My house was built in 1940. I recently pulled up the floor and subfloor in the basement and noticed this basement pit. There was about 6 inches of water in the bottom which I drained. Below that is a layer of what I think is home-heating oil. Maybe the house used to have an oil tank inside that leaked (it now has an electric boiler). I removed most of the oil; below it is clay soil. The pit is about 3 feet deep. The short pipe comes from the garage floor drain (I think). The 90 degree pipe goes to the sewer system (I can hear it). This pit has probably not been opened in at least 30 years. There are no signs of flooding or water damage in the basement.

My questions are:

  • Is the idea behind this that water drains down through the clay soil? If so, maybe the oil was blocking it, that's why there was the accumulation in the pit?
  • Is the pipe to the sewer system for overflow should the pit fill with water and not drain quickly enough through the clay?
  • Should I be thinking about putting a sump pump in the pit?

enter image description here

  • Maybe someone dumped old motor oil there for no good reason or more precisely the garage since you noted thats where the pipe is from. It looks like a sewer access point/cleanout. Given you have a drain, you could install a small pump. Obviously it doesnt see much water. (but apparently a stupid auto mechanic once)
    – noybman
    Dec 14, 2018 at 7:28

1 Answer 1


I emptied the water from this pit and after a day or so it started to smell like sewage in my basement. That's when I realized that this pit works the same way as a toilet. The water at the bottom block the fumes from the sewer. The oil-like substance at the bottom of the pit is some kind of sealant put there to prevent the water from draining into the ground below. I dumped some water down the pit and voila, problem solved. enter image description here

  • 2
    Looking at your diagram, this is probably a grease/oil separator. It allows the water to drain and the grease/oil to float and occasionally get removed. This seems consistent with the fact that the only inlet is from the garage.
    – Tim Nevins
    Jan 23, 2019 at 13:59

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