After they dig up everything to extract the gravel..... Unless there is a clause somewhere in the initial agreement from when it was built that it has to be removed and reverts to farmland or it is sold back to the original land owner.
Google Greenham Common and look at it now, the runways and taxiways were dug up and reclaimed with the gravel extracted, it was turned into a nature reserve, the cruise missile silos that still exist and were used as a film lot for one of the later Star Wars rebel bases.