Welcome to the forum, wiredit. (Trying to figure out how to pronounce that mentailly when I read it. Wire-dit? Wi-red-it? Wire-edit?)
You are sure your soil conditions will require some kind of sealing?
IIRC, according to
"Perfect Pond: Want One?" "Perfect Pond: Want One?" you should mix the bentonite into the top (6"-8") of soil. So I would spread the bentonite before tilling, and try to go deeper.
I would be sure and use enough bentonite, or be sure of whatever sealing method I went with. Doing it right the first time is gonna be lots cheaper than having to drain, dry, and redo it.