Concrete is considered “clean fill”. There is nothing toxic about it. Just dig a deep enough hole so that it does not settle after you have filled the hole back in.
Or, rent a jack hammer and break it up into smaller pieces. That is what I did with the pool concrete.
Good advice. There is a potential unwanted stream going across the property, but only after a really hard long rain. So I’m thinking a very deep French drain up at the property line — dig about seven feet down, dump a load of pea gravel in, then concrete chunks in-filled with river stones and more pea gravel, at least five feet below the surface before filling with soil.