Considered a venting issue but again, the drainage seems to be fine and haven't noticed shaking water in toilets to suggest a vacuum. The house was built in 1947. I'm not seeing cast iron pipes, only PVC. There are no trees or significant bushes between house and tank. Of note, the 3 bathrooms affected by the smell are directly in a vertical line on 3 floors, the middle main one being the worst. Also, the vent in line with those 3 bathrooms seems to be where the outdoor smell is coming from. We have a 4th bathroom at 1 end of the house in an add on attached apartment that is completely unaffected. It shares the same septic tank as the main house.

The septic people say the tank, pump, and drain field are fine. Had a plumber out who thought everything looked good, though he didn't put a camera in anything. As luck would have it, that was one of the few days when we couldn't smell much ha! :)

I'm at a loss as to what is going on or even who to call. Starting to get a little dejected. I don't know if I'm going crazy but the smells seem to get worse on nice days and better on overcast, rainy days. We don't really change our water use habits on different weather days. The more intense smells typically occur with high water usage, such as dishwasher, clothes washer, and showers. Any suggestions? Thanks all!

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    Why not get all the drain pipes clean and tested? Then you will have a known starting point.
    – Solar Mike
    Nov 16, 2023 at 7:26
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    What's going on with the old septic tank? Was it pumped and filled? Why was the new septic tank installed? Did the plumber check on your p-trap vents? How many plumbing vents punch through the roof? Can you verify that there are no blockages by dumping a bunch of water down each of them? Before the smell worsened, did you pour an unusually large amount of water down the drains as you were cleaning them? (I don't understand immediately why this could be important, but it seems like it could be.)
    – popham
    Nov 16, 2023 at 7:29
  • I would get one of those drain pipe brushes and use it to clean the house side of the p-traps. I would sniff the brush after each drain.
    – popham
    Nov 16, 2023 at 7:44
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    Your smells seem to get worse on nice days and better on overcast, rainy days suggests a connection with static air pressure differences between inside and outside. Your windows are closed on rainy days? Please edit your question to add info on air movement: exhaust fans, dryers, HVAC fresh air (anything else?) and how you operate these things differently on rainy vs sunny days. You may need to invest in a manometer with a long tube to outdoors that you can walk from room to room and measure ambient pressure. Negative pressure can suck sewer fumes from a pipe in a wall with a loose joint.
    – MTA
    Nov 16, 2023 at 13:48
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    I suggest that the septic tank was incorrectly replaced because of this issue - the source of the issue was not the septic tank, possibly the line to the tank is the issue or inside the house.
    – Tiger Guy
    Nov 16, 2023 at 14:58

2 Answers 2

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Seems like its not venting properly and or the pipes from between the house and the septic is clogged by roots most likely. The smallest cracks can lead to root damage and adding an extra ventilation pipe would help with the smell.

Copper is naturally anti-microbial which kills bacteria build up, PVC and steel pipes do not. PVC is bad for mold, bacteria build up and using ridX may help.

New vent pipe from the roof to the septic would be my next move. The pipe could be clogged or broken anywhere between the roof to the septic tank.

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  • my septic tank did not have a vent pipe.
    – Tiger Guy
    Nov 16, 2023 at 14:59
  • We do have high copper levels in the well water. Thanks
    – Bruce
    Nov 17, 2023 at 3:46
  • I fixed up a bit of formatting and did some spell check. Please make sure everything is still as you intended it and edit to make any additional fixes necessary.
    – FreeMan
    Nov 17, 2023 at 13:25
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We had a plumber scope out our main sewer lines and vents. He found this bit of creative plumbing from an abandoned sink that is now in the wall of a room in the house where the smells were the worst. Carbon filters fixed the outside smells.enter image description here

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