Waterscapes 101

Bentonite Clay for Ponds

Granular Sodium Bentonite Clay, when used appropriately, can be one of the most effective pond soil sealers available.

There are two significantly different types of bentonite clay - Sodium Bentonite and Calcium Bentonite. Sodium bentonite clay is known as "high-swell" clay. Sodium Bentonite is preferred to Calcium Bentonite Clay for pond sealing applications. Calcium bentonite has limited swelling capability and is only effective for relatively short periods of time as a sealant when it is treated with other additives. However, the additives improve the swell of the Calcium Bentonite Clay for a limited time before they start to fail. Sodium Bentonite Clay has a naturally high swell capacity and does not require additives to maintain the swelling characteristics indefinitely.

Pond Clay Sodium Bentonite

Treating a Pond Bottom with Bentonite Clay

There are 2 effective methods of using bentonite for pond sealing

  1. The blanket method. The blanket method of bentonite clay application is simply placing a uniform layer of granular bentonite between 2 confining layers of soil. In new pond construction, the pond bottom is over-excavated by 6 to 8 inches. The bentonite clay is then spread in a uniform layer before the over-excavated soil is then placed back over the bentonite clay blanket. The top layer of soil is compacted with a pad-foot roller to ensure proper sealing. When using the blanket method for bentonite the pond side slopes should less than 33 degrees (3:1) or the bentonite material will slump toward the pond bottom. This is an important detail because as the blanket of bentonite becomes saturated it takes on a gel like consistency and the bentonite can very easily slide down the pond slope.
  2. The mixed method. The mixed method for bentonite clay application provides some advantages over the blanket method. Using the mixed method, bentonite is spread at the desired application rate and then mixed with the existing pond soil using an agricultural disc harrow, rototiller, or other mixing method to be sure that the granular bentonite clay is completely mixed with the top 6 to 10 inches of pond bottom soil. Once complete, the soil mixture must be thoroughly compacted with a pad-foot roller. Very dry soil will need to have water applied to the soil/bentonite mixture before rolling to ensure complete compaction. The mixed method for applying bentonite clay does allow application on steep slopes up to 45 degrees. It is typically best to apply the bentonite at a higher rate near the top of slopes since it will tend to work down the slope when it is tilled into the soil.

Bentonite For Pond Sealing Application Rates

We get many questions about the proper application rate for bentonite clay. The answer always is the same.... "It depends". The bentonite clay application rate depends on the soil type and local soil conditions as well as the level of compaction that can be achieved in the pond bottom. In some cases, when the pond bottom simply can not be dried due to springs and groundwater inflow, the application rate must be significantly higher to achieve pond sealing. Average bentonite application rates range between 2 pounds per square foot and 6 pounds per square foot. This means that the total amount of bentonite needed to seal most pond bottoms is measured per ton rather than in pounds. For loam soils with some clay content, 2 pounds per square foot may be adequate to seal the pond bottom. Sand and gravel pond bottoms will require 6 pounds per square foot or more of bentonite clay to seal the pond bottom.

Fair Clay2 lbs per square foot
Poor Clay3 lbs per square foot
Silty Loam3-4 lbs per square foot
Sand4-5 lbs per square foot
Sand and Gravel6 lbs per square foot
Gravel & Fractured Bedrock6-8 lbs per square foot

Test before you apply bentonite clay to your pond

We recommend that you test application rates with your particular soils by treating a small area at the anticipated bentonite clay application rate. Drive a 12" or larger diameter smooth wall pipe at least 8" into the soil column. Manually compact the soil-bentonite mix within the pipe and fill with 18" or more of water. Measure the rate of decline in the pipe and adjust application rate for your soils as needed. 45 lb pails of sodium bentonite are available for testing application rates or treating small areas.

Another simple bentonite sealing test can be completed using a 5 gallon bucket.

The bentonite bucket test is completed as follows:

  1. Drill 15 - 1/4" to 3/8" holes in the bottom of a 5 gallon bucket
  2. Place 6-8 inches of soil from the pond to be sealed in the bottom of the bucket
  3. From the application rate table, select the bentonite application rate for the soil that matches the pond soils
  4. Mix soil with bentonite at that rate in lbs/square foot
  5. Thoroughly blend the soil and dry granular bentonite until the mixture is uniform
  6. Add 3 cups of water to the soil bentonite mixture and mix again until moisture is evenly distributed
  7. Firmly tamp the soil/bentonite mixture into the bottom of the bucket
  8. Using a permanent marker or paint stick mark a fill line roughly 2 inches from the top of the bucket
  9. Gently (so that the soil isn't disturbed) add water to the fill line
  10. Monitor water loss over a minimum 48 hour period
  11. If seepage continues, repeat at an increased bentonite application rate until desired results are achieved
  12. Calculate the bentonite required for your pond area as follows. Multiply the number of pounds of bentonite used in the successful bucket test by 1.75 to determine the application rate in pounds per square foot. (If using our 5 gallon pail for the test. The area of the bottom of our standard 5 gallon bucket is 0.57 square feet) If using a different bucket, determine the bottom area by the radius of the bottom (in inches) times 3.14 and divide by 144 to convert the answer to square feet.

Where Can I Buy Bentonite Clay?

Bulk bags of bentonite clay are typically equipped with drawstring discharge bottom of the bag for easily controlled emptying. The bags are shipped on pallets but include integrated lifting loops on the top which allow the bag to be lifted with heavy equipment forks.

We are often asked Can bentonite be applied through the water?   The short answer is yes, but not very effectively.

Sodium bentonite clay is a high swell clay that can expand up to 30 times it's dry size when exposed to water. When it is applied directly through the water column, in most cases the clay simply expands into a thin, low-density gel material on the pond bottom with little effectiveness for pond sealing. If you know exactly where the leak is and you apply bentonite clay at a very high rate, there is a chance of sealing the leak.... but there are more effective approaches. If you need to attempt to seal the pond through the water, we recommend a two-part polymer pond sealant such as Soilfloc