With a pond that size, you could get a 45 mil EPDM pond liner delivered for about a $1000. It needs to be EPDM for pond use -- not roof use, because of the chemical makeup. The EPDM should last quite a few years.
Here is a typical example of the type of liner I'm talking about:
Firestone 45 mil EPDM Liner Don't waste your money on PVC liners. They have a lifetime of only a few years before they breakdown from sunlight near the top edges. Also, critters such as deer will puncture it if they walk into the pond.
Good luck,
Ken
