I'd shoot for a 24" thick clay "cap" over that seepage area, but it needs to be done in 4-5 steps. You can't compact that thickness of clay at one time.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).