I'm doing a land restoration project in the Samarian highlands and am thinking about putting in a pond. There is a flat-ish ledge of land that is about 10m across that would be okay. However, our terrain is very porous (there is zero above-ground water, unless it rains for several days, and then the water is gone quickly.) I am wondering what the appropriate way to build a pond in such conditions is. I have access to used billboards, and can also bring in bentonite clay. Thoughts?
Have you seen this thread -- My Progress Gleying a Pond with Pigs Michael has been documenting his progress for the past few years. His site is very rocky and porous, but the pigs have allowed a pond to form. I haven't checked in on that thread in a while so I don't know what his progress is right now. But last I saw, it was holding water. Although from what I recall, there was a steady trickle of water into the pond. And that may be a necessity to gley a pond with animals.?
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I'm not sure if you are talking about a karst type geology, it sure does sound like it, and those areas are known for sudden changes in underground water flows, from chambers and channels dissolved in the limestone over time, like a honeycomb. Lakes appear and disappear overnight and dye tracers may appear many miles away. That sort of geology is great for filtering and infiltrating water. Once you start to dig you will have a better idea what you are dealing with. If you find that sort of honey comb structure as you dig down, then even a 1/2 meter of clay may not be enough to totally contain your pond long term.
There's all sorts of ways to seal ponds. Special high tech geofabric liners impregnated with bentonite clay. . EPDM pond liners, etc etc.
We were told that a really adequate pond seal would need 1/2 meter of clay on the bottom. That makes sense to me since doing it with much less would still allow for relatively minor disruptions in the sealed skin to cause problems. So if you have plenty of bentonite clay on hand you should be fine.
I recently dug out a pond about the size of your ledge, the top couple feet had some clay, but that gave way to sand and sedimentary rock, and at first water wouldn't stay in it more than a day or two, then I started a diversion channel from the driveway which brought in suspended clay during rain events, and it is slowly starting to seal and now holds water for many days.
I have often wondered if I could speed up the sealing process when the pond fills by adding some agitation keeping the clay suspended while the water is seeping out, since I have noticed a rather thick layer of clay on the bottom of my "pond", but higher up has taken much longer to seal. so even when the pond fills most of the way (about 2 meters deep) the top drains out pretty fast.
As mentioned in the last post, Gley is also a well known way to seal ponds also of course. A thick blanket of fresh green hay covered with carpets, ducks, cows, pigs etc, and there are also some geo polymers available that will help seal dams.
I recommend that you start by looking what you have in situ. Rocks, gravel, sand in combination with limestone or so?
Soils forming on carbonate rock can be rich in clay because clay is geochemically much more stable than carbonate that dissolves. So that soil might give you some clay. In sinkpipes you may find the more pure claystuf. That clay might be usable to hold water.
Remember you do not actually need that much clay. Any poorly sorted sediment may hold water even if the actual clay content is low. You only need some clay to block the permeability.
A poorly sorted sediment is one where sediment grains of lots of different sizes are present. The small particles or grains block the permeability between the bigger sediment particles.
Of course in an earth quake sensitive zone you might want some extra squishy clay (for want of a better term in english). Squishy clay deforms easily and can prevent your pond leaking after an earth quake.
I do not have direct experience with terra rossa. We don't have that kind of soil here. I have seen it on holidays though. I remember it being slippery when first wetted. I found out the hard way. For land restoration you want to bring in as little material and equipment as possible.
For you purpose, i would check whether destroying the permeability would work. This is the opposite of what permies ussually try to accomplish. In my garden soil - i have this ironrich soil horizon with 30% or more clay. Undisturbed it has a resonable permeability. However, if i dig down to this level and if spend some time standing on this horizon, then the permeability is reduced drastically.
I have this ongoing project to remove exotic conquerors like bamboo and yucca from my yard as well as removing bricks and such from my soil. In short i digg holes in the ground. I have this spot where i want to build a wadi (https://nl.wikipedia.org/wiki/Wadi_(infiltratievoorziening) (there is an english word - but it does not spring to mind). I want it to infiltrate excess water in the soil. In some places this really does not work - i have several smallisch ponds 'hanging' at different depths in my soil at less then 2 feet from each other. I ascribe the observation to the fact that i have worked a different depth in this soil horizon. In effect i destroyed the permeability that i want in the long term for the short time gain of eliminating bricks and bamboo from my yard.
Before you ask - handdrilling has a similar effect - the clay in the layer i want to use 'smears' the sides of the drill hole - thus reducing the permeability.
I suspect you can use this effect to get what you want - a soilhorizon that has strongly reduced permeability. So why not test this? Take some terra rossa at the water depth you want and check what it's permeability after wetting and kneading it. Then you take a transparant plastic tube. Fill it to about halfway with course sand. Add a layer of wetted and well kneaded terra rossa. There should be a tight contact between the tube and the kneaded terra rossa. After that pour water on top. If the kneaded layer holds the coarse sand dry you are in business.
'Kneading' with a digger works. I used to have this done to some extent when in an soil cleanup operation i wanted to catch some liquid pollutant - f.e. oil. That might otherwise wash out. In a pinch you might use a cement mixer and water to knead your terra rossa.
The thicker the kneaded layer-the better it will hold water. Think about how deep you want you pond to be.
In your setting (Israel ? i guess), your pond may be vulnerable to plants rooting towards the water. Another problem might be drought. The terra rossa clay i have seen, cracks when drying. If your pond dries, it might crack and overgrow with grass and/or reeds - both bad ideas if want to keep that impermeable layer going untill you can catch more water.
If you use a cementmixer you might add 'smectite-type' clay if you have that locally OR you might add commercially bought bentonite. This type of clay 'swells' when wet and thus closes up 'leaks' in your pond. That mix is kind of a 'self sealing pondliner'.
You definitely want to do some research before spending time and money. Consult local builders, local potters, .... and observe local construction sites to check if these ideas might work.
You can of course try for an organic approach but boar and swine make ponds this way where it is convenient for them. In Israel that is probably a nono?
When digging out Terra Rossa you might consider, using it to increase your rain catchment area. It sounds as if might have not enough input of water during a dry summer. If you are building nearby - filtered runoff from the building might be a good extra watersource. Be carefull about saline buildup in your ponds soil and water.
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