What little subsurface ground water is already saturated with heavy metals naturally. This isn’t sitting above or below some clean stone or alluvial aquifer that is going to get polluted. The overburden, which is the same rock, is just sitting in giant mounds all around the pit, leaching into surface water and nearby alluvial aquifers. The pit is going to need to be filled (or it will turn into a toxic lake from natural ground water seeping in) and capped and the overburden also remediated. Where do you get 1 cubic km of fill? Use the overburden. Which is crushed rock full of heavy metals. This pit is going to be filled with the same rock that came out. Garbage would be the least toxic thing in and around that pit.
Or we could use good land to bury the same garbage above clean aquifers.
