Niel, Glad to see you posted back. I almost missed your post, as it was so far down the string. Anyway, one other thought, could the pipe leading from your tank to the drainfield be plugged? these pipes are usually cast iron and it is common for them to become plugged with "black rust" after a few years. Then sometimes the effluent flows out around the pipe or out the top of the tank if the house is high enough above the tank.
Certainly a spring could be your problem, but then I would wonder how the field got installed there in the first place.
If you have to replace the field, it would probably have to be done in an alternate area, as the soil in the location where it is now is probably saturated with solids, and the water just won't soak in anymore.
If you have to replace the field check out www.infiltratorsysyems.com it may be of help to you.