We moved in to our house a couple years ago and were not told too much about a pipe that sticks out of our backyard.
We are on a septic system and this pipe is sticking out from our leech field.
The other houses on our block all have similar setups yet none (that I know) have a pipe coming up like ours.
It was suggested to me that this may be safe to remove, or at least remove, cover the whole for a few weeks, and as long as there's no issues, then remove.
It's not attached to anything and I can pretty easily pull it out and put it back in, although I've tended not to touch it much.
My professional advice is to knock it over with a lawnmower,
Then let the grass grow up around it for a few weeks until you forget it's there. Shortly thereafter, accidentally run over the pipe with the lawnmower in order to destroy at least one blade. When the lawnmower is finally fixed after several months, go mow the lawn again, which has grown to a ridiculous hight at this point. You can then forget the pipe ever existed, it's gone forever.
I've done this with several large metal objects, works well 😅
I had a blade on my riding mower snap one time, apparently it was damaged from hitting something solid at some point. The racket from that was impressive. It sounded like the mower was completely coming apart, so I was actually relieved when I saw the broken blade. Of course, then I couldn't finish mowing, because I had to wait to get new blades.
I'd take a trip to the local septic permitting agency and see if I could get copies of the permit and approved septic system design/as-built site plans. The inspector will probably be able to tell you if you need it or not.
It might be a vent for the leach field although that's not common where I'm from. I guess it could also be a marker for a distribution box or some other system component.
If you're pulling it out and putting it back in any appreciable distance, it's probably already plugged. Also if it comes out without unscrewing it might just be a marker. What does the in ground end of the pipe look like? Is it threaded? It could be someone is just having a laugh...
If it’s only 1.5 feet then get a shovel and start digging. Get a gander for yourself what it’s connecting to. Sounds like it’s not connected to anything.
We don't, at least not yet, we haven't lived here long enough to have to have dealt with it. That is a good idea though, we can see who the neighbors use.