Pond Boss Forums INTRO Questions & Observations Pond using bentonite clay

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?

Sorry for very late reply. I didn't get a notification in my email.

I don't think its sandy under the large rocks. The deeper I dig it's more and more clay it seems like.

The bottom of the pond is rocks. The two rocky sides of the pond are meeting at the bottom. So it's the two sides I have to seal.

FishinRod: Than you for your great reply. I am indeed avoiding the liner to be able to expose the rocks. I also just prefer the method using sodium bentonite. Less plastic and seems simpler.

There are many great guides to using benonite to seal a pond, but none of them are exposing rocks, so I'm not sure if its possible at all. Really seems like no one has documented this online. Would be nice to figure this out and document it online for others to see. Very often I come over other that have the same setup as me with rocks they want to expose, but end up covering it with a liner, because no one knows if its possible.

You never said how deep your finished pond will be. If less than 1 meter, then it is much easier to seal - using just tamped topsoil, clay, or a bentonite mixture.

I think your plan will work fine, except for a few leaks under your big rocks or through a pile of smaller rocks. For example, if there were any plant roots running under a big rock, that will create a leak path as those roots decay. However, you should be able to seal it in a shallow pond by undercutting the soft material beneath the big rock, and then mixing some bentonite into that material and repacking.

Likewise, if you have a rock pile, or fractured rocks you want in the pond edge, you probably need to dig out to the hard edge and then pack 10-20 cm of soil/bentonite mixture against that rocky face. You can then place any rocks you want on the pond side of your packed seal to protect the seal and create the "look" you want for your pond.

If it gets cold enough to freeze to the bottom, I am afraid bentonite sealing probably won’t work. Ice will pull apart the stones creating new leaks. Ice is very powerful at growing cracks and loosening compacted soils. I’m afraid a liner is your best bet.

For my garden pond, I dug it out enough that I could place boulders on top of the liner. I just made sure to use the felt used under the liner under the boulders ( bigger stones got 2 layers) to protect the liner.