Welcome to Pond Boss Jim.

It depends on the nature of the muck. Best to clear the possible leak/GW seep area and pack with good clay. The weight/compaction of the clay will have to be greater than the WT pressure or it will do no good. That will be the key. Suggest engineering type help as moving clay/dirt is expensive so it needs to be done right the first time. That would include factoring in the WT info and the nature of the dirt/clay to be used.