There is a leak occurring somewhere right near ground level in the main drain stack shown in the picture. The leak comes up from between the outside of the ABS 4" drain and the concrete floor, near the back of the drain (closest to the wall and underneath the 3" house drain pipe). There is no water on the outside of the drain, so the water is coming up from the concrete. When I leave a sink running, I can see the pool of water start to form at the back of the drain pipe, but then when I turn off the faucet and clean up the water, it will still continue to give a smaller amount of water overnight, even without use of anything attached to that drain.
I plugged up the drain pipe to hold water in it, and the leak seems to continue to occur when the water level is right at or just below floor level. Right at the concrete floor, the plastic drain bevels out to a slightly larger diameter. Inside the drain, I can see a gap all the way around the pipe that corresponds to that area, so it looks like there is some kind of a ring that they have manufactured into the drain. I think the leak is occurring somewhere in or at the edge of that ring. You can see the gap in the picture that shows the inside of the drain, and it is right below the opening of the 3" house drain.
What purpose does that gap provide?
I've tried to look in there with a mirror, but can't see very well, so I can't find the exact leak. I don't know if there is a good cleaner which would clean up that area without damaging the ABS, so that maybe I could see better?
Can that gap just be filled in with an epoxy or putty. Or does it allow for expansion or exist for some other purpose so that it can't be filled in to be flush with the rest of the inside of the pipe? If I could just fill in that gap, I think that would probably solve the problem.
If it can't be patched from the inside, would I need to chisel around the concrete to apply some kind of patch on the outside?