While possible, it's not common to have an outfall elevation lower than the water table, because if you did then the water table would eventually stabilize at the outfall elevation.
Basically the question goes back to whether you're trying to retain water (and then infiltrate it into the ground) or whether you're just trying to store and detain it.
If your goal is detention, storage, and release through the outlet control structure, and your water table is below your lowest available discharge elevation, then the lowest available discharge elevation is how deep you can make your pond. If the water table is higher than the lowest available discharge elevation, then you might be able to dig your pond that deep and just let it drain the water table down over time. That will be subject to local and state control.
If your goal is to capture, retain, and infiltrate water into the ground as part of your design, the bottom of the pond will need to be X feet above the water table. Often I see 3 ft, or 1 ft, for this criteria, and your first discharge control then has to be higher still in your pond, dicated by the amount of volume you have to capture. There's also infiltration math you need to do.
If your goal is to have a permanent wet pond, then you can dig plenty down below the water table, and ideally you'd set your outlet control at the water table elevation, setting the water elevation of the pond such that it interfaces seamlessly with the groundwater. If you want your wet pond significantly higher than the groundwater, you'll need to think about either pond liners or water balance equations related to your watershed. But don't take credit for volume below the permanent pool surface in your routing, because it's not available for storage.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -