Welcome to the forum. Your safest bet is to dig the pond about 24" deeper than what you want the finished depth to be. Bring in good clay and plate over the bad clay in 6" lifts, with a sheepsfoot roller or a multi-tired pan scraper filled with dirt for more ground pressure. Plate the whole pond basin, anywhere water will be.

Not only do you have to worry about the acid producing formations, you have to worry about the ponds pH (yeah, that's a function of the acid) wink. Bob Lusk ran into that problem in Texas and that's the only way that he could fix the problem.

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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).