Narrow, your situation sounds very similar to what I am dealing with. I would be very interested in any solutions that you come up with.

In my situation, I believe that I have a pond that is tied to ground water that is "perched" on a layer of shale about 5'-6' down. I only purchased the property a couple months ago and haven't done any digging. This is my best guess from looking at the soil surveys and the behavior of the pond. Anyway, this could easily be your situation as well.

I've come up with three different approaches so far:

1) Drain it and line it (clay or synthetic, whichever is cheaper).
This is probably the best option but it's also the most costly up front.

2) Find an alternate water source and pump... and pump, and pump...
This is possibly cheaper than lining the pond but you'll be spending money to run the pump forever. A well dug to the aquifer probably won't be cheap but you may be able to dig a catch basin to the shale and put a pump in it - kinda like a sump system. Upfront cost is lower but you're just circulating the ground water.

3) Live with it. You know the drop is coming so you can plan for it.
Esshup has a nice groundwater pond. I think he did this for awhile but then started pumping. If you look at some of his high/low pics you'll understand why. It's really tough to manage capacities, vegetation, etc. when your water level fluctuates so much. The only saving grace to this approach is that the upfront cost is zero. 5 feet over a few months is a lot though.

I'm pretty much locked into #3 due to budgetary constraints. I do plan to expand the surface area and bring the banks down to a 1 to 3 or 1 to 4 if I can get some free equipment. I may also try the catch basin idea down the road. I have a gravel floor barn that stays wet/damp half the year and should have some drains dug around it.

Good luck and keep us in the loop.