The Porter County NRCS, in sandy soil with excavated ponds, recommends digging a trench completely around the pond, down to the groundwater level. Then they drop a sheet of visqueen in the trench, backfilling as they go. They bring the visqueen as close to ground level as they can. They said that slows the leaking of ponds.

I don't know if that would be a viable option for you or not. It doesn't 100% seal the pond, but it slows down the leakage.

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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).