I own a 17 year old bungalow in the country. We have a bathroom and a washing machine in the basement which the waste water goes into a sewage pit then is pumped up and out to the septic tank. We also have a dry well in the back yard that collects the backwash water from the water treatment systems, the floor drain in the basement and the floor drains in the garage. The problem we are experiencing is an offensive septic odor that we can detect at the top of the basement stairway. The kicker is the odor is only present during very cold weather. Water is visible in the floor so I suspect that there is a trap. There is an odor from around the sealed septic pit unit but it is faint and smells nothing like the odor from on top of the stairs.
Answer:
This sounds more like you replaced the light fixture at in the staircase with a cheapy plastic one - they often give off the kind of odour you describe. The only other thing it can be is from that seal that is giving you some feedback and by the time it gets to the stairway. By the way, in many jurisdictions drywells are now quite illegal.
