Welcome to the forum.
To give yourself a good chance of sealing it you have to have a good supply of good clay. Dig out the area 24" deeper than you want the finished grade to be, make sure that it doesn't have standing water in it (pump it out and keep it pumped out) bring clay in and place in 6" thick layers. Roll over it with a sheepsfoot roller until the roller starts to walk up out of the ground, then bring in another 6" clay. Repeat until you have 4 layers built up for a total of 24" thickness.
If you are unsure about where the water is leaking out, do that to the whole area of the pond that will be covered with water.
| 
