Sealing your pond: A quick introduction to pond sealing

Ponds tend to be idealized as a beautiful, relaxing place where nature and men coexist peacefully and quietly. However, if you own a pond, you know the dream can quickly turn into a nightmare.

One of the most common mistakes when digging a pond is selecting a less-than-ideal location for it, because the selected location has conditions that favor excessive water seepage.

All ponds seep and lose some water through evaporation, but when the water seepage turns excessive and the pond is unable to hold a good water level for long periods of time, the dreamy pond starts to fade to make way for a high-stress situation where you start to wonder if the pond is worth the effort.

You’re not alone. This happens very often and sometimes it obeys circumstances out of our control. Whether you purchased a property that already had a pond in it, or you selected the only place available in your property to hold a pond, sometimes it just wasn’t the right place for the pond to be built.

So, what can you do to help your pond retain water?

There are processes and additions that can help you seal the bottom of the pond to prevent water leakage. Selecting the right pond sealing process and materials will depend mostly on one thing: the composition of soil in the pond area, mainly on the ratio between coarse-grained material [sand/gravel] and fine-grained material [clay/silt].

Compaction
While compaction is probably the cheapest method to help a pond retain water, it will only be effective if the ground contains about the same amount of coarse and fine particles and at least 15% of clay. Here you will find a brief summary of the 5 most common pond sealing methods and

Clay Blanket
For soils that have enough coarse grains and contain some silt but lack clay, a clay blanket can be added to the area where the pond is located. This is only recommended if you can obtain the material for the blanket in a nearby location, so the transportation costs stay reasonable. Carefully consider if the source will provide enough material for a functional clay blanket: the minimum compacted thickness of the blanket should be 12 inches and that’s only if the pond is less than 10 ft deep (about 18 inches prior to compaction), so making sure you can retrieve enough material is essential.

Bentonite
When there’s not a close source of clay material near the pond, using sodium bentonite clay becomes the next best option. Bentonite clay is a natural colloidal clay that absorbs water and swells up to 20 times its original size. When it is mixed in the right proportion, compacted and then saturated with water, it fills up the pores, thus creating a watertight seal that will prevent water seepage. You can check out this Bentonite Pond Sealer Product Guide for further information.

Chemical additives
Chemical additives are indicated in soils with high clay content, where all the particles are fine-grained and tend to rearrange themselves in porous structures. These chemicals, called dispersing agents, are only effective if more than 50% of the soil’s composition is fine-grained and if there’s at least 15% of clay. They are absolutely ineffective in coarse-grained soils. Just to name the most commonly used dispersing agents are sodium polyphosphates, sodium chloride (common salt) and sodium carbonate (soda ash). A lab test will tell which will be most effective for your soil composition and the amount required. Here is a California State Ecosystem Services Document as a reference.

Waterproof linings
Propylene, vinyl and polyethylene have gained popularity for their ability to eliminate seepage when properly installed. Their biggest weakness is their vulnerability to tear and puncture. There are certain plants (especially grasses) that have the capacity to penetrate vinyl and polyethylene, a point to be taken into account when choosing a sealant for your pond. In some cases, using these materials can turn out to be more expensive than using any other method, something else to keep an eye out for. For more information, you can check this Pond Liners for Aquaculture Guide.

As you can see, selecting the right type of pond sealing material is not a decision to be taken by chance. It will respond to the conditions of the soil in the property, where the pond is located. If you are having trouble determining the type of soil and the right method for you, you can always approach the Natural Resources Conservation Service of the USDA for further guidance, or a pond specialty service near you.