Right of way - Old drainage of unknown origin

Thank you all very much for your responses! There were already a lot of helpful tips. We are currently trying to find out more about why closing the line is technically not possible. Of course, we will keep you updated. Thanks again!

PS: Behind the properties there is partially a nature reserve and the nature conservation authority does not want a drainage pipe to be laid there (I had forgotten to answer this question earlier).

Thank you very much for all your answers! There were really a lot of helpful tips.

At least one of them you probably overlooked, if you want to follow this path ...

We are currently trying to find out why sealing the pipe is technically not possible.

... because this is not your problem, and you should not allow anyone to make it your problem. Legally, this issue concerns you at least as long as the affected parties have not seriously investigated the perpetrator. Technically, it is simply explained: if you were to simply block a dilapidated pipe, it would practically become a collection container (which would soon not withstand the pressure). The result is basically like it is now, only reduced by the contribution of your soil to infiltration. That’s why I say: you are already (and you may not care whether others are affected similarly or even worse) at least "also" victims yourself of the actual discharge through what I would call the "operation" of the dilapidated drainage. Exactly the neighbors you pity are actually the cause / perpetrators concerning you. I see you having a claim against them both for injunction and for investigation of the "preceding offender." They can’t just pass the damage on to you.

The water presumably comes from some field, which is why the nature conservation authority does not want the drainage to be laid through the nature reserve next door.

They should quietly check whether the load does not come exactly from where they don’t want it to "go." Jokers. I would not be really surprised if the drainage turns out to be a piped swamp.

The water from the unknown third party does seep, but unfortunately into the building structure of her basement

Because water flows downhill, and the improper question "could you please take our black Peter?" directed at neighbors situated higher up would be pointless, as opposed to being addressed to the OP where it is just cheekily stupid :-(

Regarding infiltration: I meant rather that the affected parties on their properties, maybe even across both, create an extra soakaway pit to which they would then lead the drainage pipe. So instead of infiltration near the basement, infiltration in the rear part of the property.
The thing with the nature reserve behind is of course unfortunate. You can’t put something like that under the sidewalk? Then the OP would also be off the hook.

We are not directly affected, but of course we still wish for the problem to be solved for the affected property owners and would even contribute financially to find another solution. It is just the solution with the easement that really scares us.

We have now been told that the pipe is probably so old that it would burst somewhere else if it were closed off.

The neighbor, from whose property the drainage leads into the new development area, according to his own statements, also has no idea where the drainage comes from and does not want to contribute to finding a solution.

We have now been told that the pipe is apparently so old that it would burst somewhere else if blocked. The neighbor, from whose property the drainage leads into the new development area, also claims to have no idea where the drainage comes from and does not want to contribute to finding a solution.

If the "perpetrator" does not want to contribute and there is no "right" registered on the properties: dig it up, close it. I wouldn't care at all if it "would burst somewhere else."