[PapaBear]

Quote: Originally Posted by Koicare ...On the soil issue - our soil is varied - first we have alayer of topsoil probably about 30 - 40cm thick - then you hit the clay for about 60 - 70cm, and then you hit shale!!! I am not really sure what shale is - it might be clay which has compacted and formed into thin sheets of rock, or a rockish type material - but all I remember is that each pond we dug here was a major nightmare in sealing - not in terms of keeping the water in - but rather keeping the water out! We had to dig a trench deeper than the ponds so that the water would go there until we had dried and sealed the concrete! If I go above the ground - which would be preferable then I do not have to get rid of the soil - I would probably have to either line the sides of the pond because of the top soil - or I would have to seal the entire pond with a bentonite. We have a variety of bentonite here - one which they call "Envirobent" (which they recommend for sealing of dams - as it swells to something ginormous!) - my only issue with it is that it has a pH of 10,5 (and I do not know enough to know whether that would be a problem and affect the pH of the pond to 10,5??????? and they have a "Calcium Bentonite" which they say does also swell, but not as much as the envirobent - it is more a water conditioner - so then the pond might leak? I am thinking about getting a bag of the envirobent and running a little test to see how much it affects the water (ie: 50 litre bucket - put envirobent in at the bottom) and test the water daily) - but I am not sure if this would be a waste of time on such a small test, and without the exact conditions - and provide inaccurate results???...

From the sound of things you should be able to follow Mickey's lead on your construction method.
I would first remove the Topsoil (30-40 cm) to the perimeter (to be re-used later)
Then excavate about 1/2 the depth of your clay (30 cm) to build up the sides of the pond above ground. That will leave you a 30 cm clay bottom and clay sidewalls of sufficient depth.
Then you can re-use the topsoil to cover the top and exterior walls of the pond for re-seeding with grass, etc... for aesthetics. (No trees or shrubs close to the pond as they will send roots throgh the clay and cause leaks).
Whatever you do don't break though to the shale. Unless it is harboring a natural spring (unlikely) you would never be able to hold water with a shale bottom.
As far as the bentonite goes, if you need it a mixture of the Calcium Bentonite and Envirobent (probably Sodium Bentonite with lime) should work well for you. Re-liming a mudpond from year to year is common practice anyway and the high ph won't last long when the pond is re-filled.
BTW, what is the ph of your well water. Tested fresh from the tap and after resting for a few hrs. In most cases it should be low from the hose and higher after it rests.