(to top that some ammo dumps were simply dumped in holes in the ground at the end of the war and are slowly diffusing there toxic compounds via under ground rivers)
That's an easier problem. The conclusion for the Onkalo nuclear fuel repository was, once it's filled and sealed, not to mark it at all. A marker would create attention, and any future society capable of mining deep enough to get in would probably know about radioactivity.
Here's the sign at the SL-1 site, which is not a deep burial.[2]
"A lot of the reason why concrete structures have failed so quickly is that the material was still fairly new when most of the structures failing now were created."
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