Shane,

First, it sounds to me like the dam was not built right and a core trench was not built in the center of the dam.

Bentonite clay, in our experience, has about a 50/50 chance of working. If a person is willing to spend the money on bentonite, it is usually our recommendation to use a liner and spend just a little more for the assurance (not gamble, with bentonite) of the pond holding water.

EPDM liners are the most expensive. We will typically use PVC liners because they are more cost effective. EPDM liners are mainly used in backyard/ornamental water garden ponds. EPDM is UV resistant whereas the PVC liners must be covered with soil when installed otherwise they will deteriorate quickly when exposed to sunlight for prolonged periods of time.

If a pond liner is done correctly, you will never know it is there...you can't see it. Another key to a good liner installation is making sure that an underlayment fabric is used in conjunction with the PVC liner. The last part to a good liner is using a liner trench on the outer portions of the pond. This is one of the most important parts to the liner. It prevents "liner float".

I hope this information is helpful. Feel free to e-mail me personally with any other questions you have.