I have a groundwater pond. 11' of sand on top of 1' of clay, then another 6' of sand on top of at least 4' of clay. (when we renovated the pond we dug down to 22' below grade and it was solid blue clay there). In a wet spring, water will sit on top of the ground until early June.
What I've found over the years is that the groundwater level will fluctuate up to 6'. During a wet year, the pond will drop about 3'. In a dry year, double that.
If I were to do it all over again, even though I put the pond in a wet spot, I would dig the pond large enough so I could line the whole pond basin with 6" lifts of clay, compacting each lift with the next one so I would end up with at least 2' of compacted clay.
I can pump 25 gpm of water into the pond and it will not reach full pool during drier years. I've never run the well during a wet year, saving on energy costs and just figuring that a 3' drop in water level was normal. If I had my druthers, I would rather have the pond full at all times vs. fluctuate in water level.
My neighbors, who have lived here about 10 years longer than I have, said that with me having the pond, they see less flooding in their back yard during wet times of the year. So, when you dig your pond, you may not see the water standing on top of the ground like you do now. The way my pond is designed, it will receive surface (top of ground) water runoff.
In hindsight, if I would have only dug the pond 11' deep so I didn't punch through the level of clay, I would end up with a pond that at least 6 months out of the year would be only 5' deep at the most. That depth is not conductive to having fish survive a long and hard winter like we just had without intensive management methods. I dug the pond down to the 22' deep mark, and normally go through a winter with 14' of water in the deepest part of the pond.
Thanks for the info. Sorry it took me a bit to reply but everything you've said seems to apply to my situation. The only thing I can say that might be a little different is the fluctuation in the water table. One thing I've noticed in the 4 years we've been there is the swamp on the opposite side of my yard never seems to vary much more than 2 or 3 ft. It actually only seems to be a few feet deep and I have never seen it dry completely up. Is it possible for the water table to only fluctuate that amount in both dry and wet seasons? If so I can only assume my pond will hold water in the same manner? If so I don't think that small amount of fluctuation would bother me much. If it were like your situation with several feet of variance in water levels I would probably prefer it to be lined as well.
Another question I have is if I were to clay line the pond how would all of the water in my yard drain to it? Wouldn't the clay lining not only seal the water in the pond but also seal the water outside the pond? If my back yard sits right at the water table wouldn't that defeat my purpose of giving it all a place to go? How would I keep it full of water? Also if I were to just dig a groundwater pond what type of soil do I need? I assume it could be anything but preferably a porous material to allow the ground water to flow into it? If I have a lot of sand in my soil how do I prevent it from simply filling in on sitself? Also if you look at the picture in the initial post you can see trees surrounding 3/4 of the sides where the pond will go. Do you think that would be a major issue? I plan on cutting down several of the ones close to where the banks will be.
I apologize for the amount of questions and I really appreciate everyone's input and advice. If I'm going to shell out this kind of money I just want to make sure I get it right the first time. Thanks again.
