I just saw Sunil posted while I was typing.
He brings up an excellent point about "groundwater" ponds.
Most of the replies to your post have been good advice for typical ponds across the U.S. However, many (like mine above) do not apply if your pond is going to be a true groundwater pond.
At my farm, it is about 6' of digging to reach the small springs in the soil. However, the topography is so flat, that the water level from the springs only pushes up about 6". If I tried to create a "groundwater" excavated pond, then the water level would always be 5 1/2 feet below the banks of my pond.
"To give an idea of how what and high my water table is, I dug a 2 foot trench by hand this summer to run electric to an outbuilding I built, and after 2 hours the entire trench filled with water."
It doesn't matter that the "whole trench" filled with water. What matters is whether the water rose up to the top of (or even flowed over) the edges of your trench. How high did the water go in your trench project?
If the static water level was high, then your contractor has an easier job than most of us thought. It also explains why he said he could do the job in February. He just needs to excavate the pond and partially seal the down-slope side of the pond enough so that the rate of downslope leak + evaporation is less than the rate of spring water inflows. (If your springs are really prolific, then he doesn't need any seal.)
That is an easier task to accomplish. Although, I am still a little leery about doing the work in February.
