@Kamelpov
purify water is really easy to do just evaporate and condensate it.
Which does absolutely nothing to remove radioactivity. You might remove some heavy metals, which is surely helpful, but the water is still contaminated. There's a reason why things like reverse osmosis is used for water treatment.
And soil not contaminated is the easiest to find dig a bit the earth you will find it especially marsh soil is the best one.
Which will still be contaminated by tilled top soil from you digging into it. And would require you to remove tons of dirt.
Notice that in Chernobyl, instead of doing that, it was easier and better to TRUCK IN FROM ELSEWHERE literal hundreds of thousands of tons of soil from elsewhere that wasn't contaminated. If just "dig up dirt below" was adequate, the Soviets would've done it since it would've been much easier.