Rainwater retention basin overflows onto the property

Does the purchase contract or the land register say anything about possible toleration obligations / encumbrances?

No, nothing at all.

Which pipe? The white one or the blue one?

And what is the white pipe for? Is there still going to be a channel/water ditch?

Is the rain retention basin even really completed and in operation?

Well, with just these two photos you really can't say much. Isn't there a development plan that shows more clearly how the whole thing is planned?

The attached site plan should be somewhat more informative than my description.
Supposedly the basin is finished, only a fence and some landscaping are missing. The development plan states that the thing should be designed to resemble a natural lake. Otherwise, nothing.

Based on the images, I suspect it is runoff rainwater – among other things from your property – which follows the slight slope and accumulates at the bottom in the collection channel and then in the puddle on the presumably heavily compacted soil. The function of the white pipe is unclear and whether it has a connection to the basin or if the basin and the puddle are at the same level.

But now please once again: This huge pipe, over which the footpath also goes, leads onto YOUR property?

What kind of property is that? Retention basin railway track industry... did you really pay anything for that? oh dear

My God, Steffen again... not everyone is lucky enough to be financially independent.

The main thing is to be "druff"... as always.

The white pipe probably runs within the green strip below 38a? Is that supposed to be some kind of natural inflow to the retention basin? Definitely looks interesting... and unfortunately nerve-wracking if it rains heavily there. Or is your property still being filled/leveled to get a bit higher?