Purchased land after demolition, construction debris in the ground

Maybe rubble from the airport??? Who is the seller, federal state, state, city, or private??

So, I will now describe the situation as well as I can.
At the beginning of the 1970s, there was a place in this municipality that was very close to a military airbase. At that time, it was decided to relocate this place to a sufficiently more distant location for the benefit of the residents. The old place was then razed to the ground on site and probably also disposed of – shame on anyone who now believes or claims under my current property -. Directly adjacent to the place newly created in the 1970s by relocation, a shooting and soccer field was established on a piece of farmland. Since pretty much every place, large or small, has its own fire station, it was decided to build a fire station at the end of the shooting range for a fire engine as well as a lounge and two garages. About two years ago, the entire shooting range including the fire station was put up for sale as building land by the municipality.
A company from the Ruhr area bought this area from the municipality and parceled it into 22 differently sized plots. The buyer company from the Ruhr area was required by the municipality at the time of purchase to fully develop this area.
21 plots were undeveloped, only ours was not. We chose this plot because it was a corner lot with a very large building window. We received from the notary the same purchase contract as all other 21 buyers. We read through it carefully (after all, it was a lot of money) and found that the contract did not include the addition concerning the fire station, etc. After consultation and notification to the notary, we had the entire development with names inserted including demolition and disposal. The purchase was then notarized about 5 weeks later. Some time later, demolition work on all buildings began. On none of the other 21 plots (well, I did not ask all, but at least half) were bricks, gravel, and concrete residues found during excavation for the base slabs or cellars, only on our plot, because the fire station etc. was located there.
I will report on how it continues.

Thank you for the description. Have you contacted the seller? Basically, in my opinion, you did everything correctly and considered everything (layman's opinion). It should be clarified whether it is new or old construction debris.

Thank you for the description. What I understood is: There was a fire station on your property, before that it was open space (sports/shooting range). However, since you also described a relocation before that, I still do not know the following:

Was there ever any other building on it besides the fire station, yes or no?

If no, it is certain that these are the remains of the fire station. Since you wisely included demolition and disposal in the contract, burying the debris on your property would be a clear case for "immediately contact a specialized lawyer," as it would be a breach of contract. It is important to get advice on what immediate measures you can/must take in this regard. There may be deadlines, which we as laypeople cannot know. Therefore my advice is to consult a professional as soon as possible, even if it initially costs money.

I still insist on pressing charges for illegal waste disposal. The investigating officer will examine the debris to see if it is connected with the fire station construction. The demolition Willy will have to present his landfill receipts. If he has none or hardly any, then... if they prove that he buried it to save money, they have him and you have the cost bearer. K.

I definitely would not do that. It takes weeks to months before the police take any action. After that, you not only have debris in the pit, but the proverbial "scorched earth," especially since a case is pending. You certainly won't be building there anytime soon.

Just try to have a clarifying conversation. Always start off nicely.