My whole tank (3+ acre tank, 11' deep) sits on top of a thick shelf of rock that we couldn't break through with a PC300 excavator. I've owned the place for 9 years (dam is 30+ years old?) and the seepage below the L-shaped dam gets worse every year. The dam itself is dry but the ground 2-10 feet behind it seeps up enough to keep my neighbor's small tank full most of the summer. The seepage extends for probably 50 yards around the curve of the dam. This summer it averaged dropping 5" per week between rains(although here in central Texas they all seem to go up and down like yo-yo's due to the sun/wind/100+ days.) After watching all these years I think the water is seeping between the dam and the rock shelf it sits on. Has anyone faced and cured a similar problem?

Hico
We deal with this problem quite often up here on older ponds it is a tough problem to deal with if you take the damm down and rebuild you have a 70% chance of creating another leak in a different spot. Most of the time we drain the pond down remove soil from area of leak and pack with hard pan or clay we have been using 30 mesh bentonite mixed with sand wash from local quarries ,the two are mixed together and applied in a depth of 8" and packed then we cover with filter fabric and pack hardpan or clay it is best to coverdouble the area that you think is leaking to prevent water from walking through. Do not be fooled by pouring bentonite from a boat over leak it will not work.This type of project always requires proper planning and equipment.
Good Luck

I don't think there is a fix to a structure problem like that unless you drain it and maybe concrete the bottom area. I once tried stopping up a creek with a gravel bottom. Never worked.

A friend of mine near you at Chalk Mountain had your situation. He broke thru the bottom to try to fix it. However, he found porous limestone below the surface and now has only a memory of a water hole.

Heck, mine goes down 5" a day in the summer when the temp gets up into the triple digits. Just hope it keeps raining enough to replace the leak and evaporation.

I had the same problem, as we dug out clay for the dam ran into a rock ledge on one side of pond. It came out about 150 feet on one side.
After the frst fill-up pond leaked about 1 inch a day. So i drained pond brought in a pc300 and rekeyed the the whole 360 foot dam as far down as it would dig. We also added about 8 feet to the widith of the whole dam. Let it refill and it leaked 1 1/2 inches a day. So once again i drained it down. Brought back in the pc300, and put a hammer on it and dug rock, rock , and more rock at the inside base of the dam. I belive we went deeper the the two previous keyways and packed it back with clay. Pond has refilled and still has a small leak of about 1/4 inch a day that drains out about 200 feet below dam.

You have a tough problem to cure. Even if you could seal soils to the rock base, rock isn't solid. It's fractured. Fractures leak.
But, I would sure seek the advice of a professional engineer, to see what practical advice he might have to solve the problem.

Thanks for all the input. Sounds like something we'll just have to live with for a while.