Huh. One of those things you think should exist…the moisture is sewer water.
Yes, some cities in the good old days (in older areas) did not separate toilet sewer and storm sewer. As a result, when it rained heavily, back pressure forced sludge into basements. Insurance companies have been demanding that residents install these backwater valves, so if the flow reverses to sewer-into-house the valve flaps shut.
Thing is that it hasn’t really rained in a while in NJ - the grass is getting brown and they’re warning of brush fire risks, but the thing is still leaking up. I spoke to the tenant this morning - she says that yesterday it dried up but today it got wet again, and it hasn’t rained since then. So there must be something going on with the sewer and it’s not from outside.
Unfortunately, the guy can’t come until next week. So the tenant is unhappy. 
Wait, is it a 10" diameter pipe, or is it a pipe that’s 10" vertically out of the floor?
If there’s a drain going from the basement floor out to the sewer (for drainage), there should be a valve somewhere along the line. It keeps water going one way (preferably not from the sewer into your basement - which is what happened to us a few years ago when our backwater valve failed. NOT a fun experience).
So how did you screw up that first quote to put my name on something said by Fotheringay-Phipps?
We had a drain in our basement floor and it was in the middle of the floor the house was build in 1940 or a little earlier.
That’s a real good question. I had multi-quoted several posts, but pared them back - I must have muddled some tags together. Sorry!
Well that symptom matches with sewerage backing up doesn’t it.
It could be as simple as blocked vent of course, that causes slow draining downward, which means that pressure can build up.
On of first thing plumber will do is see if he can find the vent that lets the air out of the pipe to the outside, so that most of the smelly air in the pipes is pushed outside, and the flow rate remains high. Sometimes people think these vents are optional and start blocking them to stop the smell …
Its all possible… actually they check out the problem with fibre optics these days…
send it down and see where the issue is. It may be some distance away from the valve itself,
Check around under the the bathroom, kitchen to see if the problem is there, it could be that a cap has become damaged or unglued at the drop , but it could become that way due to excess pressure due to a blockage further down. (all that water dropping = pressure.)
Drains in the middle are common if they feed into a sewer line that runs under the house, and in that case it would be more like a 2-inch line, not 10 inch.
Well now I’ve heard of one.
The line itself will be 2 to 4". But the access port leading down to the backflow valve installed in that line will be much larger; large enough to work in and service the valve if needed.
Is the access port often (or ever) combined with a drain? I’ve seen 10-inch access ports and I’ve seen 2-inch drains, but I haven’t seen a 10-inch pipe that serves both as access port and drain. But my experience is limited to DIY at a few houses, so I could be off on that.
I interpret the OP to mean that he has a 10" diameter hole extending vertically downwards from his basement floor. The hole is lined with a 10" pipe, probably PVC. The top of the pipe is flush with the floor and open to the air. The bottom of the pipe is some unknown depth below the floor. The pipe, at least near the top, is full of loose sand. This 10" pipe is NOT plumbing; it’s merely a hole liner.
What we’re all speculating is that some distance down beneath the sand, say 1 or 2 feet down, there is a 2-4" diameter drain pipe running horizontally. And most likely there is a backflow check valve plumbed into the drain pipe at that point. So the 10" hole and pipe form an access channel so workmen can access the valve.
A few early suggestions had folks thinking the 10" opening was just a drain open to the dirt under the basement. Like a simple sump pit in an area where sump pumps are not required. The OP’s later posts gave us more info making that an unlikely scenario.
This is correct.
Yes, especially once I spoke to the actual guy who installed it and he said it was something else.
That is always a good clue.
I was trying to be gentle to **TroutMan **who apparently skipped a few of the posts along the way.
Does the backup correspond to the tenant (or the upstairs residents) doing laundry or using a lot of water?
I’m going to guess it’s your classic tree root in the sewer line/tenants flushing tampons sort of deal, now that we know this is a sewer line. Although experiencing a sewer backup in an 8 year old house would be fairly bad luck or really bad plumbing.
Like computer users and physician patients, they lie.
Well the plumber came by and says it’s not a plumbing issue. He says the water is not coming up through this pipe but rather leaking down into it.
He says it came from the room next to this one, where the a/c units are located. Apparently the upstairs tenant has not replaced their air filter on their unit in a very long time (actually never, in the two years they’ve been living there) and the filter was completely clogged up. (He showed it to me and it was a like a solid sheet of dirt over it.) This caused some sort of ice build up which would periodically melt and drip down, and then seep under the wall to the next room. When it got soaked into the carpet it was hard to tell where it was coming from and we assumed it was that hole, which happened to be in the area.
An educational experience, though an expensive one, as the guy charged me $134 for the visit. The upstairs guy felt terrible and repeatedly offered to reimburse me, but I don’t think I am going to take him up on it. (He and his wife are very fine people and not well off financially.) Hopefully it will be an educational experience for him too.
My main concern at this point is whether the a/c unit has been damaged - I can’t imagine the lifespan has not been shortened somewhat by the extra load.
I was a bit perplexed on why there was no mention of odor in the first post. This explains it, it’s not sewage.
As for the AC, if it’s running normally after the filter change or cleaning, it is probably fine. Worst case I can think of is that the compressor may have taken a slug of liquid refrigerant on the suction side, but if it’s working OK then that probably didn’t happen.