Completely agree with John.
If you excavate into the subsoil and reach the water table, you can then still choose to make either a "surface water" pond or a "groundwater" pond.
The water level in a groundwater pond will almost always be at the level of the groundwater table. The only common exception would be when you have a big rain event. If there is enough watershed area that directs SURFACE water into your pond, then it might become full. However, over a few hours or a few days, the higher water level in the pond will slowly go down and equilibrate to the level of the water table.
Most people in areas with lower water tables build a surface water pond. If you have enough surface area of your watershed, you can collect enough runoff water to keep your pond full. This type of pond MUST BE SEALED. This type of pond would then only lose water due to evaporation and a bit of leakage. The exception would be a long term drought. In that circumstance, the sealed, surface water pond can go dry.
My worthless, long distance guess? Heavy equipment construction is very difficult once you are working in wet material, with wet clay usually being the worst. Was your contractor working on a $/hour bid, or a fixed price for the job or fixed price for the amount of material excavated? If the latter two options, then he was going to rapidly start losing money as soon as you hit the water table. (This is speculation on my part!)
One option is to call your contractor. Ask if he sealed up the connection between the pond and the groundwater table. (This usually means using good clay from your location, and compacting it with specialty compaction equipment in 6" lifts.) If he did NOT do that, then you need to say your 2.5 foot deep groundwater pond is not acceptable.
Did you have a written contract? If you did, and your reading of the contract looks like the terms were not met, then it may be better to have a lawyer do the first contact with the contractor.
Sorry for the potential bad news. Also for the lack of solid advice. It is very difficult for people not on site during the job to give expert advice in this type of situation.
Good luck on getting this resolved in a favorable fashion and getting a chance to enjoy your new pond!
