Do you have a waterfall or water feature that could be leaking? What kind of liner or pond do you have? Are you sure it is not evaporation? Unless your water feature is leaking, you should be able to find the leak at water level. With that size of a pond you should be able to find it if it is in the liner. I would try to patch first, and then if it did not work, replace the liner. No use going to the expense of replacing if it can be repaired easily.