I've been browsing through this site for the past few evenings, but haven't found a problem quite like what I've encountered yet.
I've got about a 1/2 acre pond on some property I bought
for hunting. The pond is in an area with soil mapped as Mountainburg Stony Sandy Loam. In the area of the pond, there is 18" to 24" of the loam overburden (which contains alot of sandstone fragments) sitting on top of weathered sandstone bedrock. The pond appears to have been built by scraping up and pushing the surface loam on a moderately sloping ground surface into a dam about 10 ft tall at the highest point with roughly 1.5H:1V front and back slopes. The dam itself is stable (i.e. no erosion or sloughing away of the sides). The dam contains a lot of sandstone fragments, but the surrounding soil matrix is a decent looking red clay that will ribbon to some extent. There is good drainage to feed the pond during rainfall events, but not an active creek, stream, or spring continually feeding the reservoir. After a good rain, the pond will fill up completely, but within 2-3 days of no rain, the pond only holds a couple of feet of water at the deepest point. With no further rain, this last bit of water is gone in one to two weeks. When the pond has a decent level of water in it, I have found a section of the dam that has a very evident leak. The leak is directly below where an existing 18" diameter cedar tree (now dead) was enclosed by the construction of the dam (at the point where the dam is the tallest, no less). I have no doubt that the roots are a problem and for the best fix, the huge dam in this area needs to be excavated out and the tree and rootbed grubbed out of there. This, of course, will require some serious dozer work and will be pretty expensive. I don't want to waste money on it if the pond is unlikely to ever hold water longterm anyway.
To boil all this rambling down, I can reduce this down to a few questions:
Is sandstone bedrock suitable for a pond base, or are there likely to be so many fractures it it that it will never form a good seal?
I have thought about trying to till in 1-2 pounds of sodium bentonite per square foot into the dam on a section of the dam for about 20 feet either side of the large tree and then compacting the soil as best as possible with truck or atv tires and see if this significantly decreases the rate of leakage. I've also thought about augering some 3" or 4" diameter holes at the top and along the inside face of the dam in the vacinity of the tree and filling the bore holes with bentonite slurry to try and seal in the voids and water pathways created by the tree rootbed. I've not found anything anywhere about this method, anybody here tried anything similar to this?
If the sandstone bedrock can make a pond base that will seal, is it feasible to put enough bentonite on the inside face of the dam to make it successfully hold water?? Varying info I've found on the web says anything from "1-2 psf of bentonite can seal a dam and pond bottom containing rocky soil", to "no amount of bentonite can repair a dam with rocky soil".
Any advice or experience anybody has had with ponds in sandstone bedrock would be greatly appreciated.
