Bentonite Clay Pond Liners vs. Traditional Liners

We get quite a few calls from customers asking about bentonite clay pond liners and how they stack up to traditional passive liners like RPE (reinforced polyethylene), and EPDM rubber. Bentonite is a very different means of achieving a water-tight seal compared to a passive liner, but like anything it has it’s advantages and disadvantages.

Bentonite Clay Pond Liners
Raw bentonite clay

Passive vs Active Liners

The first thing to understand when comparing bentonite to something like RPE is the difference between an active and a passive liner. A bentonite clay pond liner would be an example of an active liner. It’s called active because there’s actually an active mechanical/chemical process that must take place before the material becomes waterproof. On it’s own bentonite is just a clay and is not waterproof. However, when it gets wet, it expands many times it’s weight, absorbing water. If it has something to expand against, it can form a water-tight compressive seal and become water-tight.

In contrast, a passive liner does not rely on any ‘activation’ process to work. For example, our RPE liners are simply water-tight liners that you lay down and get instant results.

Advantages of Bentonite

The biggest advantage of bentonite from a marketing point of view is theoretically it can self-heal and expand around punctures if it’s installed correctly. For example if you were to line a pond excavation with a layer of bentonite clay. You could drive penetrations through the clay, or drive a piece of heavy machinery over top, and easily fix any damage after the fact by sprinkling down some more clay. When water is added, bentonite is designed to expand and self-heal any penetrations TO A POINT. Obviously if the hole or penetration is too large, you will be out of luck as it can only expand so much.

Disadvantages of Bentonite clay pond liner

We get quite a few phone calls from customers looking to replace a leaking bentonite clay pond liner with an RPE liner. The reason for this is usually improper installation. Often times people will mix bentonite clay with the existing soil to save on material costs. If this mix is created too thin, the pond will leak. If the mix isn’t spread out perfectly even, the pond will leak. If there’s even one single thin spot, or a spot that was ‘scuffed up’ leaving a small area with no clay, the pond will leak.

Another big disadvantage in our opinion is the methodology itself of using loose clay. Loose bentonite clay in our opinion is a bad idea if you’re looking for a 100% water-tight seal. To fix this, there are certain companies out there that make manufactured bentonite liners. By manufactured, we mean there is often a layer of bentonite clay sandwiched between two layers of geotextile fabric. This helps ‘confine’ the benotinte somewhat so that it can’t migrate away easily, leaving a uniform layer over the excavation. However, for this to work properly, you really need to confine the liner. The way that bentonite works is that it expands until it fills a void. At that point, it forms a compressive seal and stops all water from flowing through. If the bentonite is allowed to expand indefinitely without something to confine and stop it, it will NOT form a compressive seal, and it won’t be 100% water tight. That is why we would recommend bringing in a good amount of soil or other material over top of a manufactured bentonite liner to provide some confining weight. Without this weight, benotnite liners don’t seem to work well.

Advantages and Disadvantages of Passive Liners

The nice things about passive liners, like our Premium Grade RPE liners is that they work right out of the box. Just unroll them and immediately you have a water-tight pond. The only real disadvantage of passive liners compared to bentonite is that they cannot self-heal if punctured. However, in practice we usually don’t find this a not a huge issue as you can easily heat weld patches onto RPE liner to repair any small punctures if an accident happens during installation.


In the beginning people can tend to believe that bentonite is cheaper if just looking at the cost of a truck load (or multiple truckloads) of raw benonite clay. However as we explained above, using the raw clay can be asking for trouble in our opinion. So in the end, most people will go with a manufactured bentonite clay pond liner, which is usually more expensive than a passive liner like RPE. So cost alone isn’t a reason to use bentonite in most cases.

In the end, you may already have guessed what we like to use to line a pond: RPE. It’s strong, light and we can make it in just about any size you need for your project. If you have any questions about liners, or would like to discus your particular project, please give us a call!

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