Pond Boss Forums POND CONSTRUCTION Soil questions Groundwater; signpost to disaster?

In one of my other, parallel threads, we had to dig a couple of substantial holes to get down to good clay in a particular zone. Bottomscaping thread
Since their creation, there has been no significant rain (that I can remember...could be wrong?). Anyway, 6 days later I looked into their depths and there is an accumulation of water about 8" deep...not all that much. It is very clear, so it leads me to believe that it is groundwater...? The bottoms of these holes are about 10 - 12+ ft below normal existing grade. The bottoms are blue clay; it was the reason they were dug. We have no reason to believe that we are about to break thru to compromising soil...or do we? Does groundwater presence indicate pourous soil very close by, underneath? Am I looking at a potential weakness in the seal of the pond?

Brettski, my place is clay all under it. While digging the pond and (earlier) three shallow wells, we saw no sand or gravel at all. Yet two of the three dug wells produced useful amounts of water most of the year; one spot in the bottom of the pond had a very slight flow of water into the basin (under 5 gallons/day).

In all three of these places, the water ingressed through tiny veins of cracks in the clay. One well I used for a year until a deep well was drilled, the other I improved and used as the sole source of water for the main barn for 10 years. The seep in the pond bottom we just avoided because it was sloppy for a dozer to work in (post PB, I would have had it packed off with clay on general principles).

Seeps like these could potentially reverse flow in a pond bottom like larger springs in sand or gravel. But consider the size of the path that water is entering on - the reverse flow would also be very small. If it has occurred in my pond, the amount lost is greatly exceeded by evaporation.

So if you want to be sure, you could pack off the bottom (or sides, wherever the water seems to be seeping in) of those holes. But I don't think a small seepage of water requires the presence of what we normally consider undesirable soil, and if you don't repack the holes, or if such packing is ineffective, I doubt the possiblility of losing serious water through the holes exists.

Minute cracks in the dam would of course be another matter...

I agree completely w/ Theo WRT - slow-in = slow-out - ......I have constructed 100's of test (monitoring) wells in clay soils, and yes, clay can and will produce water (which is why I asked you the other day if that could be GW in your pond). This is also the basis for the phrase...all ponds leak....its just a matter of how fast. In a good clay, if it took that many days to get 8-inches, it would take almost as long to drive it back out once hydraulic pressure is put on it (full pond)......if your research is correct, and the clay continues for many tens of feet below yer pond.....you'll be fine IMHO.

i was just working on a boring log for one of my sites, where we observed a saturated sandy silt below a 10 foot clay. static GW level was well into clay, but first encountered water during drilling was not until silt layer.....so a confining condition, which you could have (and you alluded to). conversely, thick clays can over time trap and yield either perched groundwater zones or confined GW from below (where depths to more permeable layers are significantly deeper)......you may also have either of these (harmless) conditions.

if your still worried, do a test pit. dig down three to five more feet in one of your deeper holes (if excavator can reach) and evaluate soil. if you encounter a sand, silt, or silt clay mix, have excavator backfill using good clay from upper rim of hole, soften the overall pit feature, and be satisfied you averted a potential drain.

Thanks, guys. I had another update and conversation with my excavator, John. At the end, I asked him what he thought about the water at the bottom of the holes and the possibility that it may create a way for water to exit. I was quickly tuned up.
He said it was seepage from the 6 ft of topsoil above it. He called it lateral water. When he was digging down to the clay some 8 feet below, the said the stuff on the way was heavy and saturated. He has zero belief that the water came from below. He continued by explaining that the bowl that created that end of my pond basin is made out of a thick layer of very good clay. Over the hundreds of years of erosion from the surrounding forest and farm fields, this clay bowl filled up with soil. Any precip. just totally saturates whatever pourous material is being held within the clay bowl like a sponge. When he breached thru this sponge layer, the exposed edges started weeping laterally and down into the clay bottom where it stopped.

o.k.....the perched water (harmless) condition...good deal..sometimes (in clays) perched zones are formed in pockets that contained organic matter long decayed (an ancient fire zone or heavy storm season layer), it tends to leave silt or peaty zones w/ greater porosity.

i'll post a quickie on your vegetation thread so it stays up top for others to easily see.....

I think that's the situation here with the shallow wells we had/have (& maybe with the seep in the pond). They just had (upper/surface) ground water from fairly recent rainfall; I believe that's why the one we used most would go dry in August & September +/- 1 or 2 months every year. It is only 13 feet deep; we didn't hit any deep water drilling the big wells above 90 feet down.