What are these drain pipes?

[odynefit]

I have an interesting question about the plumbing in my basement. I have a 2" drain pipe that goes into the slab (presumably to the septic tank?) and upstairs. 2 feet to the right of it is another 3" stack going up. And 3 feet towards the interior there is another 3" stack that goes upstairs.

On the first drain (see photo) there is a trap and the waste side of my water softener goes into the one side, and goes up to the ceiling of the basement and appears to be in the exterior wall. It seems obvious that there is a drain just above it. I guess what is throwing me off is that there is 1 bathroom on this side of the house on the second floor and the toilet drains to the drain pipe 3 feet in the interior. I can't seem to find what the other two stacks are for. on the diagram, the 2" drain is on the bottom left, the 3" drain in the wall on the top left, and the interior drain is on the left side more towards the middle.

On the roof (see diagram) there are 3 stacks, two that seem to line up to the two basement pipes that don't seem to do anything (right). The center stack on the roof seems to line up to the one vertical drain in the basement.

Does it make sense to have 3 different drains through the slab (but most of the house drains through one (top right on the diagram)? Does it make sense to have 3 different stacks on the roof? Would it be possible for the shower and sink to have different drains from the second floor to the basement?

Hopefully these photos and diagrams help explain my situation.

 

[CT-18]

Were they roughed in for future bathroom. Are you saying they are stubbed up and capped or do they continue vertically. They could be venting something underground.

 

[odynefit]

Were they roughed in for future bathroom. Are you saying they are stubbed up and capped or do they continue vertically. They could be venting something underground.

They continue vertically. I checked the toilet, sink and shower and none of them drain to either of these two pipes. Would there be a purpose to having a stack to the roof with no drain (except the water softener)? I'd like to be sure they go to the drain because i'd like to add another fixture to one of them.

 

[voletl]

Washing machine standpipe perfect place to hook up a washing machine drain aside from the fact that it's an S trap that's what it was most likely used for

 

[odynefit]

Washing machine standpipe perfect place to hook up a washing machine drain aside from the fact that it's an S trap that's what it was most likely used for

That makes a lot of sense, at least for the 2" drain. I know a washing machine has always been in a separate laundry room, but this leaves the possibility open. There is also a roughed in toilet and shower drain on the floor (in concrete near these pipes.

But the only other question is, what would the 3" pipe for? Do houses need more than one vent, based on the number of bathrooms? Currently there are 2 1/2 bathrooms in this house. Is the additional vent for additional bathrooms?

 

[odynefit]

I forgot to mention... i figured out what the 3" PVC pipe is for. Its a passive radon remediation system. It goes from under the slab to above the roof.