I had to look into a liner for a pond that a contractor wanted to build. 1.25 acres. The EDPM liner that had to be seamed on site would have run almost $200K. I got a quote from Bend Tarp Liner BTL in Oregon. They could make 2 different thicknesses of liner that were one piece with no seaming, and an even heavier liner that needed 1 seam. The seamless liners were 1/4 the price of the EDPM liner.

No matter what liner you use, you need to cover it with soil. The BTL liner didn't need an underlayment, the ECPM liner needed an underlayment too.