When working with suspect soils it should become evident quickly. Presence of sand. gravel, or clay with poor plasticity and one needs to amend the liner, somehow, or you'll end up with a serious leak issue down the road which can cost 2-3x the original construction of the pond. I work with guys every day with similar stories - pond cost $12k to build and clay liner install or core trench rebuild is $25k+.

Bentonite disked into the basin with 2' clay liner compacted in 4-6" lifts works, but bentonite can be expensive and it's expansion qualities are usually unknown [it can be 2:1 or 32:1] so application rates is unknown unless you want to money whip it and assume the low end.

I build ponds using polymer liners in suspect areas as more of a patch approach to reduce cost and labor significantly. It's exponentially cheaper than bentonite typically due to freight cost [unless you live in TX or WY] but also in labor as you're only applying as a patch in suspect areas, and application rate never varies [always 4 oz./10 sq ft]. I've rehabbed two ponds leaking 4-6" daily due to gravel veins with a half unit of polymer [$200] and it took an hour of time. Packed lifts of high plasticity clay and boom, holds like tupperware. I save about $2,000 annually in well electricity cost and saved 10s of thousands on clay liner install rehab work.

Bottom line, treat suspect areas in a dry basin - it will save you years of heartbreak and tens of thousands in repairs later.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

[Linked Image from i1261.photobucket.com]