To provide a little back story, last year we had a soil conservation engineer come out from Tyler to give us his $.02 on our proposed pond site, which will be about 4 acres once filled. Upon inspection, we received his blessing. Since then we have accumulated an excavator and a dump truck to accompany our little D3 dozer. With the exception of a compactor we are fairly well equipped to do some damage.
My brother and I took advantage of the 4th and spent some time at the family farm. We decided to dig a test hole to see what we will encounter in this project. We dug a 16' deep hole at the deepest location of the pond site. The strata in the test hole consisted of approximately 6" of top soil, 7' of clay, 6' of red/orange clay with veins of gray/white sand (the sand contained pockets of water), then the last 3' constisted of iron ore and clay. I am not sure how deep the iron ore layer goes as the hole was filling with water . It appeared to me that the majority water was trapped above the iron ore/clay layer. Water was trickling in at a fair rate at that seam and the clay/sand strata was slaking off just above the iron ore line. According to the soil maps we have Lf soil which is Laneville loam frequently flooded, if that means anything to you.
We were planing to dig out deep areas up to 20-25' which would put us well into the iron ore/ clay layer and possibly into another unknown layer. Is it possible or feasible to dig a core through the water bearing strata and dam the under ground flow of water to aid in filling the pond? We are at the base of a sizeable hill and I assume the strata would follow the topography. Or, should we 86 the idea of having some deep water and keep it to 12ish feet and less? I obviously do not want to end up with a pond that fluctuates with the water table or has a big leak issue due to improper planning.
Sorry for the lengthy description. Please ask away as I know there are variables that I am leaving out that could be key.
Thanks.

Welcome to the forum! Deeper is always better, especially with drought conditions. The more water you can "bank" the better.

If you can, I'd push the topsoil off to one area and save it. Do the same with the 7' of clay, but in a different area.

Make the pond as deep as you want, but you'll probably have to pump out water as it flows in thru the sand veins. Once at your preferred depth, compact the clay, plate the clay at least 24" thick over the sand veins and pack it in place to stop the water from coming into the pond thru the sand. Here's why.

The sand does not have a one way valve on it, so if the sand continues downhill from the pond, it can drain the water out of the pond in dry times. Not good.

Make sure if you have to build a dam, that you make a good core trench, tie it into good soil at the ends, and compact it properly.

I have a pond dug in sand with a high water table. The level of the pond will be high, at full pool maybe 1 year out of 4. The other years it doesn't get within 3' of going over the spillway, and at times it can be 6' low. At that point I turn on the well and pump 25 gpm into the pond (or sooner than that). That can only do so much, once it gets to a certain level it leaks out as fast as I pump it in.

The problem with sand seams is that you don't know how far they go. When we dig a well, we are usually just hitting water bearing sand(aquifer) that is above some sort of clay. And, in the case of a well, we hope the seam goes a heckuva long way. Not so with ponds. As Scott says, there's no on/off valve on an aquifer.

The only way you can really know is to dig the hole. As Scott says above, if you find sand seams, cover them with good clay. Make sure the clay is well packed.