A liner is probably the best for a pond tha size.  I have a .6-.7 acre pond in my back yard that didn't hold water when we built it and was a nightmare to manage.  After the first summer we drained it and cleaned out the bottom.  Then I brought in 75,0000 lbs of bentonite.  We used an old fertilizer spreader to broadcast it evenly and a small disk on a skid loader to incorporate it.  The plan was to pack it as well but we got rained out.  I now run a garden hose about 1/2 open to keep the pond at a steady level.  Hot, windy, dry conditions in late summer/early fall are the most difficult.