There is nothing wrong with the process, just the implementation. The bottom layer is not radioactive, it just needs to be processed properly. Since there is not going to be anything there of significance which is not organic there is not reason you could not move fish from pond A to pond B, and then process the bottom layer of the pond to be fertilizer that will help grow more food to feed to the fish, etc. Since you are talking about years of use in a single location it is pretty obvious that you are not actually talking about known sustainable processes but are tagging that label onto something else.