Hi,

I want to create a pond using bentonite clay. I have read enough so that I have an idea of how to achieve it. But there is one question I can't figure out the answer to.

Is it possible to create a proper seal where bentonite meets rock? Where the bentonite only partially covers the rock, so that the top part of the rock is exposed?
Or will the water get between the rock and the bentonite, and destroy the seal?

Se pictures below.

How large is your final pond size?

Most small ponds I see in the U.S. of your apparent size, utilize a pond liner to create a perfect seal.

Are you trying to avoid a liner so that you can incorporate your beautiful, big boulders into your pond design?

One option would be to lap the liner up against your big boulders to just above your desired water level. You could then use some of your cobble-sized (or larger) stones to hold up the liner and hide the ugly edges.

However, your existing soil (in the photo) appears to have decent clay content? If so, then another option is to dig out all of the small rocks, tree roots, and sand veins. Then lightly water and compact your soil as tightly as possible. Add another few inches of soil, and then repeat the process.

The hydrostatic pressure that causes water to leak out of ponds is directly dependent upon the depth of the water. If your pond is only 2' deep, the water will not be "pushing" very hard to find a leak.

Bentonite works to seal leaks because the material expands in the presence of water. However, it is not a strong, cohesive material - like a grout or caulk. Therefore bentonite would probably NOT seal if there is a crack or fissure in your boulders, or if your boulders are sitting on other rocks. There will always be a space in the latter situation called a "point contact" that would be very difficult to seal around.

You might try digging out your pond to its final dimensions and compacting as I described above. Then fill it with water, and test your leak rate. (You may need to add water a few times because your soil will absorb some water on the initial filling - which is NOT a leak.)

If the pond holds water (or has an acceptably low rate of leakage), then proceed with your final design features. If it leaks badly, then you can consider incorporating a pond liner into your design.

Good luck on your new pond project!

Hello and welcome. Do you have reason to believe that the soil under those rocks is sandy?