Pond Boss Forums POND CONSTRUCTION Selecting a site Sealing a pond with steep slopes without liner

Hello everyone,

I’m currently working on a pond project and I’m facing a challenge. My pond is approximately 8x4 meters (26 X 13 feet) in size and almost 2 meters (79 inches) deep at the deepest end, average depth of about 80 centimeters (32 inches). I was planning to use sodium bentonite as a sealant but due to space constraints, the slopes of my pond are steeper than the max recommended 1:3 ratio (V:H) for using sodium bentonite as a sealant.

From what I’ve read, sodium bentonite is not a good option for me as it tends to creep downwards on steeper slopes. If I were to follow the recommended slope for bentonite, I would have to reduce the depth of my pond to just 67 centimeters (26 inches), which is not good since I live in a region where freezing temperatures are common, thus maintaining a deeper pond is crucial.

I came across a comment on YouTube suggesting to use 10-20% Portland cement mixed with clay or soil as an alternative method to seal ponds with steeper slopes. Has anyone here tried this method?

Please note that I’m not interested in using pond liners, ducks, or pigs for this project. Any other advice or suggestions regarding using natural materials to seal a pond with steep slopes are greatly appreciated!

This PBoss pond sealer expert should be able to help reduce your pond seepage problem. If he does not see this thread in a few days contact him directly.
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402.730.4897

Do you have a water source to easily re-fill your pond? Is it already full of fish?

How steep are your maximum slopes?. In moist clay you can work in the bentonite and re-shape the slope at pretty steep angles.

If you have the option of draining the pond and letting it mostly dry, then you can apply your bentonite on the pond slopes and the bottom and then roto-till about 8-10 cm deep to get uniform mixing. Tamp or lightly compact the entire pond area after tilling.

Bentonite is a type of clay mineral that actually incorporates water into the crystalline structure. It therefore swells substantially after it is hydrated. However, it is a little fragile as regards staying in place until it has swelled. Therefore you need to gently re-fill the pond from the bottom up such that you do NOT create any erosion channels that cut through your 10 cm blanket.

The treatment above should create a pretty good seal for a pond that is only 2m deep.

Bentonite may not be a good solution if the water levels in your pond vary over extended periods of time. If your bentonite blanket dries out, then it may suffer from large cracks as it dehydrates. Re-filling the pond may not perfectly heal these cracks.

If you can't drain and treat your pond, then bentonite can be added into a full pond. However, TJ has a good product that may be a better solution for that type of application or for the situation where your pond goes through wet/dry cycles.

I have no idea about the regulations or cost to get that product to Sweden. However, TJ is a "sealing expert" and can also just give you solid advice if you can contact him, or if he shows up in your thread.

Good luck on your new pond project!

Thank you for the reply. The pond is dry, I've just finished digging it. I have now been in contact with several distributors of bentonite and all of them say the same thing, sodium bentonite is not a good option for me as it tends to creep downwards on slopes steeper than a 1:3 ratio (V:H). Guess I'll need to look for another solution.

Sorry your bentonite application probably won't work!

Time to think outside of the box.

If you think you can seal the bottom effectively, then maybe try a pond liner on vertical sides? If you cut your sides vertical, you could have a little more pond volume within your restricted space.

You could tuck the pond liner into a trench in the pond bottom and then backfill to anchor it. I think it would be very unlikely to get punctured since no animals or humans would be able to effectively put weight on it.


Alternatively, I have seen applications of cement modified soil. However, that material was used for erosion control in emergency spillways subject to erosive water flows. I have not personally seen it used in applications where perfect sealing is required.

I have attached an engineering paper from my files to give you a little more information if you have to go the "Cement Modified Soil" route.

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://secement.org/wp-content/uploads/2017/04/EB242.pdf